The Genetic Code And Mutations Flashcards
3 features of the genetic code
Non overlapping
Degenerate
Universal
The genetic code
The sequence of base triplets (codons) in DNA which code for specific amino acids
Non overlapping
Each codon is read in sequence separate from the codon before and after it: aka, no nitrogenous base is shared between multiple codons
So each codon has its own 3 bases
Degenerate
Some amino acids are encoded for by more than one triplet and thus codon
Why is the genetic code degenerate?
There are 20 amino acids but 64 possible codons (eg combos of A U C G)
So for every amino acid, there is more than 1 codon which can encode for it
Universal
In all organisms, the same specific base triplets encode for same amino acids
How is the fact that the genetic code is universal provide evidence for evolution?
Because it shows how in all organisms, the code has been preserved from a common ancestor (eg LUCA)
What is a mutation?
When there is a change in the base sequence of DNA in terms of base
When do mutations usually occur?
During DNA replications due to to a copying error
2 types of mutation
Substitution
Deletion
Substitution
When one base is substituted for another
Does substitution mutation mean the protein synthesised will be faulty?
Not necessarily because the substituted base which leads to a different codon could still encode for same amino acid (genetic code is degenerate) possibility it changes still exists though
And has no effect on rest of amino acids encoded for because does not change order codons are read in
Deletion
One base is completely removed from the DNA sequence
Does a deletion mutation mean the protein synthesised will be different?
Yes:
Change in the order base sequence of codons are read
So changes amino acid sequence of polypeptide
Change in bonds formed between R groups so change in protein
Chromosome mutations
When errors in meiosis mean gamete(s) produced contain extra copies of certain chromosomes and the other gamete containing none of these chromosomes
Chromosome non disjunction
When chromosomes fail to separate properly: eg the homologous pairs fail to split in meiosis 1
Why can we put dna fragments of one organism from a gene into another organism to synthesise same polypeptide?
Because the genetic code is universal so same codons encode for same amino acids
Because mechanism of both translation and transcription ir the same in all organisms
Mutagenic agents
Factors that increase the rate of mutations
Advantages of mutations
Results in change in polypeptide (amino acid sequence change) which changes protein so expresses phenotype beneficial to environment
Increases chance of organism surviving to reproduce if this allele is selected
Can mutations be disadvantageous?
Yes if changes based on sequence of polypeptide so changes protein which decreases chance of organism surviving to reproduce in case of selection pressure
When do mutations NOT lead to change in phenotype?
Because the genetic code is degenerate so different codon still encodes for same amino acid : no change in polypeptide
Mutation occurs in intron so change in base sequence is spliced out of final mRNA: no change in polypeptide
New allele created is recessive so won’t influence the phenotype
No change on tertiary structure id mutation occurs in specific place etc
Frameshift mutation
All base triplets are moved downstream from mutation so changes order codons are read during translation
Because of addition/ deletion base mutation
Why is deletion worse than substitution?
Deletion of a base changes the sequence codons are read in after the mutation so affects multiple codons whereas substitution only affects 1 codon
And because dna is degenerate then the codon change might still encode for same amino acid so no change in polypeptide sequence
Why are errors in DNA replication more severe than errors in DNA transcription
Errors may be inherited if occurs in a gamete so would have a permanent effect on the whole organism of the offspring whereas in transcription, would only affect the cell
If mutation creates a recessive allele, does this mean it will never be expressed in phenotype?
No as there is still a chance it can be expressed, so only in specific circumstances it won’t