MEIOSIS
Meiosis

Division of sex cells, called gametes

Meiosis is sexual reproduction
Humans have 46 chromosomes
Numbers of Chromosomes


Diploid= 2 sets (2n)
Haploid= 1 set (n)
Meiosis vs Mitosis



Meiosis is the division of gametes (sex
cells)
Mitosis the division of somatic cells
There are two cell divisions during
meiosis and only for mitosis!
Gametes

Sperm and
egg
Fertilization

When sperm (n) and egg (n) fuse to
make a zygote (2n)
Sister Chromatids

Exact copies of DNA
Homologous Chromosomes


Pair of chromosomes have 2
chromatids each, with exact
same genes on each
Homologous pairs (tetrads)
are 2 chromosomes paired
together
Homologous Chromosomes

Each locus, where alleles or traits are
located, are in the same position on all
chromosomes
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
Phases of Meiosis
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
Interphase




Chromosomes replicate during the S
phase of the cell cycle.
Each duplicated chromosome consists
of 2 sister chromatids
atttached at their
centromere.
Interphase

Centrioles duplicate

Chromosomes are visible
Interphase

Nucleus and nucleolus visible.
chromatin
nuclear
membrane
cell membrane
nucleolus
Prophase 1
Chromosomes condense.
Synapsis occurs when 2 homologous
chromosomes are paired (Which is a
tetrad)
Prophase I - Synapsis
Homologous chromosomes
sister chromatids
Tetrad
sister chromatids
Prophase I
spindle fiber
aster
fibers
centrioles
Prophase 1

Crossing Over may occur between
chromatids of different chromosomes in
a tetrad.
Crossing Over - variation
nonsister chromatids
chiasmata: site
of crossing over
Tetrad
variation
Metaphase I

Tetrads align in the middle of the cell.
INDEPENDENT ASSORTMENT OCCURS:
1. Mixes genetic info from maternal and paternal
chromosomes
2. Leads to different combinations of chromosomes
Metaphase I
OR
metaphase plate
metaphase plate
Independent Assortment
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
Metaphase II
metaphase plate
metaphase plate
Anaphase II

sister chromatids separate,
daughter chromosomes move to
different ends of the cell
Telophase II

Nuclei form.

Cytokinesis occurs.

Four haploid daughter cells produced.
gametes = sperm or egg
Telophase II
Variation


Important to population as the raw
material for natural selection.
Question:
What are the three sexual sources of
genetic variation?
Answer:
1. crossing over (prophase I)
2. independent assortment
(metaphase I)
3. random fertilization
Remember: variation is good!
Question:

A cell containing 20 chromosomes
(diploid) at the beginning of meiosis would,
at its completion, produce cells containing
how many chromosomes?
Answer:

10 chromosomes (haploid)
Karyotype

A method of organizing the chromosomes of a
cell in relation to number, size, and type.
Karyotype
Karyotype

Diploid= 2 copies

Triploid= 3 copies


Polyploid= more
than 2 copies