CELL DIVISIONMITOSIS & MEIOSISCELL DIVISIONMITOSIS & MEIOSIS

Interphase

Mitotic

Cell Cycle 2 distinct phases

What's the most important event of interphase?

Chromosome duplicationS

G2

All chromosomes are duplicated

Do they contain identical genes?

What is significant about DNA in the S and G2 phases?

That means: two copies of each chromosome

What are the copies called?

Sister chromatids

Yes, but…

What about meiosis?

Some terminology

• Double-chromatid chromosomes

• Single-chromatid chromosomes

• Chromatin

• Homologous chromosomes

• Chromatid

When do each of these occur?

• Interphase (actually, this is not part of mitosis itself)

• Prophase• Metaphase• Anaphase• Telophase

• Interphase (actually, this is not part of mitosis itself)

• Prophase• Metaphase• Anaphase• Telophase

THE STEPS OF MITOSISTHE STEPS OF MITOSIS

Onion root tip Whitefish blastula

Interphase

• Nuclear envelop intact

• Chromosomes duplicated but not apparent

• 2 nucleoli visible in onion root tip

• Nucleoli not visible in whitefish

blastula

Mitosis

• DNA loose, uncondensed, called chromatin

Onion root tip Whitefish blastula

Prophase Mitosis

• Double-chromatid chromosomes evident

• Chromatin becomes super-coiled & compact

• Nuclear envelop breaks down

• Nucleoli disappear in onion root tip

• Centrioles migrate toward poles of cell forming the spindle

Double-chromatid chromosomes

Onion root tip Whitefish blastula

Metaphase Mitosis

• Double-chromatid chromosomes line up on equatorial plate of cell

• The spindle fibers attach to the chromosomes at the centromeres

Onion root tip Whitefish blastula

Anaphase Mitosis

• Centromeres are pulled apart

• Groups of single-chromatid chromosomes move opposite poles of the cell

• Spindle fibers pull chromatids apart

Onion root tip Whitefish blastulaTelophase Mitosis

• Cytokinesis begins with appearance of cleavage furrow

• Cytokinesis begins with appearance of cell plate

• Groups of single-chromatid chromosomes reach poles of cell

• Nuclear envelop begins to reform

• Nucleoli reform

• 2 new daughter cells formed

Onion root tip Whitefish blastula

Returning to Interphase Mitosis

• Cytokinesis completes

• Chromosomes ‘disappear’ as interphase chromatin reforms

• 2 new daughter cells enter G1 phase of the cell cycle

A SUMMARY OF MITOSIS

A SUMMARY OF MITOSIS

Mitosis QuestionsMitosis Questions

1. What does “diploid” mean?

2. We have __ total chromosomes.

3. In the term 2n, what does “n” stand for in us? In a gypsy moth?

4. Why does mitosis occur? Major functions?

5. In what cells (general term) does mitosis occur?

1. What does “diploid” mean?

2. We have __ total chromosomes.

3. In the term 2n, what does “n” stand for in us? In a gypsy moth?

4. Why does mitosis occur? Major functions?

5. In what cells (general term) does mitosis occur?

Mitosis QuestionsMitosis Questions

6. What are chromatids?7. What happens in Anaphase to result in

each new cell receiving duplicate parental DNA?

8. In a species whose diploid number is 224, what would its sperm/eggs contain?

6. What are chromatids?7. What happens in Anaphase to result in

each new cell receiving duplicate parental DNA?

8. In a species whose diploid number is 224, what would its sperm/eggs contain?

Why is this duplication of parental DNA necessary?Why is this duplication of parental DNA necessary?

Something to do with passing on genetic information?

Why undergo mitosis at all?

Why undergo mitosis at all?

Something to do with cells getting damaged, old, lost?

Something to do with the organism growing, infant to adult?

SOMATIC CELLS SOMATIC CELLS

What is the process of somatic cell duplication called?What is the process of somatic cell duplication called?

What are typical body cells called?What are typical body cells called?

These cells divide continuously

The new cells receive an exact copy of all the parent cell’s:

Mitosis

DNA

What are these somatic cells?What are these somatic cells?

Diploid or 2n

What does this mean?

They contain the full number of chromosomes

in pairs

How many in humans?

46

23 pairs

Mitosis occurs only in somatic cellsMitosis occurs only in somatic cells

• Gametes are not diploid (2n)• Instead, they are haploid (n)

• Gametes are not diploid (2n)• Instead, they are haploid (n)

What about sex cells?

Called gametes

Eggs and sperm

Produced in ovaries or testes

Our haploid (n) number is 23

Our haploid (n) number is 23

So our eggs and sperm have how many chromosomes?

Half the number

Why?

Fertilization is the union of an egg and a spermFertilization is the union of an egg and a sperm

If the egg and sperm were both diploid, what would the fertilized egg (zygote) be?

If the egg and sperm were both diploid, what would the fertilized egg (zygote) be?

It would be a genetic mess!

It would be a genetic mess!

At fertilization, n + n = 23; 23 + 23 = 46!At fertilization, n + n = 23; 23 + 23 = 46!

Instead, gametes are haploid (n).

Egg and sperm both have exactly half the number of chromosomes of somatic cells

Note how mitosis and meiosis differ:Note how mitosis and meiosis differ:

• Number of divisions?• Number of chromosomes?• Number of products?

• Number of divisions?• Number of chromosomes?• Number of products?

Meiosis !Meiosis !

Meiosis is the cell division process that enables the transformation from 2n to nMeiosis is the cell division process that enables the transformation from 2n to n

Somehow somatic cells (2n) in our ovaries or testes must produce

gametes (n)

How is meiosis more complicated than mitosis?

How is meiosis more complicated than mitosis?

Each chromosome has a partner Each chromosome has a partner

Gametes must contain precisely half the diploid number of chromosomes

They must contain one of each homologous pair of chromosomes

Remember karyotypes?

They come in pairsOne from mom

One from dad

Human KaryotypeHuman Karyotype

Which pair of chromosomes in us in not homologous?Which pair of chromosomes in us in not homologous?

All our other pairs of chromosomes are homologous

All our other pairs of chromosomes are homologous

It’s the 23rd pair in males, the XY pair

Remember what homologous means?

What are the divisions called?What are the divisions called?

How many divisions does meiosis have?

1 2 3 4

1. One pair of homologues to start1. One pair of homologues to start

2. DNA is replicated2. DNA is replicated

3. Meiosis 1 = 2n ---> n3. Meiosis 1 = 2n ---> n

4. Meiosis 2 = chromatids separate into 4 products4. Meiosis 2 = chromatids separate into 4 products

In males, how many progeny are produced?In males, how many progeny are produced?

Typically 4 viable sperm are produced following each Meiosis 2Typically 4 viable sperm are produced following each Meiosis 2

In females, how many progeny are produced?In females, how many progeny are produced?

Just one viable ovum (egg) is produced, plus 3 small polar bodiesJust one viable ovum (egg) is produced, plus 3 small polar bodies

Crossing overCrossing over

When does it occur?

Only during Prophase of Meiosis 1

Homologous chromosomes get together in temporary tetrads

Overlap (cross over) and trade their DNA

Why is this a good thing to do, generally?

Meiosis SummaryMeiosis Summary

1. Meiosis 1a. DNA replication takes place

b. A parent cell produces two daughter cells each with one member of each original pair of homologous chromosomes (to create haploid daughter cells)

c. Crossing over may occur

1. Meiosis 1a. DNA replication takes place

b. A parent cell produces two daughter cells each with one member of each original pair of homologous chromosomes (to create haploid daughter cells)

c. Crossing over may occur

Meiosis SummaryMeiosis Summary

2. Meiosis 2a. There is no more DNA replication

b. The chromatids of each chromosome separate and each daughter cell divides

c. At the end of Meiosis 2, there are 4 daughter cells from each parent cell. Each daughter cell has half the number of chromosomes as the parent cell

2. Meiosis 2a. There is no more DNA replication

b. The chromatids of each chromosome separate and each daughter cell divides

c. At the end of Meiosis 2, there are 4 daughter cells from each parent cell. Each daughter cell has half the number of chromosomes as the parent cell

1. The cells produced in meiosis are _____ (haploid or diploid?)

2. Sex cells are called ________.

_____ are produced by males, ____ by females.

3. What’s crossing over and why is it important?

1. The cells produced in meiosis are _____ (haploid or diploid?)

2. Sex cells are called ________.

_____ are produced by males, ____ by females.

3. What’s crossing over and why is it important?

Meiosis QuestionsMeiosis Questions

4. In what meiotic stage does crossing over occur?

5. Why are sex cells n, not 2n?

6. If a species’ diploid number is 50, what is n?

4. In what meiotic stage does crossing over occur?

5. Why are sex cells n, not 2n?

6. If a species’ diploid number is 50, what is n?

Meiosis QuestionsMeiosis Questions