D1.2 - the genetic code (2h)

Question 1

what are the 3 features of the genetic code?

Answer 1

• universality
• degeneracy
• triplet code

Question 2

how is the triplet code a feature of the genetic code?

Answer 2

there are four bases in DNA (adenine, cytosine, guanine and thymine) and 20 amino acids
- using triplets of bases as the basis of the genetic code, allows all 20 amino acids and ‘stop codons’ to be coded for
- a group of three bases is sufficient to code for the amino acids, as there are 64 combinations

Question 3

what is the genetic code?

Answer 3

the genetic code is how the sequence of bases in DNA determine the sequence of amino acids in the resulting proteins

Question 4

how is degeneracy a feature of the genetic code?

Answer 4

due to the 64 combinations of triplets of bases, and only 20 amino acids plus the ‘stop codons’ for which to code, some amino acids are coded for by more than one codon

• the degeneracy of the genetic code means that some point mutations will have no impact on the amino acid that is incorporated into the polypeptide, because the modified codon may code for the same amino acid as the original codon
• therefore the mutation would not impact on the protein conformation and function

Question 5

why is it possible to transfer genes between species, and still produce the original protein?
what is an example of this transfer?
what does this feature provide evidence for?

Answer 5

all organisms share the same genetic code, with the same codons coding for the same amino acids
- the universality of the genetic code allows genes to be transferred to other species during genetic engineering

(eg) the human insulin gene can be inserted into bacteria cells, which then produce human insulin protein for use in treatment of diabetes

• this feature provides evidence for the conservation of the genetic code across all life forms is evidence of universal common ancestry

Question 6

how is the genetic code expressed using the genetic table?

Answer 6

the codon table is used to deduce the sequence of amino acids coded by an mRNA strand
- the amino acids, or the stop codon, that is coded for by each mRNA codon is shown in the cells of the table

the table is read by:
1. selecting the group of four rows based on the first base in the codon
2. selecting the column based on the second base in the codon
3. selecting the row (within the selected group of four rows) based on the third base in the codon

Question 7

what is a mutation?

Answer 7

a mutation is a permanent change in the DNA base sequence

Question 8

what is a point mutation?

Answer 8

a point mutation is a change in 1 base

Question 9

what is a substitution mutation?

Answer 9

a substitution mutation is the replacement of a nucleotide base with another

Question 10

how can point mutations change protein structure?

Answer 10

changing one base on the DNA may cause a different protein to be made during translation
- the sequence of DNA bases determines the amino acid sequence of a protein, which is affected by a point mutation as follows:

1. a point mutation changes one base on the DNA, which therefore changes the triplet
2. when the mRNA molecule is made in transcription, the modified triplet will lead to a different codon at that position within the mRNA
   - this is due to the complementary base pairing between the the template strand of DNA and the free RNA nucleotides forming the mRNA strand
3. a change in the mRNA codon may code for a different amino acid to be incorporated into that position of the polypeptide during translation
   - the reason that this change “may” occur, is due to the degeneracy of the genetic code
4. if a different amino acid is incorporated into the polypeptide, this is likely to change the 3D conformation into which the polypeptide bond folds
   - if the modified codon inserts a ‘stop codon’, a truncated polypeptide will be produced
5. the resulting differently shaped protein will have a different function

Question 11

what are the 3 possible impacts of a base substitution?

Answer 11

base substitution mutations change one DNA base, therefore changing one codon, and this change will have one of three impacts following translation:
what is an example of a point mutation affecting protein structure?
1. the same amino acid is coded for, so there is no impact on protein shape or function
2. a different amino acid is coded for, so there is some impact on protein shape and function
3. a stop codon is created and a non-functioning, truncated polypeptide is produced.

Question 12

what is an example of a point mutation affecting protein structure?

Answer 12

sickle cell anaemia is a genetic condition caused by a point mutation in the gene for one of the subunits of haemoglobin
- a mutation changes the DNA triplet in the template strand from CTC to CAC, so the mRNA codon changes from GAG to GUG.
- this causes a glutamic acid to be replaced with a valine acid, and the haemoglobin molecules does not fold properly
- the sickled erythrocytes cannot transport oxygen efficiently, leading to fatigue