Meiosis

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

(complete set of chromosomes) (half the regular number of chromosomes)

• Meiosis is needed for _______________________.

• _______ divisions (____________ and __________).

Meiosis

• Reproductive cells divide to produce ______________ (___________ or ________).

• Gametes have half the regular # of chromosomes.

• This process occurs only in gonads (__________ or ___________).

Male:

Female:

• 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.

• __________________ 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

• ______________ and _____________visible.

Meiosis I (four phases)

• Cell division reduces the _______________ number in daughter cells produced by ________________.

• four phases:

a.

b.

c.

d.

Prophase I

• Longest and most complex phase (90%).

• Chromosomes condense.

• _____________ occurs: _________________________

come together to form a ________________.

• ___________ is a pair of ______________________ or four _________________ (sister and non sister

chromatids) that align in Prophase 1

Prophase I - Synapsis

Homologous chromosomes

sister chromatids sister chromatids Tetrad

Homologous Chromosomes

• Are a pair of chromosomes (____________and____________) that are similar in ________ and _______, and which carry _________controlling the same inherited ______________.

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

• Humans have _____ pairs of homologous chromosomes.

a. 22 pairs of _______________

b. 1 pair of ________________

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 ___________ during Prophase 1.

• During crossing over segments of _______________ _____________ overlap, break, and reattach to the other

• Crossing over increases ________________ in the gametes produced

• ____________ (_______________) 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

• ________________ align on the metaphase plate.

• ________________________ occurs: Orientation of homologous pair to poles is random.

Metaphase I

metaphase plate

OR

metaphase plate

Anaphase I

• __________________________separate and move towards the poles.

• Sister chromatids remain attached at their centromeres.

Anaphase I

Telophase I

• Each pole now has ___________________ set of chromosomes.

• ___________________ occurs and _____ 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 _______________________

• 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.

• _____________ form.

• __________________ occurs.

• _________ haploid daughter cells (gametes) are produced.

gametes = ________ or ____________

Telophase II

Genetic Variation

• Important to population as the raw material for __________________ (____________).

• Question:

What are the three sexual sources of

genetic variation?

Answer:

1.

2.

3.

Remember: variation is good!

Fertilization

• The fusion of a __________ and _______to form a ___________.

• A zygote is a fertilized __________

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?