MEIOSIS | BIO 11: 2ND SEM

Meiosis

- Type of cellular division that occur to produce sex cells in

the reproductive organs (ovaries and testes).

- Involves in the formation of sex cells.

- Divides the number of chromosomes into halves in the

formation of gametes.

- The process is called reduction division.

Homologous chromosomes

• separate chromosomes with one set comes from the mother and the other set from the father

• similar in terms of size, shape, length and genetic content.

Prophase 1

- Chromosomes condense and become visible.

- Homologous chromosomes pair up (synapsis).

- Tetrads form (4 chromatids).

- Crossing over happens (DNA is exchanged), site Chiasma.

- Nuclear membrane breaks down.

- Spindle fibers begin to form.

NOTE: Chromosome pairing and exchanging DNA, creates variation.

Metaphase 1

- Homologous chromosome pairs line up at the center (equator).

- Each pair is called a tetrad.

- Spindle fibers attach to each chromosome.

- Orientation is random (independent assortment).

NOTE: Chromosome pairs line up randomly, increasing variation.

Anaphase 1

- Homologous chromosomes separate.

- Each chromosome moves to opposite poles.

- Sister chromatids stay together.

NOTE: Chromosome pairs are pulled apart

Telophase 1

- Chromosomes reach opposite poles.

- Nuclear membranes may reform.

- Cell divides (cytokinesis).

- Two haploid cells are formed.

NOTE: The cell splits into two cells with half the chromosomes.

Prophase 2

- Chromosomes condense again.

- Nuclear membrane breaks down.

- Spindle fibers form.

Metaphase 2

- Chromosomes line up at the cell center.

- Spindle fibers attach to sister chromatids.

Anaphase 2

- Centromere divides and sister chromatids are pulled to the opposite poles.

- Chromatids move to opposite poles.

Telophase 2

- Chromatids reach poles.

- Nuclear membranes reform.

- Cells divide (cytokinesis).

- Four haploid cells are formed.

NOTE: The result of meiosis is four haploid cells with a recombination of chromosomes both from mother and father.

Gametogenesis

The process involving the production of gametes.

Spermatogenesis

the process by which sperm cells are produced.

Oogenesis

the process of gamete formation in females.

Independent Assortment

It is the random distribution of homologous chromosomes during meiosis

Random Fertilization

chance fusion of any sperm with any egg during fertilization

Trisomy

cells have 3 copies of a chromosome

Monosomy

cells have only 1 copy of a chromosome

Down syndrome

• Trisomy 21

• 3 copies of chromosome 21

Jacobsen syndrome

11q (chromosome 11) terminal deletion disorder

Klinefelter's syndrome

• XXY male

• one in every 2000 live births have male sex organs, but are sterile feminine characteristics

• some breast development

• lack of facial hair

• tall

• normal intelligencе

Jacob’s syndrome

• XYY male

• 1 in 1000 live male births extra Y chromosome

• slightly taller than average

• more active

• normal intelligence, slight learning disabilities

• delayed emotional maturity

• normal sexual development

Trisomy X

• XXX female

• produces healthy females

Barr bodies

all but one X chromosome is inactivated

Turner syndrome

• Monosomy X or X0

• 1 in every 5000 births

• varied degree of effects

• webbed neck

• short stature

• sterile

Deletion

loss of a chromosomal segment

Duplication

repeat a segment

Inversion

reverses a segment

Translocation

move segment from one chromosome to another