Course book: Chapter 16 "The genetic code"

Question 1

What does the fact that the DNA is degenerate mean?

Answer 1

It means that multiple codons translate to the same amino acids.

Question 2

Is there any smart evolutionary aspect to which codons result in the same amino acid?

Answer 2

Yes. Codons have evolved to be resilient to mutations. If a single nucleotide in a codon is mutated, it’s very likely that the new amino acid will have chemical characterstics similar to the amino acid which was exchanged. Some amino acids have full redundancy on the last nucleotide of the codon, which makes it highly likely to survive a mutation.

Question 3

Which is the 5th nucleotdie, which is present in tRNA, and what bases can it pair with?

Answer 3

Inosine. It can base pair with Adenine, uracil and cytosine. The reason for why it can’t pair with thymine is that the hydrogen acceptor(s) and donor(s) are too far apart.

Question 4

Is the following statement true?

The stop codons are transcriptional stop codons and will be terminated by recognition from RNApol.

Answer 4

No.

The stop codons are put into play during translation. The stop codons are bound by release factors (eRFs in eukaryotes). If these bind to the stop codon before any mutated tRNAs, the translation is stopped.

Question 5

Are codons read 3’ –> 5’ or the other way around?

Answer 5

The other way around, 5’ –> 3’.

Question 6

What three principles govern the genetic code?

Answer 6

1. Codons are read from 5’ –> 3’
2. The codons which encode an amino acid are sequential.
3. The codons are defined by a reading frame dictated by the transcription initiation site.

Question 7

What three types of DNA mutations can alter amino acid composition?

Answer 7

1. Mis-sense mutations (point mutation)
2. Non-sense / stop mutation (point mutation that generates a stop codon)
3. Frameshift mutation (deletion)

Question 8

What are suppressor genes? What kinds of suppressor genes are there?

Answer 8

Suppressor genes suppress mutations. Suppressor genes can be intra- or intergenic.

Intragenic suppression is when the suppressing mutation exists in the mutated gene.

Intergenic suppressor genes are located somewhere else on the chromosome. These suppressors usually work through mutated tRNAs.

Mutated tRNA genes can suppress nonsense mutations (fake stop codons) by recognising them as a codon for an amino acid. (In e.coli, there’s also mutant tRNAs for frame shifts and mis-sense mutations).

Mutant tRNA genes only produce a fraction of the tRNAs which can be utilized.

Mutant tRNA genes can recognise normal stop codons as long as they are bound by release factors (RFs). If the mutated tRNA binds the stop codon before any release factor, an amino acid will be translated from it.