1.6 Meiosis

Significacne of meiosis

- It keeps the chromosome number constant from one generation to the next - It generates genetic variation in the gametes and therefore the zygotes that they produce. This can happen either/both by crossing over during prophase I or by independent assortment at metaphase I (combinations of maternal/paternal chromosomes) or II (combinations of chromatids). - In the long term, if a species was to survive in a constantly changing environment and to colonise new environments, sources of variation are essential.

Metaphase I

- Pairs of homologous chromosomes arrange themselves at the equator of the spindle. - In a homologous pair, one chromosome is from the mother and the other is from the father. They lie at the equator randomly, with either one facing either pole. So, a combination of paternal and maternal chromosomes faces each pole and the combination of chromosomes that goes into each daughter cell at meosis I is random with respect to which parent they came from. This is called independant assortment of chromosomes and produces new genetic combinations, with genes from both parents going into both daughter cells.

Prophase I

- Paternal and maternal chromosomes come together in homologous pairs. This pairing of chromosomes is called synapsis, and each homologous chromosome pair is a bivalent. - The chromosomes coil up, condensing to become shorter and thicker, visible as two chromatids. - In animals and lower plants, where centrioles are present, the centrioles seperate and move to the poles of the cells. They organise the polymerisation of microtubules, which radiate out from them, and the spindle forms. - By the end of prophase, the nuclear envelope has disintegrated and the nucleolus has dissapeared

Differences between Meiosis I and II (bullet points)

- Prophase I follows DNA replication and crossing over vs Prophase II doesn't - Metaphase I homologous pairs align on either side of the equator and there is independent assortment of said pairs vs Metaphase II has chromosomes on the equator with independent assortment of chromatids - Anaphase I seperates chromosomes and produces two halpoid daughter cells vs Anaphase II seperates chromatids and produces 4 haploid cells.

Meiosis

A two-stage type of cell division in sexually reproducing organisms (plants, animals and some protoctistans) that results in the formation of four genetically distinct haploid gametes

Telophase II and cytokinesis

At the poles, the chromatids lengthen and can no longer be distinguished in the microscope. The spindle disintegrates and the nuclear envelope and nucleoli re-form. Cytokinesis takes place, producing four haploid daughter cells.

Meiosis I vs II (paragraph)

DNa replication in the interphase is followed by two divisions, called meiosis I and meiosis II. Each division goes through the same sequence of steps as mitosis. However, there is no more DNA replication between the two divisions. That only happens once, before meiosis I. By the end of meiosis I (reduction division), the homologous pairs of chromosomes have seperated, with one chromosome of each pair going into either of the two daugther cells. Each daughter cell has only one of each homologous pair, so they contain half the number of chromosomes of the parent nucleus. In meiosis II, chromatids seperate and the two new haploid nuclei divide again. Four haploid nuclei are formed from the parent nucleus, each containing half the number of chromosomes and every gamete is genetically unique.

Independent assortment

Either of a pair of homologous chromosomes moves to either pole at anaphase I of meiosis, independently of the chromosomes of other homologous pairs

Number of chromosomes

In meiosis, the diploid number of chromosomes is halved to haploid. When two haploid gametes fuse at fertilisation, the zygote formed has two complete sets of chromosomes, one from each gamete, restoring the diploid condition. If the chromosome number did not halve during gamete formation, the number of chromosomes would double every generation.

Telophase I

In some species, the nuclear envelope reforms around the haploid group of chromosomes and the chromosomes decondense and are no longer visible. But in many species, the chromosomes stay in their condensed form becasue they would disintegrate again anyway. It happens in meiosis in female mammals because there is a long time between meiosis I and II. But, in male mammals, as well as other organisms, meiosis II follows meiosis I so rapidly that the nuclear envelope does not re-form; there is no telophase I or interphase.

Prophase in meiosis vs mitosis

Prophase I differs from prophase of mitosis because the homologous chromosomes associate in their pairs, the bivalents. The chromatids wrap around each other and then partially repel one another but remain joined at points called chiasmata. At the chiasma, a segment of DNA from one chromatid may be exchanged with the equivalent part from a chromatid of the homologous chromosome. This swapping is called crossing over and is source of genetic variation, because it mixes genes from the two parents in one chromosome. This 'genetic recombination' produces new combinations of alleles. Just one cross-ver results in four different gametes, but it can happens at several places along the length of the chromatid and so there are huge numbers of different genetic combinations made.

Meiosis II

Sometimes is described as resembling mitosis, because there is no pairing of homologous chromosomes at metaphase and it is the chromatids, rather than the homologous chromosomes, that seperate at anaphase.

Prophase II

The centrioles seperate and organise a new spindle at right angles to the old spindle. Also, DNA replication never precedes prophase II, but it always precedes prophase I.

Anaphase II

The centromeres divide and the spindle fibres shorten, pulling the chromatids to opposite poles

Anaphase I

The chromosomes in each bivalent seperate and, as the spindle fibres shorten, one of each pair is pulled to one pole, and the other to the opposite pole. Each pole receives only one of each homologous pair of chromosomes and, because of their random arrangment at metaphase, there is a random mixture of maternal and paternal chromosomes.

Metaphase II

The chromosomes line up on the equator, with each chromosome attached to a spindle fibre by its centromere. Independent assortment happens because the chromatids of the chromosomes can face either pole.

Cytokinesis

The divsion of cytoplasm occurs, making two haploid cells.

chiasma (plural, chiasmata)

The site as seen in the light microscope, at which chromosomes exchange DNA in genetic crossing over.