I. DNA & RNA | 10. Types of point mutations; mechanisms of their development; possible effects of DNA sequence variations on the corresponding protein Flashcards
What is Point mutation (SNP – single nucleotide polymorphism)
genetic variation of one single basepair
What is SNP - single nucleotide polymorphism
simply points in the genome sequence where one large fraction of the human population has nucleotides, while another substantial fraction have a different variant at that particular sequence (different genotype) – yielding different a phenotype.
2 Types of point mutation:
- Substitution
- Frame-shift mutations
What is Substitution?
Point mutation that involves the replacement of one base pair with another
What are Frame-shift mutations?
insertion or deletion of bases may change the reading frame of a gene
What are 2 types of substitution?
- Transition
- Transversion:
Transition is a type of substitution mutation (point mutation)
-> What is transition?
Point mutation that changes a purine nucleotide to
another purine (A to G) OR a pyrimidine nucleotide to
another pyrimidine (C to T)
Transition is a type of substitution mutation (point mutation)
-> What is the cause of transition?
Can be caused by oxidative deamination (e.g.
conversion of methylated cytosine to thymine) and tautomerization (e.g. base pairing of guanine and thymine)
Transversion is a type of substitution point mutation
-> What is transversion?
Refers to the substitution of a (two ring) purine for a
(one ring) pyrimidine or vice versa
What is the cause of transversion mutation?
Occurs due to mistake of the DNA polymerase
Consequences of a substitution in protein-coding region:
1/ Silent point mutation
2/ Missense mutation
3/ Nonsense mutation
Consequences of a substitution in protein-coding region
-> What is Silent point mutation?
causes no change in the amino acid sequence (e.g. GAG and GAA both encode glutamate)
Consequences of a substitution in protein-coding region
-> What is Missense mutation?
Replacement of one amino acid by another.
=> This has a wide range of effects
Missense mutation: replacement of one amino acid by another. This has a wide range of effects
-> What are the 3 effects?
- If the amino acid substitution occurs at or near active site of an enzyme
= > activity of altered protein decreased or destroyed - If amino acid change is not part of the active site
= > the change does not affect the protein - Other may result in substitution of closely related amino acid (same properties).
-> Such mutations have no effect on the function of the gene and may even be undetectable
Consequences of a substitution in protein-coding region:
-> What is nonsense mutation?
base pair substitution that convert an amino acid specifying codon to a stop codon.
=> It usually destroys the function of the gene product (protein)
What are the 2 types of Frame-shift mutation?
1/ Insertion
2/ Deletion
Frame-shift mutation
-> What is insertion?
addition of an extra base pair. For example, due to the presence of DNA intercalator compounds.
-> Such compounds are able to get in between the planar bases of DNA
Frame-shift mutation
-> What is deletion
lack of base pair (loss of a purine base). Can be caused by loss from translocation, unequal crossing over, slippage of the DNA polymerase over the template etc.
3 consequences of a frame-shift (insertion/deletion) in protein coding region
Stop codon may be produced
Result in an altered amino acid sequence immediately after the change
Causes a complete loss of enzymatic activity in an enzyme
What is Polymorphism?
It is produced by changes in the nucleotide sequence or length (variation of DNA sequence). One example of polymorphism is a person having blue eyes.
