Genetic Mutations and Their Types

Mutations: Primary Subject and Tool of Genetic Analysis

• Mutations are heritable changes in base sequences, altering the information content of DNA.
• Natural genetic variations exist.
• Induced mutations can occur.
  • Random mutagens occur without target.
  • Specific target mutagens target a specific location.

Types of Mutations

• Substitution: A base is replaced by another.

  • Transition: Purine replaces purine or pyrimidine replaces pyrimidine.
  • Transversion: Purine replaces pyrimidine or vice versa.

• Deletion: One or more DNA pairs are lost.

• Insertion: One or more DNA pairs are added.

• Inversion: A 180° rotation of a DNA segment.

• Reciprocal translocation: Nonhomologous chromosome segments swap places.

• Chromosomal rearrangements: Alterations affecting multiple genes simultaneously.

Spontaneous Mutations

• Spontaneous mutations occur at a very low rate.
• Mutation rates from wild type to recessive alleles for coat color genes in mice display different frequencies.

Natural Processes Affecting DNA

• Depurination: Hydrolysis of a purine base (A or G) occurs approximately 1000 times per hour per cell. This creates an apurinic site.
• Deamination: Removal of an amino group from a base. This can change cytosine to uracil, potentially leading to a substitution to an A-T base pair after replication.

Other Factors Affecting DNA

• X-rays: X-rays break the DNA backbone.
• UV light: UV light produces thymine dimers.
• Oxidation: Oxidation from free radicals, formed by irradiation, damages individual bases.

Mistakes during Replication

• Errors during replication are rare (less than once in 10⁹ base pairs).
• Proofreading enzymes correct errors. DNA polymerase possesses a 3'→5' exonuclease activity that checks for mismatches and removes them.

Unequal Crossing Over

• Unequal crossing over creates one homologous chromosome with a duplication and the other chromosome with a deletion.

Transposable Elements

• Transposable elements (TEs) move around the genome without being subject to excision or mismatch repair.
• Two mechanisms of movement exist:
  • Copy-and-paste; or
  • Cut-and-paste.

Mutagens Increasing Mutation Rate

• Base analogs: Have similar structures to normal bases, potentially being incorporated during replication, resulting in incorrect base pairings.
  • 5-Bromouracil: A base analog that can mimic thymine or guanine.
• Hydroxylating agents: Add a hydroxyl group to a base, causing a chemical change that might disrupt pairing.
  • Hydroxylamine: A hydroxylating agent that converts cytosine to N-4-hydroxycytosine, which makes it pair with adenine instead of guanine.
• Alkylating agents: Add ethyl or methyl groups to a base.
  • Ethylmethane sulfonate: An alkylating agent changes guanine to 0-6-ethylguanine, altering its pairing.
• Deaminating agents: Remove an amino group from a base.
  • Nitrous acid: Removes an amino group from cytosine to uracil, changing the pairing capacity.

Intercalating Agents

• Intercalating agents insert between bases.
  • Proflavin is an intercalating agent causing insertion or deletion of a base pair in DNA.

Repair Enzymes

• Repair enzymes fix errors created by mutations.
  • Excision repair: Enzymes excise damaged regions of DNA and DNA polymerase and ligase repair the strand.

Direct Detection of Genotype

• Distinguishes individual genomes.

Variation in Genomes

• Members of the same species exhibit significant sequence variations in their genomes.
  • High variation in base pairing and many polymorphic loci.
• Polymorphism: Two or more alleles at a locus (location in the genome).
• Alleles: Variations in the genome at a particular location.
• DNA markers: Useful for mapping studies and disease diagnosis.
• Anonymous loci: Locations in the genome with unknown functions.

Four Classes of DNA Polymorphisms

• SNP (Single Nucleotide Polymorphism): Single base-pair substitutions (most common).
• Microsatellites: Repeated units (2 to 5 base pairs).
• Minisatellites: Larger repeats (1 to 20 kilobases)
• Small changes in DNA: Deletions and insertions.

Polymerase Chain Reaction (PCR)

• Used to rapidly isolate and amplify DNA fragments.
• Employs oligonucleotide primers to copy DNA similar to DNA replication.
• Cycles allow for exponential accumulation of target DNA.

Microsatellite Allele Detection Analysis

• Analysis focuses on the size differences of microsatellite alleles using PCR and gel electrophoresis.

Gel Electrophoresis

• Gel electrophoresis separates DNA fragments based on size.
• Used to visualize amplified DNA fragments. Fragments are stained with dye for visualization.

Deletions, Duplications, and Insertions

• Expand or contract the length of non-repetitive DNA.
• Caused by unequal crossing over. Can also be caused by transposable elements.

Restriction Enzymes

• Break down complex genomes into smaller fragments for study.
• Fragment at specific sites on the genome.
• Widely used restriction enzymes are listed.

SNP Detection by PCR

• Requires sequence present on either side of the polymorphism for amplification from PCR primers.
• Restriction enzymes are used to confirm the fragment presence.
• Gel electrophoresis is then used for fragment visualization and identification.

This quiz explores the various types of mutations, including substitutions, deletions, insertions, and inversions. It also describes natural variations and induced mutations that impact genetic analysis. Test your understanding of how these changes affect DNA and heredity.