Mutations Flashcards
What is a mutation
- Alteration in DNA or chromosome structure
- Source of new alleles and fuel for natural selection
name the 3 types of fitness effects
- Deleterious (harmful to the fitness of the individual)
- Neutral (do not have any effect on fitness
- Beneficial (increase the fitness of the individual)
Fitness effect is a __
continuum
(mutations can be neutral, weakly/midly/strongly deleterious or beneficial)
True or false: in nature, we observe more deleterious mutations than beneficial ones
True
What is the DFE (distribution of fitness effects) ?
Relative frequencies of the different types of mutations
what is a spontaneous mutation ?
Mutations that occur naturally due to errors in DNA replication, repair or because of changes in the cellular environment
What are induced mutations ?
Mutations caused by mutagen agents (external factors) such as UV radiatoin, X-rays, chemicals, viruses…
Name 3 types of mutations
- Substitution
- Indel : Deletion/insertion of 1 or more base pairs
- Chromosomal alteratoin
Name every place where mutations can occur
- Coding regions (exons)
- Non coding regions (introns, promoters, enhancers…)
- Somatic cells (non heritable)
- Germ cells (heritable)
How do we call mutations that reduce or eliminate the function of the gene product ? name one type of mutations that can cause that
Loss-of-function mutation
Indels can cause loss of function due to a change in the reading frame (frameshift)
What is a gain-of-function mutation ?
Mutations that lead to a gene product with enhanced, negative, or new functions
__ nucleotide insertion or deletion has a less dramatic impact. Why ?
3
Because will not cause a frameshift
Describe synonymous and non-synonymous mutations
Synonymous (silent) = will code for the same amino acid
Non-synonymous = will not code for the same amino acid
What type of mutations often occur in the third position ?
Synonymous
Explain why synonymous mutations can have fitness effects
- Codon usage bias (difference in the frequency of same codons in coding DNA)
- Variations in abundance/availability between tRNAs
- Increased GC content
- They can change the affinity of the promoter to the RNA polymerase
- They can change the rate and efficiency of translation
Describe why the number of GC content can affect fitness
GC content form 3 hydrogen bonds, so if the GC content is increased, the DNA is harder to denature (at high temperatures)
Substitutions on the 1st or 2nd position of the codon are often ___
Non-synonymous
Describe missense mutations
They can affect the primary, seondary and tertiary structures of a protein
Conservative = does not change the function of the amino acid
Non conservative = changes the chemical properties of the amino acid (big effect on future interactions)
What is a nonsense mutation ?
Leads to a stop codon
Describe the wobble hypothesis
During translation, the 3rd position on an anticodon in tRNA can align in several ways to allow it ot recognize more than one base in the codons of mRNA
Describe the consequences of the wobble hypothesis
- Reduces the number of tRNAs needed
- Increases translation efficiency
- Minimizes the impact of mutations
- Allows adaptation to different codon usage by favoring certain tRNAs based on codon bias
Wobble hypothesis
For the following 1st positions on the tRNA codon, name which base they can pair with at the 3rd position of the mRNA codon:
- A
- C
- G
- U
- I
- A pairs with U
- C pairs with G
- G pairs with C or U
- U pairs with A or G
- I pairs wiht A, U or C
Mutations rates vary a lot between organisms and tend to increase with genome size, expect for __
viruses (mutations are very frequent and they increase the fitness of a virus)
What’s a mismatch and what does it cause
