Notes on DNA Replication and Mutations

Cells have repair mechanisms that function during the process of DNA replication.
Mutations occur during DNA replication due to imperfections in the enzymes responsible for synthesizing and replicating DNA.
Errors made by these enzymes can lead to mutations, but they are often repaired before the cell divides, thereby preventing these mutations from being inherited by daughter cells.
If these errors are not repaired, they become mutations that persist and get classified into different categories.

Small-scale mutations refer to alterations affecting a small number of nucleotides.
Types of small-scale mutations include:
- Point mutations: A single nucleotide change.
- Insertions/Deletions: Addition or removal of nucleotides, which can lead to reading frame shifts.
The reading frame shift can have significant impacts on protein function.

Synonymous (Silent) Mutations
- A nucleotide change that does not alter the amino acid coded.
- Example: A change from GAG to GAA, both of which code for the same amino acid (glutamic acid).
- Typically occurs in the third position of the codon, where multiple codons can code for the same amino acid, minimizing impact on protein function.
Missense (Non-synonymous) Mutations
- A single nucleotide change that results in a different amino acid.
- Example: A change from GAG (glutamic acid) to GUG (valine).
- The effect on protein function depends on the similarity between the original and new amino acids. For instance, glutamic acid and valine differ significantly in properties, potentially affecting protein function, as evident in sickle cell anemia.
- Sickle cell anemia is caused by a mutation changing glutamic acid to valine in the hemoglobin beta chain, altering its structure and function, leading to impaired oxygen transport.
Nonsense Mutations
- Incorporate a premature stop codon into the protein-coding sequence.
- Example: A change from GAG (glutamic acid) to UAG (stop), resulting in truncated proteins that can severely impact function, based on where the mutation occurs within the protein.

Insertions and deletions can result in frame shift mutations depending on the number of bases altered:
- 1 or 2 base pair changes: Can lead to frame shifts, altering subsequent amino acids.
- 3 base pairs added or deleted: Introduces or removes one amino acid without affecting the rest of the protein sequence.
Cystic Fibrosis is an example of a condition resulting from a three nucleotide deletion that removes phenylalanine from the CFTR protein, significantly impacting chloride transport and leading to respiratory complications.

Result from the addition or deletion of nucleotides (1 or 2 base pairs), shifting the entire reading frame. This can create an entirely different sequence of amino acids downstream from the mutation.
Comparison metaphor: Changing a single letter in a sentence alters its meaning entirely, similar to how frameshift mutations change the protein-coding sequence.

Large-scale mutations affect chromosome structure and may influence multiple genes.
Transposable Elements: Specific regions of chromosomes can move to different locations, influencing gene expression associated with traits such as pigmentation (e.g., anthocyanin).
Large-scale mutations can include:
- Duplication: An entire section of DNA is duplicated, which can lead to gene families developing from the same original gene, potentially with varying functions.
- Inversions: A segment of the chromosome is flipped, potentially altering gene order and function.
- Reciprocal Translocations: Portions from two different chromosomes exchange places, which can disrupt both genes involved, leading to various genetic consequences.

When comparing the gene sequence for insulin between species (e.g., humans vs. rats), similarities and differences in the genetic code may yield insights into evolutionary processes and functional genetics.