Question

How does meiosis lead to genetic variation and why is this variation important?

Answer 1

Hint: The type of cell division where one diploid cell divides twice to form four haploid daughter cells is called Meiosis, a reductional division. It was first discovered by- Oscar Hertwig in 1870 in the sea urchin eggs.

Complete answer:

Meiosis occurs in the germ cells of the sexually reproducing organisms. It is responsible for reducing the ploidy level of gametes from diploid to haploid. One germ cell goes through the meiosis cycle twice to form four haploid gametes. Two nuclear divisions are followed by- two subsequent cellular divisions in this cycle. The gametes have one copy of each parent chromosome, and their chromosomal content is halved, w.r.t their parents. It occurs in two major phases- Meiosis I and Meiosis II. 

Meiosis I is further divided into 4 major stages- Prophase I, Metaphase I, Anaphase I, and Telophase I. Prophase I is further subdivided into 5 substages- Leptotene, Zygotene, Pachytene, Diplotene, Diakinesis, and Cytokinesis.

The cells in the Interphase stage undergo replication to form sister chromatids before Meiosis I. In Prophase I, the condensation of chromosomes occurs, which is followed by the pairing of homologous chromosomes. The paired chromosomes form a tetrad or a bivalent, which undergo crossing over between the non-sister chromatids of homologous chromosomes at their recombination nodules. The linkage between the chromosomes is seen on the crossing over site. Meanwhile, a synaptonemal complex forms during the chromosomal synapsis, which gets dissolved after the crossing over of the homologous chromosomes. Its dissolution leads to the separation of the tetrad and the formation of an X-shaped structure called Chiasmata. The bivalents are still linked at their crossing over site. Terminalisation of chiasmata followed by the assembly of the meiotic spindle results in the separation of the homologous chromosomes, and the disappearance of the nucleolus and nuclear membranes.

In the further stages, the tetrad arranges itself on the metaphase plate, which is followed by their separation and movement to the opposite poles through an assembly of fibers called spindle fibers. The movement of bivalents follows the law of independent assortment or the law of segregation. The sister chromatids and the centrioles remain intact. It is finally followed by cytokinesis, which results in the formation of the dyad of cells, or two cells with half of the chromosomal content.

The above process clearly explains the various steps occurring through the Meiosis I cycle. In the Prophase I stage, the crossing over and recombination occurs between the non-sister chromatids of homologous chromosomes or the maternal and paternal chromosome that results in the exchange of genetic information at the crossing over site. The exchange in the part of DNA is important for the formation of new traits and alleles. The bivalents are independently assorted during the separation. The phenomenon of crossing over, recombination, or mutation during the recombination produces genetic variability and variation in the cells.

Genetic variation is important as it introduces new traits or alleles in the population. It makes an individual different from the other and creates flexibility in the population. It acts as a major force of evolution as it helps individuals to adapt to the changing environment and undergo natural selection. It introduces a new combination of traits and variations in the population that keeps the population healthy. It results in the survival of one species over the other.

Note: Mutation in meiosis in sexually reproducing organisms can occur through the insertion, inversion, deletion, addition, or substitution of a gene sequence. The mutations can be healthy leading to the formation of a genetic variant species, or unhealthy if there is a loss or change in an important gene sequence.