Meiosis

Meiosis - meiosis is the process by which one diploid eukaryotic cell divides to generate four haploid cells often called gametes. - meiosis is essential for sexual reproduction and therefore occurs in all eukaryotes (including single-celled organisms) that reproduce sexually. - meiosis does not occur in archaea or bacteria, which reproduce via asexual processes such as mitosis or binary fission. - a few eukaryotes, notably the Bdelloid rotifers, have lost the ability to carry out meiosis and have acquired the ability to reproduce by parthenogenesis.

Meiosis - meiosis is a "one-way" process, it cannot be said to engage in a cell cycle as mitosis does. - exchange of genetic material between maternally and paternally derived chromosomes. - the preparatory steps (G 1, S and G 2 ; Interphase) that lead up to meiosis are identical in pattern and name to the interphase of the mitotic cell cycle. - Interphase is immediately followed by meiosis I and meiosis II.

Meiosis I - meiosis I consists of segregating the homologous chromosomes from each other, then dividing the diploid cell into two haploid cells each containing one of the segregates. - meiosis I consists of prophase I, metaphase I, anaphase I and telophase I. - prophase I is a complicated phase which itself is subdivided into five sections namely Leptotene, Zygotene, Pachytene, Diplotene and Diakinesis.

Leptotene/Leptonema individual chromosomes begin to condense into long strands within the nucleus. However the two sister chromatids are still so tightly bound that they are indistinguishable from one another.

Zygotene/Zygonema - homologous chromosomes are attracted and pair (synapsis). - synaptonemal complex structure starts to form between paired homologous chromosomes.

Zygotene/Zygonema synaptonemal complex structure

Pachytene/Pachynema - pairing is now completed, and the chromosomes contract further. - homologous chromosomes are closely associated (now called a bivalent).

Pachytene/Pachynema - genetic crossing over occurs with the physical exchange of DNA between maternal and paternal chromosomes. - chiasmata frequency per bivalent is directly related to chromosome length. Long chromosomes may have several chiasmata, but to ensure proper segregation at anaphase I, all bivalent must have at least one chiasmata.

Diplotene/Diplonema - chromosome contraction continues. - each chromosome is now clearly visible and acts as if it is repulsing its closely paired homologue, but they are held together at the sites of crossing over (chiasmata).

Diakinesis - contraction of the chromosomes is nearly maximal. - the nuclear membrane dissociates. - the paired chromosomes, held together by chiasmata, rotate in various planes so that they position themselves in a state of maximum repulsion and start to orientate on the metaphase plate.

Diakinesis - number of chiasmata in locust: i) bivalent, three chiasmata; ii) bivalent, two chiasmata, ring formed; iii) bivalent, one terminal chiasmata; vi) bivalent, cross-shaped, one chiasmata.

Metaphase I - the chromosomes lie on the equatorial plate, centromeres attached to the spindle fibres.

Anaphase I - the bivalents separate and the homologues are pulled to opposite poles.

Telophase I and Interphase - this is often a very rapid process such that cytokinesis may not occur. -There is no replication of DNA, so each nucleus contains half-bivalents, I.e. the haploid chromosome number.

Metaphase II - the chromosomes align on metaphase plate of newly formed spindle.

Anaphase II - the centromeres split and one daughter chromatid moves to each pole.

Telophase II - the interphase nuclei are reformed and cytokinesis occurs, forming four haploid daughter nuclei.

What is the significance of meiosis?

Significance of Meiosis - maintains chromosome number. - produces genetic variation.