Lecture 2.5: The Cell and How it Works — Cell Division

Summary

This educational script delves into the fascinating world of cell division, highlighting the two main processes: mitosis and meiosis. It explains how mitosis in body cells results in identical daughter cells, preserving cellular characteristics, while meiosis in germ cells produces genetically unique haploid cells, crucial for sexual reproduction. The script introduces key terms like chromosomes, diploid, and haploid states, and describes the steps and significance of each division process, emphasizing the importance of these mechanisms in maintaining and diversifying life.

Takeaways

• 🧬 Cells possess the unique ability to divide and replicate themselves, which is a fundamental aspect of life not shared by inanimate objects.
• 🔁 The cell division cycle consists of two main parts: DNA replication and DNA partitioning or segregation.
• 🧬📚 DNA replication results in two sets of genes, doubling the genetic material in preparation for cell division.
• 🧬➡️ DNA partitioning involves segregating the replicated DNA between the two daughter cells, ensuring each receives a complete set.
• 🧬🧬 Genes, made of DNA, are organized into structures called chromosomes.
• 🌟🧬 In human cells, each chromosome has a matching pair, resulting in a diploid state (2n), where 'n' represents the number of chromosomes.
• 🌟🧬🔄 The matching chromosomes are called homologs or homologous chromosomes, which are identical or similar to one another.
• 🌟🧬🚫 In contrast, germ cells (eggs and sperm) contain only one of each chromosome, known as the haploid state (n).
• 🔄 There are two types of cell division: mitosis and meiosis, each with distinct processes and outcomes.
• 🔄🤾‍♂️ Mitosis occurs in body cells and results in two identical daughter cells, both being diploid, maintaining the genetic continuity.
• 🔄🤾‍♀️ Meiosis leads to the production of germ cells (egg and sperm) and results in four non-identical haploid cells, contributing to genetic diversity.
• 🔄🤾‍♂️🤾‍♀️ During meiosis, chromosomes pair up and may exchange DNA segments, a process known as recombination, which adds to genetic variation.

Q & A

What is the fundamental process that distinguishes living cells from inanimate objects?

The fundamental process is cell division, which allows cells to make more of themselves, a characteristic not found in inanimate objects.

What are the two main parts of the cell division cycle?

The two main parts of the cell division cycle are DNA replication, where the cell duplicates its DNA, and DNA partitioning or segregation, where the DNA is divided between daughter cells.

What is the term used to describe the organization of genes within the nucleus?

The genes are organized in large units called chromosomes.

What is the abbreviation used for 'chromosomes' in the script?

The abbreviation used for 'chromosomes' in the script is 'chr'.

What is the term for cells that contain two of each chromosome?

Cells that contain two of each chromosome are called diploid.

What is the term used to describe the matching chromosomes in a diploid cell?

The matching chromosomes in a diploid cell are called homologs or homologous chromosomes.

What is the term for cells that contain only one of each chromosome?

Cells that contain only one of each chromosome are called haploid.

What are the two types of cell division mentioned in the script?

The two types of cell division mentioned are mitosis and meiosis.

What is the outcome of mitosis in terms of the number and type of cells produced?

The outcome of mitosis is two identical daughter cells, both of which are diploid.

What is the structure that helps in aligning and separating chromosomes during mitosis?

The structure that helps in aligning and separating chromosomes during mitosis is called a spindle, which is made of microtubules.

How does the process of meiosis differ from mitosis in terms of the outcome?

The process of meiosis results in four haploid cells that are not identical to the parent cell, whereas mitosis results in two diploid cells that are identical to the parent cell.

What is the significance of the DNA exchange that occurs during meiosis?

The DNA exchange during meiosis, known as crossing over, results in genetic variation among the offspring, contributing to the diversity of species.

What is the final result of meiosis in terms of the genetic makeup of the daughter cells?

The final result of meiosis is four haploid daughter cells, each with a unique combination of genetic material due to the DNA exchange and the distribution of chromosomes.

Outlines

00:00

🌱 Introduction to Cell Division

Hazel Sive introduces the concept of cell division, a fundamental process that distinguishes living organisms from inanimate objects. She emphasizes the cell division cycle, which includes DNA replication and partitioning. The explanation delves into the organization of DNA in chromosomes and the presence of homologous chromosomes in diploid cells, contrasting them with the haploid state of germ cells. The paragraph also introduces the two types of cell division: mitosis and meiosis, setting the stage for a deeper exploration of each.

05:01

🔬 Understanding Mitosis

This section delves into mitosis, the process of cell division that occurs in body cells, resulting in two identical daughter cells. The process is broken down into two phases: DNA replication, where chromosomes are duplicated, and the subsequent segregation of these chromosomes using a spindle made of microtubules. The outcome is two diploid daughter cells, maintaining the genetic integrity from one generation to the next. The paragraph also includes a visual representation of mitosis, illustrating the alignment of chromosomes during metaphase and their separation during anaphase.

10:04

🧬 Exploring Meiosis

The third paragraph introduces meiosis, the process that leads to the formation of germ cells, such as eggs and sperm. Unlike mitosis, meiosis results in four haploid cells, each with a unique combination of chromosomes due to DNA exchange during pairing. The paragraph explains the initial DNA replication, the pairing and potential recombination of chromosomes, and the two stages of meiosis that ultimately produce genetically diverse gametes. The description of meiosis highlights its role in genetic variation and the formation of new individuals.

15:08

📚 Summary of Cell Division Processes

The final paragraph provides a summary of the two types of cell division, mitosis and meiosis, and their significance in the life cycle of an organism. It contrasts the outcomes of both processes, emphasizing the preservation of genetic information in mitosis and the creation of genetic diversity in meiosis. The paragraph concludes with an assignment for students to practice their understanding of cell division, reinforcing the concepts learned throughout the class.

Keywords

💡Lipid Bilayer

The lipid bilayer is a fundamental component of the cell membrane, consisting of two layers of lipids that form a barrier between the cell's interior and its external environment. It is crucial for maintaining the cell's integrity and regulating the passage of substances in and out of the cell. In the script, the lipid bilayer is mentioned as part of the cell's structure that contributes to its organization and function.

💡Organelles

Organelles are specialized structures within a cell that perform specific functions necessary for the cell's survival and operation. They include the nucleus, mitochondria, and endoplasmic reticulum, among others. The script emphasizes organelles as essential parts of the cell that contribute to its identity and function.

💡Cell Division

Cell division is the biological process by which a parent cell divides into two or more daughter cells. It is a key aspect of growth, development, and reproduction in living organisms. The script focuses on cell division as the main theme, explaining how it is a distinctive feature of life and how it occurs through different processes like mitosis and meiosis.

💡Cell Division Cycle

The cell division cycle refers to the series of events that take place in a cell leading to its division and duplication. It includes the replication of DNA and the subsequent division of the cell into two daughter cells. The script discusses the cell division cycle as having two main parts: replication and partitioning of DNA.

💡DNA Replication

DNA replication is the process by which a cell duplicates its DNA before cell division. It ensures that each daughter cell receives an identical copy of the genetic material. The script describes DNA replication as the first step in both mitosis and meiosis, where the cell's DNA is copied to form two sets of genes.

💡Chromosomes

Chromosomes are thread-like structures made of DNA and proteins that carry genetic information. They are organized units of genes located in the cell nucleus. The script mentions chromosomes as the structures within the nucleus that organize DNA and play a central role in cell division, being replicated and segregated during the process.

💡Diploid

Diploid refers to the state of having two sets of chromosomes, one from each parent, which is typical of most body cells. The script explains that body cells are diploid, containing two of each chromosome, which is denoted as 2n, and this is important for understanding the starting point for mitosis.

💡Haploid

Haploid is the state of having a single set of chromosomes, which is characteristic of gametes like eggs and sperm. The script discusses haploid cells as the result of meiosis, where the germ cells contain only one of each chromosome, denoted as n.

💡Mitosis

Mitosis is a type of cell division that results in two genetically identical daughter cells, each having the same number and kind of chromosomes as the parent cell. The script describes mitosis as the process that occurs in body cells, leading to the production of identical cells that maintain the continuity of an organism's tissues.

💡Meiosis

Meiosis is a specialized type of cell division that results in four non-identical haploid cells, which are the gametes (sperm and egg cells). The script explains meiosis as a two-step process that leads to the production of genetically diverse sex cells, which is crucial for sexual reproduction and genetic variation.

💡Spindle

The spindle is a cellular structure composed of microtubules that plays a critical role in cell division, particularly in mitosis and meiosis. It helps in the alignment and separation of chromosomes during cell division. The script refers to the spindle as the structure that chromosomes line up on during mitosis, ensuring the proper segregation of genetic material.

Highlights

Introduction to the concept of cell division, emphasizing its distinction from inanimate objects.

Explanation of the cell division cycle, consisting of DNA replication and partitioning.

Importance of DNA replication in creating two sets of genes for cell division.

Introduction to the terminology of chromosomes and their organization within the cell.

Differentiation between body cells containing two of each chromosome (diploid) and germ cells with one of each (haploid).

Description of the matching chromosomes as homologs or homologous.

Introduction to the two types of cell division: mitosis and meiosis.

Mitosis occurs in body cells, producing two identical daughter cells to the parent.

Process of mitosis involving DNA replication and chromosome segregation via the spindle structure.

The spindle's role in aligning and separating chromosomes during mitosis.

Outcome of mitosis: two diploid daughter cells identical to the parent cell.

Visual representation of mitosis in sea urchin cells, illustrating the spindle and chromosome segregation.

Introduction to meiosis as the process leading to the production of germ cells (eggs and sperm).

Difference between meiosis and mitosis, particularly the outcome of four haploid cells from meiosis.

Mechanism of DNA exchange between chromosomes during meiosis, contributing to genetic variation.

Two stages of meiosis, resulting in four haploid and genetically unique cells.

Animation illustrating the process of meiosis, from DNA replication to the formation of haploid cells.

Assignment for students to practice understanding of cell division concepts.

Transcripts

HAZEL SIVE: In this part of our class organization of the cell,

you should now have some familiarity

with the notion of a lipid bilayer

and some familiarity with the organelles

that the cell contains and that indeed makes the cell a cell.

Let's go on then to the last topic

we're going to cover in our class, which

is the topic of cell division.

One of the really cool things about cells

is that they make more of themselves.

Cell division makes more cells.

And this is one of the attributes of life

that is not true for inanimate objects, of course.

When we think of cells making more cells,

you should be familiar with something

called the cell division cycle, which briefly has two parts.

This involves firstly replication

and making more of the DNA, taking

the DNA, the genes that are in one cell,

making a whole other set of them.

So you've now got two sets.

So DNA replication, which makes two sets of the genes--

since as we discussed, the genes are DNA--

and secondly, DNA partitioning or segregation.

And this partitioning divides the DNA between daughter cells.

So there are two parts to DNA--

to cell division that we've listed here that we're

going to expand upon.

But before we do, we need a bit more terminology

that you should be familiar with so

that we can talk about cell division in its correct sense.

The genes which are made of DNA are not just

floating free in the nucleus.

They're organized in large units where there are

lots of genes joins together.

And they are organized as chromosomes.

These are called chromosomes.

The genes-- reminder, DNA--

are organized in chromosomes.

And I'm going to abbreviate chromosomes

as chr for the rest of this part of our class

because it's a long word to write out.

So we're going to abbreviate this as chr.

In your cells, each chromosome actually

has a matching chromosome.

There's two of each, two of each.

So let's write body cells contain two of each chr, two

of each chromosome.

So there are two chromosome number ones, two chromosome

number twos, two chromosome number threes, et cetera.

This twofold number gets a special name.

It is called diploid.

And it's abbreviated 2n, 2 little n.

Each of the chromosomes that kind of matches the two number

ones and the two number twos and the two number threes

are called homologs or homologous chromosomes.

So the matching chromosomes are called homologs or homologous.

It means that they match.

On the other hand, the germ cells, the eggs and the sperm,

only have one of each chromosome.

Germ cells, sperm and egg, contain one of each chromosome.

This is called haploid.

And you write it n, lowercase n.

That is a little background that you

need to know before we talk about the next details of cell

division.

There are two kinds of cell division.

There's something called mitosis and something called

meiosis, mitosis and meiosis.

And they are somewhat different from one another

although they're both forms of cell division.

Let's start with mitosis.

This occurs in body cells.

And even as we're speaking, there

are thousands and millions of your cells that

are undergoing cell division right now,

hundreds, hundreds of thousands of cell divisions per second

in our bodies.

The outcome of mitosis is two cells

that are identical to the parent cell.

Outcome two cells-- they are always called daughter cells--

two cells identical to the parent cell.

The process of cell division with mitosis

has again two phases.

Firstly, the DNA replicates.

And we can think now about the chromosomes replicating

these collections of DNA segments all joined together.

Same thing, DNA replication.

We can also refer to chromosome replication.

The chromosomes then do something amazing.

They kind of line up at the equator

of the cell on a special structure called a spindle.

And then once they're lined up properly, the cell knows.

And it pulls apart one of each chromosome to the daughter

cell.

So the chromosomes line up on a special structure

called a spindle.

And again, you don't have to know exactly what this is now.

I want you to have heard of it at this point.

The spindle, for those of you who want to know,

is made of a particular type of cellular structure called

microtubules.

And what is really important is that one copy

of each homologous chromosome is going to go

to each of the daughter cells.

One copy of each chromosome is partitioned

to the daughter cell.

So you start off with a mother cell or a parent

cell that is diploid.

And you land up with two daughter cells

that are diploid also.

You get these daughter cells, finally,

because cell membrane partitions the new cells from each other.

And let's just make sure that we've written it.

The mother and the daughter cells

or the parent and the daughter cells are all diploid or 2n.

Let's look at a slide or so.

We can diagram this starting with a very simple cell that

has just one pair of chromosomes.

In our cells, we have 23 pairs or mostly pairs of chromosomes.

And in this cell, I've just shown you one pair.

The chromosomes are called A. That's

the name of the chromosome.

I've got chromosome one, but I've

called it A and the two copies I've

designated as A superscript 1 and A superscript 2.

They are homologous.

They are identical or similar to one another, largely identical.

The mother cell is diploid.

And the first thing that happens is that the cell

undergoes DNA replication.

And I've drawn the replicated chromosomes.

You can see the two A1s and the two A2s

are connected or next to one another at this point.

They will then line up on the structure of the spindle.

And one copy of A1 will go to one of the daughter cells.

One copy of A2 will go to one of the daughter cells.

So you land up with two daughter cells

that have the identical chromosome set as the mother.

They are all diploid.

And the actual chromosomes are the same

in the daughters as the mother.

This is a hugely important process.

It preserves the character of one cell

from one cell generation to the next.

And it's why during your life, your skin cells

and your intestinal cells stay much the same

because you're making more of the same

every time the cells divide.

On this slide, you can see some steps in mitosis.

I want you to have heard of mitosis at this point.

The specific steps are really descriptive of the process

of how the chromosomes are segregating.

You can see the chromosomes lining up

on the spindle at the stage called metaphase.

And then you can see them pulling apart

at the phase called anaphase.

And if you want to know what it really looks like,

what all of the spindle and the chromosomes pulling apart

look like, you can see these beautiful images

of mitosis in a sea urchin where the chromosomes have lined up

on the spindle in the top left hand image.

And then over time, you can see the chromosomes

pulling apart and being divided into two daughter cells.

And the things that look like flames or streaks in the cell

are these structures called microtubules

that make up the spindle, the part of the factory

of the cell that pulls the chromosomes into the daughter

cells.

The last type of cell division I want to tell you about

is called meiosis.

Meiosis leads to the production of the germ cells, the egg

and the sperm.

And the difference between meiosis and mitosis

is that at the end of meiosis, you get four cells.

And those four cells have each got the haploid number

of chromosomes.

They've each only got one of the normal paired chromosomes.

So the outcome, the outcome of meiosis, are four cells.

They are not identical to the parent,

not identical to the parent.

And each of these--

and they're not identical to the parent,

I should say, for a number of reasons.

But one of the reasons is that they

have the haploid number of chromosomes,

one copy of each chromosome instead of two.

How does this work?

As before, there is DNA replication,

chromosome replication so that you

get double the amount of DNA to start so that you've got DNA

that you can spread around.

And then there are some strange things

that happen during meiosis.

The chromosomes pair up together.

So you've got, at the end of DNA replication,

you've actually got two copies of chromosome A1

and two copies of chromosome A2 as I showed you in our diagram

on the screen.

And I'll show you another diagram in a moment.

And those two copies come really close together.

And they pair with one another.

And I mentioned that those copies

of chromosome 1 or chromosome A were similar but not

entirely identical.

They can exchange bits of DNA with one another.

So a bit of DNA from the chromosome A1

can now jump on to chromosome A2.

And the matching bit from A2 will go into A1.

So you land up with chromosomes that are not

exactly the same as the parental chromosome, either.

That's another difference with meiosis.

So the DNA, there's DNA replication.

The chromosomes pair and very often exchange DNA segments.

It is crazy, but this is what they do.

And then there are two steps to meiosis.

In meiosis I, the same pair, one replicated chromosome pair,

goes to the daughter cell.

And you land up with two cells that are diploid,

but they're diploid not like the parent.

So you get the outcome here is two cells.

And then there is a second division.

Those two cells in meiosis II will divide again.

And a single copy of each chromosome

will go to the daughter cells.

So they divide again, and a single homolog

goes to each daughter cell.

You do not need to know the details of this at this point.

I want you to have the general idea.

So let's go through this animation, which

will give you the general idea.

We start again with a mother cell that is diploid.

It undergoes DNA replication.

You have now two copies of chromosome A1,

two copies of chromosome of A2.

And these four copies, these four chromosomes,

pair with one another.

They come real close together.

And they pair with one another.

And that's when they may exchange pieces of DNA

in a process that is very complicated.

Meiosis I then happens.

And look.

And see you'll be able to contrast this with mitosis.

You've got both copies of chromosome A1 going

to one daughter cell and both copies of chromosome A2

going to another daughter cell.

So that is really different from mitosis.

But then what's also different is

that there's another division.

And you now land up with four cells

such that each of the cells has got one copy of chromosome A1,

or one copy of chromosome A2.

These cells are haploid.

They are different from the parent because of this haploidy

but also because they've had this funny DNA exchange

during the process of meiosis.

So these are the two kinds of cell division

that occur throughout life.

And now you've heard of them.

And you know some differences between the processes.

For your last exercise in class today, I'd

like you to look at this assignment and practice what

you know about cell division.