12.3 - Mutation

Mutation refers to spontaneous and random changes that happen in the DNA of an organism. Agents that cause mutation are called mutagens. The new genetic materials formed are known as mutants. Mutants can exist in the form of mutant gene, mutant cell, mutant organelle or mutant individual.

Types of Mutagen

Mutagens can be categorized into 3 agents.

• Physical agents
• Chemical agents
• Biological agents

Physical agents

• Ultraviolet rays from sunlight
• Ionizing rays such as X-ray, alpha ray and beta ray

Chemical agents

• Carcinogens such as cigarette smoke
• Food preservatives
• Formaldehyde
• Benzene

Biological agents

• Viruses
• Bacteria

Types of Mutations

Gene mutation

• Changes that happen at the sequence of the nucleotide bases in genes.
• Also known as point mutation.
• This mutation can cause genetic code for the synthesis of amino acids to change. This in turn affects the final structure of the protein.
• Leads to genetic diseases such as thalassemia, cystic fibrosis, sickle cell anemia, albinisme and hemophilia.
• Gene mutation can occur in 3 ways which are base substitution, base deletion and base insertion.

Figure 5 - Base substitution, base insertion and base deletion. Sickle cell anemia is a type of disease caused by base substitution. Cystic fibrosis is caused by base insertion. Thalassemia disease happens due to base deletion.

• Sickle cell anemia

  - Gene mutation affects the synthesis of **hemoglobins**.
  - As a result, the shape of the hemoglobin formed is sickle-shaped.
  - Blood circulatory in the body can be hindered when the sickle-shaped red blood cells are stuck in smaller blood vessels.
  

• Albinisme

• Individual who has albinisme is called an albino.

• Gene mutation causes the failure of synthesis of melanin pigments.

• Consequently, the skin, hair and eyes appear white.

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Chromosomal mutation

• Refers to the changes that happen at the chromosomal structure or chromosomal numbers.

• Chromosomal aberration means the abnormal changes of the structure of the chromosomes.

• There are 4 types of changes in the chromosomal structure which are deletion, duplication, inversion and translocation. These four changes can result in mutant chromosomes.

• Deletion

  - One of the ends or segments of chromosome is broken and lost.
  

• Duplication

  - Parts of the chromosome is copied, resulting in repeated gene sequence.
  

• Inversion

  - A segment of chromosome is broken, reversed at 180° then rejoined at the chromosome.
  

• Translocation

• Part of the chromosome is broken and join to another chromosome.

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• Changes in chromosomal numbers can be losing one or more chromosome, or gaining one or more extra chromosome.
• This condition happens due to nondisjunction during anaphase I and anaphase II of meiosis.

• Nondisjunction during spermatogenesis or oogenesis can lead to abnormal number of sex chromosomes. As a result, fertilization of abnormal gametes produces zygote with certain abnormal characteristics.

Figure 6 - Mutation of chromosomal number.

• Abnormalities in autosomes:

  • Down syndrome
  • Cri du chat syndrome

• Abnormalities in sex chromosomes:

  • Klinefelter syndrome
  • Jacob syndrome
  • Turner syndrome

Down syndrome

• 47 chromosomes (three chromosome number 21).
• Individual with Down syndrome has slant eyes, protruding tongue, broad forehead and mental retardation.

Cri du chat syndrome

• Structure of chromosomes changes due to partial deletion of the short arm of chromosome number 5.
• Babies with Cri du chat syndrome let out cry of a meowing kitten.
• Delayed development, mental and physical retardation.
• Most patients die during childhood.

Klinefelter syndrome

• 47chromosomes (44 + XXY).
• Individuals with Klinefelter syndrome are infertile with small testes that cannot produce sperms.
• Sound and have breasts like women.
• Long legs and hands.

Jacob syndrome

• 47 chromosomes (44 + XYY).
• Individuals with Jacob syndrome are very tall (taller than normal males).
• Pimples problem on his face.
• Slow speech and learning disability.
• Weak muscle problem (hypotonia).

Tuner syndrome

• 45 chromosomes (44 + XO).
• Individuals with Turner syndrome lack the female secondary sexual characteristics.
• Breasts and ovaries do not develop.
• Weblike neck.
• Low IQ.

Mutation of Somatic Cells and Gametes

• Similarities

  • Both mutations produce variation in a population.

• Differences

  • Mutation in somatic cells

    • Cells involved: Body cells like skin cell and eye cell
    • Inheritable: No
    • Individuals involved: Disease only happens to individuals who have the mutation
    • Examples of disease: Disease related to nervous system

  • Mutation in gametes

    • Cells involved: Germ cell
    • Inheritable: Yes
    • Individuals involved: Disease can occur in individuals having the mutation and those inherited from generations
    • Examples of disease: All inherited diseases such as thalassemia