Mutations, D1.3

• What are mutations? A mutation is a change in the DNA sequence of a gene.
• What are the three types of genetic mutation? Substitution (replaced)

  Insertion (added)

  Deletion (removed)
• What is a mutagen? A mutagen increases the rate of mutation, and are either radiation or chemicals.
• What are radiation mutagens? Increases mutation rate if it has enough energy to cause chemical changes in DNA

  e.g. gamma rays, x-rays, short-wave UV radiation in sunlight, alpha particles from radioactive elements
• What are chemical mutagens? Increases mutation rate if cause chemical changes in DNA

  e.g. polycyclic aromatic hydrocarbons, nitrosamines (both found in tobacco), mustard gas
• What does it mean that genetic mutations are random? Mutations are random chemical changes- they are unpredictable and cannot be directed by a living organism to achieve an intended outcome. This randomness is key to genetic variation, driving evolution through natural selection.

  Mutations can occur anywhere in the base sequences of a genome, but some bases have a higher probability of mutating than others, due to positioning or susceptibility to chemical changes. There is no known natural mechanism by which an organism can deliberately target and change its own DNA sequence in a directed way to achieve a specific purpose. All mutations arise through random or environmentally induced processes.
• What are insertion and deletion mutations? Both insertions and deletions are less likely than substitutions to have beneficial consequences. An insertion is when a nucleotide is inserted, adding an extra base in the sequence of the gene. A deletion is when a nucleotide is removed, so there is one less base in the sequence of the gene.
• What is a frameshift mutation? Insertions or deletions of one or two nucleotides are known as frameshift mutations if they result in the total loss of function of a polypeptide. This is because they change the reading frame for every codon from the mutation onwards in the direction of transcription and translation.

  Insertion or deletion of a multiple of three nucleotides are not frameshift mutations, but still have consequences for the polypeptide.
• What is the BRCA1 gene? The BRCAI gene codes for the BRCA1 protein in humans. Its function is DNA repair, as it mends double strand breaks and helps to correct mismatches in base pairing.

  If the gene mutates and the protein cannot carry out its function, the consequence is an increased risk of other genetic mutations due to the lack of DNA repair.

  In body cells, this results in an increased risk of tumour formation, especially breast, ovarian and prostate cancer. Not all mutated variants of BRCA1 cause this though.
• What is a base substitution mutation? This type of genetic mutation is where a base in the DNA sequence is replaced by a different base.

  It is most common for a G to be replaced by a T.
• What is a same-sense mutation? Change one codon for an amino acid into another codon for the same amino acid

  Possible due to the degeneracy of the genetic code

  Degeneracy means not all mutations are harmful, which is vital for genetic stability and evolutionary flexibility.

  e.g. AGC to AGT still codes for Serine

  Do not affect the phenotype
• What is a nonsense mutation? Change one codon (that codes for an amino acid) into a stop codon

  Stop codons are: ATT, ATC or ACT

  Translation is therefore terminated before a polypeptide is completed, hindering the polypeptide's functionality.
• What is a mis-sense mutation? Change one codon for an amino acid into another codon for a different amino acid

  If the new amino acid has a similar structure and properties to the originally intended one, this is known as a synonymous substitution. This does not have much effect.

  If the new amino acid has a different structure and properties to the originally intended amino acid, then this can have severe and even lethal deaths.

  e.g. sickle cell disease

  Very few improve the functionality of a protein, most are deleterious (harmful)
• What are SNPs? A single-nucleotide polymorphism (SNP) is a DNA sequence variation that occurs when a single nucleotide (A, T, C, or G) in the genome differs between individuals.

  Most SNPs are found in noncoding regions

  They help explain why people are different

  Most SNPs don't cause problems, but some can affect how genes work or how the body responds to illness or drugs.
• What is the impact of a mutation in a germ cell vs a somatic cell? Germ cells are reproductive cells, their function is to pass genetic information to offspring

  Mutation in these cells is passed to offspring

  Does not always affect the parent

  Somatic cells are body cells, their function is to build and maintain an organism.

  Mutation in these cells are not passed to offspring

  Affects the organism (can cause diseases such as cancer)
• How is genetic mutation essential for genetic variation? New alleles (versions of a gene) arise through genetic mutation.

  This increases genetic variation- the differences in DNA sequences between individuals in a population. Thsi is essential for evolution and natural selection.

  All genetic variation originally comes from mutations, but within a species, variation may result from Gene flow, meiosis and sexual reproduction. While meiosis/reproduction generates new combinations of alleles, mutations are the only way by which entirely new alleles are formed.

  Without mutations, there would be no new alleles, and evolution would not occur. this is because species cannot evolve without variation in heritable characteristics.

  One example of a beneficial mutation is lactose tolerance
• What does a beneficial mutation do?


  Increases survival
• What does a neutral mutation do? Has no effect (usually on a non-coding strand of DNA, or a same-sense mutation)
• What does a harmful mutation do?


  Decreases proteins' functionality