Download 8.4 - Meiosis L5

Cellular Control
Meiosis
Prophase
Spindle fibers attach to the two sister chromatids
of each chromosome contract and separate
chromosomes which move to opposite poles of the
cell.
Metaphase
Spindle apparatus attaches to sister chromatids of
each chromosome. All the chromosomes are line
up at the equator of the spindle
Anaphase
DNA and proteins start to condense. The two
centrioles move toward the opposite end of the
cell to form a spindle. The nuclear envelope and
nucleolus also start to break up.
Telephase
the chromosomes become less condensed and
reappear as chromatin. New membrane forms
nuclear envelopes and the nucleolus is reformed.
Can you think of 3 reasons why
mitosis is important?
Meiosis
Aims:
Why is meiosis necessary?
What happens during meiosis?
How does meiosis create genetic
variation?
1) Match up
Meiosis
Produces 4 daughter cells each with half the
amount of DNA as the parent cell.
In sexual reproduction 2 gametes fuse to give
rise to new offspring. This requires 2 haploid
cells to join together to make 1 diploid cell.
In order to maintain a constant number of
chromosomes in the adults of species, the
number of chromosomes needs to be halved
during meiosis.
SNA 14/12/04
Meiosis
Every diploid cell of an organism has 2 sets of
chromosomes, one from each parent.
During meiosis, the homologous pairs of
chromosomes separate, so that only one
chromosome from each parent enters each
gamete.
These are know as haploid cells.
SNA 14/12/04
Cellular Control
Meiosis
2n
2n
diploid
2n
2n
4n
Cellular Control
Meiosis
2n
2n
diploid
1n
haploid
1n
2n
haploid
Meiosis
• only found in sexual reproduction
• produces haploid gametes
(egg/sperm, pollen/ovule)
• takes place in the sex organs
(ovary/testes, stamens/carpel)
•
Introduces genetic variation
Meiosis - Involves 2 nuclear divisions
During interphase each chromosome replicates
 each chromosome is made up of 2 chromatids
Meiosis I
• Homologous chromosome form bivalents
• Chiasmata form & crossing over occurs
• Homologous chromosomes separate
Meiosis II
• Chromatids separate
Significance of meiosis
• Halves the diploid chromosome number
 haploid gametes
• Diploid cells are formed on fertilisation
• Increases variation
Independent segregation of maternal & paternal
chromosomes
crossing over so that genes on the same chromosome
are not always inherited together
Meiosis I
• Homologous chromosomes separate
Interphase
• Thread-like chromosomes
• Chromosomes replicate (DNA synthesis)
• Centrioles replicate
Prophase I
• Chromatin condenses and coils 
chromosomes become visible
• Nuclear membrane breaks down &
nucleolus disappears
• Spindle starts to form
• Homologous pairs of chromosomes form
bivalents
• Chiasmata form  crossing over
• Homologous chromosomes repel but remain
attached at chiasmata
Metaphase I
• Bivalents line up along the equator
attached to the spindle by their
centromeres
Anaphase I
• Homologous chromosomes separate
and are pulled by the spindle fibres
towards opposite poles of the cell
• Independent segregation (random
separation of maternal & paternal
chromosomes)
Telophase I
• Chromosomes reach poles
Often the cell goes straight into meiosis II
• Spindle disappears
• Nuclear membrane starts to reform
• Cell divides by cytokinesis
Meiosis II
chromatids separate
Prophase II
• Spindle forms
(at right angles to original spindle)
• (nuclear membrane disappears if it had
reformed)
Metaphase II
• Chromosomes line up at
equator attached to spindle
by their centromere
Anaphase II
• Centomeres divide &
chromatids separate
Telophase II
• Chromosomes reach poles
& uncoil
• Spindle disappears
• Nuclear membrane
reforms
• Cytoplasm divides
(cytokinesis)
• Tetrad of 4 haploid cells
see
http://www.sumanasinc.com/webcontent/anis
amples/biology/biology.html
For animation of meiosis
Role of chiasmata
• Hold homologous chromosomes together
as a bivalent
• Separate alleles on same chromosome 
increased genetic variation in gametes
Cellular Control
Metaphase I
2n
1 cell
4n
1 cell
Genetic
Recombination
replication
Prophase I
Meiosis
Prophase II
Anaphase II
Telephase I
Telephase II
2 cell
2 cell
1n
4 cell
Metaphase II
Anaphase I
2n
2n
Cellular Control
Apoptosis
Identify one
difference between
mitosis and Meiosis
Cellular Control
Meiosis
Without genetic recombination:
How many genetically different
gametes could a human make?
• 23 Chromosomes
• 2 genetically
distinct chromatids
• Maternal stays with
maternal
• Paternal stay with
paternal
2
Cellular Control
gene for a
characteristic
(protein)
Meiosis
gene for a
characteristic
(protein)
Inherited
from male
Inherited
from female
Paternal Chromatid
Maternal Chromatid
Genes have a position on a
chromosome (in a genome) that is
conserved among species
LOCUS (loci)
Cellular Control
Meiosis
Different versions of a gene, that code for different
versions of a characteristic, are called alleles
Cellular Control
2
Meiosis
Evolution via natural selection
requires variation within a population
Cellular Control
4
Meiosis
Evolution via natural selection
requires variation within a population
Sexual
Reproduction
Combining gametes from 2
organisms doubles genetic variation
Cellular Control
529
Meiosis
Evolution via natural selection
requires variation within a population
Sexual
Reproduction
Independent
assortment
Combining gametes from 2
organisms doubles genetic variation
Maternal and paternal chromatids
distributed to gametes independently
from each other (randomly)
8
Cellular Control
Meiosis
Evolution via natural selection
requires variation within a population
Sexual
Reproduction
Independent
assortment
Crossing Over
Combining gametes from 2
organisms doubles genetic variation
Maternal and paternal chromatids
distributed to gametes independently
from each other (randomly)
Alleles on Chromatids are randomly
distributed between sister chromatids
Cellular Control
Apoptosis
Homologous
pairs
Chromatids
Loci
Alleles
Chiasmata
Crossing
over
Cellular Control
Apoptosis
Errors in Meiosis
Mutations
Mutation rate of DNA is
higher during meiosis
and those mutations
are passed on to
offspring
Polypoidy
Multiple copies
of genes leads to
variability in
gene expression
and adaptability
Increases genetic
variability in plants
Genetic Abnormalities
in most animals
Multiple copies
of genes leads to
inbalance of
gene products
and
abnormalities
Cellular Control
Apoptosis
Errors in Meiosis
Kleinfelter’s
Syndrome
• XXY, XXXY, XXXY
• Sterile or low fertility
Polypoidy
Increases genetic
variability in plants
• Hormonal imbalances
• Learning difficulties
• Increase risk of
cancers
Genetic Abnormalities
in most animals
Multiple copies
of genes leads to
inbalance of
gene products
and
abnormalities
Meiosis
Aims:
Why is meiosis necessary?
What happens during meiosis?
How does meiosis create genetic
variation?
1) Match up
Cellular Control
loci
Homologous
pairs
Gametes
Centromeres
Meiosis
Chiasmata
Crossing
over
Chromatin
Cytokinesis
Centrioles
Chromatid
Spindle
fibres