# Mutations A *gene mutation* is a permanent change in the DNA sequence that creates a new allele in the population. Mutations can range from a single nucleotide change (point mutation) to alterations in the structure or number of entire chromosomes. Agents causing mutations are called **mutagens**, and the affected organism is a **mutant**. ## Sources and Types of Mutation Mutations can be classified based on their source (mutagen) and their origin (mutagenesis). **Sources of Mutagens:** - **Physical mutagens:** High-energy agents that can damage DNA. Examples include UV radiation, gamma rays, X-rays, and high temperatures. - **Chemical mutagens:** Substances that can chemically alter DNA. Examples include colchicine, mustard gas, nitrous gas, acridine orange, and reactive oxygen particles. - **Biological mutagens:** Biological entities or processes that cause mutations. Examples include certain viruses, transposons (jumping genes), and errors during DNA replication or meiosis. **Types of Mutation based on Origin (Mutagenesis):** - **Natural Mutations:** Occur spontaneously due to internal factors (e.g., errors in DNA replication) or natural external factors. - **Induced Mutations:** Caused artificially by exposing an organism to mutagens, often for research or to create new varieties of organisms. **Types of Mutation based on Scale:** - **Point Mutation:** A change in a single nucleotide or a few nucleotides within a gene. - **Chromosomal Mutation (Aberration):** A large-scale change affecting the structure or number of chromosomes. ## Importance of Mutation Mutations can have harmful, neutral, or beneficial effects on an organism. **Harmful Aspects:** - A mutation that decreases an organism's fitness. - Examples include congenital abnormalities (e.g., microcephaly, cleft palate), cancer, hereditary disorders (e.g., sickle cell anaemia, phenylketonuria), and chromosomal abnormalities (e.g., Down syndrome). - Nature eradicates harmful mutations through **natural selection**, where affected individuals may have reduced reproductive success or premature death (a genetic death). **Useful Aspects:** - Mutations are the ultimate source of genetic variation and are the driving force of **evolution**. - Beneficial mutations that increase an organism's fitness tend to accumulate in the gene pool. - Major changes in developmental genes can lead to large-scale evolutionary transformations, such as the evolution of wings or the transition of mammals from land to water. ## Chromosomal Mutations (Aberrations) These involve changes in the number or structure of chromosomes, often caused by errors during meiosis, such as non-disjunction (the failure of chromosomes to separate properly). | Syndrome | Karyotype | Chromosomal Change | Key Symptoms | Cause | | :--- | :--- | :--- | :--- | :--- | | **Down Syndrome** | 47, +21 | Trisomy 21 (extra copy of chromosome 21) | Smaller head, flattened nose, single palm crease, developmental delays | Autosomal non-disjunction during maternal meiosis | | **Klinefelter Syndrome** | 47, XXY | Extra X chromosome in males | Sparse body hair, enlarged breasts, small testes, infertility | Non-disjunction of sex chromosomes during meiosis | | **Turner Syndrome** | 45, XO | Monosomy X (missing one X chromosome in females) | Shorter than average height, webbed neck, infertility, abnormal bone development | Non-disjunction of sex chromosomes during meiosis | **Karyotypes of Chromosomal Syndromes:** ## Gene/Point Mutations These mutations arise from a change in the base sequence of an individual gene. **Types of Point Mutations:** - **Base Substitution:** Replacement of one base with another. - **Insertion:** Addition of one or more bases. - **Deletion:** Removal of one or more bases. - **Frameshift Mutation:** Caused by insertions or deletions that are not in multiples of three. This shifts the reading frame of the codons, resulting in a completely different amino acid sequence and usually a non-functional protein. ### Examples of Genetic Disorders from Point Mutations **Sickle Cell Anaemia (Hb SS Disease):** - **Cause:** A point mutation (base substitution) in the gene for the beta chain of haemoglobin. The DNA code CTT is changed to CAT. This results in the 6th amino acid, *glutamic acid*, being replaced by *valine*. - **Effect:** The abnormal haemoglobin (HbS) causes red blood cells to become sickle-shaped, especially under low oxygen conditions. These cells are rigid, block blood flow, and carry less oxygen. - **Inheritance:** Autosomal recessive. - **Symptoms:** Painful episodes, fatigue, jaundice, damage to organs. - **Treatment:** Blood transfusions, pain medication, bone marrow transplants. **Phenylketonuria (PKU):** - **Cause:** A point mutation in the gene for the enzyme *phenylalanine hydroxylase*. - **Effect:** The defective enzyme cannot convert the amino acid *phenylalanine* into *tyrosine*. Phenylalanine builds up and is converted into toxic phenylketones, which damage the central nervous system. - **Inheritance:** Autosomal recessive. - **Symptoms:** Delayed mental and social development, seizures, mental retardation, lighter skin and hair. - **Treatment:** A lifelong diet that is extremely low in phenylalanine. Special formulas like Lofenalac are used for infants. ## Possible Questions/Answers **Q:** What is a mutation? **A:** A permanent change in the DNA sequence that makes up a new allele in the population. **Q:** What is the difference between a mutagen and a mutant? **A:** A *mutagen* is a physical, chemical, or biological agent that causes a mutation. A *mutant* is the organism or cell in which the mutation has occurred. **Q:** How does a frameshift mutation occur? **A:** It occurs when one or more nucleotides are inserted into or deleted from the DNA sequence, altering the reading frame of the codons and typically producing a non-functional protein. **Q:** What is non-disjunction and what can it cause? **A:** Non-disjunction is the failure of homologous chromosomes or sister chromatids to separate properly during meiosis. It can lead to aneuploidy (abnormal number of chromosomes), causing conditions like Down syndrome, Klinefelter syndrome, and Turner syndrome.