MEIOSIS

Meiosis

• The form of cell division by which gametes, with half the regular number of chromosomes, are produced.

diploid (2n) haploid (n) (complete set of chromosomes) (half the regular number of chromosomes)

• Meiosis is needed for sexual reproduction.

• Two divisions (meiosis I and meiosis II).

Meiosis

• Reproductive cells divide to produce gametes (sperm or egg).

• Gametes have half the regular # of chromosomes.

• This process occurs only in gonads (testes or ovaries).

Male: spermatogenesis

Female: oogenesis

• Meiosis is similar to mitosis with some chromosomal differences.

Spermatogenesis

llll llll

human

sex cell

diploid (2n)

llll

llll

meiosis I

ll

ll

ll

ll

sperm

haploid (n)

meiosis II

haploid (n)

Interphase I

• Similar to mitotic interphase.

• Chromosomes replicate (S phase).

• Each duplicated chromosome consist of two identical sister chromatids attached at their centromeres.

• Centriole pairs also replicate (in animal cells).

Interphase I

• Nucleus and nucleolus visible.

nuclear

membrane

nucleolus

cell membrane

chromatin

Meiosis I (four phases)

• Cell division reduces the chromosome number in daughter cells produced by one-half.

• four phases:

a. prophase I

b. metaphase I

c. anaphase I

d. telophase I

Prophase I

• Longest and most complex phase (90%).

• Chromosomes condense.

• Synapsis occurs: homologous chromosomes come together to form a tetrad.

• Tetrad is a pair of homologous chromosomes or four chromatids (sister and non sister chromatids) that align in Prophase 1

Prophase I - Synapsis

Homologous chromosomes

sister chromatids sister chromatids Tetrad

Homologous Chromosomes

• Pair of chromosomes (maternal and paternal) that are similar in shape and size, and which carry genes controlling the same inherited traits.

• Each locus (position of a gene) is in the same position on homologues.

• Humans have 23 pairs of homologous chromosomes.

a. 22 pairs of autosomes

b. 1 pair of sex chromosomes

Homologous Chromosomes

Paternal Maternal

eye color

locus

eye color

locus

hair color

locus hair color

locus

Sex Chromosomes

XX chromosome - female XY chromosome - male

Crossing Over

• Crossing over may occur between non sister chromatids at the chiasmata during Prophase 1.

• During crossing over segments of nonsister chromatids overlap, break, and reattach to the other

• Crossing over increases variation in the gametes produced

• Chiasmata (chiasma) are the sites of crossing over.

Crossing Over - variation

nonsister chromatids

chiasmata: site

of crossing over variation

Tetrad

Prophase I

centrioles spindle fiber

aster

fibers

Metaphase I • Shortest phase

• Tetrads align on the metaphase plate.

• INDEPENDENT ASSORTMENT OCCURS: Orientation of homologous pair to poles is random.

Metaphase I

metaphase plate

OR

metaphase plate

Anaphase I

• Homologous chromosomes separate and move towards the poles.

• Sister chromatids remain attached at their centromeres.

Anaphase I

Telophase I

• Each pole now has haploid set of chromosomes.

• Cytokinesis occurs and two haploid daughter cells are formed.

Telophase I

Meiosis II

• There is no interphase II

(or, if it does occur, it is very short)

• There is no more DNA replication

• Remember: Meiosis II is similar to mitosis

Prophase II

• same as prophase in mitosis

Metaphase II

• same as metaphase in mitosis

metaphase plate metaphase plate

Anaphase II

• same as anaphase in mitosis

• sister chromatids separate

Telophase II

• Same as telophase in mitosis.

• Nuclei form.

• Cytokinesis occurs.

• Four haploid daughter cells (gametes) are produced.

gametes = sperm or egg

Telophase II

Genetic Variation

• Important to population as the raw material for natural selection (evolution).

• Question:

What are the three sexual sources of

genetic variation?

Answer:

1. crossing over (prophase I)

2. independent assortment (metaphase I)

3. random fertilization

Remember: variation is good!

Fertilization

• The fusion of a sperm and egg to form a zygote.

• A zygote is a fertilized egg

n=23

egg

sperm

n=23

2n=46

zygote

Question:

• A cell containing 20 chromosomes (diploid) at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes?

Answer:

• 10 chromosomes (haploid)

Meiosis Video

• http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter28/animation__how_meiosis_works.html