CELL DIVISION MITOSIS & MEIOSIS

Interphase

Mitotic

Cell Cycle 2 distinct phases

What's the most important

event of interphase?

Chromosome duplication

S

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

THE 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 blastula

Telophase 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

Mitosis 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?

Mitosis 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?

Why is this duplication of parental DNA necessary?

Something to do with passing on genetic information?

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

What is the process of somatic cell

duplication 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? 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 cells

• 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

So our eggs and sperm have how many

chromosomes?

Half the number

Why?

Fertilization is the union of an egg and a sperm

If the egg and sperm were

both diploid, what would the

fertilized egg (zygote) be?

It would be a genetic mess!

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:

• Number of divisions?

• Number of chromosomes?

• Number of products?

Meiosis !

Meiosis 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?

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 pairs

One from mom

One from dad

Human Karyotype

Which pair of chromosomes in us in not 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?

How many divisions does meiosis have?

1 2 3 4

1. One pair of homologues to start

2. DNA is replicated

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

4. Meiosis 2 = chromatids separate into 4 products

In males, how many progeny are produced?

Typically 4 viable sperm are produced

following each Meiosis 2

In females, how many progeny are produced?

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

Crossing 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 Summary

1. Meiosis 1

a. 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 Summary

2. Meiosis 2

a. 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?

Meiosis 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?

Meiosis Questions

Cyclin

• protein

• regulates the timing of the cell

cycle in eukaryotic cells

Regulatory Proteins

• Two types

1. Internal Regulators

2. External Regulators

Internal Regulators

• proteins that respond to events

inside the cell

• signals cell cycle to continue

ONLY WHEN OTHER THINGS

ARE COMPLETE

External Regulator

• proteins that respond to events

outside the cell

• direct cells to speed up or slow

down the cell cycle

• example: growth regulators

Uncontrolled Cell Growth

• Cancer

– body loses ability to control

growth

– does not respond to the regulator

proteins

– result = large masses of cells

Tumors • masses of cells

• damages the surrounding tissues

• cells break off and spread

throughout the body

Cause of cancer • Brought to you by carcinogens

–Cigarette smoke

–Burnt meat

–Preservatives, etc

• Radiation exposure

• Viral infection

• Others

Review Questions

1. Why is it important for cells to have

regulated cell growth?

2. What is the purpose of the protein

cyclin?

3. Compare and contrast internal

regulators and external regulators.

4. What is cancer? What causes

cancer?