Week 1 L2 - Meiosis and Segregaton Flashcards
How many chromosomes do humans have?
- 46 chromosomes - 22 pairs (diploid) autosomes (non-sex chromosomes) plus 1 pair of X/X or X/Y (sex-chromosomes)
- About 2m in length per cell
What is a karyotype?
An image / flat lay of all the chromosomes of an organism.
What are Mendelian factors?
Genes
What is the chromosome theory of inheritence?
The theory that Mendelian factors (genes) are located on chromosomes. And it’s the chromosome as a unit that is inherited from generation to generation.
The different outcomes of meiosis vs mitosis?
Mitosis ensures genetic identity between cells,
and meiosis contributes to genetic variability by forming
new combinations of chromosomes.
The 5 stages of meiosis 1
‘IPMAT’
* Interphase 1 - each single chromosome of a pair (not joined by a centromere) duplicate and split, but do remain joined by a centromere (called sister chromatids).
* Prophase 1 - homologous chromosome pairs find each other and align to form tetrads, held together by covalent links at homologous junctions (called chiasmata - sequences where strand breaking and re-joining occurs). Very precise, allows ‘swapping over’ to occur between non-sister chromatids down to the nucleotide.
* Metaphase 1 - The nuclear envelope dissipates and spindle fibres attach to kineticores, moving the paired
chromosomes to the spindle equator/metaphase plate in
preparation for metaphase. Unlike mitosis, chromosomes assemble independently. The sister chromatids of one homologue orient toward one pole, and those of the other homologue to the opposite pole (randomly).
* Anaphase 1 - Homologous chromosomes separate and migrate to opposite poles (arms don’t split as in mitosis). Each chromosome sorted independently, i.e. chromosome 1 can be from the father, chromosome 2 from the mother, etc. Note that one pole will contain the father’s allele, the other the mother’s allele.
* Telophase 1 - The cell divides, producing two cells each with just one set of chromosomes (haploid cells). Original cells were diploid - 2 pairs of chromosomes, one from the father, the other from the mother. The cells produced at telophase I are haploid, because they contain either the mother’s or the father’s chromosome, not both (excluding of course elements that crossed over).
Meiosis II
Very similar to mitosis, but no replication of chromosomes
• At metaphase, chromosomes assemble independently on the metaphase plate (no homologous pairs present)
• Centromeres split, each arm goes to opposite poles during anaphase
• 4 haploid daughter cells formed during telophase
In meiosis, identify the differences between metaphase I and metaphase II.
In metaphase I, homologous pairs align on the metaphase plate, whereas in metaphase II, the chromosomes assemble independently and split.
In meiosis, identify the difference between anaphase I and anaphase II.
In anaphase I, the homologous pairs of chromosomes are split apart and pulled to opposite poles, whereas in anaphase II, the independent chromosomes line up and split in two, with each half being pulled toward opposite poles.
Independent assortment of homologous chromosomes
and genetic diversity
Due to the way in which chromosomes are independently sorted during meiosis, no chromosome pair has any influence on any other.
• Allows for tremendous number of chromosome combinations in different gametes
- 2n where n = the haploid chromosome number
- i.e. in humans there are 2^23 = 8,388,608 possible chromosome combinations
• Crossing-over generates even more variation
• Advantageous alleles, and combinations of alleles, lead to evolutionary changes.
Mendel’s first law - Segregation
- Two members of a gene pair (alleles) segregate from each other in the formation of gametes
- Half the gametes carry one allele, the other half carry the other allele
Mendel’s second law - Independent assortment
• Genes for different traits segregate (assort) independently of one another
• For example, when considering a cross between individuals with 2 traits, calculate probabilities of expected offspring phenotypes as with single traits
Consider a cross between: AA Bb x aa Bb
- All offspring will have dominant A phenotype (Aa)
- 3/4 offspring will have dominant B phenotype (1/4 BB, 1/2 Bb),
- 1/4 will have recessive b phenotype (bb)
Dihybrid crosses
• For dihybrid crosses, use the product rule - the probability of independent events occurring simultaneously is the product of their individual probabilities.
Consider a cross between: Aa Bb x Aa Bb
- Four possible phenotypes: AB, Ab, aB, and ab
- The probability of A = 3/4, a = 1/4
- The probability of B = 3/4, b = ¼
- The expected proportion of AB:Ab:aB:ab is 9:3:3:1
