The Genetic Code

Question 1

What is the central dogma of molecular biology?

Answer 1

DNA —(transcription)—> RNA —(translation)—> Protein

Question 2

What is a gene?

Answer 2

A gene is a specific sequence of nucleotides along a DNA molecule which codes for a specific sequence of amino acids in a polypeptide chain.

**more accurate to say gene products are polypeptides as compared to proteins

Question 3

What is the genetic code?

Answer 3

1. A code for the sequence of amino acids in a protein
2. Stored in sequences of nucleotides in DNA
3. Transcribed to the specific sequence of nucleotides in the mRNA
4. Translated into a sequence of amino acids in a polypeptide

Question 4

What are the features of the genetic code?

Answer 4

1. The code is a triplet code.
2. The code is degenerate.
3. The code is punctuated.
4. The code is non-overlapping.
5. The code is nearly universal.

Question 5

How is the genetic code a triplet code?

Answer 5

1. Every amino acid in a protein is coded for by a sequence of three bases along a DNA molecule.
2. Codons are triplets of bases on mRNA, which are complementary to the triplets of bases found on the DNA template.
3. Sequence of mRNA is read as codons, each codon specifies a particular amino acid.

• • DNA - triplet code
• • RNA - codons

Question 6

How is the genetic code degenerate?

Answer 6

1. More than 1 codon can code for the same amino acid
2. In many cases, codons that are synonyms for a particular amino acid differ only in the third base of the triplet
3. Hence, mutation changing only a single nucleotide in DNA —> changes only a single nucleotide in corresponding mRNA, often has no effect on amino acid sequence of the encoded polypeptide

Question 7

How is the code punctuated?

Answer 7

1. AUG: start codon —> signals initiation of an mRNA into polypeptide
2. UAA/ UAG/ UGA: stop codons —> signal termination of translation

Question 8

Why is the code non-overlapping?

Answer 8

1. Codons in the genetic code do not overlap.

2. Each nucleotide in a triplet code is used only once.

Question 9

Why is the code nearly universal?

Answer 9

Same triplet of bases codes for the same amino acid in almost all organisms

Question 10

Since RNA consists of 4 types of nucleotides, how many possible codons are there?

Answer 10

4^3 = 64

** 61 out of 64 code for amino acids, remaining three codons do not code for amino acids but signal termination (UAA, UGA, UAG)

Question 11

Describe the consequence of the degeneracy of the genetic code to the likely effect of a change to one base in a triplet.

Answer 11

A mutation that occurs in the last base of the codon may still result in a codon that codes for the same amino acid, and thus have no effect on the function of the protein.