During interphase :-
- DNA Replication occurs
- ATP Synthesis
- Protein Synthesis
- Cell size increases
- Centrioles divide
- Chromatin is unwound
When the cell is not dividing it is said to be in interphase. During this phase the DNA content of the cell doubles and new organelles are also formed.
2 Cell Division occur.
Homologous chromosomes pair up and their chromatids wrap around each other and equivalent portions of these chromatids may be exchanged in a process called Crossing Over.
By the end of this stage, the homologous pair seperate with 1 chromosome of each pair going into one of the 2 daughter cells.
This results in 2 daughter nuclei containing half the number of chromosomes.
Homologous = a pair of chromosomes, 1 maternal and 1 paternal, that have the same gene loci and therefore determine the same feature.
Chromatids = One of the two copies of a chromosome that are joined together by a single centromere prior to cell divison.
The chromosome become shorter and thicker and split into 2 chromatids. The centrioles move to opposites poles of the cell and microtubules begin to radiate from them forming asters which form spindles.
Homologous chromosomes associate in pairs and each pair is called a bivalent. Each bivalents consists of 4 starnds, made up of 2 chromosmes, each splits into 2 chromatids which wrap around each other and then partially repel each other but remain joined at certain points called Chiasmata.
This exchange of pieces of chromosomes is called Crossing Over and is a source of Genetic Variation. At the end of Prophase the Nuclear membrane disintegrates and the nucleolus disppears.
Homologous Chromosomes carry the same genes at the same position on chromosomes but one Allele is from the mother and other from the father. Both Alleles may be Dominant or Recessive. It is the different forms of the same Alleles that are exchanged during crossing over.
Spindle fibres are formed.
When the pairs of homologous chromosomes align themselves on the equator of the spindle, the maternal and paternal chromosomes are arranged randomly.
Centromeres on bivalents join to the spindle fibres.
This random distribution and consequent independent assortment of chromosomes produces new genetic combinations.
The chromosomes in each bivalent seperate and 1 of each pair is pulled to one pole, its sister chromosome to the opposite pole.
Thus each pole receives only one of each homologous pair of chromosome and because of their random arrangement at metaphase these will be a random mixtue of maternal and paternal chromosomes.
This is called Independent Assortment of chromosome and produces new genetic combinations
The chromosomes reach the opposite poles and the nuclear envelope reforms around each group of haploid chromosome.
Usually the Chromosomes stay in their condensed form and Meiosis II follows on immediately.
In animal cells Cytokinesis occurs, that is, the division of the cytoplasm to give two haploid cells.
Many plant cells go straight into Meiosis II with no formation of the spindle.
Cytokinesis = Division of the cytoplasm to give 2 haploid cells.
Many plant cells go straight tinto Meiosis II with no reformation of the spindle.
- Prophase II
Chromosomes condense and become visible as 2 chromatids (not identical) joined by a centromere.
- Metaphase II
Chromosomes line up seperately on the equator of the spindle, with each chromosome attached to a spindle fibre by its centromere.
- Anaphase II
The centromeres divide and the chromatids are pulled to opposite poles.
- Telophase II + Cytokinesis
On reaching the poles the chromatids lengthen and are indistinct. The spindle disappears and the nuclear membrane re-forms. After Two Meiotic Divisions four haploid daughter cells are formed and the genetic makeup of each cell is different.
Significance of Meiosis
Meiosis is the reduction division that occurs during gamete formation in sexually reproducing organisms. In this Division the Diploid number of Chromosomes (2n) is reduced to the Haploid (n) .
Thus, when two gametes join together at fertilistaion the zygote that is formed has two complete sets of chromosomes returning to the diploid condition.
However, meiosis does more than halve the chromosomes into a cell, it also introduces genetic variation into gametes and therefore the zygotes that are produced.
The two events that take place during meiosis that help to produce genetic variation are :
- Independent Assortment of homologous chromosomes
- Crossing over which happens between the chromatids of homologous chromosomes.
When these genetically different gametes fuse, randomly, at fertilistaion, more variation is produed amongst the offspring.
Significance of Meiosis Continued.....
In the long term, if a species is to survive in a constantly changing environment and to colonise new environements, sources of variation are essential.
Three ways of creating variety:
- Each of the chromosomes making up a homologous pair carries different genetic material. During sexual reproduction the Genotype of one parent is mixed with that of the other when haploid gametes fuse.
- The different pairs of homologous chromosomes arrange themselves on the spindle during metaphase I of Meiosis. When they subsequently seperate they do so entirely independently of each other, so the daughter cells contain different combinations of maternal and paternal chromosomes.
- Crossing Over during chiasmata formation during Prophase I of meiosis. Equivalent parts of Homologous Chromosomes may be exchanged thus producing new combinations and the separation of linked genes.