2: The Genetic Code

Question 1

Why do codons have to be 3 groups of 3 bases and not more or less?

Answer 1

definitely couldn’t be less than 3 because:

• there are 4 bases
• 4^2 = 16, 16 different base combinations
• 20 is the number of amino acids needed to make most polypeptides, so 16 is too few base combinations

shouldn’t be more than 3 because:

• 4^3 = 64, 64 different base combinations
• 64 possible codons easily covers 20 diff amino acids
• occam’s razor: it would be wasteful/pointless for a codon to be bigger

Question 2

How many codons are there for each amino acid?

Answer 2

two or more codons for most amino acids

- exceptions: methionine (start codon), tryptophan

Question 3

Know how to read a ‘meaning of codon’ table. (p31)

Answer 3

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Question 4

What is human insulin? Why would scientists want to transfer the gene that codes for insulin (in humans) to the bacterium E.Coli and other organisms? How come a bacteria produces the same thing (insulin) as humans?

Answer 4

• a protein that contains just 51 amino acids
• to produce the insulin that is needed to treat diabetes
• amino acid sequence of insulin produced in E.Coli = identical to the sequence produced in humans. This is because of the: universality of the genetic code

Question 5

Are there any exceptions to ‘the universality of the genetic code’?

Answer 5

yes: e.g.
- in some yeasts, CUG codes for serine rather than leucine
- in some organisms, a stop codon is used for a non-standard amino acid

Question 6

What is polymerase chain reaction (PCR) used for?

Answer 6

used for copying DNA artificially

Question 7

Why might polymerase chain reaction (PCR) be useful?

Answer 7

• for gene transfer procedures: many copies of the desired gene are needed
• forensic analysis: small sample (from crime scene) can be replicated as larger amount is needed for analysis

Question 8

What are eppendorfs?

Answer 8

small tubes in which DNA is copied (in PCR)

Question 9

How many copies of a gene might there be in an eppendorf by the end of PCR?

Answer 9

100mil+ in a 0.2ml eppendorf

Question 10

How can PCR be sped up? What change is necessary for this to condition to work?

Answer 10

• by carrying it out at high temperatures
• special type of heat-stable DNA polymerase has to be used: Taq DNA polymerase. Obtained from Thermus aquaticus, a bacterium adapted to hot springs