Meiosis (3.3, 10.1) Flashcards
Aneuploidy
An abnormal number of chromosomes in a cell.
Types of aneuploidy
- Autosomal aneuploidy (eg. trisomies)
- Sex chromosome aneuploidy
- Polyploidy
Give two examples of autosomal aneuploidy
Downs syndrome (Trisomy 21)
Edwards syndrome (Trisomy 18)
Give two examples of sex chromosome aneuploidy
Klinefelter syndrome (extra X chromosome in men) eg. XXY
Turner syndrome (missing X chromosome in women) eg. XO
Non-disjunction
Refers to the chromosomes failing to separate correctly in meiosis, resulting in gametes with one extra, or one missing chromosome.
Trisomy
A genetic disorder where an individual has an additional copy of a chromosome (i.e. three instead of two).
Monosomy
A zygote with only one homologue.
What can influence non-disjunction?
The age of the parents. There is a particularly strong correlation between maternal age and non-disjunction.
What is Meiosis?
Cell division for the production of gametes (sex cells). It occurs in the ovaries and testes and involves the division of a diploid cell into four genetically distinct haploid nuclei.
Stage that precedes Meiosis
Interphase - Here the DNA is replicated to produce chromosomes consisting of two sister chromatids.
Meiosis 1
The division of homologous chromosomes into two intermediate cells (i.e. diploid -> haploid).
This stage is where mutations may occur that result in an abnormal number of chromosomes eg. Downs Syndrome.
First stage of Meiosis 1
Prophase 1 (P1) - The chromosomes condense and the nuclear membrane dissolves. The homologues chromosomes pair up in synapsis to form bivalents. Crossing over can occur.
Second stage of Meiosis 1
Metaphase 1 (M1) - The bivalents line up at the equator randomly. This results in independent assortment.
Third stage of Meiosis 1
Anaphase 1 (A1) - The bivalents separate and the homologous chromosomes move to opposite poles of the cell.
Last stage of Meiosis 1
Telophase 1 (T1) - Chromosomes decondense. The nuclear membrane may reform. Cell divides in cytokinesis to form two haploid (n) daughter cells.
Meiosis 2
The separation of the sister chromatids with the number of chromosomes remaining the same.
First stage of Meiosis 2
Prophase 2 (P2) - Chromosomes condense. Centrosomes move to opposite poles. Nuclear membrane dissolves.
Second stage of Meiosis 2
Metaphase 2 (M2) - Spindle fibres from centrosomes attach to the chromosomes at the centromere and align them in the centre of the cell.
Third stage of Meiosis 2
Anaphase 2 (A2) - Spindle fibres separate the sister chromatids which move to opposite poles.
Last stage of Meiosis 2
Telophase 2 (T2) - Chromosomes decondense. Nuclear membrane reforms. Cells divide in cytokinesis to form four haploid daughter cells.
Meiosis creates genetic variation by;
- Allele segregation (Mendel’s law of segregation) (Anaphase 1)
- Independent assortment (Metaphase 1)
- Crossing over (Prophase 1)
Explain how allele segregation in meiosis creates genetic variation
An individual that is a diploid has a pair of alleles for a particular trait. Each allele is carried on separate homologous chromosomes. In Meiosis 1, these are separated. So pairs of alleles are sorted into different gametes which creates variation.
Explain how independent assortment in meiosis creates genetic variation
Independent assortment occurs in Meiosis 1 (Metaphase). The bivalents are oriented randomly in the centre of the cell, so the chromosomes are sorted independently of each other.
Explain how crossing over in meiosis creates genetic variation
Crossing over occurs in Meiosis 1 (Prophase). Homologous pairs of sister chromatids undergo synapsis, where they line up side by side. During synapsis, the chromatids can tangle (chiasmata formation) and exchange sections of DNA. So all of the chromatids will be genetically different from each other. This process is called crossing over.
