3.3 Meiosis Flashcards
Meiosis is a ___________ of the nucleus to form _____________.
reduction division, haploid gametes
Meiosis I overview
- Diploid nucleus (2n) 🡪 haploid nucleus (n)
- Homologous chromosomes separate
Meiosis II overview
- Haploid nucleus (n) 🡪 haploid nucleus (n)
- Sister chromatids separate
Product of meiosis
4 haploid (n) daughter cells
Meiosis occurs in
GONADS (testes or ovaries)
SPERMATOGENESIS - creates sperm cells
OOGENESIS - creates egg cells
To compensate for the chromosome doubling during fertilisation gametes undergo …
meiosis, which halves the chromosomes present in gametes compared to the parent
Sexual reproduction involves ….
fertilisation, the fusion of gametes (sex cells), one from each parent.
Interphase
In the S-phase of the interphase before meiosis begins, DNA replication takes place.
Chromosomes are replicated and these copies are attached to each other at the centromere.
The attached chromosome and its copy are known as sister chromatids.
Following S-phase, further growth and preparation take place for meiosis.
Prophase I
DNA supercoils and chromosomes condense
Nuclear membrane dissolves
Centrioles migrate to the poles of the cell
The homologous chromosomes associate with each other to form bivalents (synapsis)
Chiasma = point of contact between the two non-homologous sister chromatids
Crossing-over between non-sister chromatids can take place. This results in recombination of alleles and is a source of genetic variation in gametes.
Metaphase I
The bivalents line up at the equator.
Random orientation occurs - each bivalent aligns independently and hence the daughter nuclei get a different mix of chromosomes.
This is a significant source of genetic variation
Anaphase I
- Spindle fibres contract.
- Homologous pairs pulled apart (separate)
- This is the reduction division – the bivalent is split and half the chromosomes move towards each pole.
Telophase I
The nuclei are now haploid (N) not diploid (2N): they each contain one pair of sister chromatids for each of the species’ chromosomes.
The cytoplasm begins to divide by cytokinesis.
New nuclei form
Chromosomes decondense
n.b. If crossing-over and recombination has occurred then the sister chromatids will not be exact copies.
Prophase II
Nuclear membranes dissolves
Chromosomes, consisting of two sister chromatids, condense
No crossing-over occurs
Centrioles move to opposite poles
Metaphase II
Spindle fibres form and attach at the centromeres.
Pairs of sister chromatids align at the equator.
Anaphase II
The sister chromatids are separated. The chromatids (now called chromosomes are pulled to opposite poles.
Spindle fibres contract and the centromeres are split.
Telophase II
Nuclear membrane reforms
Chromosomes decondense
Cytokinesis begins, dividing the cells.
The end result of meiosis is four haploid gamete cells.
Fertilisation of these haploid gametes will produce a diploid zygote.
Crossing-over between non-sister chromatids results in…
recombination of alleles
Random orientation of the homologous chromosomes means…
there are 2n possible orientations in metaphase I. That is 223 in humans – or 8,388,068 different combinations in gametes!
Because both crossing-over and random orientation occur during meiosis the result is…
effectively infinite genetic variation in the haploid gamete.
mitosis vs meisis
- mitosis
One division
Diploid cells produced
No crossing-over in prophase
No chiasmata formation
Homologous pairs do not associate and line up at the equator in metaphase
Sister chromatids separate in anaphase - meiosis
Two divisions
Haploid gametes produced
Crossing-over in prophase I
Chiasmata form
Homologous pairs associate as bivalents and lined up at the equator in metaphase I
Homologous pairs separate in anaphase I
Sister chromatids separate in anaphase II
For a new organism to arise sexually meiosis must occur in…
both parents followed by fusion of the gametes (fertilisation)
Meiosis in a single individual produces near infinite variation, but genetic variation is further increased by:
Meiosis occurs in two individuals
Alleles from two organisms combine in novel ways
benefits of genetic variation
Increased genetic variation produces a more resilient population that is more likely to withstand environmental change such as a disease. Genetic variation is essential for successful change by evolution.
non-disjunction
homologous chromosomes fail to separate at anaphase
- Disjunction is the term used to describe the separation of chromosomes
The result of non-disjunction is…
gametes that either have one chromosome too many or one too few.
Trisomy
organisms with one extra chromosome (three of one chromosome instead of pair)
Monosomy
organisms with one less chromosome (one chromosome instead of a pair)
Trisomy 21
commonly known as Down Syndrome. The individual possesses three of chromosome 21 and 47 chromosomes in total.
Nuchal translucency scan
non-invasive test, carried out between 11 and 13 weeks, an ultrasound scan looks at the nuchal fluid behind the neck of the fetus.
A large amount of nuchal fluid is strongly correlated with Down Syndrome, and the mother may be advised to have an amniocentesis or chorionic villus sample.
These tests are more invasive, but result in a karyotype of fetal cells so the parents can be sure.
Amniocentesis
“amniotic fluid” & “puncture”
A sample (20 mL) of amniotic fluid is taken using a syringe.
The needle is guided by using an ultrasound.
Amniotic fluid contains cells from the fetus, and these cells are used for karyotyping.
Can be carried out in the 16th week of the pregnancy with around a 1% chance of a miscarriage
Chorionic Villus Sampling
The chorion is a membrane which is a part of the placenta.
It contains many cells with villi, and some of these are taken and then cultured.
The needle is guided by using an ultrasound.
When sufficient cells have grown, a karyotype is carried out.
Can be carried out in the 11th week of the pregnancy with around a 2% chance of a miscarriage
