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Cell DivisionBiology 105

Laboratory 8

THE MAJOR

STEPS OF CELL

DIVISION:

When does DNA replicate?

The first step of cell division is DNA replication: This occurs just prior to cell division.

Cells need to divide for multicellularorganisms to grow or to replace old cells, or for one-celled organisms to reproduce.

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Chromosomes

Chromosomes are made of DNA.

DNA contains the code that makes proteins.

Examples of chromosomal locations that correspond with various pathologies.

Chromosomes

DNA is wound up in coils and folded to form chromosomes.

When the DNA replicates, it forms a chromosome in a “duplicated state”.

This “duplicated state” has two strands: each strand is called a chromatid – they are “sister chromatids”.

The chromatids are held together at the centromere.

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Chromosome Pairs

After DNA replication, each chromosome will be made of two sister chromatids.

But remember that chromosomes come in pairs!

These are two chromosomes,

each with two chromatids:

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Cell Cycle: Stages of Cell Division

Two main phases: interphase and the mitotic phase Interphase prepares for cell division – DNA

replicates during interphase.

Mitotic phase: chromosomes separate and cell divides.

In an average animal cell, the cell cycle takes about 24 hours.

Mitosis take only about 30 minutes.

Most of the time is spent in G1, which is when the cell grows in size (about 12 hours).

Interphase

During this phase:

The cell grows.

The cell makes new cellular components.

DNA is replicated.

Check points will stop cell division if there is a problem.

When the check points in interphase do not work properly, the cell divides when it should not!

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End of Interphase: DNA has replicated

Mitosis

Divided into four phases:

1. Prophase

2. Metaphase

3. Anaphase

4. Telophase

= PMAT!

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Think about how hard it would be to divide the DNA into two equal parts if it was in the

uncondensed form!

Prophase

1. Chromosomes condense.

2. Nuclear membrane breaks down.

3. Two centrioles (one centrosome) begin to move to opposite ends of cell.

4. Microtubules form.

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Microtubules

Prophase

Metaphase

1.Microtubules (mitotic spindles) attach to the chromosomes.

2.The microtubules line up the chromosomes at the center of the cell (the equator).

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Metaphase

Anaphase

1.Sister chromatidsare pulled apart by the microtubules.

2. Chromatids move toward the poles of the cell. Now the chromatids are called chromosomes.

Anaphase

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Telophase

1.The chromosomes begin to uncondense.

2. Nuclear membrane forms.

3. Cell begins to divide in half.

Telophase

Cytokinesis

The cell divides completely, forming two separate cells.

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Cytokinesis

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Videos

Mitosis overview:

http://www.youtube.com/watch?v=AhgRhXl7w_g

Mitosis in an embryo

http://www.youtube.com/watch?v=m73i1Zk8EA0

Mitosis seen using contrast microscope:

http://www.youtube.com/watch?v=DD3IQknCEdc

Where does DNA replication take place?

1. Cytosol

2. Nucleus

Cyt

osol

Nucl

eus

50%50%

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When does DNA replication take place?

1. Metaphase

2. Anaphase

3. Prophase

4. Telophase

5. Interphase

Cyt

osol

Nucl

eus

50%50%

During this stage the nuclear membrane breaks down:

1. Metaphase

2. Anaphase

3. Prophase

4. Telophase

Met

apha

se

Anap

hase

Pro

phase

Tel

ophas

e

25% 25%25%25%

During this stage chromosomes line up at the equator:

1. Metaphase

2. Anaphase

3. Prophase

4. Telophase

Met

apha

se

Anap

hase

Pro

phase

Tel

ophas

e

25% 25%25%25%

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During this stage the chromosomes begin to uncondense:

1. Metaphase

2. Anaphase

3. Prophase

4. Telophase

Met

apha

se

Anap

hase

Pro

phase

Tel

ophas

e

25% 25%25%25%

At the end of mitosis, how many cells are there?

1. One

2. Two

3. Three

4. Four

One

Tw

o

Thre

e F

our

25% 25%25%25%

At the end of mitosis, how many chromosomes are there in each cell?

1. 23 chromosomes

2. 46 chromosomes

23

chro

mos

omes

46

chro

mos

omes

50%50%

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At the end of mitosis, are the chomosomes in the duplicated state?

1. Yes

2. No

Yes N

o

50%50%

Cell Types

Mitosis occurs in all the body’s cells, except the cells that are responsible for reproduction.

Gametes: cells that are responsible for reproduction.

All the rest of the body’s cells are called somatic cells.

Gametes = Reproductive Cells

Sperm and eggs are reproductive cells called gametes. They do not divide like the somatic cells.

Instead, gametes undergo a process called meiosis.

Recall that we have 23 pairs of chromosomes = 46 total chromosomes. If gametes (sperm and egg) combine with all

these chromosomes, then the offspring will have 92 chromosomes!

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How do gametes overcome this problem?

Before the gametes combine, they need to reduce their number of chromosomes by half.

So instead of 23 pairs (46 total chromosomes) they need to have 23 total chromosomes.

The answer to this problem is meiosis – this process halves the number of chromosomes.

Terminology

Diploid = cells that have 46 chromosomes or 23 pairs of chromosomes. All somatic cells are diploid (2n).

Haploid = cells that have 23 chromosomes. Gametes are haploid (1n).

Meiosis is when a diploid cell divides to produce haploid reproductive cells.

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Meiosis

First the DNA replicates during interphase. The chromosomes duplicate. Remember that there are pairs of chromosomes,

and each chromosome has two chromatids after DNA replication.

Then the chromosome pairs separate and the cell divides = 1st cell division.

Then the chromatids separate and the cells divide = 2nd cell division.

The figures are going to show only two pairs of chromosomes – but there are really 23 pairs!

1st step: DNA replicates –chromosomes duplicate, the cell is diploid (2n).

2nd step: homologous chromosomes separate and the cell divides, resulting in two haploid cells (1n).

3rd step:the chromatids separate and the cells divide, resulting in four haploid cells (1n).

Meiosis occurs in order to form gametes – the reproductive cells (sperm and eggs).

The cells that produce the gametes start out diploid before meiosis, and will end up haploid.

There are two stages of meiosis: meiosis I and meiosis II.

Each stage of meiosis has prophase, metaphase, anaphase, and telophase.

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Crossing Over

End of interphase: DNA has replicated

Prophase I: Duplicated chromosomes condense and intertwine, and this produces genetic variation = crossing over Genetic material is exchanged between the

homologous chromosomes.

Prophase I:1. Duplicated

chromosomes condense and crossover – this produces genetic variation.

2. Nuclear membrane begins to break down.

3. Two centrioles(centrosomes) begin to move to opposite ends of cell.

4. Microtubules form.

Metaphase I:1. Microtubules (mitotic spindles)

attach to the chromosomes.

2. Microtubules line up the chromosomes at the center of the cell (along the equator).

3. The chromosomes arrange themselves randomly, and this also gives genetic variation = independent assortment.

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Anaphase I:Chromosomes in the pairs separate.

*** This is different from mitosis! Here the chromosomes in a pair are separated from each other.

So now there are 23 duplicated chromosomes on each side.

Telophase I:

1. Nuclear membrane forms.2. Cell divides in half.

We now have two haploid cells (1n), which means there are 23 chromosomes total.

The chromosomes are in the duplicated form.

Now the cell enters meiosis II.

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Meiosis II

Meiosis II

Prophase II: the 23 chromosomes are already condensed, and the nuclear membrane dissolves.

Metaphase II: chromosomes line up at the equator.

Anaphase II: chromatids separate.

Telophase II: cells separate.

Now there are four haploid cells: each has 23 chromosomes (not in the duplicated state).

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Videos

Meiosis overview:

http://www.youtube.com/watch?v=D1_-mQS_FZ0

At the end of meiosis I, how many cells are there?

1. One

2. Two

3. Three

4. Four

One

Tw

o

Thre

e F

our

25% 25%25%25%

At the end of meiosis I, are these cells haploid or diploid?

1. Haploid

2. Diploid

Hap

loid

Dip

loid

50%50%

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At the end of meiosis I, how many chromosomes are there in each cell?

1. 23

2. 46

23

chro

mos

omes

46

chro

mos

omes

50%50%

At the end of meiosis I, are the chomosomesin the duplicated state?

1. Yes

2. No

Yes N

o

50%50%

At the end of meiosis II, how many cells are there?

1. One

2. Two

3. Three

4. Four

One

Tw

o

Thre

e F

our

25% 25%25%25%

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At the end of meiosis II, are these cells haploid or diploid?

1. Haploid

2. Diploid

Hap

loid

Dip

loid

50%50%

At the end of meiosis II, how many chromosomes are there in each cell?

1. 23 chromosomes

2. 46 chromosomes

23

chro

mos

omes

46

chro

mos

omes

50%50%

At the end of meiosis II, are the chromosomes in the duplicated state?

1. Yes

2. No

Yes N

o

50%50%

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Genetic Diversity through Meiosis

There are two points in this process that contribute to the genetic diversity of the offspring.

Prophase I: the pairs of chromosomes undergo crossing over.

Metaphase I: the way the chromosomes line up on the equator is random = independent assortment.

Meoisis and Gender

The gametes now contain 23 chromosomes (not in the duplicated form).

One of these chromosomes is a sex chromosome.

Eggs contain an X chromosome.

Sperms contain either an X or a Y chromosome.

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Spermatogenesis

In the male testes, sperm are produced. One cell produces 4 sperm.

Each sperm has 23 chromosomes, and they are not in the duplicated form.

The sperm can have either an X or a Y sex chromosome.

The sperm have a small head and a long tail (= flagellum) for locomotion.

Spermatogenesis

The sperm need to contain the genetic material and deliver it to the egg.

The head contains the chromosomes and lots of mitochondria to power the flagellum.

About 400 million sperm are produced each day.

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Egg Formation

The ovaries in females produce eggs. One cell will produce one egg and three non-

functioning “polar bodies”. The one egg gets most of the cytoplasm,

leaving the other three cells unable to survive. It contains 23 chromosomes, with an X sex

chromosome. It is large enough to support the embryo.

Egg Formation

All of the cells that produce the eggs are made before the female is even born.

When a female is born, her ovaries contain all the cells that produce her eggs.

Each month one of these cells will leave the ovary and go on to mature – and produce the egg and polar bodies.

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Review of Mitosis versus Meiosis

Mitosis and meiosis both start with a diploid cell (46 chromosomes, 23 pairs).

Before both mitosis and meiosis, the DNA replicates. Duplicated chromosomes are formed, each

containing two chromatids.

Mitosis occurs in somatic cells (cells other than those that produce the gametes), and meiosis produces gametes.

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Mitosis

During mitosis:

The chromatids separate to produce two cells, each with 46 chromosomes = 23 pairs of non-duplicated chromosomes.

These cells are diploid (2n) cells.

There is no exchanging of genetic material.

Meiosis – Two Stages

Meiosis I: the pairs of chromosomes line up and the chromosomes separate, resulting in 2 cells, each with 23 chromosomes in the duplicated state.

Meiosis II: the chromatids separate, producing two haploid cells that each contain 23 non-duplicated chromosomes.

One original cell produces four haploid cells.

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Important Concepts

Know the two phases of the cell cycle (interphase and mitosis) and what happens in each phase.

For mitosis: know each stage, the order of the stages, and what happens in each stage.

Be able to recognize each stage of mitosis from pictures, models, and microscope slides.

Understand the importance of check points in interphase.

Important Concepts

Know what state the cell and the chromosomes are in at the beginning and end of mitosis, meiosis I, and meiosis II. For example: Are the cells haploid or diploid?

Are the chromosomes duplicated, or not duplicated? How many chromosomes are there in the cell? Are they in pairs?

Know which cells undergo mitosis vs meiosis.

How is genetic diversity introduced during meiosis? What are the events that add diversity and when

does each event occur?

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Important Concepts

How is the gender of the offspring determined?

How many functioning sperm are produced from one spermatocyte? Which sex chromosomes can a sperm have? How many functioning eggs are produced from one oocyte? Which sex chromosomes do eggs have?

Definitions

Gametes, somatic cells, diploid, haploid, crossing over, independent assortment, cytokinesis, centromere, centrioles, centrosome, chromatid, polar bodies, spermatogenesis, oogenesis