Molecular Basis of Inheritance Test - 45

Exon is the nucleotide sequence of mRNA that carries information for the synthesis of a protein while the interrupting sequences that do not code for any protein but are present between exons in mRNA, are called as introns. The nucleotide sequence of exon determines the sequence and type of amino acids of a polypeptide. Cistron is defined as DNA segment that carries the genetic code for a single polypeptide and therefore functions as a hereditary unit, i.e., cistrons are transcribed as a unit into a single polynucleotide sequence of mRNA. 

Francis Crick and James D. Watson published an article entitled as "Molecular Structure of Nucleic Acids: A Structure for Deoxyribose Nucleic Acid" in the scientific journal Nature dated 25 April 1953 which described the discovery of the double helix structure of DNA. The above quote refers to the fact the complementary base pairing is key to the double helical structure of DNA and the same pairing facilitates the synthesis of new DNA strands using parental DNA strand as a template.
As we know that new DNA molecules are synthesized during replication which is a semi-conservative mechanism and forms a daughter DNA duplex carrying one parental and one new strand. This copying mechanism ensures inheritance of genetic information, present on DNA, from parents to offspring. 

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. A code of single nucleotides would give only four amino acids to be translated. The doublet code would code for 16 amino acids (4 x 4). Option D is correct answer.

According to Watson and Crick model, DNA has four nitrogenous bases, that includes two purines and two pyrimidines, in its nucleotide chains. The purines are adenine and guanine and pyrimidines are namely cytosine, thymine. In a dsDNA, a purine always pairs with a pyrimidine and a pyrimidine base pairs with purine base only; thus, adenine pairs with thymine (A-T) and guanine pairs with cytosine (G-C). A DNA strand with sequence "AGCTAAGC" will have a complementary strand with the sequence "TCGATTCG". Uracil (U) is a pyrimidine base that is present in RNA in place of T; it is not found in DNA. The sequence given in option B and D have "U", which makes them wrong. A DNA strand cannot have the same base sequence in its complementary strand; thus option C is wrong.

As we know that a set of three bases of mRNA, called as triplet codon, specify one amino acid. Also genetic code is commaless and non-overlapping which means that one base can not be shared by two sequential triplets and the codons are read sequentially. A naturally occurring coding strand composed of alternating C and U residues would have codons as: CUCUCUCUCUCUCUCUCUCUCUCUCUCUCUCUCUC and so on. If read as triplet, it gives two alternating codons "CUC" and "UCU". 

DNA polymerase I serve clean-up functions during replication, recombination, and repair. It has 3' --> 5' proofreading exonuclease activity and 5' --> 3' exonuclease activity. The 5' --> 3' exonuclease activity of DNA polymerase I can replace an error containing the segment of DNA/RNA, paired to the template strand which is then simultaneously replaced by polymerase activity of the same enzyme. The process is called as nick translation. DNA polymerase III with its ten types of subunits has 5' --> 3' polymerization and 3' --> 5' proofreading activities in its $$\alpha$$ and $$\epsilon$$ subunits, respectively. 

A set of three nucleotide bases specify particular amino acid; the set is called as triplet codon. The triplet "UUA" codes for leucine while valine is encoded by GUA, GUG, GUC, GUU; option A is wrong. The triplet "AAA" codes for lysine; option B is right. The triplet "AUG" codes for methionine while cysteine is encoded by UGU and UGC; option C is wrong. The triplet "CCC" codes for proline while alanine is encoded by GCA, GCC, GCG, GCU; option D is wrong.

The DNA-dependent RNA polymerase of is a large, complex, holoenzyme. It has five core subunits (Mr 390,000) and a sixth subunit. The five core units are two copies of alpha subunit, one copy of each beta, beta prime and omega subunit. The sigma subunit carries active site of the enzyme and binds transiently to the core to direct the enzyme to specific binding sites on the DNA. Thus the core enzyme ($${\alpha _2\beta {\beta}'}$$) lacks active site. Correct answer is D.

Quaternary structure of the protein refers to the number and arrangement of multiple folded protein subunits with respect to each other to form a multi-subunit complex; it does not include any molecule other than polypeptide chains. Quaternary structure of DNA refers to the organization of DNA into chromatin via histone octamers to arrange it into a condensed structure; statement A is incorrect. Each nucleotide base of one strand is paired, via a hydrogen bond, in the same plane with a base of the other strand thereby joining two strands together; statement B is incorrect. The same orientation of all phosphodiester linkages along the chain imparts each linear nucleic acid strand a distinct polarity and distinct 5’ and 3’ ends; statement C is incorrect. The 5’ end lacks a nucleotide at the 5’ position thereby having free phosphate group and the 3’ end lacks a nucleotide at the 3’ position thereby having a free hydroxyl group. The two DNA strands are antiparallel means that 5’ end of one strand faces 3’ end of other strand and vice versa. Thus, option D is correct.