Molecular Basis of Inheritance Test - 44

The genetic code is degenerate which means that most amino acids are specified by more than one codon. As we know that there are only four nucleotides namely deoxyadenosine, deoxyguanosine, deoxythymidine and deoxycytidine. Further, there are 20 essential amino acids that are to be encoded by these four nucleotides. A triplet code could make a genetic code for 64 different combinations (4 X 4 X 4) genetic code. Thus, there are ample of codons to specify 20 amino acids. Wobble hypothesis is based on the observation that, the base in the first position of anti-codon on tRNA is usually an abnormal base, like inosine, pseudouridine, tyrosine etc. These abnormal bases pair with more than one type of nitrogenous base in the third position of the codon on mRNA; Inosine (I) can pair with A, C or U. This base is called Wobble base which is present at position 1 of the anti-codon and position 3 of the codon. According to Wobble hypothesis, the third position of the codon on mRNA and the first position of the anticodon on tRNA are bound less tightly than the other two pair and provide unusual base combinations. Due to genetic code degeneracy, many amino acids share the third base and differ in first two bases only thereby, making the third base less important. Base triplets that do not code for amino acids are called as stop codons; there are three stop codons TAG, TAA and TGA. Option D is correct.

DNA and RNA both have four bases, that includes two purines and two pyrimidines, in the nucleotide chains. There are two purines namely adenine and guanine and three pyrimidines namely cytosine, thymine and uracil. The purines adenine and guanine and the pyrimidine cytosine are common to both DNA and RNA. DNA has the pyrimidine thymine; RNA has the pyrimidine uracil. Thus, the correct answer is option B.

A nucleoside has a nitrogenous base and a 5-carbon sugar joined via a $$\beta$$-glycosidic linkage. nucleoside of DNA has deoxyribose sugar wheras that of RNA has ribose sugar. On the basis of different bases, there are five types of nucleoside; cytidine, uridine, adenosine, guanosine, thymidine. When a phosphate group is covalently linked at the 5' hydroxyl of adenosine, it is called as adenylic acid which is a nucelotide. Adenine is a purine base, found in both DNA and RNA, base pairs with thymine via two hydrogen bonds. Deoxyribose is a pentose sugar present in DNA. Correct answer is A.

The triplet UUU codes for phenylalanine; Leucine is encoded by CUU,CUG,CUC and CUA; Methionine is encoded by AUG; Glycine is encoded by GGU,GGA,GGC,GGG. Correct answer is C.

Codons are the nucleotide base triplets that specify a particular amino acid in proteins. The codons that do cause termination of protein synthesis are known as stop codons. RNA stop codons are UAG, UAA and UGA. The start codon specify the first codon of mRNA transcript that is translated by a ribosome. It codes for methionine in eukaryotes and a modified Met in prokaryotes. "AUG" is the most common start codon. UCG codes for serine; UUU-codes for phenylalanine; UGU codes for cysteine and UAC codes for tyrosine. Thus, correct answer is D.

The genetic code is degenerate which means that most amino acids are specied by more than one codon. As we know that there are only four nucleotides namely deoxyadenosine, deoxyguanosine, deoxythymidine and deoxycytidine. Further, there are 20 essential amino acids that are to be encoded by these four nucleotides. A triplet code could make a genetic code for 64 different combinations (4 X 4 X 4) genetic code. Thus, there are ample of codons to specify 20 amino acids. Genetic code degeneracy was identified by Lagervist. He showed that codons GAA and GAG specify glutamic acid and exhibit degeneracy. Correct option is E.

Jerard Hurwitz isolated the RNA Polymerase enzyme from E.coli in 1960. Hurwitz demonstrated a marked stimulation of RNA synthesis by externally added DNA in his E.coli preparation.

Arthur Kornberg purified and characterized DNA polymerase from E. coli cells which are encoded by the polA gene. It catalyzes phosphoryl group transfer. The 3-hydroxyl group of the nucleotide at the 3'-end of the growing strand serves as a nucleophile for nucleophilic attack at the $$\alpha$$-phosphorus of the incoming deoxynucleoside triphosphate. Inorganic pyrophosphate is released in the reaction. 

The codon degeneracy means that most amino acids are specified by more than one codon. This is because in genetic code table, a box is determined by the first and second positions; e.g., the AAX box, in which X can be any of the four bases. Therefore, if one consider 16 boxes; eight of the sixteen boxes have just one amino acid per box. Thus, the codon is read in the first two positions, for these eight amino acids, as the same amino acid will be represented irrespective of the third base of the codon. Correct answer is D.