1. Homologous Chromosomes
In diploid organisms, each parent, who possesses 2 complete sets of chromosomes, contributes a haploid gamete to their offspring. Fusion of the egg and sperm results in the formation of a diploid offspring.
In human, 1 complete set of chromosomes consists of 22 autosomes and 1 sex chromosome. Each autosome has been numbered as 1 to 22 and he sex chromosome is either X or Y.
A paternal chromosome 1 and a maternal chromosome 1 is referred to as a pair of homologous chromosome or homologue.
The similarities of each pair of homologues (1) has the same length and same size, (2) has centromeres in the same positions, (3) has the same number of genes, (4) has the same genes, (5) has genes arranged linearly in the same order at the corresponding positions.
They may have different alleles.
2. Significance of Meiosis
There are consequences of meiotic division.
The chromosomal number is halved. There is a reduction in the diploid number of chromosomes from 2n to n.
Genetic variation is increased. Each diploid parental cell divides to form 4 haploid daughter cells / gametes which are genetically different / non-identical.
There are biological significances of meiosis.
Sexual Reproduction. To stabilize the chromosome number i.e. chromosome number remains constant in each generation of a species. Meiosis occurs in all organisms carrying out sexual reproduction. Meiosis reduces the number of sets of chromosomes by half, so that when gametic recombination occurs, the ploidy of the parents will be reestablished. If meiosis did not occur, fusion of gametes would result in doubling of the chromosomes for each successive sexually reproduced generation.
Introduce genetic variation. Meiosis also provides opportunities for new combinations of existing alleles to occur in the gametes. This leads to genetic variation in the offspring produced by random fusion of the gametes. Genetic variation has evolutionary significance. It is the genetic basis for natural selection to occur in the process of evolution and speciation. Genetically identical individuals may be wippped out if the environment is no longer favourable for their survival. However, if a population of organisms is genetically varied, there is a greater possibility that some individuals may survive the adverse changes in the environment and continue to perpetuate the species.
Meiosis allow a mix of paternal and maternal alleles in daughter cells in 2 ways. Crossing over between non-sister chromatids of homologous chromosomes at prophase I, and independent assortment and segregation of chromosomes at metaphase I.
3. Stages in Meiosis I
3.1 Interphase I
DNA replication followed by supercoiling of chromatin into chromosomes as in mitosis.
3.2 Prophase I
Chromatin condenses into chromosomes. The nucleolus disappears. Nuclear membrane disintegrates. The centrioles migrate to the poles. The spindle forms. Involves the pairing of homologous chromosomes to form a bivalent or tetrad by synapsis. Corresponding segments of non-sister chromatids exchange at the chiasma. Hence, the sister chromatids are no longer genetically identical.
3.3 Metaphase I
Bivalents / tetrads randomly align around the equator of the spindles. Spindle fibres attach to the centromere region of each member of the homologues.
3.4 Anaphase I
The bivalents / tetrads separate and are drawn to opposite poles, centromeres first, by the spindle fibres resulting in 2 haploid sets, one set at each pole. The centromeres in anaphase I remain intact.
3.5 Telophase I
The arrival of chromosomes at opposite poles marks the end of meiosis I. Chromosome number has been halved but each of the chromosomes is still composed of 2 chromatids. Spindles and spindle fibres usually disassemble. Chromatids usually uncoil. Nuclear envelope re-forms at each pole. Nucleus enters interphase. Cleavage or cell wall formation then occurs.
4. Stages in Meiosis II
4.1 Interphase II
This stage is present usually only in animal cells and varies in length but it may be absent altogether. No further DNA replication
4.2 Prophase II
The nucleoli disperse. Nuclear envelope disappears. The chromatin shortens and thickens. Centrioles move to opposite poles of the cells and at the end of prophase II. New spindle fibres appear and are arranged at right angles to the spindle of meiosis I.
4.3 Metaphase II
Chromosomes align singly at the equator of the cell. Spindle fibres attach to the centromeres.
4.4 Anaphase II
The centromeres separate and the 2 chromatids are segregated to opposite poles, centromeres first.
4.5 Telophase II
The chromosomes uncoil, lengthen and become very indistinct. The spindle fibres disassemble/ Nuclear envelopes re-form around each nucleus. Cytoplasmic cleavage or cell wall formation will produce 4 haploid genetically non-identical daughter cells from the original single diploid parent cell.
5. Sources of Genetic Variation
5.1 Pairing of Homologous Chromosomes
Homologous chromosomes bridged by the synaptonemal complex which is made up of protein and RNA is called a bivalent / tetrad. This process is called synapsis. The function of the synaptonemal complex is unknown but is absent in homologues that do not crossover.
5.2 Non-sister Chromatids of Homologues Crossover
Each member of the homologues comprises of 2 genetically identical sister chromatids. The 2 homologues are visibly joined at several points along their lengths where non-sister chromatids overlap and intersect. This is called crossing over. The points of intersection are called chiasmata.
5.3 Mutual Exchange and Recombination of corresponding Segments of Non-Sister Chromatids of Homologues
Genes are arranged linearly along the length of a chromosome and the genes on the same chromosome are linked because they are transmitted as a unit to daughter cells during mitosis. However, to increase genetic variation, permutations of existing allelic combinations are allowed so that the offspring may have a different combination of alleles from the genetic makeup of its parents. New combinations of existing alleles may arise without changing the original gene sequence / loci on a particular chromosome.
5.4 How is this achieved?
Each chiasma becomes a site of breakage and union of non-sister chromatids of homologues. The phosphoester linkage of DNA is broken at inter-genic regions so that there is no loss of genetic information This produces a breakage between linked genes on a single chromatid.
The breakage occurs simultaneously in both the paternal and maternal homologues allowing mutual exchange of corresponding segments of genes. This is immediately followed by reunion / reattachment of segment of chromatids leading to new combinations of alleles in the resulting recombinant chromatids.
The chromatids of homologous chromosomes continue to repel each other and bivalents assume particular shapes depending on the number of chiasmata. Bivalents having a single chiasma appear as open crosses, 2 chiasmata produce a ring shape and 3 or more chiasmata produce loops lying at right angles to each other.
After crossing over and recombination, the resulting 2 of the 4 chromatids present in the tetrad end up with a mixed set of alleles, some paternal some maternal. This means that the sister chromatids of each member of the holomogous chromosomes is no longer identical. The recombinant chromatids will have both paternal and maternal alleles. When the sister chromatids segregate, each gamete will receive a unique and scrambled combination of parental alleles. However, the original position of genes an the number of genes in each chromosome has not changed. The more heterozygous the gene loci of the parental cell, the more varied the allelic combinations. Therefore, outbreeding is usually preferred as it leads to hyterozygosity. Crossing over and recombination will not give rise to any variation at all of all the gene loci of homologous chromosomes are homozygous. As such, inbreeding is discouraged as it leads to increasing homozygosity of all gene loci. Crossing over and recombination occur during prophase I of meiosis.
5.5 Independent Assortment and Segregation of chromosomes
Independent assortment refers to the random alignment of bivalents / homologues at the metaphase / equatorial plate. Each bivalent orientates independently of others. Given a simple situation in which there are only 2 bivalents, there are therefore only 2 possible orientations at the metaphase plate. The number of different gamete types in heterozygotes resulting from independent assortment of the homologues during meiosis I can be calculated using the formula 2n where n is the haploid chromosome number. This is true in most cases except when most or all the gene loci are double homozygous i.e. pure bred lines. In human, he number of gamete types can be produced on the basis of independent assortment alone is about 8.5 million possibilities. This is the basis of Mendel’s 2nd law.
