The Genetic Code And Mutations

Question 1

3 features of the genetic code

Answer 1

Non overlapping
Degenerate
Universal

Question 2

The genetic code

Answer 2

The sequence of base triplets (codons) in DNA which code for specific amino acids

Question 3

Non overlapping

Answer 3

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

Question 4

Degenerate

Answer 4

Some amino acids are encoded for by more than one triplet and thus codon

Question 5

Why is the genetic code degenerate?

Answer 5

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

Question 6

Universal

Answer 6

In all organisms, the same specific base triplets encode for same amino acids

Question 7

How is the fact that the genetic code is universal provide evidence for evolution?

Answer 7

Because it shows how in all organisms, the code has been preserved from a common ancestor (eg LUCA)

Question 8

What is a mutation?

Answer 8

When there is a change in the base sequence of DNA in terms of base

Question 9

When do mutations usually occur?

Answer 9

During DNA replications due to to a copying error

Question 10

2 types of mutation

Answer 10

Substitution
Deletion

Question 11

Substitution

Answer 11

When one base is substituted for another

Question 12

Does substitution mutation mean the protein synthesised will be faulty?

Answer 12

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

Question 13

Deletion

Answer 13

One base is completely removed from the DNA sequence

Question 14

Does a deletion mutation mean the protein synthesised will be different?

Answer 14

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

Question 15

Chromosome mutations

Answer 15

When errors in meiosis mean gamete(s) produced contain extra copies of certain chromosomes and the other gamete containing none of these chromosomes

Question 16

Chromosome non disjunction

Answer 16

When chromosomes fail to separate properly: eg the homologous pairs fail to split in meiosis 1

Question 17

Why can we put dna fragments of one organism from a gene into another organism to synthesise same polypeptide?

Answer 17

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

Question 18

Mutagenic agents

Answer 18

Factors that increase the rate of mutations

Question 19

Advantages of mutations

Answer 19

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

Question 20

Can mutations be disadvantageous?

Answer 20

Yes if changes based on sequence of polypeptide so changes protein which decreases chance of organism surviving to reproduce in case of selection pressure

Question 22

When do mutations NOT lead to change in phenotype?

Answer 22

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

Question 23

Frameshift mutation

Answer 23

All base triplets are moved downstream from mutation so changes order codons are read during translation
Because of addition/ deletion base mutation

Question 24

Why is deletion worse than substitution?

Answer 24

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

Question 25

Why are errors in DNA replication more severe than errors in DNA transcription

Answer 25

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

Question 26

If mutation creates a recessive allele, does this mean it will never be expressed in phenotype?

Answer 26

No as there is still a chance it can be expressed, so only in specific circumstances it won’t

Question 27

Why does most of DNA not code for a polypeptide?

Answer 27

Between genes in a chromosome, there are non coding multiple repeats of base sequence Within a gene, 1 or 2 non coding sequences exist called introns exist between exons

Question 28

Exons

Answer 28

Within a gene, the base sequence that actually encodes for amino acid sequence in a polypeptide