Question

How does meiosis change chromosome number?

Answer 1

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 eggs of sea urchin.

Complete answer:
Meiosis is a special type of cell division in germ cells of sexually reproducing organisms. It occurs to maintain the ploidy level and to reshuffle- the genetic content of the organism, which generates the genetic diversity in the offspring. The maternal and paternal genetic material are crossed and are recombined to produce gametes- sperm or egg cells. 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. Meiosis II is also divided into 4 major stages- Prophase II, Metaphase II, Anaphase II, and Telophase II. The cells in the Interphase stage undergo replication to form sister chromatids before Meiosis I.

In this process, a single cell divides to form 4 daughter cells each having- one paternal and one maternal chromatid, and the DNA content and the number of chromosomes- are reduced to half. 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.

In the Anaphase I phase, the two homologous chromosomes are pulled apart and moved to the opposite ends of the cells. By the end of Meiosis I, two daughter cells are formed that have half the chromosomal content. These two daughter cells enter the process of Meiosis II.
In the Anaphase II phase, the sister chromatids also separate- and are pulled by the microtubules at the opposite poles. By the end of Meiosis II, four haploid daughter cells are formed.
The reduction in the number of chromosomes occurs due to the two nuclear and cytokinesis divisions and the separation of the homologous chromosomes.

Note: The reduction of the chromosomes during meiosis depends on two key factors- the attachment of the chromosomes to the spindle and the cohesion or attachment between the sister chromatids. The pattern of attachment differs between the two meiosis as the kinetochores are present in different positions.