Meiosis is a specialized form of cell division fundamental to sexual reproduction. It creates gametes, such as sperm and egg cells, with a reduced number of chromosomes. Meiosis precisely halves the chromosome count, preparing for the combination of genetic material from two parents during fertilization. This reduction maintains the correct chromosome number across generations.
Chromosomes and Ploidy
Chromosomes are thread-like structures carrying genetic information in the form of genes. Cells are categorized by their chromosome count as diploid (2n) or haploid (n). Diploid cells contain two complete sets of chromosomes, one from each parent. For instance, human body cells are diploid, with 46 chromosomes in 23 pairs.
Haploid cells contain only one complete set of chromosomes. These are reproductive cells, like human sperm and egg cells, each with 23 unpaired chromosomes. In diploid cells, homologous chromosomes are pairs carrying the same genes at corresponding locations, though they may have different versions of these genes, called alleles.
The First Meiotic Division
Meiosis I, the first stage of meiosis, is a “reductional division” because it halves the number of chromosome sets. During this division, homologous chromosomes, each replicated into two sister chromatids, pair up in synapsis. These paired homologous chromosomes then align and separate, with one chromosome from each pair moving to opposite poles.
The separation of homologous pairs reduces the chromosome number per cell. During this process, segments of DNA can be exchanged between non-sister chromatids through crossing over, contributing to genetic variation. Sister chromatids remain attached throughout Meiosis I.
Chromosome Number After Meiosis I
After Meiosis I, the original diploid cell divides into two daughter cells. Each resulting cell is haploid (n). However, each chromosome within these haploid cells still consists of two sister chromatids. While the number of chromosome sets is halved compared to the original cell, the genetic material within each chromosome remains duplicated.
For human cells, a diploid cell with 46 chromosomes (23 homologous pairs) undergoes Meiosis I to produce two cells. Each cell then contains 23 chromosomes, with each chromosome still composed of two sister chromatids. Thus, while the chromosome number is halved, the DNA content per cell is not yet fully halved to its final gamete state.
Completing the Meiotic Process
Following Meiosis I, the two haploid cells transition into Meiosis II, which is similar to mitosis. Meiosis II is an “equational division” because it does not further reduce the chromosome number. Its function is to separate the sister chromatids that remained joined during Meiosis I.
Each of the two cells from Meiosis I undergoes this second division, separating sister chromatids into individual chromosomes that move to opposite poles. The entire meiotic process ultimately forms four haploid cells, each with a single set of unduplicated chromosomes. This final haploid state is necessary for sexual reproduction, ensuring that when two gametes fuse during fertilization, the resulting zygote restores the characteristic diploid chromosome number.