Meiosis is a specialized form of cell division reserved for germ cells, the precursors to sperm and egg. This two-part process is foundational to sexual reproduction, ensuring that the resulting gametes possess the correct amount of genetic material. Meiosis transforms a single parent cell into four genetically distinct daughter cells. This process is often called a reduction division because it cuts the total number of chromosomes by half.
Defining Ploidy: Diploid vs. Haploid
Understanding the outcome of meiotic division requires defining ploidy, the number of chromosome sets a cell contains. A cell is considered diploid ($2n$) if it possesses two complete sets of chromosomes, receiving one set from each parent. Nearly all non-reproductive cells, or somatic cells, in the human body are diploid, containing 46 chromosomes organized into 23 homologous pairs.
Conversely, a cell is designated as haploid ($n$) when it contains only a single set of chromosomes, meaning it has one representative from each homologous pair. Mature gametes, such as sperm and egg cells, are the primary examples of haploid cells. In humans, this means a haploid cell contains 23 total chromosomes, one from each type.
The Mechanism of Meiosis I
Meiosis begins with a diploid cell that has already replicated its DNA, resulting in each chromosome consisting of two identical sister chromatids joined at a centromere. Meiosis I is categorized as the reductional division because it is responsible for halving the chromosome number. During Prophase I, the homologous chromosomes physically pair up and exchange segments of DNA in a process called crossing over, which introduces genetic variation.
During Anaphase I, the spindle fibers pull the paired homologous chromosomes apart. Unlike in mitotic division, the sister chromatids remain attached to each other. One full, replicated chromosome from each homologous pair moves toward a pole of the cell. This separation of homologous pairs reduces the chromosome number.
The State of the Cell After Meiosis I
The classification of cells after Meiosis I is based on the chromosome number, which is determined by the number of centromeres present. Because the homologous pairs separated in Anaphase I, the resulting two daughter cells contain only one chromosome from each original homologous pair. This reduction means that the cells are now classified as haploid ($n$) in terms of their chromosome number.
A significant distinction must be made regarding the composition of these chromosomes. While the cell is haploid in number (e.g., $n=23$ in humans), each chromosome is still composed of two sister chromatids. The genetic material has been reduced by half in terms of chromosome sets, but the DNA content is still high because the chromosomes are in a replicated state. The cell has achieved the correct number of chromosomes for a gamete, but the genetic material is still doubled within each chromosome structure.
Why Meiosis II is Required
The replicated state of the chromosomes after Meiosis I necessitates a second round of division to produce functional gametes. If the process stopped after Meiosis I, the resulting cells would have the correct chromosome number ($n$) but would carry twice the required DNA content for fertilization. A sperm and egg with replicated chromosomes would fuse to create a zygote with four copies of every gene, which is genetically unstable.
Meiosis II, often referred to as the equational division, resolves this issue by separating the remaining sister chromatids. This division closely resembles mitosis, but it starts with a haploid set of replicated chromosomes. During Anaphase II, the sister chromatids finally separate at the centromere and move to opposite poles of the cell. The final result is four cells, truly haploid ($n$), containing a single, unreplicated chromosome for every type, making them ready to combine during fertilization.