Ploidy refers to the number of complete sets of chromosomes within a cell. Cells are commonly described as either haploid (n), containing a single set of chromosomes, or diploid (2n), possessing two sets, typically one from each parent.
Meiosis produces gametes for sexual reproduction. It halves the chromosome number, ensuring that when two gametes combine during fertilization, the offspring receives the correct diploid chromosome count for the species.
Understanding Ploidy Before Meiosis
Cells begin meiosis in a diploid state, containing two sets of chromosomes. These chromosomes exist as homologous pairs, meaning each pair consists of one chromosome inherited from each parent. For instance, a human somatic cell is diploid, with 46 chromosomes arranged in 23 homologous pairs.
Before meiosis, the cell undergoes interphase, which includes the S phase. During the S phase, the cell’s DNA undergoes replication. Each chromosome duplicates itself, resulting in two identical sister chromatids that remain attached at a central point called the centromere.
Despite this doubling of DNA content, the cell’s ploidy remains diploid (2n). This is because the number of chromosome sets has not changed; each replicated chromosome is still considered a single chromosome for ploidy. However, the amount of DNA effectively doubles, often represented as 4c, where ‘c’ signifies the DNA content of a haploid cell.
Ploidy Reduction in Meiosis I
Meiosis I is the first major division, termed the “reductional division” because it halves the cell’s ploidy. This stage is characterized by the separation of homologous chromosomes.
Prophase I begins with homologous chromosomes pairing up (synapsis). Within these paired homologous chromosomes, segments of genetic material can be exchanged through a process known as crossing over, which contributes to genetic diversity.
In Metaphase I, the homologous pairs align along the cell’s central plate. During Anaphase I, the paired homologous chromosomes separate and move to opposite poles of the cell, with each chromosome still consisting of two sister chromatids.
At the completion of Telophase I and cytokinesis, the original diploid cell divides into two new cells. Each of these new cells is now considered haploid (n) because it contains only one chromosome from each homologous pair, even though each chromosome still comprises two sister chromatids. This reduction from a diploid (2n) to a haploid (n) chromosome number is the defining event of Meiosis I.
Ploidy Status in Meiosis II
Meiosis II follows Meiosis I without further DNA replication. This second meiotic division is known as the “equational division” because it resembles mitosis and does not further reduce the chromosome number. The cells’ ploidy remains haploid (n) throughout Meiosis II.
The main event in Meiosis II is the separation of sister chromatids. In Prophase II, the nuclear envelope breaks down and chromosomes condense. During Metaphase II, the chromosomes, each still made of two sister chromatids, align individually at the equatorial plate.
Anaphase II sees sister chromatids pull apart and move to opposite poles, becoming individual chromosomes. Finally, Telophase II and cytokinesis result in the formation of four daughter cells. While the chromosome number per cell remains haploid (n), the DNA content per cell is further reduced, from 2c to 1c, as the sister chromatids have separated.
Outcome of Meiosis
Meiosis culminates in four genetically distinct haploid cells. These cells (sperm or egg) each contain a single set of chromosomes.
When two haploid gametes fuse during fertilization, the species’ diploid chromosome number is restored in the zygote. This mechanism ensures the chromosome count remains stable across generations. Crossing over in Meiosis I and the random assortment of homologous chromosomes contribute significantly to genetic variation among offspring.