Transcript MEIOSIS


Homologous: term used to refer to
chromosomes that each have corresponding
chromosome from opposite-sex parent
› Example: Your # 12 chromosome from your mom
is homologous to your # 12 chromosome you
received from your dad

Diploid: refers to a cell that contains both
sets of homologous chromosomes
› Referred to as 2n
› 2n in humans = 46

Haploid: refers to cells that have one set of
chromosomes
› Referred to as n
› In humans n = 23

Meiosis is a process of reduction division in
which the # of chromosomes per cell is cut
in half through the separation of
homologous chromosomes in a diploid cell.
› Only occurs in sex cells
› Involves 2 distinct divisions:
 Meiosis I
 Meiosis II

Prior to:
› each chromosome has replicated

Homologous
chromosomes are
paired forming a tetrad


Chromatids (#4) in
tetrad exchange
portions with their
homologous partner =
crossing-over
Results in new
combinations of
alleles

Metaphase I
› Homologous pairs line
up on metaphase plate

Anaphase I
› Homologous
chromosomes separate
and move to opposite
poles

Telophase I
› Same as telophase in
mitosis

Cytokinesis I
› 2 daughter cells
› 1set of duplicated
chromosomes (n)
› Chromosomes are
different from parent
cell
2 cells made in Meiosis I immediately enter
Meiosis II
 Except there is no replication of
chromosomes prior to Prophase II


Prophase II
› Each chromosome has
›
›
›
›
2 sister chromatids
No crossing over
Spindles start to form
Nuclear envelope &
nucleolus fade away
(a lot like prophase in
mitosis)

Metaphase II
› Chromosomes line up
on metaphase plate
› Spindles attach to
centromeres
› (just like metaphase in
mitosis)


Centromeres split
separating sister
chromatids
Individual
chromosomes move
toward poles of cell



Nuclear envelopes and
nucleoli reform
Spindle disappears
Cleavage furrow forms

4 daughter cells
formed each with
haploid # (n) of
chromosomes
 Parent
cell
produces 2
genetically
identical daughter
cells
 Parent
cell
produces 4
genetically
different daughter
cells
 Males:
› Even cell
divisions at end
of Meiosis I & II
 Females:
› uneven cell
divisions in
Meiosis I & II
(most of
cytoplasm goes to
1 cell)

Females:

http://highered.mcgrawhill.com/sites/0072495855/student_view0/
chapter28/animation__how_meiosis_wor
ks.html

http://www.biology.arizona.edu/cell_bio
/tutorials/meiosis/page3.html