What is meiosis
process by which sex cells (gametes) are made in the reproductive organs
Meiosis 1
The 1st meiotic division is a reduction division*(diploid→haploid)in which homologous chromosomes are separated
meiosis I: the number of cells is doubled but the number of chromosomes is not. This results in 1/2 as many chromosomes per cell.
Meiosis 2
The 2nd division separates sister chromatids (these chromatids may not be identical due to crossing over in prophase I)
Prophase 1
- homologous chromosomes undergo a process called*synapsis→ synapsis: chromosomes pair up to form abivalent
(or tetrad)
homologous chromosomes are held together at points calledchiasmata(singular: chiasma)
- Crossing over of genetic material between non-sister chromatids occurs at chiasmata
- result of exchanging genetic material → new gene combinations are formed on chromatids (recombination)
- Once chiasmata are formed, the homologous chromosomes condense as bivalents and then are separated in meiosis
- If crossing overoccurs then all 4 haploid daughter cells will be geneticallydistinct (sister chromatids are no longer identical)
where does crossing over occur
at the chiasmata
What promotes genetic variation
- crossing over
- random orientation
- fusion of gametes from different parents
In what phase does crossing over occur?
prophase 1
When does random assortment of chromosomes occur
metaphase 1
The failure of chromosomes to separate may occur via:
- Failure of homologues to separate in Anaphase I (resulting in four affected daughter cells)
- Failure of sister chromatids to separate in Anaphase II (results in only 2 daughter cells being affected)
synapsis
when chromosomes pair up to form a bivalent
what does crossing over result in
new gene combinations are formed on chromatids (recombination)
monosomy
chromosomal abnormality - when there is 1 less chromosome
nuchal translucency
- non-invasive
- ultrasound looking at nuchal fluid behind the neck of the foetus - large about is strongly correlated w Down syndrome
chronic villus sampling
- invasive
- sample of chronion (villi - placenta tissue) via tube inserted through the cervix
- cells are cultured & then karyotypes
- 1% risk of miscarriage
amniocentesis
- invasive
- sample of amniotic fluid (contains foetal cells) with a needle guided by camera/ultrasound
- 0.5% of miscarriage
why is chromosome number halved in gametes?
to prevent the total chromosome numbers doubling each generation
Metaphase 1
- Spindle fibres connect to bivalents which align along the equator
- random orientation occurs - chromosomes line up in 1 of 2 arrangements
- maternal copy left/right I paternal copy left/right
- this orientation is random, as is the assortment of chromosomes into gametes
- final gametes will differ depending on whether they got the maternal or paternal copy of a chromosome
- random assortment will occur for each homologous pair - number of possible gamete combinations depends on number of homologous pairs, therefore 2^23 possible gamete combinations
This means an allele on one chromosome has an equal chance of being paired with, or separated from, any allele on another chromosome (their inheritance is independent of one another)
Random assortment
describes how pairs of alleles separate independently from one another during gamete formation
- According to independent assortment, inheritance of one gene/trait is independent to the inheritance of any other gene/trait
- Independent assortment is due to the random orientation of pairs of homologous chromosomes in meiosis I
- Independent assortment willnot occur if two genes are located on the same chromosome (linked genes)
Random orientation vs independent assortment
- random orientation is the behaviour of chromosomes in meiosis
- independent assortment is the behaviour of alleles of unlinked genes as a result of gamete productions - due to random orientation, the alleles of these unlinked have been independently assorted into the gametes
Anaphase 1
- spindle fibres contract and split the bivalent
- homologous chromosomes move to opposite poles of the cell
- reduction division - homologous pair is separated but sister chromotids remain attached
Telophase 1
- Chromosomes decondense
- nuclear membranemayreform
- cell divides (cytokinesis) forming two haploid daughter cells each containing one pair of sister chromatids
Prophase 2
- Chromosomes condense
- nuclear membrane dissolves
- centrosomes move to opposite poles (perpendicular to prophase I)
- no crossing-over occurs
Metaphase 2
spindle fibres attach to chromosomes (at centromere) and align them along the equator
Anaphase 2
- spindle fibres contract - centromeres split
- sister chromatids are separated.
- now called chromosomes
- pulled to opposing poles
