Lecture 10 notes

Question 1

how is genetic code deciphered?

Answer 1

DNA -> transcription -> mRNA -> translation -> polypeptide

Question 2

DNA (and RNA) notation

Answer 2

• the sequence of one strand of DNA dictates the sequence of the other strand
• if only one strand is show, 5’ is to the left unless otherwise noted
• if only one strand is shown the DNA is assumed to be double stranded unless otherwise noted

Question 3

translation

Answer 3

proteins are synthesized by ‘reading’ mRNA

-tRNAs, guided by mRNA, align amino acids for peptide bond formation in the ribosome

Question 4

DNA is collinear with encoded protein

Answer 4

approach:

• isolate many mutants of one gene
• map the mutants to establish position within gene
• isolate mutant proteins, determine amino acid sequence
• linear order of DNA mutations correlates with linear order of amino acid changes

Question 5

how many different amino acids could be specified by a two letter code?

Answer 5

16; 4^2

Question 6

codons are triplets

Answer 6

• use Phage T4 rll mutants to define the size of a codon
• many ‘frameshift’ mutants were isolated
• combining 3 different mutations with the same sign (+ or -) can yield rll+

Question 7

deciphering the genetic code

Answer 7

basic approach:

• synthesize RNA of known sequence
• translate the RNA into protein (in vitro)
• determine the sequence or composition of protein

Question 8

nonsense codons

Answer 8

“stop” codons, ends a sequence of amino acids

• UAA
• UAG
• UGA
• amber mutants
• truncated proteins end before an amino acid whose codon differs from UAG at one position

Question 9

combination of +1 and -1 mutants

Answer 9

yield wildtype