Meiosis

Stages of meiosis

Meiosis begins with a diploid somatic cell which contains 46 chromosomes, or 23 pairs of homologous chromosomes. One pair of homologous chromosomes is composed of a maternally- and paternally-derived chromosome, each of which has the same genes at the same loci but differing alleles, which are different versions of the same gene.

The end product of meiosis is four genetically different daughter cells, all of which are haploid. The steps taken to arrive at this end-stage requires two nuclear divisions, meiosis I and meiosis II. Below, we will discuss these steps in detail. Note that there are many similarities between meiosis and mitosis, another form of cellular division. Later on in this article, we will compare the differences between the two.

Meiosis I

Meiosis I is composed of the stages:

• Prophase I

• Metaphase I

• Anaphase I

• Telophase I

However, we cannot forget about the stage preceding cell division, interphase. Interphase is divided into the G1 phase, S phase and G2 phase. To understand the changes in chromosome numbers during meiosis, we must first know what happens during interphase.

• During G1, normal metabolic processes occur,, including cellular respiration, protein synthesis, and cellular growth.
• The S phase Involves the duplication of all DNA in the nucleus. This means after DNA replication, each chromosome will comprise of two identical DNA molecules, each of which are called sister chromatids. These sister chromatids are attached at a site called the centromere. The chromosome structure appears as the characteristic 'X-shape' that you are perhaps familiar with.
• Finally, the G2 phase continues G1 in the cell that grows and undergoes normal cellular processes in preparation for meiosis. At the end of the interphase, the cell contains 46 chromosomes.

Prophase

In prophase I, the chromosomes condense, and the nucleus breaks down. The chromosomes arrange themselves in their homologous pairs, unlike mitosis, where each chromosome acts independently. A phenomenon called crossing over occurs at this stage, which involves the exchange of corresponding DNA between the maternal and paternal chromosomes. This introduces genetic variation!

Metaphase

During metaphase I, the homologous chromosomes will align on the metaphase plate, driven by spindle fibres, in a process called independent assortment. Independent assortment describes the array of the different chromosomal orientations. This also increases genetic variation! This is different to mitosis where individual chromosomes line up on the metaphase plate, not pairs.

Anaphase

Anaphase I involves the separation of the homologous pairs, meaning each individual from a pair is pulled to opposite poles of the cell through the shortening of spindle fibres. Although the homologous pair is broken, the sister chromatids are still attached together at the centromere.

Telophase

In telophase I, the sister chromatids decondense and the nucleus reforms (note that two sister chromatids are still referred to as a chromosome). Cytokinesis is initiated to produce two haploid daughter cells. Meiosis I is usually referred to as the reduction division stage as the diploid number has halved to the haploid number.

Fig. 2 - Crossing over and independent segregation/assortment

Meiosis II

Much like the previous stage, meiosis II is composed of

• Prophase II
• Metaphase II
• Anaphase II
• Telophase II

Interphase does not occur prior to meiosis II so the two haploid daughter cells enter prophase II immediately. The chromosomes condense and the nucleus breaks down once again. No crossing over occurs, unlike in prophase I.

During metaphase II, spindle fibres will align individual chromosomes on the metaphase plate, much like in mitosis. Independent assortment occurs during this stage as the sister chromatids are genetically different due to the crossing over events in prophase I. This introduces more genetic variation!

In anaphase II, the sister chromatids are pulled apart to opposite poles due to the shortening of the spindle fibres.

Finally, telophase II involves the decondensing of chromosomes and the reforming of the nucleus. Cytokinesis creates a total of four daughter cells, all of which are genetically unique due to the genetic variation that was introduced during both cellular divisions.

Differences between mitosis and meiosis

Some of the differences between the two cellular divisions were explained in the previous section, and here, we will clarify these comparisons.

• Mitosis involves one cell division, whereas meiosis involves two cell divisions.
• Mitosis produces two genetically identical daughter cells, whereas meiosis produces four genetically unique daughter cells.
• Mitosis produces diploid cells, whereas meiosis produces haploid cells.
• In the metaphase of mitosis, individual chromosomes align on the metaphase, whereas homologous chromosomes align in metaphase II of meiosis.
• Mitosis does not introduce genetic variation, whereas meiosis does through crossing over and independent assortment.

Types of mutations

Mutations describe random changes in the DNA base sequence of chromosomes. These changes usually occur during DNA replication, where there is the potential for nucleotides to be incorrectly added, removed or substituted. As the DNA base sequence corresponds with an amino acid sequence for a polypeptide, any changes may affect the polypeptide product. There are four main types of mutations:

• Nonsense mutations
• Missense mutations
• Neutral mutations
• Frameshift mutations

Although mutations arise spontaneously, the presence of mutagenic agents can increase the rate of mutations. This includes ionizing radiation, deaminating agents and alkylating agents.

Nonsense mutations

These mutations result in a codon becoming a stop codon, which terminates the polypeptide synthesis prematurely. Stop codons do not code for an amino acid during protein synthesis, preventing further elongation.

Missense mutations

Missense mutations result in the addition of an incorrect amino acid in place of the original amino acid. This will harm the organism if the properties of the new amino acid are significantly different from the original amino acid. For example, the amino acid glycine is a nonpolar amino acid. If serine, which is a polar amino acid, is incorporated instead, this mutation may alter the polypeptide structure and function. Conversely, if alanine, another nonpolar amino acid, is incorporated, the resulting polypeptide may remain the same because alanine and glycine have very similar properties.

Silent mutations

Silent mutations occur when a nucleotide is substituted, but the resulting codon still codes for the same amino acid. The genetic code is described as 'degenerate' as multiple codons correspond with the same amino acid—for example, AAG codes for lysine. However, if a mutation occurs and this codon becomes AAA, there will be no change as this also corresponds with lysine.

Frameshift mutations

Frameshift mutations occur when the 'reading frame' is altered. This is caused by the addition or deletion of nucleotides, causing every successive codon after this mutation to change. This perhaps is the most lethal kind of mutation as every amino acid may be altered, and therefore, the polypeptide function will be dramatically affected. Below are examples of the different types of mutations that we have discussed.

Fig. 3 - The different types of mutations including deletions and insertions