CELL DIVISION

MITOSIS:

Reproducing the nucleus of a somatic cell.

MEIOSIS:

Producing sex cells (gametes).

CYTOKINESIS:

Division of cytoplasm and organelles.

MITOSIS & MEIOSIS, con’t.

Both mitosis and meiosis consist of a sequence of phases:

• interphase (preparation for division)

• prophase

• metaphase

• anaphase

• telophase

MITOSIS & MEIOSIS, con’t.

Result of Mitosis:

•formation of two daughter nuclei

•have the same chromosome number as original (parent) cell

MITOSIS & MEIOSIS, con’t.

Result of Meiosis:

•formation of 1 to 4 daughter nuclei

•have half the chromosome number as original (parent) cell

Concludes with cytokinesis

•division of the cytoplasm and organelles

I. MITOSIS

A. Terminology

1. Chromatin

2. Chromosome

3. Diploid (46 or “2N” in humans)

4. Haploid (23 or “N” in humans)

5. Homologous chromosomes

6. Somatic cells

MITOSIS, con’t.

B. PHASES: arbitrarily assigned

1. INTERPHASE:

a. NO division. Cell appears to be “resting”

b. the metabolic phase of the cell

Interphase, continued …

c. Chromatin visible

d. Nucleus, nuclear membrane, nucleolus visible

e. Centrioles located at poles

f. Longest phase

1. ~96% of the cell’s life cycle

2. average of 23/24 hours

Interphase of Mitosis

MITOSIS, con’t.

2. Prophase

a. Centrioles move to opposite poles of cell

b. Chromatin condenses into chromosomes

c. Nucleoli disappear

d. Spindle apparatus is organized

Late Prophase of Mitosis

MITOSIS, con’t.: Prophase

e. Toward end of phase, nuclear membrane dissipates

f. Phase ends as spindle fibers attach to centromeres

g. Each chromosome now consists of two chromatids

Chromosome at Metaphase

A single metaphase chromosome with two chromatids

MITOSIS, con’t.

3. Metaphase

a. Chromosomes arranged around equatorial plate

b. “Single file”

c. Two spindle fibers attached at each centromere

MITOSIS, con’t.: Metaphase

d. A relatively short phase

e. Chromosomes easily counted (KARYOTYPE)

f. Phase ends as chromatids separate

g. Centromere splits as DNA replication is completed

Metaphase of Mitosis

MITOSIS, con’t.

4. ANAPHASE

a. Begins as chromatids separate

b. Spindles move chromosomes to opposite poles

c. Polar spindles elongate cells

d. Phase ends when chromosomes have arrived at poles

Anaphase of Mitosis

MITOSIS, con’t.

5. Telophase

a. “Reverse” of prophase

b. Identical sister chromosomes have arrived at opposite poles

c. Nuclear membrane reassembles

d. Spindle apparatus dissipates

e. Nucleolus reassembles

MITOSIS, con’t.: Telophase

f. Chromosomes relax chromatin

g. Centrioles replicate

h. Each chromosome consists of a single complete strand of DNA

i. Phase ends when nuclear membrane is completely reassembled

Telophase of Mitosis

MITOSIS, con’t. …

C. CYTOKINESIS

1. Begins during anaphase

2. Completed during telophase

3. Result: two daughter cells

a. Genetically identical to parent cell

b. Contain ~1/2 the cytoplasm, organelles

Mitosis

An overview of Mitosis showing all stages

II. MEIOSIS:• A special case of mitosis

• Occurs during maturation of sex cells (gametes)

• Result: daughter nuclei receive one half the number of chromosomes of somatic cells

• Makes sexual reproduction possible

II. MEIOSIS, con’t.

A. Terminology

1. Haploid: having a single set of chromosomes per

individual or cell (N)

2. Diploid: having two full sets of chromosomes per individual or cell

(2N)

II. MEIOSIS, con’t.: Diploid

a. Usually find sexual reproduction in diploid organisms

b. Provides enormous adaptive advantages:

1. allows for population variations

2. Increases chances for survival

II. MEIOSIS, con’t.: Terminology

3. Homologous chromosomes:

a. Chromosomes that pair during meiosis

b. Have the same morphology

c. Contain genes governing the same characteristics

MEIOSIS, con’t.: Terminology

4. Allele: one of the two or more alternate forms of a

gene

a. dominant, recessive, co-dominant

b. examples:

B = Brown hair (dominant)

b = blonde hair (recessive)

c. Different alleles arise by mutations

MEIOSIS, con’t.: Terminology

5. Gene:

a. sequence of DNA nucleotides

b. unit of heredity transmitted in the chromosome

6. Chromosome: sequence of genes

II. MEIOSIS, con’t.: Terminology

7. One gene – One enzyme theory:

• A gene is that part of a DNA molecule responsible for the synthesis of one enzyme (polypeptide)

II. MEIOSIS, con’t.:

B. Phases of Meiosis1. Two “sets” of events

2. Same names as mitosis

3. Must indicate phase ‘I’ or ‘II’

I = Reductional DivisionII = Equational Division

II. MEIOSIS, con’t.:

C. Reductional Division

1. Interphase I: Like mitosis

2. Prophase I:

a. Long, complex

b. Three subdivisions

II. MEIOSIS, con’t.: Prophase I

1. Early: like mitosis2. Middle: important

a. homologs pair

b. crossing over occurs

c. exchange of genetic info

d. called “tetrads”

3. Late: like mitosis

Prophase I of Meiosis

II. MEIOSIS, con’t.:

3. Metaphase Ia. Similar to mitosisb. Homologous pairs alignc. One spindle

fiber/centromered. Ends as one of each pair

is pulled to opposite polese. NO DNA replication!!

II. MEIOSIS, con’t.:

4. Anaphase I

a. Similar to mitosis

b. Members of homologous pairs move to poles

c. Moved by spindle fibers

Anaphase II of Meiosis

II. MEIOSIS, con’t.:

5. Telophase I

a. Nuclear membrane reorganizes

b. Chromosomes “relax”

6. Cytokinesis: may not occur

Telophase I of Meiosis

II. MEIOSIS, con’t.:

7. End result

a. Nuclei are haploid (N)

b. Chromosome number is reduced by one-half

II. MEIOSIS, con’t.:

D. Equational Division:

1. Interphase II

a. NO DNA replication!!

b. “Coffee break” for nuclei

c. May not occur

Interphase II of Meiosis

II. MEIOSIS, con’t.:

2. Prophase II

a. Chromosomes condense

b. Spindle apparatus forms

c. Two fibers/centromere

d. Membrane dissipates e. Like mitosis

II. MEIOSIS, con’t.:

3. Metaphase II

a. Like mitosis

b. Chromosomes on equator

c. Single file

d. Ends with centromere replication

Metaphase II of Meiosis

II. MEIOSIS, con’t.:

4. Anaphase II

a. Centromeres replicate

b. Chromatids pulled to opposite poles

Anaphase II of Meiosis

II. MEIOSIS, con’t.:

5. Telophase II

a. Nuclear membranes

reorganize

b. Nucleolus reorganizes

c. Chromosomes relax

d. Centrioles replicate

e. Like mitosis

Telophase II of Meiosis

II. MEIOSIS, con’t.:

5. Cytokinesis occurs

6. Result:

a. Females: unequal division of cytoplasm

-one ovum, 3 polar bodies

b. Males: equal division of cytoplasm

-four spermatozoa

Telophase II of Meiosis: Four new cells

II. MEIOSIS, con’t.:

7. Cells produced are:

a. Different from parent cellb. Different from each otherc. Haploid

Spermatogenesis

Spermatogenesis results in four spermatozoa

Oogenesis

Oogenesis results in one Ovum and two or three polar bodies