2025-2026 MCAT General Biology, Chapter 2- Reproduction (Part 1, Cellular)

Okay, once again, hello everyone. We are

live. Let's get started. Um, thank you guys for being here. Really appreciate

everyone. We have a huge, very, very full room. Kind of intimidating, can't

full room. Kind of intimidating, can't lie. Um,

lie. Um, hope you guys are settled in. We're

going to have we have a pretty difficult lecture to give today. Um, this is notoriously one of my least favorite chapters to teach. Number one, because there's a lot of content. Number two, I struggled with it when I was a student.

And number three, it just kind of feels like me lecturing about something I don't truly know about. Um, and that'll become evident as we go on. This is

chapter two, general biology, reproduction.

Every large thing is made up out of what?

small parts. Every large thing, rocks, carpets people anything, every large thing is made up of small parts. And if you look back in

time, there was a point in time where religion, philosophy, science sort of all came together. And you ask the question like, what are we?

And from a reductive perspective, it seems easy to say that we're just human beings. But then you look inside human

beings. But then you look inside human beings when they died, right? Or when

they're alive and they got hurt and you see there's things inside of us. We're

not just like these empty cases of flesh that move around, right? So they're

organs and tissue beds and muscles and this and that and blah blah blah, whatever it is. And when we eat the things just disappear, right? Where does

it all go? What happens? So the question becomes like you are a human but you are also a brain and a heart and

skin and eyes and in the eyes are proteins and in the proteins are atoms and it's like how far does this go really? And in order for reproduction to

really? And in order for reproduction to occur, you can't just reproduce one thing and nothing else, right? Some

things get produced, some things get reproduced, and some things get constantly destroyed and constantly recycled. And this happens throughout

recycled. And this happens throughout the body, right? Let me clue you into something. Your heart,

something. Your heart, the cells in your heart, the cardiac myasytes, right? You okay?

myasytes, right? You okay?

Am I like >> your your cardiac myasytes, right? If

they die, let's say you had a heart attack, by the way, what's a heart attack? Does anyone know? Anyone know

attack? Does anyone know? Anyone know

how a heart how a heart attack works?

>> Yeah. But why?

Your heart, let's take the let's take the box model of the heart. Your heart

is split up into four chambers and they're arteries that run along here in order to give your heart the circulation that one wraps around behind. Right? So

they're sort of these arteries that come like this. And over time because of diet

like this. And over time because of diet and stress and smoking and this and that and blah blah blah you get a little bit of a blockage and that blockage can turn into complete blockage and then that will kill the oxygen supply downstream

from that artery which will choke out those cells lead to inflammation. Those

cells will die right and eventually they will scar over it. When you get cut, you scar over pretty well on your skin because your skin is capable of

reproducing skin cells. I have a very large cut on my leg right here uh from when I was running track and field. a

track spike went right through my leg um during a race, but it scarred over pretty well. I have a kloid scar there,

pretty well. I have a kloid scar there, right? It's like a hyper hypertrophic

right? It's like a hyper hypertrophic scar, right? But it did scar over and I

scar, right? But it did scar over and I still have mobility and I can still like sort of feel the part around my leg and things like that. But when your heart scars over, it doesn't really regain function at any point. Why is that? Is

because heart cells, and this is something that we didn't speak about before when we talked about cell types, these are permanent.

They're permanent cells. They don't

actually replicate, divide, grow in the classic way. The heart cells you have

classic way. The heart cells you have are the heart cells you get. You're

that's it, right? Same thing with what to a point 25 years old, right? What do

I have that I have for the rest of my life?

My brain. Neuronoplasticity

is the ability for neurons to actually generate connections between one another. But do neurons duplicate and

another. But do neurons duplicate and grow and regenerate? No. Well, I hope not because there's so such a limited defined region of space inside the skull

that if the neurons started dividing, you would actually start to outgrow the space that you have inside your skull.

That would lead to an increase in the incraanial pressure and that comes with a bunch of different problems, right?

But there are cells in the body and the majority of cells in the body I would be willing to say that are constantly dividing. And this division that makes

dividing. And this division that makes up the different tissue beds and organs and systems and blah blah blah whatever it is. This is what we know as cellular

it is. This is what we know as cellular reproduction.

And it is the cellular reproduction that we must talk about in order to look at the big picture, right? Because almost

picture, right? Because almost everything inside of anatomy, physiology, this that the other thing is taught from a perspective where it's like, well, here's the big picture and let's look small. The reason for that is

because they assume that you come in with the knowledge of what does the small look like, right? And that's why we started with the cell in the first place. And that's why the chapter before

place. And that's why the chapter before this was the cell, right? By the way, for the people watching online, this chapter gets very nittygritty into

endocrinology at the end when we talk about reproduction from the perspective of the menstrual cycle, which is going to be a completely different part of the video because I like to post that by itself because it's so heavy in content.

Um, I would heavily suggest that you watch my fundamentals of endocrinology lecture, which I will link in the description. And if you can't find it in

description. And if you can't find it in the description, please just go looking for it. It's should be right there. It

for it. It's should be right there. It

is imperative that you watch that in order to fully understand everything that's going on in this lecture as well as chapter five of biology, which is the endocrine system.

Okay.

In order to talk about cellular reproduction, we need to talk about the cell cycle.

Cell cycle and mitosis.

Correct.

This is a cell from my cheek.

A cheek cell, right? Scrape the inside of my cheek with a wand and then I put it under a microscope. Right? If I were to get a strong enough microscope to sort of look at the inside of this cell

and let's just say that it was in the process of replicating, right? Or at any point a cheek cell, right? I don't know what sort of cell we call this, what

subtype. This is a kind of weird

subtype. This is a kind of weird question, but what type of cell do we call this? We have two different types

call this? We have two different types of cells in the body which have two different numbers of chromosomes within them.

the sematic cell, right? As somatic

autotoal cell, correct? You'll find that there's

cell, correct? You'll find that there's a lot of like sort of double names inside of medicine and biology and things like that for whatever reason.

Bless you. Um, and these cells are diploid, meaning

diploloyid. Listen very closely. Of the

diploloyid. Listen very closely. Of the

necessary genetic material to code for the genome, it has two of the necessary genetic material to

code for the genome. It has two. We are

not talking about the stranding. We are

not talking about the way that it does that. We're not talking about what genes

that. We're not talking about what genes are turned on and turned off. Just

saying that for the necessary completion of the genome, it has two of those copies. Correct. So in humans, how many

copies. Correct. So in humans, how many chromosomes are needed to code for the genome?

23.

23 chromosomes are needed to code the entire human genome. Am I talking about single double stranded? No, I am not.

23 chromosomes And if you get one copy of 23 from your mom and one copy of 23 from your dad, that would make 46.

This is what we call the 2n number.

The 2n number is 46. Correct? Yes.

So when cells exist within the body and they're not dividing or they're not turning into germ cells which are sexual cells which pass on and recombine and

create offspring, right? They have how many chromosomes in them?

Louder.

>> No. How many? 46. They have two copies of each of the 23. I just told you somatic cells are diploid.

And if we need 23 in order to code for the genome and diploid means you have double the amount needed to code for the genome.

That means that you have 46.

Take a fly for example. Let's say that there's a fly out there who has two chromosomes in its entire genome. That

fly floats around with four chromosomes inside of it. One of pair one, one of pair two. I I like to do this sort of

pair two. I I like to do this sort of demonstration. It takes a while, but

demonstration. It takes a while, but let's say that you have chromosome 1 and chromosome 2. That's not a good way to

chromosome 2. That's not a good way to show it. Chromosome 1, chromosome 2, and

show it. Chromosome 1, chromosome 2, and chromosome 3 and chromosome 4 and chromosome 5, blah blah blah. All the

way down to 23. Correct? You have a pair of each one.

I have a pair of chromosome 1 and a pair of chromosome 2 and a pair of chromosome 3. One came from my mom, one came from

3. One came from my mom, one came from my dad. Does that make sense?

my dad. Does that make sense?

We need to lock this in in order to understand what's going to happen later.

Correct.

I'm going to leave that there.

Ukarotic cells replicate through the cell cycle.

Correct?

And this is bas basically a series of phases that the cell goes through.

where DNA is replicated, split and the cell divides.

You hear me?

Yes.

Yes. I need validation, guys. I didn't

get enough as a child. Come on.

Am I in frame?

There are much better diagrams of this in your textbook in the Kaplan textbook.

What smart person can tell me what M stands for here exactly?

There are two things you need to remember. The cell is either doing one

remember. The cell is either doing one of two things. It's either dividing or it's not dividing.

Mitosis is the act of cell division. So

everything that is not mitosis is not mitosis.

Very simple. We have a name for not mitosis. What is it?

mitosis. What is it?

Interphase. Very good. Interphase. So we

have interphase and we have mitosis. And

judging by the diagram that tells us that G1 S and G2 are collectively known as loud.

>> Come on guys, there's a lot of you in here.

>> Interface.

>> Good. Very good. Fantastic. Interphase

is the longest part of the cell cycle.

it goes through these motions. An

interphase when a cell is not preparing for so when a cell is preparing for division aside from when it's not doing that which we'll talk about later is the longest part of the cell cycle it prepares for mitosis and basically

that's all it does by the way for anyone who's new here you guys can get up leave this is a very very free environment I

don't care if you're eating in front of me just enjoy the lecture that's it all I'm here for I'm here for for comfort and accessibility. All right,

and accessibility. All right, cool. Interphase, like I said, is the

cool. Interphase, like I said, is the longest part of the cell cycle. Correct.

Cells that are not dividing will exit this cycle and go into what is G0? It is basically just

where the cell hangs out, right? People

sometimes when they're older, they're like, especially doctors, doctors do this a lot because they have the liberty to do so. They sort of just like pick up and pack up whenever they want. Um

hopefully if you have that sort of schedule, uh they're like, "Yeah, well, I just quit my last job." They're like, "Oh, what are you doing now?" It's like, "Nothing. Just chilling, living my life,

"Nothing. Just chilling, living my life, right?" G0 is the cell chilling and

right?" G0 is the cell chilling and living its life, right? Does all the cellular processes, does everything it has to do, does its job to maintain itself, right? But it's just not working

itself, right? But it's just not working towards dividing right now. When you're

in G1, S and G2, you're kind of working towards dividing. Correct?

towards dividing. Correct?

And the reason for that distinction is not random. It's because of how

not random. It's because of how important mitosis is, right? Because of

how important mitosis is. The entire

time leading up to my educational journey to medical school is leading up to what's happening this upcoming fall, applying to residency.

every single of one of the past 21 years whether I knew it or not was leading up to this fall. That's what it was all leading up to. Right?

Okay. So, it's almost like that.

So we can say that the combination of G1 S and G2 is interphase and within interphase

individual chromosomes for example let's look at the fly again with two chromosomes in its entire genome correct which means that it has a

pair pair of two chromosomes each.

Sorry, it has two pairs of chromosomes which makes it four. Right? These

individual chromosomes are what?

Not visible because they exist in a less condensed state.

They're not actually going to look like that. They're going to be this weird

that. They're going to be this weird tangled mess of DNA that you're not really going to see inside the cell.

Correct?

And the reason for that, who can sort of like deduce the reason that that happens? Why isn't the DNA as tightly

happens? Why isn't the DNA as tightly wound as it usually is?

What's what part of the cell cycle are we in right now?

Interphase. And during interphase, we're preparing for mitosis. And in order to prepare for mitosis, what do we have to do to the DNA? We have to replicate it.

You have to make it over again. Imagine

you're trying to transcribe a book, right? I made this distinction last

right? I made this distinction last time, and I think it's a distinction that uh doesn't go recognized by a lot of students. The difference between

of students. The difference between transcription and translation.

Anyone here ever had like a teacher for another language like at home where they would make you like copy lines of like different, you know, uh, like stories

and and poems and things like that and you just copy it. You look at it, you copy, you look at it, you copy it, right? You've done that before to train

right? You've done that before to train your hand in writing that, right?

Transcription is when you look at a language and you copy that same language.

But translation is when you go from this language to one that other people understand.

That's the difference between transcription translation. So in DNA

transcription translation. So in DNA synthesis, transcription is actually done in order transcribe the DNA and replicate it to get ready to divide

later because the thing is we want everyone to have the same amount of DNA, right? Let's say that I had a kid and I

right? Let's say that I had a kid and I had $100,000 saved up for this kid and I I was like, you know what? I'm gonna

make sure that each of my kids has $100,000. I have another kid. What do I

$100,000. I have another kid. What do I have to do with my money? I gotta double it, right? I gotta double my money. So,

it, right? I gotta double my money. So,

if you want every cell to have that same amount of DNA, you got to double your DNA, right? Okay. Now, here's the

DNA, right? Okay. Now, here's the question.

If I said 2 N= 46, meaning that the diploid number is 46.

We have 46 chromosomes in the body. When

that when that DNA replicates, how many chromosomes are we going to have?

Huh?

92 sounds like the right answer. Not

really. It's still going to be 46.

And we're going to talk about why, right? Remember in the beginning I said

right? Remember in the beginning I said I'm not talking about single or double stranded. I'm talking about the number

stranded. I'm talking about the number of chromosomes. Later on when we talk

of chromosomes. Later on when we talk about mitosis, we are going to talk about single and double stranded. So if

this trips you up, we'll clear the air.

Okay. All right.

Cool.

Everyone good on this? Any questions?

Don't be shy. Promise.

>> Yeah, less condensed. So, let's say that I held up a hundred strands of human hair like this from across a football field. Would you be able to see

football field. Would you be able to see them? Probably not. If I took them and I

them? Probably not. If I took them and I crumpled them up into a ball and I held them on my hand, you might be able to maybe because they're going to take up a lot of dense space. Correct? Something

like that. So, the more condensed the DNA is, the better you're actually going to see it through the microscope.

Correct. Okay, cool.

By the way, brothers, it's Thursday night.

We're going to dinner.

Any questions about the chapter, about life, about your deepest, darkest fears and secrets, anything.

G1, refer back to that diagram that I hope you drew or copied from the book, right?

And if you guys need a professional diagram to mark up, like to put on your iPads, I want you to look up cell cycle

Kaplan MCAT. That image will pop up.

Kaplan MCAT. That image will pop up.

Cell cycle Kaplan MCAT. Right.

Is that for me?

>> I I'll take it in a second. Yeah,

I I would really really love to, but we're in the middle. So, I will I will forego the pleasure of eating for all of you because you guys mean that much to me.

Listen, if you guys if someone means more than food to me in my life, y'all are tripping. You don't understand. I'm

are tripping. You don't understand. I'm

a big back.

Listen, when I say when I say we're going out to dinner every Thursday, y'all better not flake on me, man. And

if you do, I'm eating by myself.

All right.

This beginning of the chapter is a little bit boring because I know that like we're kind of split. Like half the people know this really really well as like cellular biology majors and you have done this like a hundred times. But

promise stick through. It gets better.

And the fact that we're doing this now is actually going to make the end of the chapter a lot easier. Right? The MCAT is a lot of patience and waiting. It's a

lot a lot a lot of watchful waiting. You

just look at yourself. You say, "Do I know it? Do I know it? Do I know it?"

know it? Do I know it? Do I know it?"

And when the hammer comes down, you do.

Right? Okay. G1 is known as the preynthetic gap.

The fact that is known as preynthetic clues you into something about the diagram. G1 comes before

diagram. G1 comes before S. So it's literally just the pres

S. So it's literally just the pres and the G actually refers to to

growth.

So this is preynthesis growth phase one.

G1 presynthetic gap, right?

This is where all of the bulking is done. This is where you're putting on

done. This is where you're putting on the pounds, right? The cell replicates mitochondria which have what type of replication?

What are the genetics of the mitochondria?

Has its own DNA? Has it own DNA given to you by the Mom.

Every single mitochondria in every single cell in this room can be traced back to one female ancestor. A single

one. The first female human ever.

Correct. Okay.

The mitochondria replicate. You also

have the ribosomes, the endopplasmic reticula, etc. All these important organs, right?

And because of all of this growth, you get an increase in size. This is the bulking right?

We have about an hour to unlock, right?

Roughly.

Okay, that should be good.

And the passage from G1 into S is actually gated.

You guys might remember my thing about the gated community. It's like, "Hey, listen. Like, I know where you live in

listen. Like, I know where you live in the suburbs, but you're just not on my level, right?"

That's what the S-phase is saying to G1.

It's like, "Yo, I know you put in the work, you got rich, you got your money, you made sure that everything was done right, but you're just, this is a different tax bracket right here, right?

There is a restriction point.

Are any of you guys married?

Anyone? You're married. Okay. When your

husband does something, do you double check what he does sometimes? 100%.

Right? And it's not that you don't trust them. He's probably done it a 100,000

them. He's probably done it a 100,000 times for her before just out of his love for her. Inshallah. May Allah bless them. Say a mean. Right? He's probably

them. Say a mean. Right? He's probably

done it a million times. He knows how to do it to the tea. But it's not that you don't trust them. It's that you know what can happen if you don't check.

You know what could go wrong, right? Oh,

patients labs were fine. We just checked four times in the past 24 hours. Check

again, again, again. Again, again, again,

again, again. Again, again, again, because if you get it wrong here, you're done for.

Right? I need you guys to really understand this point. It's critical,

absolutely critical that you understand the reason that a restriction point is there. A restriction point is there not

there. A restriction point is there not because the cell is inefficient, but because it has so many things to do and it executes them all so perfectly and there's so much ahead that if one thing

is out of place, then we're all done for.

Right? There's a process in Islamic juristprudence studies known as each.

It reflects a serious and concentrated study and focus. There is extreme amounts of diligent prep work that goes

into formulating a law.

For example, a lot of modern Muslim jurists are asked, "What is the ruling on investing in Bitcoin? Is it allowed?

Is this something that we have to pay charity on? Is this something that we

charity on? Is this something that we can put into our wills? Is this

something people can inherit from us? Is

it allowed in the first place? Right?

There is an immense amount of that goes behind this. A serious study. This is

behind this. A serious study. This is

the cell's the deep and serious study of what's happened. And you need to see if we can

happened. And you need to see if we can move forward from there. Right? And the

reason I'm making such a big point about this because we're going to see what happens when that thing goes wrong.

When this fails, horrible things. Okay.

horrible things. Okay.

Are you all with me?

Good. It's more of an answer that I like.

This restriction point is basically to ensure that the cell has the proper amount

type and accommodations of slash for the DNA.

You want to have a kid?

Do you have food at home? Do you have money? Can you afford diapers? Do you

money? Can you afford diapers? Do you

have somewhere to keep the kid? Do you

have good family support? Are you ready?

That's the question, right? You want to replicate. You want to make a new cell.

replicate. You want to make a new cell.

You have enough mitochondria, enough ribosomes, enough space, enough DNA. You

have all the genes, all the genes properly matched up. Is there anything wrong? Is this cell going to survive?

wrong? Is this cell going to survive?

And more importantly, is this cell that you're about to make going to kill us?

That's the question.

And who's us? Does the cell know that it's working as part of a whole body?

No, not always. Doesn't always know that fact, right? Through hormonal regulation

fact, right? Through hormonal regulation and through communication, it knows that there are other cells around and there are other things going on, but it doesn't always know the fact that it's part of something bigger, right?

S-phase synthesis.

So remember how I said that the gphase

the DNA is open for transcription.

Did I say replication?

No, I did not. There's a difference between why the DNA is open in the G1 phase and why it's open in the Sphase.

In the G1 phase, it's open to code for the genes and the proteins that are going to make all of those materials that you're prepping. In the Sphase, it's now open to be transcribed and

replicated.

This is the DNA synthesis phase.

Correct.

This is where the cell replicates genetic material.

Make sense?

And this is so that therefore each daughter cell contains

what?

Identical DNA.

You guys know how whenever there's someone who scores really well in the MCAT, everyone's just like, "What was your study schedule? What did you do?

How'd you do it? Let me do what you do."

So when this cell that's lived its whole life and never had anything go wrong is about to divide, it's giving up exactly

what it had because they said it worked for me. It's going to work for you.

for me. It's going to work for you.

Identical same thing, same circumstance, same DNA, same looks, same habits, same environment, same everything. All the

same. Correct? Why wouldn't it work?

Correct. Right.

I'm going to introduce you guys to a new set of abbreviations I use all the time.

I use them everywhere.

The A with the arrow and the P with the arrow.

This means a and this means post.

A means before and post means after.

So the P with the arrow means after correct. Okay. So after replication

correct. Okay. So after replication each chromosome how many chromosomes human?

46 consists of two blank blank bound

by a blank.

Two sister chromatids bound by a centromeir.

Centro mirror.

You have a chromosome top and bottom. a chromosome

that in the G1 phase was open and replicating and doing whatever it had to do. When it enters the Sphase, let's say

do. When it enters the Sphase, let's say the Sphase occurs, it happens to it.

This G1 chromosome that was singlestranded now looks like this.

And it's not that those are crossing over the centromeir. They're pinched

together, right? So, this is one side and this is

right? So, this is one side and this is the other. Can you guys see that? You're

the other. Can you guys see that? You're

taking this, you're taking, you're pinching them together. So, this is one of them and this is the other one.

They're pinched at the center. Correct?

And they're held together like glue with the centromeir.

Does that make sense? Are you guys with me? Are you following? Because it's

me? Are you following? Because it's

about to get very complicated.

Sphase happens.

>> Sphase happens. You guys all right?

Good.

So, these are your sister chromatids.

And I want to include these right here because I don't want you to get confused. Sister chromatids.

confused. Sister chromatids.

Correct?

Yes. Good.

Copy that down. It'll take a second.

We'll move on.

This is very slow, very boring stuff. I

know you guys probably aren't having fun right now. You will. In just a few short

right now. You will. In just a few short minutes, you'll be having a lot of fun.

Too much fun even.

Yes, Sam. How are we doing?

>> You good? Okay, perfect. Can I erase this?

>> Yes.

>> Yes. So, G1, let's just say that G1 happened, the Sphase happened, and then that got spat out. G1 happened,

Sphase happened, now that got spat out.

And not even that you would actually see it in G1. It's not going to look like that in G1. I'm just showing you that it's single stranded.

Does anyone remember the name of the proteins that DNA wraps around in order to tighten up?

>> Histones, which I previously mistakenly said contain histadine, but they actually contain the other two basic amino acids, lysine and

>> arginine. Very good. You guys are smart.

>> arginine. Very good. You guys are smart.

smarter than I was.

Can I erase this?

So, now we have sister chromatids.

Now what?

I'm going to tell you guys something that I'm going to say once and I will take no questions about it because when people started asking questions, then I started to mess up.

I'm going to say this one time and for the people it clicks with, it clicks with. And for the people it doesn't it

with. And for the people it doesn't it doesn't.

In the G1 phase we have 46 chromosomes that are single stranded.

In the after the Sphase after the Sphase we have 46 chromosomes that are double stranded.

That means individual DNA chromatids we have 92 individual individual

and I got into a heated argument with a student because I once and I stopped doing this but I once called this

4N and I stand by that. I stand by myself calling that that 4N because if N is 23

single stranded and 2 N is 46 uh single stranded then 4N would be 46 double stranded you're doubling it

talking about the number of specific DNA chromatids 92 from 46 doublestranded.

Does that make sense? Okay. I'm not

going to answer questions about that. If

it clicked for you, it clicked for you.

If it didn't, it didn't. It's okay. It's

not an important point.

Welcome. How are you guys doing? Okay.

doing? Okay.

From S to G2, we have another restriction point. Do you think that

restriction point. Do you think that this restriction point is easier or harder to get through than the last one?

Why? Why harder?

>> Much more difficult to get through than the last one. Every single time you take an exam, right, you take the MCAT, take step one, you take step two, you take all the shelf exams, you do this, you do

that. As you get closer and closer and

that. As you get closer and closer and closer, the exams become more critical.

They're twisting the knife deeper into your head being, "No, answer. Answer.

Answer. This is the question. answer.

Now that you've replicated the DNA, anything could have happened there.

Anything could have happened there. When

you replicated the DNA, you could have swapped a C for an A. And now we're all screwed. Everyone's screwed. There is a

screwed. Everyone's screwed. There is a scrutinizing, incredibly difficult position to get through right there.

That is much more difficult than the last. As difficult as the last one was,

last. As difficult as the last one was, this one's harder. Correct.

So the G2 stage occurs after the and we call it SG2

checkpoint.

Correct. Yes.

How am I doing? Good.

I mean that seriously, by the way.

occurs after the SG2 checkpoint. So the DNA has been duplicated.

Have you guys ever noticed how I write the word 'the'?

It's like one big stroke.

That to me that says 'the'.

My mom saw me do that one time. She's

like she's like reading something I wrote in philosophy and I wrote the word 'the' and I tell yeah the you know there's your T, your H, and your E.

The My friends describe my handwriting as a mix of script and garbage.

And I say it's almost as though you took a spider, dipped its legs in ink, and let it run over the paper.

I hope you guys tell me when I should rewrite or reexlain something.

Okay, I am not a perfect individual. I

might be very close, but I'm not perfect.

Okay.

And the cell here, the cell has a job.

It's going to check the house.

Like I said, when you're about to have a child, right, you're going to check the house. You're going to check the

house. You're going to check the heating, check the walls, check to see if there's lead in the paint, check the room, check the money, check the bank account, check the food, check the this, check the that. Make sure that people are on standby in case you need them,

right? Checking everything. Organels,

right? Checking everything. Organels,

DNA space this that membranes vesicles, every everything. You're

checking everything. You're doing your whole mental checklist. Anyone here ever moved houses before? Not like for college, but like actually moved homes.

It's hard. It's really difficult. It's

really difficult. And as a human being, you're always going to forget something.

Always. Every single time. What did I forget back in New York when I came to Dallas? Forgot my running shoes. Had to

Dallas? Forgot my running shoes. Had to

buy a new $200 pair of running shoes.

Annoying, right? But the cell doesn't forget.

right? But the cell doesn't forget.

Definitely not.

purposeless, not without purpose.

>> I want you to think that the cell is always capable of making mistakes. It

just doesn't really happen all too frequently. And what I mean by all too

frequently. And what I mean by all too frequently, I mean one in a billion trillion times.

it'll happen.

And then when it does make a mistake, it's caught by something else.

Finally, after the G2 phase, you've checked the house, everything's ready, all the synthesis done, restriction points have been passed, this, this, that. What do you think it's time for?

that. What do you think it's time for?

It's time to party.

I have a habit. I don't know why. It's

just like a sort of little ritual of mine. I never erase the title of the

mine. I never erase the title of the chapter unless I need the whole board, right? In which case, you will see me

right? In which case, you will see me dramatically wipe off the title of the chapter. Very dramatic. I love I love

chapter. Very dramatic. I love I love the drama. I'm with all the drama. If

the drama. I'm with all the drama. If

you guys like being dramatic, do it.

Mitosis.

This is the Mphase.

You guys are always laughing about something.

It's a very good question.

Question was if the cell is constantly replicating the DNA in the Sphase and attaches them by the centromeir which is a protein or something that cell needs to make. Does

that also mean that while replicating it is creating proteins and doing this and doing that? If the cell let's let's take

doing that? If the cell let's let's take this for a ride. If the cell wasn't producing proteins at the time, what would happen? All the cell function

would happen? All the cell function would actually halt. You're still having transcription factors come in, hormones, signaling, this that the other thing.

The cell can't stop for anything. So

even though there is DNA synthesis going on, there is still DNA activity. That

activity is producing a lot of changes at the cellular level that are helping to maintain the cellular and physiological homeostasis. So yes, both

physiological homeostasis. So yes, both things are happening. To the best of my knowledge, mitosis is split up into four phases.

We have prophase, metaphase, anaphase and telophase or tilophase.

Would you guys like me to speed up?

Yes. A general consensus of yes. If one

person says no, I'm not speeding up. Say

no or just shake your head no if you don't want me to.

I'm not going to out you. Okay,

I got the message I needed. Prophase is

the condensation of genetic material.

And this is where you have the visibility of the chromosomes.

So, you actually won't be able to see them until you get to the point where they're just about to divide. You won't

even be able to see them until you get right here.

And this is also where you have centrialsles.

Anyone remember what centrialsles are?

They're those little protein things that move to the opposite poles of the cell while things are going on and they're going to sort of throw those fibers to pull the chromosomes apart later, right?

Okay. So, the centrialsles form and they start to move away, right?

In metaphase chromosomes and sometimes I just like to remind people that chromosomes look like this right now.

Right? Chromosomes

line up at the center of the cell.

Does anyone know what that's called? I

know this is called an amperand.

What's that called? Is it just an at sign?

I don't know. If there's is there there's a name for that, someone find out and tell me.

Not right now.

And this center of the cell actually has a name. It's called the

a name. It's called the what?

It's named after the part of the cell cycle that it's in. Part of mitosis called the metaphase plate.

Metaphase plate.

The metaphase plate also known as the equatorial plate because it's going to split the cell in half.

Audio video. We're okay.

Yes. Perfect.

And the centrialsles that we formed before now are at opposite poles of the cell.

Can I draw over here?

>> Okay.

right here. Okay,

proase we can physically see the chromosomes and we have these little things called centrialsles which are later going to play a very big role

that are starting to move to the opposite sides of the cell while the chromosomes move to the middle.

This is prophase metaphase.

metaphase right?

The chromosomes now line up in the middle of the cell at the equator, the equatorial or metaphase plate

and these centrialsles are at the edge of the cell.

You guys all know how this story ends, right?

We're going over it because we're actually going to talk about this next time as well. Yes.

Little louder.

Yes.

Do you remember how in the cell there was a nucleus?

We said goodbye to the nucleus and the chromosomes are exposed.

That's one thing I forgot to mention before prophase in prepar forgot to mention that because that is something the MCAT loves to harp on that

the nuclear envelope has to disappear in order for mitosis to take place. Please

write that down. That the nuclear envelope must disappear before mitosis takes place.

I saw your brother yesterday. I was

looking for him after the talk, but I couldn't find him. I was I was going to tell him that we met.

in anaphase.

These sister chromatids separate.

How does that happen?

These centrialsles throw things called.

You guys know it. You guys heard a hund times loud.

>> Spindle fibers.

Uh, I like to draw it the proper way because actually shows the animation of it going away and pulled that way.

Pulled this way.

Boom. And boom. Opposite ends of the cell.

Correct? And just for completion sake, Anyone here from Boston or ever lived in Boston? Ever? Yeah. My sister left to

Boston? Ever? Yeah. My sister left to Boston at the time that I was studying like ninth grade biology. So I used to just send her this picture. I was like, "Oh my god, it's you and me." We're like

going opposite ways.

My sister lived in Boston for 11 years.

She loves that place. I love that city, too. Boston's a great city. Boston's a

too. Boston's a great city. Boston's a

really nice city.

Chicago, Boston, New York City, Dallas, San Diego. Great cities.

San Diego. Great cities.

Anyone here from like a weird city? Like

nobody's ever heard of this place before, huh? Where you from?

huh? Where you from?

Never heard of it. I did one time pray on a local like sort of highway roadish area in Arkansas in the middle of the

night because it was like fudger time was running out and it was super cloudy and we were driving down this highway back from Dallas to New York. Me and my friends were driving back to New York the first time I visited Dallas. And

there's a photo of me uh with my hands wrapped in prayer and you can just see like the hill underneath me with like all the fog while I'm praying cuz we took turns praying because we were just like surrounded by arsonians and we had

no idea whether we were going to get shot or not for being in the middle of the road praying. Uh, and the thing is like we were just coming back from Dallas, so we were like dopeed up. Got

I've got like my Dean over Duna hoodie on. Like I was a prime suspect at the

on. Like I was a prime suspect at the moment.

All right. And that's basically the only thing that's important about anaphase.

Those spindle fibers get thrown. Like I

said, these are called spindle fibers that come from the centrialsles.

Right?

You have these spindle fibers that come from the centrialsles. Correct.

Anyone know what spindle fibers are made of?

Microtubules. Yes. That's also an MCAT fact that these spindle fibers are actually made of microtubules.

And another MCAT fact, what are microtubules made of? Tubulin protein.

Very good. So the spindle fibers in the centrialsles are made of microtubules which are made of tubulin.

Right? And this is why I introduced the idea of what did we come from in the beginning. You break it down and break

beginning. You break it down and break it down, break it down, break it down.

And after tubulant, you have to get into the amino acid chain. Then you have to get into the DNA and blah blah. And you

that goes in a cycle, right? Okay.

And finally for telophase the spindle apparatus disappears.

The nuclear membrane reforms. The chromosomes uncoil.

Yes, I like the nods of approval.

These guy I'm just going to put a little bit of haze on them.

Chromosomes start to sort of disappear, right?

The membrane starts to involute inwards.

Correct?

You have a sort of pattern, huh?

>> Uncoil script plus garbage, right? You have a little sort of thing going down the middle that I'm going to ask you to name in a second. And the nuclear envelope is

beginning to return. And you actually no longer see that spindle apparatus that we spoke about before.

Take a picture if you need this for future reference.

I'm sure you don't, but shut up.

Does anyone need to take a picture of that? I feel like we know this pretty

that? I feel like we know this pretty well. Okay, cool. What is that called in

well. Okay, cool. What is that called in the middle?

It's called cleavage furrow. That's

where the cell is going to split from, right? The cleavage of the cell happens

right? The cleavage of the cell happens from there. This process of the cleavage

from there. This process of the cleavage of the cell and into two different cells is known as cytochinesis.

Cytoinesis. Correct?

And this makes two cells which are both 2N single stranded which is exactly what

we started with.

We have successfully replicated, moved, split and divided the genetic material inside the cell. Correct? And this is all within what phase of the cell cycle?

The Mphase.

The Mphase. So G1, G2S was just to get G1 SG2 was just to get ready for this.

That was it.

Understood? Good.

If you guys understand this, then knowing meiosis actually won't be that difficult because now I feel like in previous years I spent a

long time going over mitos meiosis when I could have just said one or two things and it would have been clear.

There are rules to meiosis.

Number one, it starts at the same place.

Starts at the same place. Now we are talking about meiosis.

What is pause reel it back. What is

meiosis?

Meiosis is the production of hloid cells.

If diploid is 46, hloid is 23.

enough to code the genome.

I want you to remember that very clearly in your head. Enough to code the genome.

The whole genome is coded inside that cell. Can the cell grow by itself?

cell. Can the cell grow by itself?

No.

This cell cannot turn into an organism by itself.

For reasons you will learn in embryology specifically, you need two copies of the chromosomes in order to sustain life.

What happens if you have three copies of all the chromosomes?

That is something known as a molar pregnancy. Very scary stuff. You can

pregnancy. Very scary stuff. You can

look it up. There are partial moles and complete moles and blah blah blah and this and that. It's creepy.

a hloid cell 23 single stranded right I'll make the distinction single stranded right

why would we want to make hloid cells so that when you combine two hloid cells you get a diploid cell and that diploid cell can undergo undergo what

mitosis not meiosis That's diploid cell can undergo mitosis and divide and divide and divide and divide and turn into a

into a organism turns into an embryo and a fetus and a baby and an organism and an adult. We came from one cell all of

an adult. We came from one cell all of us. Everyone in the room, right? Unless

us. Everyone in the room, right? Unless

one of you is a robot. I would not be surprised.

suspicious, right? Okay. The production of hloid

right? Okay. The production of hloid cells. So, let's let's take a look at

cells. So, let's let's take a look at this for one moment with me. I know I know it's so boring, but I need to drill these concepts into your head early on

so you don't get lost later. Right? Look

at this. Let's go back to our example.

The two chromosomes, that's the diploid number, right? The hoid number would be

number, right? The hoid number would be one.

Hloid number would be one. And if you were to combine these two, you would get a cell that is capable of sustaining life.

The hloid number is how much you need to code the genome. The diploid number is how much you need to sustain life.

That's a very important distinction.

You guys feel me?

Hopefully not literally. That'd be

weird.

Unless you're that guy or his brother.

By the way, I have to have dinner with the two of you soon.

Meiosis is split into two parts, correct? Because we have the same

correct? Because we have the same starting point, right? So let's take a look at our two diploid chromosome. Correct. Starts at

diploid chromosome. Correct. Starts at

the same place.

Remember how it looked like this?

Yes.

How can you get that to look like this in one go?

You can't. You can't do one divisive move and make that look like that. Has

to be two. Either you split these apart and then split the sisters apart or you split the sisters apart and then split those apart. Correct? Which one do we

those apart. Correct? Which one do we do?

Let me define these for you. These two

chromosomes, I'm color blind.

Okay.

My 23rd pair of chromosomes has a defect.

I have an X chromosome and a Y chromosome. My Y chromosome has a

chromosome. My Y chromosome has a defect. Right?

defect. Right?

Maybe I'm not the best example of this.

Let's assume that these two were quote unquote the same. What would you call these two

same. What would you call these two chromosomes with respect to one another?

They're part of the same pair, which means they are homologous. Let's take a look at my 13th pair of chromosomes.

One of them looks like that and one of them looks like this. These two are considered I just said it homologous chromosomes.

Now take my 13th and divide it after Sphase in preparation for mitosis. These

two are considered still homologous chromosomes. The number

doesn't matter on either of them. They

code for the same things on the same pair with the same genes from different parents. Those are homologous

parents. Those are homologous chromosomes. So my question to you is

chromosomes. So my question to you is there's two steps of meiosis. Like I

said question, do we separate the homologous chromosomes first or do we separate the sisters first?

Homologous chromosomes. Separating

siblings is difficult.

Separating siblings is difficult. Right.

Okay. So, we're going to look into that as we continue to do this. And that is a fact. If you didn't know that about me,

fact. If you didn't know that about me, I am color blind.

I will explain the genetics of how that works at the end of the lecture because it's important. You have to know it. It

it's important. You have to know it. It

has a very specific inheritance pattern that you must know.

>> Does anyone know who had to be color blind for me to be color blind?

>> Is my mom color blind?

>> No. My mom's dad is color blind.

Figure that one out in your head.

My mom's dad is color blind, so I'm color blind.

But neither of my sisters are because who is not color blind?

My dad.

Starts at the same place. Number two,

it's two events.

back to back.

Number three, guys, creates hloid cells for humans. That's 23.

for humans. That's 23.

And number four, we dance.

We do a dance in the middle of meiosis.

Got to go.

I need to tell you guys about the dance.

I will demonstrate the dance. No, I will not. No way. No shot. All right. The

not. No way. No shot. All right. The

dance is called what? Who knows what I'm referring to?

What's the dance?

Crossing over. Crossing over is the dance.

Can I erase this? Okay.

The stage of meiosis that you're in is denoted by a number.

And the stages actually have the same steps as mitosis. Correct? So there is meiosis one and meiosis 2.

I always thought meiosis was spelled kind of weird.

Would have thought it was meiosis, but that means something else in medicine, right? What does myo mean? My yo.

right? What does myo mean? My yo.

muscle. Good.

What does myioitis mean?

Inflammation of the muscle. Right. Good.

Infection was a good guess. Who said

that?

Muscle infections are nasty, disgusting things. Is if is anyone not

disgusting things. Is if is anyone not queasy and want to see like some Is anyone like looking up like really gross medical stuff? I want you to look up

medical stuff? I want you to look up for gang green.

Forier gang green.

Make sure you turn off your safe search.

Meiosis one starts with prophase one.

Shouldn't have said that right now.

>> Yeah.

>> Oh, that's terrible.

>> That's like a running joke.

>> That's awful.

If you guys need to wash up for prayer, I suggest you do it now. We're going to stop in just a little bit. And also,

this part of the lecture gets a little bit repetitive. It's just a little bit

bit repetitive. It's just a little bit of a difference from whatever it is.

Actually, after I'm done with crossing over, then go. Prophase 1 is the same thing as prophase and mitosis except

mitosis proase plus crossing over.

What was what was mitosis prophase? The

nuclear envelope disappears. The

chromosomes become visible. These move

to different sides. The centrialsles,

right? Everything all of that happens.

Crossing over occurs. Guys, this is the part of the lecture that I really need you to dial in. This is when things starts getting interesting because this

specific phenomenon describes all of genetic variation amongst human beings that isn't described by spontaneous mutation. Right?

mutation. Right?

The reason that I don't look like a carbon copy or mix of half my parents and half this half that is because of crossing over, right?

And it is a point at which problems can happen. And if problems can happen, the

happen. And if problems can happen, the USMLE will ask you about it. That's the

important part here.

I have a cell.

Correct.

I have a cell prophase. This is the prophase of

prophase. This is the prophase of mitosis.

Right?

As these cells begin to move towards the as the chromosomes begin to move towards the center. Correct?

the center. Correct?

You guys know how those are homologous.

They're the same pair, part of the same thing. One's from mom, one's from dad,

thing. One's from mom, one's from dad, right? My mom has deep dark black eyes.

right? My mom has deep dark black eyes.

My dad has very light brown eyes, right?

There's not going to be any other color.

I'm Bengali, right? I know one Bengali guy with green eyes. Handsome man. Very

handsome man. Right?

Let's zoom into these for a second.

Right?

I want you to consider them like blocks.

I'm going to shade in this domain.

And on this one, I'm going to shade in everything except that domain.

On that domain, I'm going to put some dots, not ribosomes.

Crossing over is the phenomenon by which these homologous chromosomes kiss.

Mom, dad, the homologous chromosomes kiss. And what do they do?

kiss. And what do they do?

They swap a tiny bit of complimementary genetic material.

Meaning that something like that takes place.

You guys see what happened?

You guys see that? Right. Well,

actually, what I'm going to do, I'm going to I think it looks better if I erase this, right? Because now you can see that it's different, right?

You see how that happened? Why did I say that this impacts all of the genetic variation?

Because what's going to happen to each of these? This is one, two, three, four

of these? This is one, two, three, four different things inside of one cell. All

of those are going to go four different ways, right? Let's say I happen to pick

ways, right? Let's say I happen to pick up number two. Guys, lock into this. If

I pick up number two before crossing over, I get straight mom, right? That's

it. All everything that's in my mom I get. But because of crossing over, I get

get. But because of crossing over, I get 80% of my mom and just 20% of my dad.

And then when I grow up and I have a child, right?

Yes.

That's one of my chromosomes. But

there's another one of those two, right?

Inside of me. This is happening inside of one person, one of my parents, from his mom and his dad. Let's see the same happens in my mom, right? From her mom and her dad. And those two things mix.

Now I have four people's DNA inside of me. And this same thing happens again in

me. And this same thing happens again in me. So what was 80% my mom's mom and 20%

me. So what was 80% my mom's mom and 20% my mom's dad now becomes 80% my mom and 80% 20% my dad. And it keeps going and going and going and going and going. And

that's how genetic variation happens.

You guys see this? This happens in my parents, then it happens in me, then it happens in my kid, then it happens in their kids. And you get a mix of

their kids. And you get a mix of thousands of different people of genetic material in one single cell. That's how

it happens. Do you guys kind of see it?

It's like building blocks, right?

You guys get that?

If you don't get it, there are beautiful animations online that do a very good job of showing you how crossing over works. One thing that I did to s sort of

works. One thing that I did to s sort of show it is homologous chromosomes crossing over.

homologous chromosomes crossing over, but this one came from there and this one came from here.

Meaning that every single time you swap over, it's coming from a different place. Does that make sense? Because

place. Does that make sense? Because

once you swap, it's not like you're the same chromosome anymore. You're not 100% from one

anymore. You're not 100% from one person. Does that make sense? It's a

person. Does that make sense? It's a

little bit difficult to wrap your head around if you don't see it. If you don't physically see it in front of you. Might

do a demonstration. I might like make something to do a demonstration with.

Okay. The crossing over happens in prophase 1. Yes. In prophase 1.

prophase 1. Yes. In prophase 1.

After that it is business as usual.

Metaphase one is the same. Anaphase

one is the same, but the homologous chromosomes are pulled apart.

The homologous chromosomes are pulled apart.

Yes.

So, what does that look like?

Metaphase.

Anaphase spindles are pulling them opposite directions.

Yes, understood. Does anyone have questions?

Did everyone understand crossing over?

Please tell me if you didn't. I'm

begging you.

>> Okay. After this we will once these split you will get two cells.

One of which looks like this and one of which looks like this.

Does that look familiar guys?

That is the beginning of basically mitosis. But instead of having

basically mitosis. But instead of having a pair of every single one, you just have one.

So this is a diploid cell with 46 doublestranded chromosomes.

This is a hloid cell with 23 doublestranded chromosomes. But the

doublestranded chromosomes. But the chromatid number here is 92 and here is still 46.

Do you guys see that? Do you see the distinction I made earlier? Did it click with some of you? Yes. Just nod your heads. Fine if you didn't. Okay, good.

heads. Fine if you didn't. Okay, good.

You still have 46 chromatids there. Why

am I making that distinction? Because

I'm telling you that even though you have 23, you're capable of making two cells with the entire genome from each

of them. That's what I'm saying.

of them. That's what I'm saying.

That from here 92, if I tell you that the whole genome is 40 is uh 23, you get it. 92, I can make four of the whole

it. 92, I can make four of the whole genome. 23 46 I can make two of the

genome. 23 46 I can make two of the whole genome because all you need for the genome is a single strand.

And that's exactly what happens in meiosis 2.

And the reason I never explain meiosis 2 is because it's just what?

It's just mitosis.

It's mitosis with one single chromosome.

Take mitosis, copy it over, replicate for one chromosome. That's it.

Do you guys get that?

>> Huh?

Meaning that specific chromatids, I have 46 double stranded chromosomes.

That means I have 92 chromatids. And

since in meiosis, every single cell ends up with a single chromatid of each chromosome. I'm showing you that

chromosome. I'm showing you that specific thing.

You look like you were crying. I'm so

scared.

>> She like looked up and I just saw like tears in her eyes. I was like, "Oh my god, this lecture really sucks." Guys, um, I'm going to reexplain crossing over real quick. I'm going to erase this.

real quick. I'm going to erase this.

We're going to reexlain crossing over in a more detailed fashion. Now would be a good time to watch out for prayer if you understood that whole discussion.

Okay, everyone's good to erase this. Yeah,

sorry I didn't ask before.

Oh no, I erased the title. That's okay.

I didn't want to do that. That was

unintentional. That was not for the trauma.

When I was born, it took an intense level of precision between two people.

Let's say that humans in general have a single chromosome.

one. Take the 23, put them into one.

Inside my mother, there are two pairs.

There's one pair of that single chromosome.

This chromosome she got from her mother.

This chromosome she got from her father.

The chromosome she got from her mother is completely black and the chromosome she got from her father is completely white. Correct?

During prophase 1 of meiosis, there is a crossing over event that leaves the chromosomes looking like this.

Yes.

that is inside my mother.

And when these split, the homologous chromosomes will split leaving this.

Yes. And those sister chromatids will split, leaving the egg cell that created me. Let's say that this is the one with

me. Let's say that this is the one with my name on it. This egg cell now contains a single

version of that chromosome that is a mix between my mother's mother and my mother's father.

The same thing happens inside my dad.

In my dad, he has cells that are a mix of his mother and his father.

Same thing happens crossing over in meiosis before anything. This makes

what?

And just to show you that different parts of it can cross over. Oops. Sorry.

Still stuck in my old ways.

I'm just going to follow this one. You

understand? The same thing happened over there. This splits

there. This splits mix between his mom and his dad. This

also has my name on it.

So when these two combine, you now get a cell that formed me. And

inside of my pair of chromosomes, I have a little bit from my paternal grandfather. Sorry, my maternal

grandfather. Sorry, my maternal grandmother, my maternal grandfather, and both my par mo both my dad's parents

from him.

Yes. But now when I have children, the same thing's going to happen with these. So now look,

these. So now look, it's no longer as simple as one style on one chromosome.

So now when you do the crossing over between two pairs of those, right, you'll get something that looks a little bit like this.

And four people are packed into a single chromosome that goes onto my child. And

whoever I have a child with, the same thing happens with four completely different people.

But from those four people came the four people before them and the four people before them and the four people before them and before them and before them.

And this is why every single individual in any room ever is a genetic anomaly of billions of people.

Does that make sense? Does that clarify any confusion about crossing over? You

guys see how it works? Very good.

We will take a break and we will cover the reproductive system and the menstrual cycle. Thank you guys so much

menstrual cycle. Thank you guys so much for watching. Please support the

for watching. Please support the channel. Link in the description below.

channel. Link in the description below.

50% of all money made off this channel actually goes towards Doctors Without Borders, right? And Zade, can you please

Borders, right? And Zade, can you please turn off the video for me?