Question

For the peptide sequence (from N to C terminus) Lys-Arg-Ser-Cys-Tyr-Tyr-Trp-His-Lys(KRSCYYWHK), which of the following is the correct coding sequence of the gene encoding this peptide?

• A. $3^{\prime }$-aaa tac ggt tat tat cgt cga cgc aaa-$5^{\prime }$
• B. $3^{\prime }$-aaa cgc agc tgc tat tat tgg cat aaa-$5^{\prime }$
• C. $3^{\prime }$-ttt gtagta gtc gta gca act gcg ctt-$5^{\prime }$
• D. $3^{\prime }$-ttc gcg tca acg ctg ctg att gcg ttc-$5^{\prime }$

Answer 1

The correct option is B $3^{\prime }$-aaa cgc agc tgc tat tat tgg cat aaa-$5^{\prime }$

There are 20 amino acids which are coded by 61 codons. Lysine is encoded by 2 codons, AAA and AAG. Arginine is encoded by 6 codons, AGG, AGA, CGU, CGC, CGA, and CGG. Serine is encoded by 6 codons, AGU, AGC, UCU, UCC, UCA and UCG. Cystine is encoded by UGU and UGC. Tyrosine is endoced by UAU and UAC. Tryptophan is encoded by single codon, UGG. Histidine is encoded by 2 codons, CAU and CAC. Hence, the correct coding sequence of the given peptide Lys-Arg-Ser-Cys-Tyr-Tyr-Trp-His-Lys is 5' - AAA CAT TGG TAT TAT TGC AGC CGC AAA - 3'. In this, uracil present in mRNA is replaced by thymine in DNA.

Thus, the correct answer is option B.