5. The genetic code

Question 1

what is the thermodynamic process of translation

Answer 1

endergonic- driven by cleavage of high energy phophoanyhydride bonds

Question 2

How can sequences of 4 residues specify up to 20 amino acids?

Answer 2

triplet code- allows 64 different triplet of bases

Question 3

a codon is

Answer 3

several bases specifying the amino acid

Question 4

degenerate

Answer 4

amino acids can be specified by more than one codon `

Question 5

how are codons read

Answer 5

sequentially

Question 6

reading frame

Answer 6

a sequence of nucleotide triplets that codes for a polypeptide

Question 7

how are DNA bacteriophages different

Answer 7

overlapping genes that have different reading frames
bacteria display overlapping of ribosomal initiation sequence of one gene in a polycistronic mRNA

1 polynucleotide can encode up to 3 polypeptides

Question 8

benefits of overlapping

Answer 8

same nucleotide can encode multiple genes

more likely to have genes that have an evolutionary advantage

Question 9

anticodon

Answer 9

region of tRNA that has 3 bases complementary to a codon on mRNA

Question 10

features of the genetic code

Answer 10

1. the code is degenerate
2. arrangements are non-random- 1st position can specify for aa. 2nd encodes for hydrophobic aa. 3rd encodes polar aa
3. UAG, UAA and UGA are stop codons
4. AUG and GUG start codons specify Met and Val

Question 11

the standard code

Answer 11

is not universal 
differ in ciliated protozoa 
in mammals: AUA and AUG start codons 
UGA specifies Trp, not stop 
AGA and AGG are stop codons

Question 12

common features of tRNA cloverleaf structure

Answer 12

1. 5’ terminal phosphate group
2. 7bp stem acceptor containing a 5’ terminal nucleotide non watson-crick bp
3. D arm ends in 7nucleotide loop containing dihydrouridine
4. anticodon arm- 5bp stem ending in loop with anticodon
5. T psi C arm
6. 3’ CCA sequence with free 0H- 3rd carbon of adenosine

Question 13

structure of tRNA

Answer 13

complex tertiary

maintained by stacking interactions and base pairing within and between helical stems

Question 14

what is needed for amino acids to attach to tRNA

Answer 14

aminoacyltRNA-synthetases

1. amino acid is activated- forms aminoacyl-adenylate
2. aminoacyl AMP reacts with tRNA form aa-tRNA

Question 15

classes of aminoacyl-tRNA synthetase

Answer 15

class 1 and 2 
differ in: 
structural motif
anticodon recognition
site of aminoacylation 
amino acid specificity 

a tRNA may recognise more than one codon

Question 16

Non- Watson base pairing

Answer 16

occur at third codon-anticodon position

Question 17

the wobble hypothesis

Answer 17

accounts for codon degeneracy
first two bp are watson and crick
third U anticodon pairs with A or G

Question 18

how many tRNAs are there

Answer 18

31 required to translate 61 coding triplets

32 at minimal for translation initiation