Gene Mutations and DNA Repair Study Notes

Gene Mutations and DNA Repair

Learning Goals

• Understand what mutation is.
• Distinguish between somatic mutations and germline mutations.
• Identify and categorize beneficial, neutral, and deleterious mutations.
• Define point mutations and their types.
• Understand the causes of mutations.
• Familiarize with DNA repair mechanisms.

What is Mutation?

• Definition:
  • A mutation is a process that results in a change in DNA or chromosome structure.
  • It can change an allele into a new allele (new gene form), potentially leading to new genes or traits.
  • The scope of mutations can range from a single nucleotide change to entire chromosome alterations.
  • Essentially, anything that alters chromosome structure is considered a mutation.

Types of Mutations

Somatic Mutation

• Definition:
  • A somatic mutation occurs in any cells of the body except gametes (sperm and egg).
  • Heritability:
  • Somatic mutations are not passed on to offspring and are therefore not heritable.
  • Characteristics:
  • The mutation is only found in the originating cell and subsequent cells derived from it.

Germline Mutation

• Definition:
  • Germline mutations occur during meiosis in gametes (sperm and egg).
  • Heritability:
  • Unlike somatic mutations, germline mutations can be passed on to offspring.
  • Outcome:
  • When passed on, the mutation exists in all cells of the offspring.

Evolutionary Implications of Mutations

• Notable Point:
  • Germline mutations have greater implications in evolutionary biology due to their heritability.
  • These mutations could either benefit or be detrimental to the offspring.

Molecular Types of Mutations

Point Mutations

• Description:
  • A point mutation involves a change in one nucleotide, altering the identity of the nucleotide.

Categories of Point Mutations:

1. Silent Mutation

   • Definition: A change in nucleotide that does not alter the amino acid specified by the codon.

2. Missense Mutation

   • Definition: A change in nucleotide that results in a different amino acid, thus changing the phenotype.

3. Nonsense Mutation

   • Definition: A change in nucleotide that introduces an early stop codon, thus stopping translation prematurely. This can have significant effects on protein functioning.

4. Frameshift Mutation

   • Definition: Either the addition or deletion of a single nucleotide, leading to a shift in the reading frame and altering the entire downstream code. This typically results in the destruction of the protein.

Summary Table of Point Mutations

Example: Sickle-cell Anemia

• Description:
  • The first seven codons of the coding strand of the β-globin gene (HBB) are critical. Hemoglobin is composed of four subunits: 2 α globin chains and 2 β globin chains, coded by multiple genes.
• Mutation Type:
  • Involves a missense mutation where the incorporation of a mutant β globin chain causes the production of sickle-shaped red blood cells.
• Codon Example:
  • A missense mutation affects the codon sequence involved in hemoglobin structure, leading to clinical effects like sickle-shaped morphology of red blood cells.

Cancer and Mutations

• Many cancers are linked to mutations that affect critical pathways involved in DNA repair mechanisms.
• For example, mutations linked to breast cancer often involve the BRCA1 gene, which is crucial for the proper repair of double-stranded breaks in DNA.

Causes of Mutations

Spontaneous Mutations

1. Errors in DNA Replication
2. Errors in mRNA splicing
3. Deamination - this involves the removal of an amino group from nucleotide bases, disrupting base pairing.
4. Oxidation - alterations in nucleotide structure due to oxidative damage, also disrupting base pairing.

Induced Mutations

1. Alkylating Agents - Chemicals that modify nucleotide structures.
2. Base Analogs - Molecules that insert themselves into DNA and act as bases.
3. Intercalating Agents - Molecules that insert between nucleotides, causing replication skipping.
4. Radiation: UV and X-ray exposure can cause thymine dimers and various similar alterations.

Mutation Rates

• The error rate of human DNA polymerase III is approximately 1 in 100,000 ($1.0 imes 10^{-5}$).
• Considering there are roughly 3 billion base pairs in the human genome, this results in approximately 300,000 mistakes every time a cell divides.
• Notably, the number of mutations accumulates in spermatogonium/sperm with age, doubling around every 16.5 years.

UV Radiation and Mutations

• UV radiation can induce the formation of pyrimidine dimers, primarily between adjacent thymine or cytosine bases, leading to significant mutations.
• A specific example of these changes is thymine dimers that can result due to UV light exposure.

DNA Repair Mechanisms

Overview of DNA Repair

• Cells can employ various mechanisms to repair mutations:
  1. Proofreading - carried out by DNA polymerases during replication.
  2. Direct Repair - mechanisms to correct DNA damage directly.
  3. Mismatch Repair (MMR) - recognizing and fixing incorrectly paired nucleotides.
  4. Base Excision Repair (BER) - removes and replaces damaged nucleotides on single DNA strands.
  5. Nucleotide Excision Repair (NER) - removes stretches of damaged DNA.
  6. Double-Stranded Break Repair (DSB) - repairs significant breaks in DNA backbone.
  7. Homologous Recombination Repair (HRR) - uses a homologous template for accurate repair.
  8. Non-Homologous End Joining (NHEJ) - directly joins break ends without a template.

Proofreading by DNA Polymerases

• Description:
  • DNA polymerases possess a 3'-5' exonuclease domain to correct mismatched DNA base pairings during DNA replication, such as converting A-G → A-T.

Nucleotide Excision Repair (NER)

• Definition:
  • NER functions to remove and replace damaged nucleotides that result in distortions within the DNA strand, such as thymine dimers.
  • Proteins such as UvrA, UvrB, UvrC, and UvrD are involved in this repair mechanism for E. coli; eukaryotes have a more complex process.

Xeroderma Pigmentosum (XP)

• Condition:
  • A disorder resulting from inability to repair nucleotide dimers caused by UV exposure, resulting in increased skin cancer risk.
• Implications:
  • Individuals with XP demonstrate different mutations in NER-associated proteins, indicating varied repair capacities.

Double-Stranded Break Repair

• Challenges:
  • Double-stranded breaks (DSBs) are more challenging to repair than single-strand mutations because the template or correct sequence may not always be accessible.
  • DSBs can lead to chromosome rearrangements, which are often associated with cancers.

Repair Mechanisms for DSBs:

1. Homologous Recombination Repair (HRR)

   • Utilizes homologous DNA as a template for repair.
   • Most accurate repair method, but only available during S or G2 phase.

2. Non-Homologous End Joining (NHEJ)

   • Repairing DSBs by directly joining the ends without template DNA.
   • More error-prone, typically occurring in the G1 phase.
   • Notably involves the BRCA1 protein, which is crucial in this pathway.

CRISPR/Cas9 Technology in Mutations and DNA Repair

• CRISPR technology enables researchers to create mutations or add new DNA fragments at specific genomic locations.
• Functionality:
  • Cas9 serves as an endonuclease to create a double-stranded break (DSB).
  • Mutations introduced using CRISPR typically take advantage of the NHEJ repair mechanism, where repairs can result in genetic incorporation of alternative template DNA provided at the cut site via HRR.