Meiosis


Meiosis requires two divisions, meiosis I and meiosis II, which in humans reduce the 46 chromosomes (23 pairs) in a diploid cell to 23 chromosomes in each haploid gamete. In the first meiotic division, meiosis I, prophase I includes five substages:

  1. leptotene-earliest condensation;
  2. zygotene-synapsis, or pairing up;
  3. pachytene-crossing over;
  4. diplotene-X-shaped chiasmata form; and
  5. diakinesis-homologue separation.

 

During synapsis, homologues (homologous chrmosomes) are bridged by the synaptonemal complex which is made up of protein and RNA. In crossover events, enzyme complexes called recombination nodules overlie the homologues. Crossovers (regions along chromatids which break and cross over to produce new arrangements of chromatids) are a major source of genetic recombination, the creation of new gene associations in the chromosomes.

The highly condensed (coiled) chromosomes are aligned on the metaphase plate. Centromeric spindle microtubules from one pole attach to only one side of each centromere. In anaphase I, no centromeric division occurs, and each set of homologues separates, going to opposite poles (reduction division). During telophase I, the reorganization of the nucleus occurs, followed by cytoplasmic division.

 


Meiosis II closely resembles mitosis, and in anaphase II, centromeres divide and single chromosomes (former chromatids) are drawn to opposite poles.

In Prophase II, the chromosomes condense, with each containing two chromatids still attached by their centromeres. The centromeric microtubules become attached in standard mitotic fashion.

In metaphase II the chromosomes line up on the metaphase II plate in preparation for separation. The centromeres divide and the chromatids separate into individual chromosomes, which are then pulled apart in anaphase II.

In telophase II the nuclear envelope is formed around the four groups of decondensing chromosomes. Cytokinesis results in gametes--egg or sperm.

 

Meiosis halves the chromosome number of eggs and sperm, making fertilization feasible. It also provides a means of shuffling and reorganizing chromosomes, increasing genetic variation in offspring.