Transcript Meiosis - DiBiasioScience

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).
Interphase I
• Similar to mitosis interphase.
• Chromosomes replicate (S phase).
• Each duplicated chromosome consist of two
identical sister chromatids attached at their
centromeres.
• Centriole pairs also replicate.
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%).
• Chromosomes condense.
• Synapsis occurs: homologous chromosomes
come together to form a tetrad.
• Tetrad is two chromosomes or four
chromatids (sister and nonsister chromatids).
Prophase I - 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
Crossing Over
• Crossing over (variation) may occur between
nonsister chromatids
• Crossing over: segments of nonsister
chromatids break and reattach to the other
chromatid.
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
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
Meiosis 1
First division of meiosis
• Prophase 1: Each chromosome dupicates
and remains closely associated. These are
called sister chromatids. Crossing-over can
occur during the latter part of this stage.
• Metaphase 1: Homologous chromosomes
align at the equatorial plate.
• Anaphase 1: Homologous pairs separate
with sister chromatids remaining together.
• Telophase 1: Two daughter cells are formed
with each daughter containing only one
chromosome of the homologous pair.
Meiosis II
Second division of meiosis: Gamete formation
• Prophase 2: DNA does not replicate.
• Metaphase 2: Chromosomes align at
the equatorial plate.
• Anaphase 2: Centromeres divide and
sister chromatids migrate separately to
each pole.
• Telophase 2: Cell division is complete.
Four haploid daughter cells are
obtained.
Meiosis KM
27
Meiosis KM
28