Download Cell Division (Meiosis)

Cell Division
(Meiosis)
Meiosis
• The form of cell division by which
gametes, with half the number of
chromosomes, are produced.
• Diploid (2n)
 haploid (n)
• Meiosis is sexual reproduction.
• Two divisions (meiosis I and meiosis II).
Meiosis
• Sex cells divide to produce gametes (sperm or
egg).
• Gametes have half the No. of chromosomes.
• Occurs only in germ cells
Meiosis is similar to mitosis with some
chromosomal differences.
Spermatogenesis
n=23
human
sex cell
sperm
n=23
n=23
2n=46
haploid (n)
n=23
diploid (2n)
n=23
n=23
meiosis I
meiosis II
Interphase I
• Similar to mitosis interphase.
• Chromosomes replicate (S phase).
• Each duplicated chromosome consist of two
identical sister chromatids attached at their
centromeres.
Interphase I
• Nucleus and nucleolus visible.
chromatin
nuclear
membrane
cell membrane
nucleolus
Meiosis I (four phases)
• Cell division that reduces the chromosome
number 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%).
• It has 5 sub stages:
• Leptotene,Zygotene ,Pachytene,Diplotene&
diakinesis
• Synapsis occurs: homologous chromosomes
come together to form a tetrad.
• Tetrad is two chromosomes or four
chromatids (sister and nonsister chromatids).
Zygotene - Synapsis
Homologous chromosomes
sister chromatids
Tetrad
sister chromatids
Homologous Chromosomes
• Pair of chromosomes (maternal and paternal) that are
similar in shape and size.
• Homologous pairs (tetrads) 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. 01 pair of sex chromosomes
Homologous Chromosomes
eye color
locus
eye color
locus
hair color
locus
hair color
locus
Paternal
Maternal
Pachytene - Crossing Over
• Crossing over (variation) may occur between
nonsister chromatids at the chiasmata.
• Crossing over: segments of nonsister
chromatids break and reattach to the other
chromatid.
• Chiasmata (chiasma) are the sites of crossing
over.
Crossing Over - variation
nonsister chromatids
chiasmata: site
of crossing over
Tetrad
variation
Prophase I
spindle fiber
aster
fibers
centrioles
Metaphase I
• Shortest phase
• Tetrads align on the metaphase plate.
• INDEPENDENT ASSORTMENT OCCURS:
1. Orientation of homologous pair to poles is random.
2. Variation
3. Formula: 2n
Example: 2n = 4
then n = 2
thus 22 = 4 combinations
Metaphase I
OR
metaphase plate
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
• No interphase II
(or very short - 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.
• Remember:
four haploid daughter cells
produced.
gametes = sperm or egg
Telophase II
Significant results of meiosis:
1.
Haploid cells are produced because two rounds of division follow one
round of chromosome replication.
2.
Alignment of paternally and maternally inherited chromosomes is
random in metaphase I, resulting in random combinations of
chromosomes in each gamete. Number of possible chromosome
arrangements = 2n-1.
3.
Crossing-over between maternal and paternal chromatids during
meiosis I provides still more variation. Moreover, the crossing-over
sites vary from one meiosis to another.
Meiosis
sex cell
n=2
sperm
n=2
n=2
2n=4
haploid (n)
n=2
diploid (2n)
n=2
n=2
meiosis I
meiosis II