Week Eight Mutations in DNA and their Repair Flashcards
What is DNA mutation?
The process by which a sequence of base pairs in a DNA molecule is altered, meaning they are changed at the sequence level of chromosome organisation
What is the difference between gene and chromosomal mutation?
Gene = alteration of nucleotide sequence of a gene
Chromosome = alteration of number of chromosomes or their structure
Compare and contrast spontaneous and induced mutations
Spontaneous = occurs naturally due to errors in DNA replication
Induced = occur when organism is exposed to physical or chemicals agents that interact with DNA to cause mutation
Induced mutation = occurs when organism is exposed to physical or chemicals agents that interact with DNA to cause mutation
Provide 2 detailed examples for spontaneous mutations
- Errors during meiosis e.g. non-disjunction
- can occur in meiosis I or II to result in change in number of chromosomes - non-homologous (unequal) cross over
- results in insertions and deletions of large sections of DNA with no regard for functional boundaries
Provide 2 detailed examples for induced mutations
- ionising radiation
- electrons are knocked out of orbit, causing ions to be created. these ions break covalent bonds and through this can break DNA in multiple places, leading to chromosome rearrangement - UV light and thymine dimers
- UV light increases chemical energy of pyrimidines. covalent bonds then are made between T bases (creating thymine dimer). DNA struggles to read this dimer because it doesn’t fit smoothly in active site. DNA pairs wrong base with them, causes mutation. if this gene controls cell growth, can lead to cancer
Difference between germ line and somatic cell mutations
Both can lead to genetic conditions which affect one’s health
Somatic = occur in single body cell and cannot be inherited
Germline = occur in gametes or cells that produce gametes, can be passed on to offspring. All cells are affected, half of all gametes will carry mutation
Causes of DNA mutation in both genes and chromosomes
Spontaneous - deamination and depurination, non-disjunction (C), non-homologous crossover (C), transposable elements (G),
Induced - DNA damage (G), ionising radiation (G), UV light and thymine dimers (G)
A point mutation changes a codon from UCG to UAG. What will happen to the resulting polypeptide?
UCG - Ser
UAG - STOP codon
Results in premature termination of the polypeptide, often resulting in a non-functional protein
A mutation during DNA replication causes a G to be inserted after the first base of the codon for tryptophan. How will this affect the growing polypeptide chain?
Codon for tryptophan = UGG
In this case G is inserted between U and G
Example of frameshift mutation
- will lead to changes in how reading frame is read and therefore will change codons from that point onwards
- could generate shorter or longer polypeptide
Typically genetic material is not lost in an inversion or a translocation event. How might these mutations be problematic?
Some translocations may result in trisomy e.g. Down’s syndrome
Describe chromosomal translocation and outline the different types
Occurs when one piece of chromosome breaks off and attaches to another chromosome. No gain or loss of genetic material.
1. Robertsonian = entire C attaches to another @centromere
2. Reciprocal = two different Cs exchange segments with each other
What is the likely effect of the following on the AA sequence of the protein?
Missense - base pair change causes different AA to be inserted
Nonsense - AA changed to stop codon, resulting in non-functional protein
Silent - base pair change results in altered codon but specifies same AA
Neutral - exchange for chemically equivalent AA, AA becomes different
Frameshift - addition or deletion of one or a few base pairs leads to change in reading frame from that point onwards
DNA polymerase occasionally introduces the wrong base when synthesising DNA. How does DNA polymerase correct such errors?
Contains 3’-5’ exonuclease which removes mistmatched bases and allows the correct base to be inserted
Describe a mechanism that permits prokaryotes to distinguish the parental DNA strand from the daughter strand. Why is this important?
Methyl-directed mismatched repair. Corrects errors that DNA polymerase misses during proofreading. Recognises parent strand as the original, non-mutated, and the daughter strand as containing new the mutations. It is strand specific, meaning that excision and resynthesis aren’t approriately targeted to parental strand, where mismatch would be converted to mutation.
How do nucleotide excision repair and base excision repair differ?
Base = repairs damaged DNA bases and replaces them with normal ones
Nucleotide = repairs constantly changing DNA due chemical reactions that cause lesions
