Cell Division Biology 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 multicellular

organisms to grow or to replace old cells, or for one-celled organisms to reproduce.

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.

Presenter

Presentation Notes

Figure: 19-04 Title: Changes in chromosome structure because of DNA replication during interphase. Caption: Changes in chromosome structure because of DNA replication during interphase.

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:

Presenter

Presentation Notes

Figure: 09-07 Title: A Karyotype Displays a Full Set of Chromosomes Caption: One member of each chromosome pair comes from the individual’s father and the other member from the mother. Each paired set of chromosomes is said to be “homologous,” meaning the same in size and function. (The two chromosomes over the number 1 are a homologous pair, the two over number 2, and so forth.) Homologous chromosomes are not exactly alike, however; the genes on them may differ somewhat, meaning the effects they produce will differ. Notice that this karyotype set is from a human male; there are 22 pairs of homologous chromosomes and then one X and one Y chromosome (which are not homologous). A female would have two X chromosomes. All the chromosomes are in the duplicated state.

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).

Presenter

Presentation Notes

Figure: 09-09 Title: The Cell Cycle Caption: The three main stages of cell division—DNA replication, mitosis, and cytokinesis—can be seen here in the context of a complete cell cycle. This cycle traditionally is divided into two main phases, interphase and mitotic (or “M”) phase, which are in turn divided into other phases. Note that DNA replication—and hence chromosomal duplication—takes place within the S phase of interphase. The average cell spends most of its time in G1 phase, growing and carrying out its normal cellular functions. Some types of cells, however, cease going through the cell cycle at all, while others progress through it repeatedly and rapidly.

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!

End of Interphase: DNA has replicated

Mitosis

Divided into four phases: 1. Prophase 2. Metaphase 3. Anaphase 4. Telophase

= PMAT!

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

uncondensed form!

Presenter

Presentation Notes

Figure: 19-07 Title: From chromatin to chromosomes. Caption: During prophase, duplicated chromosomes condense from a threadlike form called chromatin to compact packages of DNA.

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.

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).

Metaphase

Anaphase

1.Sister chromatids are pulled apart by the microtubules.

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

Anaphase

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.

Cytokinesis

Presenter

Presentation Notes

Figure: 09-11 Title: Cytokinesis in Animals Caption: Cytokinesis in animal cells begins with an indentation of the cell surface, a cleavage furrow, shown here in a dividing frog egg.

Presenter

Presentation Notes

Figure: 19-05l Title: An overview of mitosis. Caption: Left side: DNA replication and first stages of mitosis.

Presenter

Presentation Notes

Figure: 19-05r Title: An overview of mitosis. Caption: Right side: later stages of mitosis.

Videos

Mitosis overview: http://www.youtube.com/watch?v=AhgRhXl7w

_g Mitosis in an embryo http://www.youtube.com/watch?v=m73i1Zk8E

A0 Mitosis seen using contrast microscope: http://www.youtube.com/watch?v=DD3IQknC

Edc

Where does DNA replication take place?

1. Cytosol 2. Nucleus

Cyto

sol

Nucle

us

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When does DNA replication take place?

1. Metaphase 2. Anaphase 3. Prophase 4. Telophase 5. Interphase

Cyto

sol

Nucle

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During this stage the nuclear membrane breaks down:

1. Metaphase 2. Anaphase 3. Prophase 4. Telophase

Meta

phase

Anap

hase

Prophas

e

Telophas

e

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During this stage chromosomes line up at the equator:

1. Metaphase 2. Anaphase 3. Prophase 4. Telophase

Meta

phase

Anap

hase

Prophas

e

Telophas

e

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During this stage the chromosomes begin to uncondense:

1. Metaphase 2. Anaphase 3. Prophase 4. Telophase

Meta

phase

Anap

hase

Prophas

e

Telophas

e

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

1. One 2. Two 3. Three 4. Four

One

Two Three Four

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

1. 23 chromosomes 2. 46 chromosomes

23 ch

romos

omes

46 ch

romos

omes

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

1. Yes 2. No

Yes No

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!

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.

Presenter

Presentation Notes

Figure: 09-07 Title: A Karyotype Displays a Full Set of Chromosomes Caption: One member of each chromosome pair comes from the individual’s father and the other member from the mother. Each paired set of chromosomes is said to be “homologous,” meaning the same in size and function. (The two chromosomes over the number 1 are a homologous pair, the two over number 2, and so forth.) Homologous chromosomes are not exactly alike, however; the genes on them may differ somewhat, meaning the effects they produce will differ. Notice that this karyotype set is from a human male; there are 22 pairs of homologous chromosomes and then one X and one Y chromosome (which are not homologous). A female would have two X chromosomes. All the chromosomes are in the duplicated state.

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.

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.

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.

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.

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).

Presenter

Presentation Notes

Figure: 19-10 Title: Overview of meiosis. Caption: Meiosis reduces the chromosome number from the diploid number to the haploid number. Meiosis involves two cell divisions.

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

Two Three Four

25% 25%25%25%

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

1. Haploid 2. Diploid

Hap

loid

Diploi

d

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

1. 23 2. 46

23 ch

romos

omes

46 ch

romos

omes

50%50%

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

1. Yes 2. No

Yes No

50%50%

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

1. One 2. Two 3. Three 4. Four

One

Two Three Four

25% 25%25%25%

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

1. Haploid 2. Diploid

Hap

loid

Diploi

d

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

1. 23 chromosomes 2. 46 chromosomes

23 ch

romos

omes

46 ch

romos

omes

50%50%

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

1. Yes 2. No

Yes No

50%50%

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.

Presenter

Presentation Notes

Figure: 10-04 Title: The X and the Y Caption: The human X chromosome can be seen on the left, while the human Y chromosome is on the right. The difference in size between them is a reflection of the difference in the number of genes each of them contains. The Y chromosome only has about 50, while the X has about 1,500. (Magnified X 10,000)

Presenter

Presentation Notes

Figure: 09-07 Title: A Karyotype Displays a Full Set of Chromosomes Caption: One member of each chromosome pair comes from the individual’s father and the other member from the mother. Each paired set of chromosomes is said to be “homologous,” meaning the same in size and function. (The two chromosomes over the number 1 are a homologous pair, the two over number 2, and so forth.) Homologous chromosomes are not exactly alike, however; the genes on them may differ somewhat, meaning the effects they produce will differ. Notice that this karyotype set is from a human male; there are 22 pairs of homologous chromosomes and then one X and one Y chromosome (which are not homologous). A female would have two X chromosomes. All the chromosomes are in the duplicated state.

Presenter

Presentation Notes

Figure: 10-05 Title: Sex Outcome in Human Reproduction Caption: Early in meiosis I, the mother’s two X chromosomes line up at the metaphase plate (XX). Meanwhile, in the father’s meiosis, his X and Y chromosomes line up (XY). The X-X and X-Y pairs then separate into different cells. In meiosis II, the chromatids that made up the duplicated chromosomes separate, yielding individual eggs and sperm. In fertilization, should a Y-bearing sperm from the male reach the egg first, the child will be a boy; should the male’s X-bearing sperm reach the egg first, the child will be a girl.

Presenter

Presentation Notes

Figure: 10-06 Title: Sperm and Egg Formation in Humans Caption: Sperm and Egg Formation in Humans.

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.

Presenter

Presentation Notes

Figure: 10-07 Title: Mobile Cells Caption: Human sperm, colored to show detail.

Presenter

Presentation Notes

Figure: 19-11 Title: Comparison of spermatogenesis and oogenesis. Caption: Meiosis results in haploid cells that differentiate into mature gametes. Spermatogenesis produces four sperm cells that are specialized to transport the male’s genetic information to the egg. Oogenesis produces three polar bodies and one ovum that is packed with nutrients to nourish the early embryo.

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.

Presenter

Presentation Notes

Figure: 10-08 Title: Big Difference in Size Caption: A human egg surrounded by much smaller human sperm.

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.

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?

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