Lecture 7: Genetic code

Question 1

What is function of the genetic code

Answer 1

A set of rules which controls how the information is translated from genetic material like RNA in translation into the amino acid sequence

Question 2

What is the genetic code

Answer 2

Triplet of nucleotides or codon. We have 64 possible combinations of three bases to specify for 20 amino acids

Question 3

How did scientists decipher the genetic code - find what amino acid is coded for by each codon

Answer 3

They used a cell free system for protein synthesis - only had ribosomes, amino acids, tRNA molecules. Then they synthesised artificial mRNA so only one codon available. Eg, UUU and found that only amino acid was made eg. Phe.

Question 4

What did more elaborate experiments for deciphering genetic code show

Answer 4

It identified the amino acids specified by mixed triplets. It also demonstrated mRNA is read sequentially. it is read in 5’ to 3’ direction. Each codon is read one after the other and this dictates the order off amino acid added to the polypeptide

Question 5

What does the redundacy of the genetic code mean

Answer 5

Most amino acids are specified by more than one triplet/codon. Hence there is >1 tRNA for some amino acids

Question 6

What is wobble

Answer 6

Some tRNA molecules require accurate base pairing only at the first two positions of the codon and can tolerate a mismatch at the third. Eg. GGX = glycine

Question 7

What is the start codon and the amino acid it translates. is it always start?
How does this differ with stop codons

Answer 7

AUG, initiates translation if at the start but elsewhere it will also code for Methionine so not always start.
Stop codons UAA, UAG and UGA always terminate translation and don’t code for any amino acid

Question 8

What does it mean when genetic code is universal

Answer 8

Genetic code is shared by most organisms from bacteria to complex organisms. Therefore the same product (protein) can be made from using a gene from outside a species.

Question 9

Define mutation, and main causes

Answer 9

Changes in nucleotide sequence which can be permanent and inheritable.
Caused by external agents that damage DNA (eg UV light), mistakes in replication by DNA polymerase, or spontaneously in low frequency random errors., spontaneous chemical reactions in cells.

Question 10

What are the two types of point mutations

Answer 10

Nucleotide pair substitutions (silent, missense and nonsense) and Nucleotide pair insertions or deletions (causes frameshift resulting in missense or nonsense).

Question 11

Compare the silent, missense and nonsense outcome

Answer 11

• Silent is where the one base pair change results in the same amino acid being added so no effect on sequence.
• Missense is where a different amino acid is translated for so depending on where it is it can effect the folding and function of the final protein.
• Nonsense is where the stop codon is made prematurely. Earlier = more damage.

Question 12

Where in the polypeptide are amino acids that influence folding.

Answer 12

The earlier in the polypeptide the amino acid is the more likely it is involved in the folding.

Question 13

What causes more damage, frameshift at beginning or end

Answer 13

beginning. causes extensive missense or immediate nonsense

Question 14

what is the case of no frameshift for deletion/ insertion

Answer 14

3 nucleotide base pair if it corresponds to the codon.

Question 15

DNA damage by UV causes

Answer 15

covalent bonding between two thymine bases (thymine dimer that distorts DNA molecule)

Question 16

What are the 3 steps of excision repair of DNA

Answer 16

1. Nuclease enzyme comes in and cuts teh damaged DNA strand at two point and the damaged section is removed.
2. DNA polymerase fills in the missing nucleotides by complementary base pairing.
3. DNA ligase seals the free end of the new DNA to the old DNA making the strand complete.

Question 17

What does Dolly tell us about totipotent cells

Answer 17

Dolly was made from the genetic information in a mammary cell (differentiated cell) being inserted into an egg. This shows that the information for all genes is in a differentiated cell.