MEIOTIC CELL DIVISION JANUARY 17, 2013

MEIOSIS Meiosis is the cell division which occurs only in reproductive organs during gamete production. Meiosis results in the formation of four genetically unidentical cells. Each of these cells contain half the number of chromosomes as the parent cell (the haploid or n number).

Meiosis ensures that: Each daughter cell has the haploid number of chromosomes so that the diploid number (and no more) can be restored within the zygote at fertilization.

Meiosis ensures that: Each daughter cell has a different combination of genes which leads to variation amongst offspring.

Please remember that gametes must be different from regular body cells in that they have half the chromosome number (haploid or n) so that when the mother and father gametes meet, they produce a zygote with the correct diploid number (2n) of a normal body cell.

Meiosis (meio, “ reduce”) is a form of nuclear division in which the chromosome number is halved from the diploid (2n) to the haploid number (n).

Like mitosis, it involves DNA replication during interphase in the parent cell, but this is followed by two cycles of nuclear divisions and cytoplasmic divisions, known as meiosis I (the first meiotic division) and meiosis II (the second meiotic division).

Thus a single diploid cell gives rise to four haploid cells. Meiosis occurs during the formation of sperms and eggs (gametogenesis) in animals and during spore formation in plants.

Like mitosis, meiosis is a continuous process but is conveniently divided into two prophase, two metaphase, two anaphase and two telophase. These stages occur in the first meiotic division and again in the second meiotic division.

CHROMOSOMES AND MEIOSIS

HOMOLOGOUS CHROMOSOMES

MEIOSIS

PROPHASE I The longest phase. Homologous chromosomes pair up in a process called synapsis. One of the pair comes from the father and the other from the mother.

PROPHASE I

CHIASMATA FORMATION Homologous chromosomes pair up. Chromatids now visible. Chromosomes are joined at several places along their length (chiasmata). Each chiasma is the site of an exchange between chromatids.

It is produced by the breakage and reunion between any two of the four strands present at each site. As a result, genes from one chromosome may swap with genes from the other chromosome leading to new gene combinations in the resulting chromatids. This is called crossing over (slides 33-36).

METAPHASE I

ANAPHASE I

TELOPHASE I

INTERPHASE II This stage is present usually only in animal cells and varies in length. No futher DNA replication occurs.

PROPHASE II

METAPHASE II

ANAPHASE II

TELOPHASE II

MEIOSIS IN ACTION !

MEIOSIS IN SUMMARY

Recall: MITOSIS SUMMARY

RELATIONSHIP BETWEEN MITOSIS AND MEOSIS

SIGNIFICANCE OF MEIOSIS Sexual reproduction Genetic variation Independent assortment of chromosomes Crossing over

GENETIC VARIATION ..\..\..\Documents\ImTOO\Download YouTube Video\GENETIC DIVERSITY.flv ..\..\..\Documents\ImTOO\Download YouTube Video\GENETIC DIVERSITY.flv

INDEPENDENT ASSORTMENT Bivalent arrange themselves randomly on the equator of the spindle fibres. Independent assortment refers to the fact that the bivalents line up independently and so the chromosomes in each bivalent separate (assort) independently of those in other bivalents during anaphase I.

ANAPHASE I

CROSSING OVER Chromosomal crossover (or crossing over) is an exchange of genetic material tween homologous chromosomes. It is one of the final phases of genetic recombination, which occurs during prophase I of meiosis (pachytene) in a process called synapsis. Crossover usually occurs when matching regions on matching chromosomes break and then reconnect to the other chromosome.

Consequences of cross-over In most eukaryotes, a cell carries two copies of each gene, each referred to as an allele. Each parent passes on one allele to each offspring. An individual gamete inherits a complete haploid complement of alleles on chromosomes that are independently selected from each pair of chromatids lined up on the metaphase plate.

Without recombination, all alleles for those genes linked together on the same chromosome would be inherited together. Meiotic recombination allows a more independent selection between the two alleles that occupy the positions of single genes, as recombination shuffles the allele content between homologous chromosomes.

Rarely, crossovers can also occur between similarities in sequence at other regions. The result is mismatched alignments. These processes are called unbalanced recombination.

Severe problems can arise if a gamete containing unbalanced recombinants becomes part of a zygote. The result can be a local duplication of genes on one chromosome and a deletion of these on the other, a translocation of part of one chromosome onto a different one, or an inversion.

COMPARISON OF MITOSIS AND MEIOSIS

MITOSIS AND MEIOSIS Mitosis is necessary for: Growth of cells Repair of cells Asexual reproduction (Involves only one division of genetic material). Meiosis is necessary for: The production of male and female gametes for sexual reproduction. These are the egg and sperm cells. (Involves twp divisions of genetic material).