Lecture 7: Genetic code Flashcards
What is function of the genetic code
A set of rules which controls how the information is translated from genetic material like RNA in translation into the amino acid sequence
What is the genetic code
Triplet of nucleotides or codon. We have 64 possible combinations of three bases to specify for 20 amino acids
How did scientists decipher the genetic code - find what amino acid is coded for by each codon
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.
What did more elaborate experiments for deciphering genetic code show
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
What does the redundacy of the genetic code mean
Most amino acids are specified by more than one triplet/codon. Hence there is >1 tRNA for some amino acids
What is wobble
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
What is the start codon and the amino acid it translates. is it always start?
How does this differ with stop codons
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
What does it mean when genetic code is universal
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.
Define mutation, and main causes
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.
What are the two types of point mutations
Nucleotide pair substitutions (silent, missense and nonsense) and Nucleotide pair insertions or deletions (causes frameshift resulting in missense or nonsense).
Compare the silent, missense and nonsense outcome
- 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.
Where in the polypeptide are amino acids that influence folding.
The earlier in the polypeptide the amino acid is the more likely it is involved in the folding.
What causes more damage, frameshift at beginning or end
beginning. causes extensive missense or immediate nonsense
what is the case of no frameshift for deletion/ insertion
3 nucleotide base pair if it corresponds to the codon.
DNA damage by UV causes
covalent bonding between two thymine bases (thymine dimer that distorts DNA molecule)
