Lecture 8 - Genetic Code

Question 1

What is the function of aminoacyl tRNA synthetase?

Answer 1

It attaches specific amino acids to their corresponding tRNAs, ensuring accurate translation of the genetic code.

Question 2

Why is the genetic code described as degenerate?

Answer 2

Multiple codons can encode the same amino acid, providing redundancy in the genetic code.

Question 3

What does a “wobble base” allow in tRNA?

Answer 3

It allows flexibility in base pairing at the third position of the codon, enabling a single tRNA to recognise multiple codons.

Question 4

What is a frameshift mutation?

Answer 4

It occurs when a nucleotide insertion or deletion shifts the reading frame of the genetic code, altering the downstream amino acid sequence.

Question 5

How can a suppressor mutation restore gene function?

Answer 5

By compensating for a frameshift or nonsense mutation, such as a tRNA mutation that allows read-through of a stop codon.

Question 6

Why do cells tolerate suppressor tRNAs?

Answer 6

Cells have multiple copies of tRNA genes, allowing suppressor tRNAs to function without severely disrupting normal translation.

Question 7

What is the significance of the triplet binding assay in deciphering the genetic code?

Answer 7

It demonstrated the specificity between codons and their corresponding amino acids by using tRNA-ribosome complexes.

Question 8

Define “nonsense mutation”.

Answer 8

A mutation that converts a codon into a stop codon, leading to premature termination of translation.

Question 9

How is the universality of the genetic code advantageous?

Answer 9

It allows genes from one species to be expressed in another, enabling advancements in biotechnology and medicine.

Question 10

What are the start and stop codons in prokaryotes?

Answer 10

The start codon is AUG, while stop codons are UAA, UAG, and UGA.

Question 11

How does tRNA structure contribute to its function?

Answer 11

The anticodon loop pairs with mRNA codons, while the 3’ end binds to the corresponding amino acid.

Question 12

What is the role of release factors in translation?

Answer 12

They recognise stop codons and terminate translation by releasing the polypeptide from the ribosome.

Question 13

Why does RNA use uracil instead of thymine?

Answer 13

Uracil is less chemically stable but sufficient for the shorter lifespan of RNA compared to DNA.

Question 14

What feature of the genetic code ensures that codons are read correctly?

Answer 14

The code is non-overlapping and read in a fixed reading frame starting at the start codon.

Question 15

What are the consequences of mutations in the genetic code?

Answer 15

Depending on the mutation type, it can result in silent mutations, missense mutations, nonsense mutations, or frameshift mutations.

Question 16

What was the significance of the triplet binding assay in deciphering the genetic code?

Answer 16

The triplet binding assay allowed researchers to determine which amino acids correspond to specific codons. It relied on ribosome-bound RNA triplets recruiting complementary aminoacyl tRNAs to reveal the codon-amino acid relationships.

Question 17

How does “wobble” in the genetic code affect translation accuracy?

Answer 17

Wobble refers to the flexibility in base pairing at the third position of the codon. It allows some tRNAs to recognise multiple codons, increasing the efficiency of translation and reflecting the degeneracy of the genetic code.

Question 18

What is the difference between intragenic and intergenic suppressor mutations?

Answer 18

Intragenic suppressor mutations: Occur within the same gene as the original mutation, restoring function (e.g., correcting a frameshift).

Intergenic suppressor mutations: Occur in a different gene, often in tRNA genes, to suppress nonsense or frameshift mutations by modifying translation machinery.

Question 19

What makes nonsense mutations particularly challenging for cells?

Answer 19

Nonsense mutations introduce premature stop codons, truncating proteins and potentially disrupting function. They can also cause read-through issues if suppressor tRNAs are present, leading to abnormal polypeptides.

Question 20

Why is the near-universality of the genetic code important for biotechnology?

Answer 20

The near-universality allows genes from one organism to be expressed in another (e.g., GFP from jellyfish in mice). It is critical for advancements like recombinant protein production and mRNA vaccine technology.

Question 21

How do frameshift suppressor mutations prove the triplet nature of the genetic code?

Answer 21

Frameshift suppressor experiments showed that inserting three nucleotides restored the reading frame, demonstrating the genetic code’s triplet nature. This was pivotal in understanding how codons specify amino acids.

Question 22

What mechanisms prevent errors when suppressor tRNAs are present?

Answer 22

tRNA gene redundancy ensures enough normal tRNAs are available.

Suppressor tRNAs compete with release factors at stop codons but are less efficient, reducing widespread misreading.

Rare stop codons (e.g., UAG in E. coli) minimise interference.

Question 23

Why does RNA use uracil instead of thymine?

Answer 23

Uracil is structurally simpler and energetically cheaper for RNA to synthesise, as RNA does not require the extra stability provided by thymine, which is critical for the long-term storage role of DNA.

Question 24

What makes the genetic code “degenerate”?

Answer 24

Degeneracy refers to multiple codons coding for the same amino acid. For example, serine is specified by six different codons, allowing for redundancy and minimising the impact of certain mutations.