The Genetic Code

Question 1

Codon def?

Answer 1

Unit specifying formation of one aa

Question 2

Features of genetic code

Answer 2

4 nucleotides encode 20 aa’s
Read in 5-3 direction
Non overlapping
Degenerate 
Almost universal
Almost ambiguous

Question 3

Theoretically: how do we know that theee nucleotides encode one aa?

Answer 3

4^2= 16 (not enough)
4^3 = 64 (enough)

Question 4

In practice: how do we determine that three nucleotides encode one aa?

Answer 4

Poly a/u/g/c encodes the same aa
Two alternating bases encode 2 aa
Alternating sets of 3 bases encode one aa

Question 5

Describe reading frame

Answer 5

Basically which base we look at determines the aa’s made

Question 6

Describe wobble binding

Answer 6

3’ codon can mismatch - accounting for degeneracy.

Question 7

DESCRIBE tRNA structure

Answer 7

small - 75-90 nucleotides
has modified bases, accounting for unique structure
3’ end attaches to AA
has anticodon, which is complementary to mRNA codon

Question 8

describe the number of tRNAs

Answer 8

one per codon - hence potentially multiple for a single aa.

Question 9

describe reactions in which aa is added to tRNA

Answer 9

aa + ATP –> aminoacyl AMP + Pi
aminoacyl AMP + tRNA –> aminoacyl tRNA + AMP

both catalysed by aminoacyl tRNA synthetase

Question 10

how many aminoacyl tRNA synthetases?

Answer 10

one per aa.