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Cell Division & Cell Cycle

Comparing Mitosis and Meiosis

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Cell Division•Cell Division-when a parent cell divides into two daughter cells.

•All body cells are somatic cells and divide by mitosis

•Sperm and egg, which are gametes (sex cells) divide by meiosis

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Cell Division• Mitosis-the

nucleus of the cell divides equally

Parent Cell

Two identic

al daughter cells

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Eukaryotic Cell Cycle

Daughter Cells

DNA is Copied

Cells Matur

e

Organelles form & Cells prepares to

Divide

Cell nucleus divides equally

Gap 1 phase

Synthesis phase

Period between cell division

Gap 2 phase

Cytoplasm Divides

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Cell CycleRemember: I P M A T C

• I = Interphase (G1, S, G2)• P = Prophase• M= Metaphase• A= Anaphase• T= Telophase• C= Cytokinesis

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Interphase: Part 1 of Cell Cycle

• Interphase-cell prepares for cell division.

• Genetic Material (DNA)-is in the form of chromatin-uncoiled chromosomes

• Has 3 phases: G1, S, G2

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Eukaryotic Chromosomes• Eukaryote cells

store genetic info.in tightly coiled DNA segments or chromosomes

• Human body cells =46 chromosomes Chromosomes can’t be seen when cells aren’t dividing and are called chromatin

• Chromatin-uncoiled chromosomes

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Chromosome Pictures• Duplicated

chromosomes are called sister chromatids & are held together by the centromere-found at the center of the chromosome

Sister Chromatids

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Mitosis: Part 2 Of Cell Cycle• Mitosis-is

equal division of the nucleus of eukaryote cells

Has four stages:

• P rophase• Metaphase• Anaphase• Telophase

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Prophase• Prophase-first and longest

phase of mitosis• Nucleolus and nuclear

envelope will disappear• Chromosomes are now

visible and are attached to the spindle

• The Spindle-(a fan-like structure that pulls the chromosomes apart) appears.

• Centrioles-(two tiny structures in the cytoplasm that travel toward opposite sides of the nucleus) in animal cells help to form the spindle

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Metaphase• Second, shortest phase of mitosis; lasts a

few minutes• Chromosomes are now lined up at the

equator or center of the cellEquator of Cell

Pole of the Cell

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Anaphase

Sister Chromatid

s being separated

Spindle is pulling the sister chromatids apart Spindle

sister chromatids

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Telophase• Sister chromatids

are at opposite poles

• Spindle breaks apart

• Nuclear envelope forms around each set of sister chromatids

• Nucleolus reappears

• Chromosomes reappear as chromatin

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Cytokinesis• Means division

of the cytoplasm

• Division of cell into two, identical halves called daughter cells

• In plant cells, cell plate forms at the equator to divide cell

• In animal cells, cleavage furrow forms to split cell

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Identical Daughter Cells

Chromosome number is maintained or is the same (Diploid), but cells

smaller than parent cell

What is the 2n or diploid

chromosome number in humans?

4646

46

46

Mitosis-goes from diploid to diploid

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MeiosisFormation of

Gametes (Eggs & Sperm)

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Facts About Meiosis• Interphase occurs before

meiosis; chromosome replication occurs

• Two meiotic divisions-Meiosis I and Meiosis II

• Called Reduction Division-chromosome # is reduced by ½ (4623)

• Original cell is diploid (2n)=46• Four daughter cells produced at

the end of meiosis are monoploid (n) or haploid (chromosomes reduced by ½)

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Facts About Meiosis• Daughter cells contain half (23)

the number of chromosomes as the original cell (46)

• Meiosis produces gametes (eggs & sperm)

• Occurs in the testes in males (Spermatogenesis)

• Occurs in the ovaries in females (Oogenesis)

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Why Do we Need Meiosis?

• It is the fundamental basis of sexual reproduction

• Two haploid (n) gametes are brought together through fertilization to form a diploid (2n) zygote

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Meiosis Forms Haploid Gametes

• Meiosis must reduce the chromosome number by half

• Fertilization then restores the 2n number

meiosis reducesgenetic content

from mom from dad child

toomuch!

The right number!

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Prophase I

Early prophase• Homologs pair.

• Crossing over occurs.

Late prophase• Chromosomes condense.

• Spindle forms.• Nuclear envelope fragments.

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Crossing-Over• Homologous

chromosomes in a tetrad cross over each other

• Pieces of chromosomes or genes are exchanged

• Produces Genetic recombination in the offspring

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Metaphase I

Homologous pairs of

chromosomes align along the equator of the

cell

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Anaphase I

Homologs separate and move to opposite poles.

Sister chromatids remain attached at their centromeres.

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Telophase I

Nuclear envelopes reassemble.

Spindle disappears.

Cytokinesis divides cell into two.

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Meiosis IIOnly one homolog of

each chromosome is present in the cell.

Meiosis II produces gametes with one copy of each chromosome and thus one copy of each gene.

Sister chromatids carry identical genetic information.

Gene X

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Meiosis II: Reducing Chromosome Number

Prophase II Metapha

se IIAnaphas

e II

Telophase II 4 Identical

haploid cells

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Prophase II

Nuclear envelope fragments.

Spindle forms.

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Metaphase II

Chromosomes align along equator of cell.

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Anaphase II

Sister chromatids separate and move to opposite poles.

Equator

Pole

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Telophase II

Nuclear envelope assembles.

Chromosomes decondense.

Spindle disappears.

Cytokinesis divides cell into two.

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Results of MeiosisGametes (egg & sperm) form

Four haploid cells with one copy of each chromosome

One allele of each gene

Different combinations of alleles for different genes along the chromosome

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Spermatogenesis• Occurs in the

testes• Two divisions

produce 4 spermatids

• Spermatids mature into sperm

• Men produce about 250,000,000 sperm per day

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Spermatogenesis in the Testes

Spermatid

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Oogenesis• Occurs in the ovaries• Two divisions produce 3 polar

bodies that die and 1 functional egg

• Polar bodies die because of unequal division of cytoplasm

• Immature egg is called an oocyte

• Starting at puberty, one oocyte matures into an ovum (egg) every 28 days

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Mitosis MeiosisNumber of

divisions 1 2

Number of daughter cells 2 4

Genetically identical? Yes No

Chromosome # Same as parent Half of parent

Where Somatic (body) cells Germ cells

When Throughout life

At sexual maturity

Role Growth and repair

Sexual reproduction

Comparing Mitosis and Meiosis

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Karyotype• A picture of the

chromosomes from a human cell arranged in pairs by size

• First 22 pairs are called autosomes

• Last pair are the sex chromosomes

• XX female or XY male

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Nondisjunction• When chromosomes fail

to separate during Anaphase I or Anaphase II of meiosis

• Chromosome number can be more (47) or less (45), than the normal chromosome number (46)

• Example: Down’s Syndrome-has extra chromosome on #21 also called Trisomy 21

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Quiz1. If a cat has 38 chromosomes in each

of its body cells, how many chromosomes

will be in each daughter cell after mitosis?

F 11G 19H 38J 76

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Quiz2. If a cat has 38 chromosomes in each

of its cells, how many chromosomes will be in each gamete after meiosis?

F 11G 19H 38J 76

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Quiz3. Which of these symbols represents the normal karyotype of a human female?

F XXYG XXH XOJ XXX

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Quiz4. This human karyotype is unusual because chromosome set —

A 5 has chromosomes of different shapesB 10 is missing genetic materialC 14 has enlarged centromeresD 21 has extra genetic material

Unusual Human Karyotype

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Quiz5. The diagram represents

the chromosomes of a person with a genetic disorder caused by nondisjunction, in which the chromosomes fail to separate properly. Which chromosome set displays nondisjunction?

F 2G 8H 21J 23

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Quiz6.

45

Quiz7.

46

Quiz8.

47

9.

48

Quiz10.