Genetic Mutations and Cell Division

Genetic Mutations

  • Mutations are errors or changes in DNA, genes, or chromosomes that can have a variety of consequences.
  • Mutations can occur by chance (spontaneous) or due to a specific cause (induced).

Spontaneous vs. Induced Mutations

  • Spontaneous Mutations: Occur when the cause of the mutation cannot be identified.
  • Induced Mutations: Occur when the cause of the mutation can be identified.

Mutagenic Agents

  • Mutagens or mutagenic agents are factors that trigger mutations in cells.
  • Examples include:
    • Radiation: Ultraviolet radiation, nuclear radiation, and X-rays.
    • Chemical Substances: Formalin, asbestos, tobacco, and benzene.
    • Infectious Agents: Human papillomavirus (HPV).
  • Mutations may result in cancerous growths (tumors).

Types of Mutations

Gene Mutations
  • Two main types: substitution and frameshift.
    • Substitution:
      • One base is substituted for another.
      • Only one nucleotide is replaced (e.g., a T is replaced by an A).
      • Affects only the codon that contains the altered nucleotide.
      • Example: A C replaced by a T, resulting in one amino acid change (e.g., Glu > Lys).
    • Frameshift:
      • Involves the addition (insertion) or deletion of one or two nucleotides.
      • Causes a shift in how the DNA strand is read.
      • All amino acids are affected after the place where the mutation took place.
      • Example:
        • Original sequence: THE FAT CAT SAT
        • Insertion: THE EFA TCA TSA T
        • Deletion: THE ATC ATS AT
Chromosomal Mutations
  • Involve altering the structure of a chromosome by inserting, deleting, or swapping a cluster of nucleotides during meiosis.
  • Can impact multiple genes.

Chromosomal Abnormalities

  • Examples:
    • Cri du Chat Syndrome:
      • Loss of a section of chromosome 5.
      • Infants have a high-pitched cry that sounds like a cat.
    • Turner's Syndrome:
      • Female with one X chromosome instead of two (XO).
      • Characteristics: Short stature, low hairline, shield-shaped thorax, widely spaced nipples, shortened metacarpal IV, small fingernails, characteristic facial features, fold of skin, constriction of aorta, poor breast development, elbow deformity, rudimentary ovaries, gonadal streak, brown spots (nevi), and no menstruation.
    • Klinefelter's Syndrome:
      • Males with XXY chromosomes instead of XY.
      • Characteristics: Taller than average height, reduced facial hair, reduced body hair, breast development (gynaecomastia), osteoporosis, feminine fat distribution, and small testes (testicular atrophy).
    • Down's Syndrome: Represented by Trisomy 21.

Inheritance

  • The passing on of characteristics from parents to children is called inheritance or heredity.
  • Humans get 23 chromosomes from each parent, forming 23 homologous pairs.
  • Homologous chromosomes are the same size and carry the same genes.
  • The position of a gene on a chromosome is called the gene locus.

Alleles, Genotype, and Phenotype

  • Alleles: Different forms or variants of a gene, resulting in different versions of a trait.
  • Genotype: The combination of alleles inherited for a particular characteristic (e.g., BB, Bb, bb).
  • Phenotype: The outward expression, appearance, or description of a characteristic, resulting from genotype plus the environment (e.g., purple flowers).
  • Genes are expressed when the DNA codes for a protein.

Homozygous vs. Heterozygous

  • Homozygous: Having the same alleles for a gene (e.g., BB or bb), also known as pure breeding.
  • Heterozygous: Having different alleles for a gene (e.g., Bb), also known as hybrid.

Dominant vs. Recessive

  • Dominant Allele: Masks or hides the expression of the recessive allele in the phenotype. Always expressed in the heterozygous genotype. Represented by upper-case letters (e.g., B for brown eyes).
  • Recessive Allele: Hidden or masked by the dominant allele in the heterozygous genotype. Only expressed in the phenotype if there are two copies of the recessive allele.

Carriers

  • A carrier has a recessive gene but does not express it because of a dominant gene.
  • If the recessive gene codes for a genetic disease, the person is a carrier.
  • Example: Cystic fibrosis requires two copies of the recessive gene for the disease to manifest.

Mitosis vs. Meiosis

  • Cells divide to create new cells through two different processes: mitosis and meiosis.
    • Mitosis:
      • Makes most body cells.
      • Cells are diploid, meaning they have a pair of each type of chromosome (46 in total).
    • Meiosis:
      • Makes gametes (sperm or egg cells).
      • Cells are haploid, meaning they only have one of each type of chromosome (23 in total).
      • When a sperm and egg meet, they create a new cell with 46 chromosomes.

Cell Division Leading to Mutations

  • Uneven division of chromosomes during cell division can lead to new cells with too many or not enough chromosomes (e.g., one egg cell gets 22 chromosomes, and the other gets 24).
  • If such an egg cell is fertilized, the child will have too many or not enough chromosomes.
  • The structure of individual chromosomes can also be affected during cell division.