Molecular Basis of Inheritance Test - 24

The genetic code is the set of rules by which information encoded within genetic material (DNA or mRNA sequences) is translated into proteins by living cells. 

Nirenberg and Matthei found one enzyme called polynucleotide phosphorylase that when used in cell free system made uridine diphoshates into a poly-U- messenger RNA (mRNA). This was then used for cracking the genetic code by them, i.e, to understand which codon codes for which amino acid. They used a cell-free system in a test tube where they put together all the things they needed for protein synthesis - RNA template (poly U mRNA), ribosomes, nucleotides, amino acids, stabilizing agents and energy. They found phenylalaline residues being incorporated into the polypeptide as mentioned in the question. Ribosomes read the information in the mRNA and incorporate the specific amino acid. Each three letter nucleotide code is read as one codon and a specic amino acid is added. No amino acid is incorporated for the three stop codons. Similarly a 34 poly adenine mRNA ( i.e (A-A-A-A-A-A-----) sequence will encode a 11 amino acid peptide. Since 34 when divided by 3 gives 11 (approx), therefore it shows the genetic code for lysine is a triplet i.e AAA. Thus, the correct option is C. 

Degeneration of a genetic code is attributed to the third member of the codon. Degeneracy of codons is the redundancy of the genetic code. Degeneracy results because there are more codons than encodable amino acids. The codons encoding one amino acid may differ in any of their three positions; however, more often than not, this difference is in the second or third codon.

DNA was first identified and isolated by Friedrich Miescher and the double helix structure of DNA was first discovered by James Watson and Francis Crick in the year 1953, using experimental data collected by Rosalind Franklin and Maurice Wilkins. 

Genetic code consists of a set of five nitrogenous bases is used in the construction of nucleotides, which in turn build up the nucleic acids like DNA and RNA. These nitrogenous bases are adenine (A), guanine (G), thymine (T), and cytosine (C). These nitrogenous bases have hydrogen bond between opposing DNA strands to form a "twisted ladder" or double helix of DNA or a biological catalyst that is found in the nucleotides. Adenine is always paired with thymine, and guanine is always paired with cytosine. These are known as base pairs. Uracil is only present in RNA, replacing thymine. Pyrimidines include uracil, thymine, cytosine. They have a single ring structure. Purines include adenine and guanine. They have a double ring structure. 

• The complementary strand will have the sequence CTA, ATC, GTA, CTG because in DNA, the bases adenine and guanine form hydrogen bonds with their complementary pyrimidines thymine and cytosine, respectively. 
• This is called a complementary base pairing.
  

In B-DNA, the most common double helical structure, the double helix is right-handed with about 1010.5 nucleotides per turn.The double helix structure of DNA contains a major groove and minor groove, the major groove being wider than the minor groove. The length of one loop of B-DNA is 3.4nm.

The genetic code expresses in a way that 64 codons constitute it, as it occurs in triplets. According to the genetic code, three bases must be employed to encode the 20 standard amino acids used by living cells to build proteins. With four different nucleotides, a code of 2 nucleotides would allow for only a maximum of 42 = 16 amino acids. A code of 3 nucleotides could code for a maximum of 43 = 64 amino acids.

TAC is correctly matched to tyrosine. A codon is a sequence of three adjacent nucleotides constituting the genetic code that determines the insertion of a specific amino acid in a polypeptide chain during protein synthesis or the signal to stop protein synthesis.