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Cell Division

Biol 105Laboratory 10

Overview of cell division:

The process starts with DNA replication

Chromosomes

Chromosomes are made of DNA DNA contains the code that makes

proteins

When does DNA replicate?

Just prior to cell division

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

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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.

Chromosome Pairs

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

But these chromosomes come in pairs. These are two chromosomes, each with two

chromatids:

Remember DNA is in the nucleus

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Cell Cycle: Stages of Cell Division

Two main phases: Interphase and Mitotic phase

Interphase prepares for cell division and DNA is replicated

Mitotic phase: Chromosomes separate and cell divides

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

Mitosis take only about 30 min

Most of the time is spent in G1 which is where the cell grows in size – 12 hours

Interphase

During this phase: The cell growsMakes new cellular components DNA is replicated Check points will stop cell division if

there is a problem When the check points in Interphase don’t

work properly the cell divides when it should not

End of Interphase: DNA has replicated

Mitosis

Divided into four phases:

1. Prophase2. Metaphase3. Anaphase4. Telophase

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Microtubules

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

Prophase

1.Microtubules (mitotic spindles) attach to the chromosomes

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

Metaphase

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MetaphaseAnaphase

1.Sister chromatid are pulled apart by the microtubles

2. Chromatidsmove toward the poles. Now the chromatidsare called chromosomes

Anaphase Telophase

1.The chromosomes begin to uncondense

2. Nuclear membrane forms

3. Cell begins to divide in half

TelophaseCytokinesis:

The cell divides completely forming two separate cells

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Mitosis overviewMitosis overviewWhere does DNA replication take place?

1. Cytosol2. Nucleus

Cyto

sol

Nucle

us

50%50%

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When does DNA replication take place?

1. Metaphase2. Anaphase3. Prophase4. Telophase5. Interphase

Cyto

sol

Nucle

us

50%50%

During this stage the nuclear membrane breaks down

1. Metaphase2. Anaphase3. Prophase4. Telophase

Meta

phase

Anap

hase

Prophase

Telophase

25% 25%25%25%

During this stage chromosomes line up at the equator

1. Metaphase2. Anaphase3. Prophase4. Telophase

Meta

phase

Anap

hase

Prophase

Telophase

25% 25%25%25%

During this stage the chromosomes begin to uncondense

1. Metaphase2. Anaphase3. Prophase4. Telophase

Meta

phase

Anap

hase

Prophase

Telophase

25% 25%25%25%

At the end of Mitosis how many cells are there?

1. One2. Two3. Three4. Four

One

Two

Three Four

25% 25%25%25%

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

1. 23 chromosomes2. 46 chromosomes

23 chro

mosomes

46 ch

romoso

mes

50%50%

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

1. Yes2. No

Yes No

50%50%

Cell Types

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

Gametes: are the cells that are responsible for reproduction

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

Gametes = reproductive cells

Sperm and eggs are reproductive cells –gametes

They will not divide like the somatic cells

Instead gametes undergo meiosis

Remember that we have 23 pairs of chromosomes = 46 chromosomes

If gametes (sperm and egg) combined with all these chromosomes then the offspring will have 92 chromosomes

How do gametes overcome this problem?

Before the gametes come together they need to reduce their number of chromosomes in half.

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

The answer to their problem is meiosis –halving their number of chromosomes

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Terminology

Diploid = cells that have 46 chromosomes or 23 pairs, 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

Meiosis

First the chromosomes (DNA) are duplicated during Interphase

Then there are two cell divisions

Remember that mitosis had chromosome (DNA) duplication followed by one cell division

Remember that there are pairs chromosomes, each chromosome has two chromatids after DNA replication

Meiosis

First the DNA replicates – the chromosomes become duplicated, this happens during Interphase

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

Then the chromatids separate and cell divide = 2cd cell division

The figures are going to show only two pairs of chromosomes – but there are 23 pairs at the start

1st: DNA replicates –chromosomes become duplicated, the cell is diploid (2n)

2cd: homologous chromosomes separate and the cell divides resulting in two haploid cells (1n)

3rd: The chromatids separate and then the cell divides resulting in four haploid cells (1n)

Remember Meiosis happens 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 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 – 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:Microtubules (mitotic spindles)

attach to the chromosomes

The microtubles line up the chromosomes at the center of the cell (the equator)

The chromosomes arrange themselves randomly – this also gives genetic variation = independent assortment

Anaphase I:

chromosomes in the pairs are separated

This is different from Mitosis – here the chromosomes in a pair are separated from each other

So now 23 chromosomes are on each side

Telophase I

1. Nuclear membrane forms2. 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 cell enters Meiosis II

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Meiosis II Meiosis II

Prophase II: The 23 chromosomes are already condensed. 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)

At the end of Meiosis I how many cells are there?

1. One2. Two3. Three4. Four

One

Two

Three Four

25% 25%25%25%

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

1. Haploid2. Diploid

Hap

loid

Diploid

50%50%

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

1. 232. 46

23 chro

mosomes

46 ch

romoso

mes

50%50%

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At the end of Meiosis I, are the chomosomes in the duplicated state?

1. Yes2. No

Yes No

50%50%

At the end of Meiosis II how many cells are there?

1. One2. Two3. Three4. Four

One

Two

Three Four

25% 25%25%25%

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

1. Haploid2. Diploid

Hap

loid

Diploid

50%50%

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

1. 23 chromosomes2. 46 chromosomes

23 chro

mosomes

46 ch

romoso

mes

50%50%

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

1. Yes2. No

Yes No

50%50%

Genetic diversity through meiosis

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

Prophase I: The pairs of chromosomes crossing over.

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

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Meoisis and Gender

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

One of these chromosomes will be a sex chromosome

Eggs will contain a X chromosome Sperms will contain either a 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, 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

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

The heads contain the chromosomes and lots of mitochondria to power the flagella

About 400 million sperm are produced each day

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 cell not able to survive

The one egg has 23 chromosomes, with a X sex chromosome

The one egg is large enough to support the embryo

Egg Formation cont

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

So when a girl 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 vs Meiosis

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

Before both Mitosis and Meiosis the DNA replicates, forming duplicated chromosomes, each containing two chromatids

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

Mitosis During Mitosis:

The chromatids are separated 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 are separated, resulting in 2 cells, each with 23 chromosomes, in the duplicated state Meiosis II: The chromatids are separated

producing two haploid cells that contain 23 non duplicated chromosomes. One original cell produces four haploid

cells

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Important concepts

The two phases of the cell cycle – Interphase and Mitosis - 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.

The importance of check points in interphase

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Important Concepts

Know what state the cell and the chromosomes are in at the beginning and end of Mitosis, Meiosis I and at the end of 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 into

meiosis – what are the events that add diversity and when does each event occur

Important Concepts

How is the gender of the offspring determined. How many functioning sperm are produced from

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

Definitions

Gametes, Somatic cells, Diploid, Haploid, Crossing over, Independent assortment, Cytokinesis, centromere, centrioles, centrosome, chromatid, polar bodies, spermatogenesis, oogenesis