S phase: The period during which DNA is synthesized. In most cells, there is a narrow window of time during which DNA is synthesized; even though DNA synthesis is confined to a narrow window, the synthesis of organelles occurs throughout interphase.
G2 phase: The period after DNA synthesis has occurred but prior to the start of prophase. The cell synthesizes proteins such as kinase, which is necessary for cell division at this time and continues to increase in size. The chromosome begins to condense and the proteins necessary for construction of the mitotic spindle also are synthesized. When the chromosomes become visible the cell enters the first stage of mitosis, prophase.
In the last part of interphase, the cell still has nucleoli present. The nucleus is bounded by a nuclear envelope and the cell's chromosomes have duplicated but are in the form of chromatin. In animal cells, two pair of centrioles formed from the replication of one pair are located outside of the nucleus
Prophase I
This stage is very different form the mitosis prophase and in meiosis, this is the stage where gene variation occurs, it is a lengthy stage and a number of actions take place during it.
The early prophase is almost exactly like it is during mitosis: the DNA condenses, and each chromosome becomes stainable and is seen as two chromatids attached at a centromere; the nuclear membrane is still present; the centrioles begin to move towards the poles of the cell and microtubules assemble around each of them. In mid prophase the homologous chromosomes pair up and lie alongside each other becoming bivalents, this is called synapsis (note, this does not happen at all during mitosis).
During the last part of prophase crossing over occurs, this is stage were variation really occurs:
Crossing-over
Each chromatid contains a single molecule of DNA. So the problem of crossing over is really a problem of swapping portions of adjacent DNA molecules.
It must be done with great precision so that neither chromatid gains or loses any genes. In fact, crossing over has to be sufficiently precise that not a single nucleotide is lost or added at the crossover point if it occurs within a gene. Otherwise a frameshift would result and the resulting gene would produce a defective product or, more likely, no product at all.
Note that each recombinant DNA molecule includes a region where nucleotides from one of the original molecules are paired with nucleotides from the other. The need for a smooth double helix guarantees that each exchange takes places without any gain or loss of nucleotides. So long as the total number of nucleotides in each strand and the complementarity (A-T, C-G) is preserved, this "heteroduplex" region (which may extend for hundreds of base pairs) will only rarely have genetic consequences.
Metaphase I
In comparison with the length and complexity of prophase I, this stage is relatively straightforward.
- the nuclear membrane breaks down
- the nuclear spindles form
- the bivalents move to the equator of the spindle and each chromosome becomes attached to a spindle fibre by its centromere
The orientation of paternal and maternal homologues at the cell equator is random. Therefore, although each cell produced by meiosis contains only one of each homologue, the number of possible combinations of maternal and paternal homologues is 2n, where n = the haploid number of chromosomes. In this diagram, the haploid number is 3, and 8 (23) different combinations are produced.
Random assortment in humans produces 223 (8,388,608) different combinations of chromosomes. Furthermore, because of crossing over, none of these chromosomes is "pure" maternal or paternal. It is safe to conclude that of all the billions of sperm produced by a man during his lifetime (and the hundreds of eggs that mature over the life of a woman); no two have exactly the same gene content.
Anaphase I
The movement and contraction of the spindle fibres causes whole chromosomes to move apart towards opposite poles of the cell, so homologous chromosomes separate. At this stage of meiosis I the centromeres do not separate: chromatids within one chromosome stay together; this is very different from the anaphase in mitosis.
Telophase I
Generally a nuclear membrane forms around each group of chromosomes, the spindle fibres disappear, and the centrioles divide into two. However, sometimes telophase and interkinesis do not occur. In some organisms these phase are even skipped; this implies that no new nuclear membrane is built around the two nuclei after anaphase I and that the cell directly proceeds to meiosis II. In other organisms telophase I and interkinesis last very shortly; the chromosome temporarily decondense and are less visible for a period, while a nuclear membrane is formed around each new nucleus.
Meiosis II begins immediately or straight after interkinesis if it occurs; it is very similar to mitosis and starts with prophase II.
Prophase II
The centrioles move towards opposite poles of the cell; this stage is generally characterized by the by the presence of a haploid number of chromosomes that condense again.
Metaphase II
The chromosomes move again to the equatorial plane between the poles. However, this plane is perpendicular to the equatorial plan of Metaphase I.
Anaphase II
The centromeres divide and sister chromatids move towards opposite poles.
Telophase II
Each chromatid is now called a chromosome; a nuclear membrane forms around each group of chromosomes. Spindle fibres may disappear and the centrioles may divide into two.
In conclusion; meiosis consists of two parts (after interphase) – the first part, consisting of prophase I, metaphase I, anaphase I, and Telophase I, involves the cell splitting into two diploid cells; during prophase I, crossing over occurs and variation is brought about. This the second part of meiosis – prophase II, metaphase II, anaphase II, and telophase II; each diploid cell splits to become two haploid cell; yielding the end result of four haploid gametes.
