Question

1. The “one gene–one enzyme” hypothesis of George Beadle and Edward Tatum was an oversimplification because:

   		A. mutations may not completely inactivate an enzyme.
   		B. some enzymes contain more than one polypeptide.
   		C. some enzymes can be phosphorylated.
   		D. auxotrophs can be corrected.
   		E. some RNAs are independently catalytic.

2. Describe the 5 events that convert an mRNA into a cDNA.

Answer 1

Ans: The “one gene–one enzyme” hypothesis was proposed by George Beadle and Edward Tatum in 1941. The idea states that each gene encodes a single enzyme. Beadle and Tatum used the bread mold Neurospora crassa and performed genetic and biochemical studies. This hypothesis was soon found to be oversimplified since many enzymes are made up of multiple polypeptide chains (or subunits) and the concept was changed into “one gene–one polypeptide" hypothesis.

So, the correct alternative is option-B.

2. cDNA or Complementary DNA is copied from a specific mRNA for cloning.

1) For this, a short oligonucleotide primer strand is first hybridized with the 3'-terminal nucleotides of the mRNA (the poly A tail).

2) Reverse transcriptase isolated from a retrovirus is then used for reverse transcription of the mRNA into a complementary DNA strand by adding successively to the 3'-end of the hybridized primer such deoxynucleotides as are complementary in bases to the ribonucleotides of the template mRNA.

3) The cDNA strand is next freed from the mRNA template strand by removing the latter with an alkali.

4) The 3'-end of each cDNA strand is then elongated by a terminal deoxynucleotidyl transferase which adds successively several residues of an identical nucleotide (e.g., deoxycytidine nucleotides) at the end to form a 3'-tail, say, a poly-C tail.

5) An oligonucleotide strand with complementary bases (e.g., a poly-G strand) is next hybridized with that tail (e.g., the poly-C tail) to serve as a short primer strand which is stepwise elongated by DNA polymerase using the cDNA strand as the template. This results in the formation of a cDNA duplex.

These are the 5 events that convert an mRNA into a cDNA.