5.2.1 - Meiosis Flashcards
Meiosis
DNA
deoxyribonucleic acid
* Strucutred in a double helix (circular in prokaryotes and linear in eukaryotes)
* Chromatin - DNA wrapped around histone proteins. This is how data is contained in the nucleus.
Human Karyotype
There are 23 pairs of chromosomes
* Pairs because one is from the mother, one from the father.
The 23rd chromosome is the sex chromosome (fem: XX, mal: XY)
The other 22 are called autosomes
* **In all body cells of organisms that reproduce sexually, there are is a maternal and paternal copy of each chromosome **
Homologous pairs
Genetically identical chromosomes:
* Joined by a centromre in the same location
* Length of the chromatids are the same
* Banding are the same
Genes that are the same, but alelles that are different (from different parents)
When to use sister chromatids/chromatids
Chromosome has two sister chromatids:
* Called sister when the chromosomes condense and to describe the chromatids as they are genetically identical
When they separate (centromere breaks), just call them two chromosomes (1 chromatid each)
Mitosis and Meiosis similarities
Mitosis and meiosis go through the same phases.
* Interphase occurs before nuclear division.
* Chromatin material transforms into chromosomes in the same way during prophase in the first meiotic division.
* Break down of nuclear material and formation of the spindles are the same.
* Cytokinesis in meiosis takes place in the same manner.
Meiosis - basic definition
Meiosis is the process that forms haploid gametes.
* 4 haploid daughter gamets are formed from each parent (diploid) cell.
* Genetic variation from crossing over and independent assortment.
Occurs in sex cells to produce gametes - in ovaries or testes
Undergoes two cell divisions
* Meiosis I: reduction division as a diploid cells results in haploid cells.
* Meiosis II: haploid forms two haploid.
Interphase in Meiosis
- Before entering meiosis 1, the cell must go through interphase
- DNA replicates, goes through G1, S, G2 just the same.
Prophase I
- Chromatin condenses and chromosomes separate into homologous pairs.
- Nuclear membrane breaks down
- Crossing over occurs (father and mother DNA exchanged) - genetic variation introduced
- Centrioles forms and moves to poles
Metaphase I
- Homologous chromosomes align in paris in the middle of the cell
- Each pair lines up indepentdently of next (independent assortment) so various combination of paternal and matermal chromosomes alignment are possible. Mother/Father chromosome can be on either side, randomly - genetic variation
- Law of independent assortment: the inheritance of one trait genetically does not affect the inheritance of another, as alleles of different genes assort independently during metaphase.
Anaphase I
- Homologous chromosomes pairs separate and each moves to opposite end of the cell by spindle fibres from the centrioles.
- Law of segregation: each allele in a gene pair separates (segregates) from the other to form gametes to each gamate receives only one allele for each gene.
- Difference with mitosis: the centromere does not split, whole chromosomes go to cell poles.
Law of segregation
Anaphase I: homologus chromsome pairs separate with each chromosome going to a different daghter cell. Although these chromosomes have two chromatids they are identical, thus following the law of segregation.
Anaphase II: sister chromatids separate, ensuring that each gamete has only one allele for each gene (chromosomes consist of identical chromatids)
Telophase I
- Two daughter cells form
- Chromosome number is halved
- Chromosome combinations in cells differ
- Spindle fibres break
- Nuclear membranes may or may not appear.
Cytokinesis I
Membrane divides by a contractile ring.
Daughter cells are not identical to each other and have half the original number of chromosomes.
Prophase II
Spindle fibres starte forming again - but at a rotated angle of 90 degrees from original poles.
Metaphase II
Chromosomes align at the equator
