3.3+ 10.1 MEIOSIS
considered to be sister
CHROMOSOME REPLICATION chromatids until the
- Chromosomes replicate in interphase before meiosis ↓ splitting of the
centromere at the start
- In S phase, DNA replicated so each chromosome has 2 chromatids of anaphase II
- At start of meiosis, chromosomes condense
and are visible as two chromatids (sister
chromatids) joined by a centromere
- Then synapsis occurs (pairing), where
homologous chromosomes align beside each
other - this is referred to as a tetrad (as
composed of 4 chromatids)/ also referred to
as bivalent (as it is a homologous pair)
- Synaptonemal complex (SC) - protein that
connects paired homologous chromosomes -
mediates synapsis
- Hormones/other stimuli trigger testes for
meiosis
- Gamete producing cells can do mitosis and
meiosis:
—> Mitosis to regenerate itself and growth of
tissue in testes (for future sperm production)
—> Meiosis to produce sperm
- Meiosis: gives rise to genetically different cells
# of chromosomes = # of centromeres and is used to produce gametes
present —> One diploid nucleus divides by meiosis to
produce 4 haploid nuclei
CROSSING OVER meiosis was discovered by microscope examination
of dividing germ-line cells.
Prophase I
1) Chromosomes condense and is visible as a pair of sister chromatids
2) 2 homologous chromosomes align alongside each other
3) Synapsis forms bivalent/tetrad (pair of homologous chromosomes)
4) Crossing over occurs (exchange of DNA between non-sister homologous chromatids), two non-
sister chromatids form a junction (become entangled) - here, the two chromatids break and rejoin
with each other (due to the stress caused by the entanglement)
—> Crossover occurs at exactly same loci on the 2 non-sister chromatids - allows same
genes to be exchanged
—> Recombination: breaking and re-joining of DNA to create new combinations of genetic
material - forms recombinant DNA
—> Non-sister chromatids are homologous but not genetically identical - so some alleles will
be different
—> Breaking and rejoining is catalysed by endonuclease and DNA ligase
5) The crossing point is called the chiasma
—> Crossing over can occur to either/both chromatids adjacent to the homologous pair -
chiasmata formation between non-sister chromatids results in exchange of alleles
- This is a significant source of genetic intraspecific variation
, - Likely to occur further down the
chromosome, away from the centromere
—> as further away DNA flails around
more = more likely to get entangled
—> So gene locus can affect genotype
spread - based on likelihood of crossing
over
- Infinite combinations of how DNA can
recombine in crossing over due to random
nature of where chiasmata form
- Other than crossing over, in prophase I:
—> Centrioles migrate to opposite poles and spindle fibres are formed
—> Nuclear envelope breaks down and nucleolus disintegrates
RANDOM ORIENTATION (INDEPENDENT
ASSORTMENT)
Metaphase I
1) Homologous chromosomes line up at the cell equator as they prepare to separate
2) Spindle microtubules grow out from the poles and attach to the centromeres of the
chromosomes
2) Each pair can be arranged with either chromosome on top, this is completely random
—> Orientation of one homologous pair is independent
—> Orientation of bivalent determines which pole each chromosome gets attached to
3) The combination of alleles that will end up
in each daughter cell depends on how the
pairs of homologous chromosomes were
lined up (variation)
- Number of different chromosome
combinations = 2ⁿ = 2²³
- Independent assortment: the alleles of two
(or more) different genes get sorted into
gametes independently of one another (due
to random orientation)
FINISHING MEIOSIS I
Anaphase I
- Homologous pairs of chromosomes are separated (bivalents separate in a process
called disjunction)
—> microtubules pull whole chromosomes to opposite ends (centromeres don’t split)
Telophase I
- Chromosomes arrive at opposite poles and uncondense
- Spindle fibres break down
- Nuclear envelope forms around 2 groups of chromosomes and nucleolus reforms
considered to be sister
CHROMOSOME REPLICATION chromatids until the
- Chromosomes replicate in interphase before meiosis ↓ splitting of the
centromere at the start
- In S phase, DNA replicated so each chromosome has 2 chromatids of anaphase II
- At start of meiosis, chromosomes condense
and are visible as two chromatids (sister
chromatids) joined by a centromere
- Then synapsis occurs (pairing), where
homologous chromosomes align beside each
other - this is referred to as a tetrad (as
composed of 4 chromatids)/ also referred to
as bivalent (as it is a homologous pair)
- Synaptonemal complex (SC) - protein that
connects paired homologous chromosomes -
mediates synapsis
- Hormones/other stimuli trigger testes for
meiosis
- Gamete producing cells can do mitosis and
meiosis:
—> Mitosis to regenerate itself and growth of
tissue in testes (for future sperm production)
—> Meiosis to produce sperm
- Meiosis: gives rise to genetically different cells
# of chromosomes = # of centromeres and is used to produce gametes
present —> One diploid nucleus divides by meiosis to
produce 4 haploid nuclei
CROSSING OVER meiosis was discovered by microscope examination
of dividing germ-line cells.
Prophase I
1) Chromosomes condense and is visible as a pair of sister chromatids
2) 2 homologous chromosomes align alongside each other
3) Synapsis forms bivalent/tetrad (pair of homologous chromosomes)
4) Crossing over occurs (exchange of DNA between non-sister homologous chromatids), two non-
sister chromatids form a junction (become entangled) - here, the two chromatids break and rejoin
with each other (due to the stress caused by the entanglement)
—> Crossover occurs at exactly same loci on the 2 non-sister chromatids - allows same
genes to be exchanged
—> Recombination: breaking and re-joining of DNA to create new combinations of genetic
material - forms recombinant DNA
—> Non-sister chromatids are homologous but not genetically identical - so some alleles will
be different
—> Breaking and rejoining is catalysed by endonuclease and DNA ligase
5) The crossing point is called the chiasma
—> Crossing over can occur to either/both chromatids adjacent to the homologous pair -
chiasmata formation between non-sister chromatids results in exchange of alleles
- This is a significant source of genetic intraspecific variation
, - Likely to occur further down the
chromosome, away from the centromere
—> as further away DNA flails around
more = more likely to get entangled
—> So gene locus can affect genotype
spread - based on likelihood of crossing
over
- Infinite combinations of how DNA can
recombine in crossing over due to random
nature of where chiasmata form
- Other than crossing over, in prophase I:
—> Centrioles migrate to opposite poles and spindle fibres are formed
—> Nuclear envelope breaks down and nucleolus disintegrates
RANDOM ORIENTATION (INDEPENDENT
ASSORTMENT)
Metaphase I
1) Homologous chromosomes line up at the cell equator as they prepare to separate
2) Spindle microtubules grow out from the poles and attach to the centromeres of the
chromosomes
2) Each pair can be arranged with either chromosome on top, this is completely random
—> Orientation of one homologous pair is independent
—> Orientation of bivalent determines which pole each chromosome gets attached to
3) The combination of alleles that will end up
in each daughter cell depends on how the
pairs of homologous chromosomes were
lined up (variation)
- Number of different chromosome
combinations = 2ⁿ = 2²³
- Independent assortment: the alleles of two
(or more) different genes get sorted into
gametes independently of one another (due
to random orientation)
FINISHING MEIOSIS I
Anaphase I
- Homologous pairs of chromosomes are separated (bivalents separate in a process
called disjunction)
—> microtubules pull whole chromosomes to opposite ends (centromeres don’t split)
Telophase I
- Chromosomes arrive at opposite poles and uncondense
- Spindle fibres break down
- Nuclear envelope forms around 2 groups of chromosomes and nucleolus reforms
