MUTATION.

This is the greatest game changer as far as variation is concerned. Mutation can be defined as the changes in the amount, arrangement or structure of the DNA of an organism. Mutations cause changes in the genotypes of organisms which can be inherited by their progeny or offspring. This directly modifies a particular trait amongst organisms of a given population. It sometimes adds a new characteristic (this has been highly exploited by the entertainment industry). To better understand the various types of mutations mentioned and studied below, it’s important you remember that DNA is a chemical (Deoxyribonucleic acid). Hence, bonds can be broken and reconstructed fairly easily, especially when the right amount of energy enters or leaves the nucleolus (where the genetic material is located) in a eukaryotic cell and cytoplasm for a prokaryotic cell.

A multicellular organism like man is made up of two types of cells. The sex cells also known as germ cells and cells of the rest of the body known as somatic cells. Mutations occurring in gamete cells (spermatozoa and oocytes) are inherited by offspring from parents during sexual reproduction. These mutations are called Germ Cell Mutations. Mutations that occur in somatic cells are only inherited during mitotic cell division. They are referred to as Somatic Mutations.

Causes of Mutation.

Mutation generally occurs spontaneously. When and how they will occur is unpredictable even by modern science. Its occurrence though, is sometimes argued to be regular (during meiosis). Notwithstanding, the following may provoke or increase the frequency of mutation.

• High energy radiation like UV rays, X rays, gamma rays etc.
• Cosmic rays from outer space.
• Radioactive radiations.
• Chemicals like mustard gas, caffeine, formaldehyde, cocaine etc.
• Tobacco, pesticides, drugs etc.
• Artificial or man induced.

Types of Mutation.

A gene may contain 1200 to 1500 base pairs. The sequence of these nucleotides in DNA determines the sequence of amino acids in proteins. Changes occurring in the sequence of nucleotides occurring in the DNA are called gene mutation. These changes will eventually (in most cases) lead to a different or wrong sequencing of amino acids in proteins. This will in-turn lead to a different 3-D conformation for the protein. If the protein is an enzyme, the enzyme will be defective. The various ways in which gene mutation can occur include;

• Duplication: This is a situation where a portion of the polynucleotide (DNA) doubles itself along the same strand. This leads to an overall increase in the length of the nucleotide and even the resulting protein. In the figure below is a simplified view of a section of a gene. The first bar shows all the letters from A to H without any repetitions. On the second bar, we notice it is longer. This is because a section of the gene (D-E-F) has repeated itself. This may lead to an alteration in the final structure of the protein resulting from the gene.

• Insertion or Addition: Here, an extra nucleotide sequence is added into the DNA sequence. This form is mostly observed artificially in genetic engineering. For example, a human gene could be introduced into a bacteria (Esheisha coli). This can have the same effect as duplication to a particular protein but if well controlled (in the case of artificially induced mutation), a whole new protein will be produced by the GMO (Genetically Modified Organism). Below, a brand new set of nucleotides, X-Y-Z is introduced into an already existing strand.

• Inversion:. This is the gene mutation where a part of the DNA sequence cleaves, rotates itself by 180o and then rebinds again. Observe below how D-E-F-G becomes G-F-E-D.

• Substitution: This is a complete replacement of a part of a gene sequence. It could be a sequence of nucleotides or just a single nucleotide. This can be done either naturally or artificially. As a matter of fact this is the purpose of crossing over during meiosis. This is the type of mutation that leads to sickle cell anaemia. Below, we observe how D-E-F was completely replaced by X-Y-Z.

• Deletion: This is a situation where a sequence of nucleotides in a gene or probably just a nucleotide is completely removed, making the gene shorter than normal. This can consequently lead to a relatively shorter protein after protein synthesis. Of cause, the 3D shape or conformation of the protein will be altered. Below, notice how the original gene structure (simplified) from A to H is significantly shorter after deletion of some nucleotide base pairs (D to F).

It results from the changes in the structure or number of chromosomes in the cells. They may affect several genes and hence have a more profound effect on the phenotype than gene mutations. Changes in the number of chromosomes usually result from errors during meiosis and may involve the loss or gain of chromosomes – Aneuploidy or simply a change in the entire haploid set of chromosomes – Euploidy/Polyploidy.

Aneuploidy of n+1, (2n+1)

For example, humans have 23 pairs of chromosomes making 46 chromosomes. In a case of n+1 or (2n+1) the person will end up with 47 chromosomes.

Euploidy à (3n)(4n)

Here, a diploid organism could become a triploid (3n) or tetraploid (4n). This mean a person with 46 chromosomes might end up with 46 + 23 = _____ chromosomes.

Aneuploidy involving sex chromosomes.

Below are some important cases of aneuploidy that you should keep in mind.

1. Klinefelter’s syndrome: This is a situation resulting from the fusion of a diploid ovum (XX) with a haploid spermatozoa (Y). The zygote end up with a double XY genotype (XXY). This individual will have as characteristics; be a male, halfway grown testes, produce no sperms (hence impotent) and is usually taller than normal.
2. Triple X syndrome: This is a situation resulting from the fusion of a diploid ovum (XX) and a haploid sperm (Y). The zygote ends up with a triple X chromosome (XXX). This individual will have as characteristics; be a female, do not menstruate (this does not mean infertile)
3. XYY: This is a male individual, generally taller than normal, aggressive and prone or associated to criminality.
4. Turner’s syndrome: this is a female with only 1 X chromosome. These female generally have child-like sex organs and develop a thick fold of skin on both sides of the neck.

Understanding the deviations to Mendel's laws of inheritance helps to answer and explain the most pertinent questions of variation and evolution which will be discussed in the next part of our studies, Part 5.

You can equally find some in past Cameroon GCE Advanced and Ordinary level Biology Papers.

Find Cameroon GCE Ordinary level papers here.

Find Cameroon GCE Advanced level papers here.

Links to previous parts:

A level Genetics Part 1 of 7.

A level Genetics Part 2 of 7.

A level Genetics Part 3 of 7.

A level Genetics Part 4 of 7.

A level Genetics Part 5 of 7

Note written by BUO GENESIS KELLY

Last edited on the 27/07/2023

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