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Outline the various stages involved in meiotic division and explain clearly how variation is brought about relating this clearly to the relevant stage of meiotic division.

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Introduction

Outline the various stages involved in meiotic division and explain clearly how variation is brought about relating this clearly to the relevant stage of meiotic division. This is a special type of cell division that occurs in the production of the gametes in animals and also in the formation of spores in plants. Meiosis results in the chromosome number being halved from the normal diploid, to haploid number. The mechanism of cell division is very similar to that of mitosis but in addition, two divisions occur resulting in the production of four cells. Mitotic cell division produces new cells genetically identical to the parent cell. Meiosis increases genetic variation in the population. Each diploid cell undergoing meiosis can produce 2n different chromosomal combinations, where n is the haploid number. In humans the number is 223 which is more than eight million, due to this genetic mixing which provides variation within offspring, this species has an advantage when dealing with environmental change. Meiosis consists of interphase followed by two parts each consisting of four phases: Prophase I, Metaphase I, Anaphase I, Telophase I, Prophase II, Metaphase II, Anaphase II, Telophase II. Interphase As this is the cell's time to prepare for splitting into two completely separate cells, this is quite an intensive period of work. ...read more.

Middle

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. ...read more.

Conclusion

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. ...read more.

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