01b - DNA Mutations

1. DNA Basics

  • DNA = deoxyribonucleic acid

  • Found in all living organisms

  • Made of nucleotides (linked together chemically)

Each nucleotide has 3 parts:

  1. Sugar → deoxyribose

  2. Phosphate group

  3. Nitrogenous base

Bases:

  • Adenine (A)

  • Thymine (T)

  • Cytosine (C)

  • Guanine (G)

Base pairing: A ↔ T  C ↔ G

Example sequence:
 AGACTGCTATTACGGTTCA
TCTGACGATAATGCCAAGT

2. Genes

  • Genes are DNA segments that code for proteins

  • The order of bases determines which protein is made

  • Proteins determine traits (phenotypes)

Example:

  • Brunette hair → normal pigment protein

  • Red hair → mutation makes protein nonfunctional



3. From DNA to Protein (Gene Expression)

Step 1: Transcription (DNA → mRNA)

  • DNA stays in the nucleus

  • mRNA (messenger RNA) is made as a copy of the DNA

  • RNA uses uracil (U) instead of thymine (T)

    • A ↔ U

    • T ↔ A

    • C ↔ G

    • G ↔ C

  • mRNA carries the code to the ribosome

Step 2: Translation (mRNA → Protein)

  • Occurs at the ribosome

  • tRNA (transfer RNA) reads the mRNA code in codons (3 bases = 1 amino acid)

  • Amino acids link to form a protein chain

  • The sequence and shape of the protein determine its function

4. Mutations

  • Mutation = permanent change in DNA sequence

  • Can alter protein structure or function → changes phenotype

Example:

  • Redhead gene = mutated brown-hair gene → no pigment made

5. Causes of Mutations

External causes:

  • Radiation (UV light, X-rays)

  • Chemicals (carcinogens, free radicals)

  • Viruses (e.g. HPV)

  • Only mutations present in germ cells will be passed on to the offspring

Internal causes:

  • Errors during DNA replication (S-phase)

  • Most fixed by proofreading enzymes, but some persist

6. Inheritance

  • Germ cell mutations (sperm or egg) → passed to offspring

  • Somatic cell mutations (body cells) → affect only the individual

7. Effects of Mutations on Populations

  • Neutral (silent): no change in phenotype, no effect on population

  • Negative: harmful, lowers survival or reproduction; removed by natural selection

  • Positive: helpful, provides advantage (e.g. disease resistance); can spread in population