One of the techniques used to decipher the genetic code was to synthesize polype
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Question
One of the techniques used to decipher the genetic code was to synthesize polypeptides in vitro, with the use of synthetic mRNA with various repeating base sequences- for example, (AGA)n, which can be written out as AGAAGAAGAAGAAGA…. Sometimes, the synthesized polypeptide contained just one amino acid (a homopolymer), and sometimes it contained more than one (a heteropolymer), depending on the repeating sequence used. Furthermore, sometimes different polypeptides were made from the same synthetic mRNA, suggesting that the initiation of protein synthesis in the system in vitro does not always start on the end nucleotide of the message.
The table below shows the actual results obtained from the experiment done by Khorana.
Synthetic RNA
Polypeptide(s) synthesized
(UC)n
(Ser-Leu)
(UG)n
(Val-Cyst)
(AC)n
(Thr-His)
(AG)n
(Arg-Glu)
(UUC)n
(Ser-Ser) and (Leu-Leu) and (Phe-Phe)
(UUG)n
(Leu-Leu) and (Val-Bal) and (Cys-Cys)
(AAG)n
(Arg-Arg) and (Lys-Lys) and (Glu-Glu)
(CAA)n
(Thr-Thr) and (Leu-Leu) and (Tyr-Tyr)
(UAC)n
(Thr-Thr) and (Leu-Leu) and (Tyr-Tyr)
(AUC)n
(Ile-Ile) and (Ser-Ser) and (His-His)
(GUA)n
(Ser-Ser) and (Val-Val)
(GAU)n
(Asp-Asp) and (Met-Met)
(UAUC)n
(Tyr-Leu-Ser-Ile)
(UUAC)n
(Leu-Leu-Thr-Tyr)
a.Why do (GUA)n and (GAU)n each encode only two homopolymers?
b.Assign an amino acid to each triplet in the following list. Bear in mind that there often are several codons for a single amino acid and that the first two letters in a codon usually are the important ones (but that the third letter is occasionally significant). Also remember that some very different looking codons sometimes encode the same amino acid.
AUG GAU UUG AAC
GAG UUC UUA CAA
GUU CUC AUC AGA
GUA CUU UAU GAG
UGU CUA UAC GAA
CAC UCU ACU UAG
ACA AGU AAG UGA
Synthetic RNA
Polypeptide(s) synthesized
(UC)n
(Ser-Leu)
(UG)n
(Val-Cyst)
(AC)n
(Thr-His)
(AG)n
(Arg-Glu)
(UUC)n
(Ser-Ser) and (Leu-Leu) and (Phe-Phe)
(UUG)n
(Leu-Leu) and (Val-Bal) and (Cys-Cys)
(AAG)n
(Arg-Arg) and (Lys-Lys) and (Glu-Glu)
(CAA)n
(Thr-Thr) and (Leu-Leu) and (Tyr-Tyr)
(UAC)n
(Thr-Thr) and (Leu-Leu) and (Tyr-Tyr)
(AUC)n
(Ile-Ile) and (Ser-Ser) and (His-His)
(GUA)n
(Ser-Ser) and (Val-Val)
(GAU)n
(Asp-Asp) and (Met-Met)
(UAUC)n
(Tyr-Leu-Ser-Ile)
(UUAC)n
(Leu-Leu-Thr-Tyr)
Explanation / Answer
(a). The polymer of (GUA)n is, -GUAGUAGUAGUAGUA-. Of this, GUA codes for valiene, UAG codes for stop codon and AGU codes for serine.
The polymer of (GAU)n is, -GAUGAUGAUGAUGAU-. Of this, GAU codes for aspartic acid, AUG codes for methionine and UGA codes for stop codon.
So, the (GUA)n and (GAU)n each encode only two homopolymers.
(b). A triplet code (codons) with four bases (adenine (A), thymine (T), guanine (G), and cytosine (C)) gives 64 combinations (Eg: AUG, UAG, UGA, etc.), which code for 20 specific amino acids. A single amino acid may be represented by more than one codon, so the genetic code is “redundant,” or also known as “degenerate.” As the genetic code is redundant, a single amino acid may have more than one codon. For example, the amino acid proline can be coded by four different codons such as CCA, CCG, CCU, and CCG.
AUG - Meth GAU -Asp UUG- Leu AAC-Asn
GAG- Glu UUC -Phe UUA -Leu CAA-Gln
GUU-Val CUC -Leu AUC - Ile AGA-Arg
GUA-Val CUU -Leu UAU -Tyr GAG-Glu
UGU -Cys CUA -Leu UAC -Tyr GAA-Glu
CAC-His UCU -Ser ACU -Thr UAG- Stop codon
ACA-Thr AGU -Ser AAG -Lys UGA- Stop codon
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