Genetic Variation

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.

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.

Differences between mitosis and meiosis

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.

Causes of genetic variation

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!

Independent segregation

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.

Random fertilisation

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
• aneuploidy

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.

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