By Eric Bank, MBA, MS Finance
Meiosis is the process by which most eukaryotic organisms, those with cells having an organized nucleus, produces sex cells, the male and female gametes. Cells normally have two copies of each chromosome, one donated from each parent. This is the diploid chromosome condition and the paired chromosomes are called homologous. Meiosis transforms a diploid cell into four haploid granddaughter cells, each having a single copy of each chromosome. The process helps increase the genetic diversity of a species.
Meiosis occurs over two generations of cells. During normal cell division, or mitosis, each chromosome is copied, resulting in chromosomes containing twin sister chromatids. These joined chromatids later separate in mitosis to form the next generation of identical chromosomes. In meiosis I, the chromatids do not separate, which means each daughter cell receives only one copy of each chromosome, the haploid number, and each copy contains two chromatids. In meiosis II, the chromatids separate and are distributed to each resulting gamete.
During prophase of meiosis I, the double-chromatid homologous pairs of chromosomes cross over with each other and often exchange chromosome segments. This recombination creates genetic diversity by allowing genes from each parent to intermix, resulting in chromosomes with a different genetic complement. The exchange occurs between non-sister chromatids. Because genes often interact with each other, the new combination of genes on a chromosome can lead to new traits in offspring.
Because the duplicated chromatids remain joined during meiosis I, each daughter cell receives only one chromosome of each homologous pair. This reduces the diploid number to haploid, and the distribution of each chromosome is random. This means that it is equally likely for a given chromosome to be distributed to either of the two daughter cells. By shuffling the genetic deck in this way, the gametes resulting from meiosis II have new combinations of maternal and paternal chromosomes, increasing genetic diversity.
A third source of genetic diversity occurs during meiosis II, in which the sister chromatids separate and are randomly distributed to the daughter cells, the gametes. Crossing over in meiosis I leads to non-identical chromatids in meiosis II chromosomes. During anaphase of meiosis II, the centromere joining each chromatid pair dissolves, creating two chromosomes of each type. The outcome of which chromosome will go to which gamete is random, so that each gamete has a potentially unique combination of genetic material.
Fertilization creates genetic diversity by allowing each parent to randomly contribute a unique set of genes to a zygote. While fertilization is not part of meiosis, it depends on meiosis creating haploid gametes. The fertilized cell restores the diploid number. Without meiosis, the number of chromosomes per cell would double in each generation of offspring, leading to unstable conditions that could threaten the viability of a species.
Genetic variation describes the differences in our DNA and how the resulting offspring will be genetically different from the parents. Mutation events, meiosis and random fertilisation, cause genetic variation. You might have read our article on gene mutations and learned how changes in the DNA base sequence causes genetic variation. Here, you will learn about the importance of meiosis and random fertilisation in generating genetic variation.
Genetic variation in meiosis
Genetic variation is introduced during meiosis - a form of cellular division. This process produces genetically different sex cells, called gametes, for sexual reproduction. Meiosis is extremely important in evolution. It is a key driver of natural selection (the process by which organisms with traits that favour their survival live on to reproduce is and only made possible with genetic variation). Meiosis also ensures that the resulting zygote (a fertilised egg) will contain the correct amount of chromosomes upon fertilisation.
Phases of meiosis
A detailed explanation of different phases in meiosis is outlined in another article on meiosis, but we will revisit the steps here briefly. Recall that meiosis involves two cellular divisions, meiosis I and meiosis II. Each division has four common stages
- prophase
- metaphase
- anaphase
- telophase
Meiosis I begins with a single diploid cell containing 46 chromosomes due to DNA replication which occurred during interphase. Prophase I involves homologous chromosomes pairing up. The homologous (similar position) chromosomes undergo crossing over, which is a recombination event involving the exchange of DNA. The bivalents line up along the metaphase plate, and spindle fibres drive this action during metaphase I. Independent assortment occurs during metaphase I, and we will explore this process in the following section. Anaphase I describes the separation of the homologous chromosomes, while telophase involves the assembly of the chromosomes at each cell pole. At the end of meiosis I, cytokinesis is initiated to produce two genetically different haploid cells.
Crossing over: a recombination event in which sections of DNA are swapped between homologous chromosomes.
Independent assortment describes the random orientation of the homologous chromosomes on the metaphase plate and the different combinations of alleles inherited.
You might see homologous chromosomes be referred to as bivalents because the chromosomes are pairs.
Meiosis II is the second cellular division. During prophase II, the cell prepares for division by condensing the chromosomes and breaking down the nucleus. Metaphase II involves spindle fibres assembling individual chromosomes along the metaphase plate and independent assortment. Anaphase II results in the separation of sister chromatids, and telophase II describes the decondensing of the chromosomes at opposite cell poles. After cytokinesis is complete, four genetically unique haploid gametes are left.
One chromosome after DNA replication is composed of two identical sister chromatids. This means that there are a total of 4 chromatids in a pair of homologous chromosomes.
Mitosis is another form of cellular division but, unlike meiosis, involves only one cellular division. The purpose of mitosis is to produce genetically identical cells to replace damaged cells and asexual reproduction. Conversely, meiosis aims to produce genetically unique cells for sexual reproduction. We will explore the differences between these different cell division types.
Table 1. Differences between mitosis and meiosis.
mitosis | meiosis | |
Cellular divisions | One cellular division | Two cellular divisions |
Daughter cells | Diploid | Haploid |
Number of daughter cells | Two | Four |
Genetic variation | No genetic variation - all daughter cells are genetically identical | Genetic variation - all daughtercells are genetically unique |
Chromosome alignment | Individual chromosomes assemble at the metaphase plate | Homologous chromosomes assemble at the metaphase plate (meiosis II) |
Genetic variation during meiosis is caused by crossing over and independent segregation. After meiosis is complete, random fertilisation also contributes to genetic variation. We'll take a detailed look at each of these events here.
Crossing over
Crossing over is a process that occurs only in meiosis I during prophase I, and this involves the exchange of sections of DNA between homologous chromosomes. A section of a chromatid wraps around the corresponding chromatid of the other chromosome, which effectively allows these sections of DNA to 'break' off and swap between the pair to produce recombinant chromatids. Alleles are swapped, or new alleles are created as new gene combinations are made!
A chromatid is one molecule of DNA. Before DNA replication, each chromosome is composed of one chromatid. After DNA replication, each chromosome is composed of two chromatids.
Chiasmata is the term given to the point at which the chromatid section breaks off and exchanges.
Crossing over occurs between two non-sister chromatids from a pair of homologous chromosomes!
Independent segregation occurs in meiosis I and meiosis II (metaphase I and metaphase II). This describes how the chromosomes can assemble along the metaphase plate, which gives rise to immense genetic variation. This process is entirely random, and to illustrate how much genetic variation is introduced, we use some maths.
A pair of homologous chromosomes is composed of two individual chromosomes. Therefore, the number of possible alignments along the metaphase plate is 2n, where n is the number of pairs of homologous chromosomes in a cell. This gives us 223, which is over 8 million possible combinations in a human cell.
In meiosis I, individual segregation occurs between homologous chromosomes. In meiosis II, individual segregation happens between individual chromosomes.
Random fertilisation similarly gives rise to genetic variation as sexual reproduction involves the random fusion of two gametes, all of which are genetically different due to crossing over and individual segregation. This leaves organisms that reproduce sexually with tremendously vast combinations of genetic uniqueness. Again, we use maths to calculate the number of different chromosome combinations that can arise from random fertilisation.
After crossing over and independent segregation, we calculated over 8 million possible chromosome combinations. As sexual reproduction involves the fusion of two gametes, this gives us (223) 2 combinations, which is 70 trillion!
Chromosomal mutations
Chromosomal mutations describe changes to the chromosome structure or chromosome number. One of the most common chromosome mutations occurring during meiosis is non-disjunction. Non-disjunction is the failure of chromosomes to split equally during the anaphase stage of nuclear division. This is a spontaneous event, and it means the resulting gametes will not have the expected number of chromosomes.
The two main outcomes of this are:
Polyploidy is caused by the failure of homologous chromosomes to separate during meiosis. This gives rise to gametes containing more than two sets of chromosomes, including triploid cells (three sets of chromosomes) or even tetraploid cells (four sets of chromosomes). Polyploidy is a common phenomenon in plants, and this leads to an increase in gene expression and morphological changes, such as cell enlargement. In humans, polyploidy is extremely rare and lethal, but polyploid cells can occur in some cases.
The majority of babies with polyploidy, unfortunately, end as miscarriages or shortly after birth. In some cases, liver and bone marrow cells can undergo abnormal cell division and become polyploid.
Aneuploidy is caused by the failure of sister chromatids to separate during meiosis, and this gives rise to gametes containing one extra or one less chromosome. This often leads to genetic disorders, as is the case for Down syndrome. Down syndrome occurs when a gamete with one extra chromosome at position 21 fuses with a normal gamete, giving rise to a zygote containing three copies of chromosome 21.
Genetic Variation - Key takeaways
Meiosis is essential for sexual reproduction as it generates gametes. This form of cellular division is also a key driver in natural selection.
Genetic variation is introduced in meiosis during crossing over, independent segregation, random fertilisation and mutations. These events create immense genetic variation.
Chromosomal mutations can give rise to polyploid and aneuploid cells. Polyploidy results in cells containing more than two sets of chromosomes. Aneuploidy results in a cell containing one extra or one less chromosome.