Meiosis
• animation
How are the Goals of MEIOSIS Different
from the Goals of MITOSIS?
MITOSIS
MEIOSIS
1) Produces identical
daughter cells
(genetically)
1) Produces cells that
are NOT genetically
identical
2) Produces daughter
cells with the same
number of chromosomes as mother cell
2) Cuts chromosome
number of mother cell
in half
Homologous
chromosomes DO NOT
interact
Homologous
chromosomes pair up,
align and separate
Homologous Chromosomes
Homologous Chromosomes work in a similar
way:
• Chromosome 19 from your dad will have
the same genes on it as Chromosome 19
from your mom
• the information on these genes may be
slightly different
Ex. On this chromosome, you might get a
brown hair version of the gene from your
dad and a blond hair version of the gene
from your mom
• are similar in shape and content
• they carry genes for the same
proteins.
• This means that you have two copies
for any given gene on a non-sex
chromosome (not X or Y).
• Remember, organisms with two
copies of each gene (and/or
chromosome) are called diploid
Encylopedia Analogy• Imagine you get a set of encylopedias from
each parent.
• Each has 23 volumes present similar
information in a similar format
• Ex. Each has a listing for “kangaroo",
but may say slightly different things
about “kangaroos".
• You have inherited two versions of
“kangaroo".
How does the process of meiosis accomplish
the previously mentioned goals?
• two meiotic divisions
without DNA
replication in between
them.
• Independent
assortment and
crossing over
MEIOSIS I
– Prophase 1:
• Homologous sister chromatids find
each other. Crossing Over can occur
during the latter part of this stage.
– Metaphase 1:
• Homologous chromosomes align at
the equatorial plate. Independent
assortment happens here.
– 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
– Prophase 2:
• DNA does not replicate.
Sister Chromatids prep for
division
– Metaphase 2:
• Sister chromatids 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.
Summary
• One parent cell produces four daughter cells.
– Daughter cells have half the number of chromosomes
found in the original parent cell and with crossing over,
are genetically different.
• Meiosis differs from mitosis primarily because
there are two cell divisions in meiosis, resulting in
cells with a haploid number of chromosomes.
Meiosis Square Dance
• animation
• Independent Assortment– The process of random segregation
and assortment of chromosomes
during anaphase I of meiosis
resulting in the production of
genetically unique gametes.
• Chiasmata– A cross-shaped structure commonly
observed between homologous
chromosomes during meiosis; the
site of crossing-over.
• Random Fertilization– A mating system in which there is an equal
opportunity for all male and female gametes to join in
fertilization
– This means that an egg cell representing one of eight million different
genetic possibilities will be fertilized by a sperm cell representing one of
eight million genetic possibilities and the resulting zygote can have any of
the possible 64 trillion diploid combinations of genes.
Making Gametes
• Spermatogenesis– The process of sperm formation in male
animals
– Happens in the testes
– Produces 4 haploid cells which go on to
develop tails and are called
“spermatozoa”
• Oogenesis– The process of ovum formation in
female animals
– Happens in ovaries
– Cytoplasm divides unequally after
Meiosis 1
– This “ovum” grows into a mature egg
cell and carries a rich supply of
nutrients for the developing egg.
– The other 3 cells become polar bodies
and die