Molecular Basis of Inheritance Test - 91

DNA is a double helical polymer of nucleotides in which two antiparallel strands are joined to each other by complementary base pairing between nitrogenous bases. DNA has four bases; two purines and two pyrimidines; purines are namely adenine and guanine and the pyrimidines are cytosine, thymine. Adenine in one DNA strand makes two hydrogen bonds with thymine in other DNA strand and guanine makes three hydrogen bonds with cytosine. The rate of DNA strand separation depends on of A-T/G-C ratio; this makes option A wrong. Thus, a DNA strand with the higher A-T ratio is separated faster as compared to those having higher G-C content. In the question, student A got DNA with higher A-T residues and hence, will be separated faster (this makes option D correct) while student B has DNA with higher G-C content which needs comparatively more energy input to break three hydrogen bonds and thus will be separated slowly (this makes option C wrong). The base ratio given in question clearly shows the separation speed; this makes option B wrong.

The nucleotide sequence of DNA serves as the template for RNA synthesis during transcription. The mRNA sequence of triplet serves as the template for amino acid sequence in proteins; thus an amino acid sequence of the protein is indirectly determined by DNA; option A is wrong. In genetic code table, each box is specified by the first and second positions (for example the AAX box, in which X is any of the four bases) therefore eight of the sixteen boxes contain just one amino acid per box. This means codons need only be read in the first two positions for these eight amino acids because the same amino acid will be represented regardless of the third base of the codon. That’s why multiple codons specify single amino acid and make the genetic code degenerate. Thus, DNA segments with 45% homology can code for the protein with 60% homology; option B is correct. There is a specific base pairing between first two bases of mRNA codon and tRNA anticodon, thus two triplets differing in the second base cannot code for same amino acid; option C is wrong. Codons are specific and same codons do not specify multiple amino acids; option D is incorrect. The correct answer is B.

• DNA polymerase enzyme helps in the synthesis of DNA molecules from deoxyribonucleotides, the building blocks of DNA. They are essential for DNA replication and usually works in a pair to create identical DNA strands.
  
• DNA polymerases require 3' OH group for the initiation of synthesis of DNA strand. Therefore, it can synthesize in only one direction by extending the 3' end of the pre-existing nucleotide chain. Hence, DNA polymerase moves along the template strand in 3' - 5' direction and the daughter strand is formed in a 5' - 3' direction.
  

The bacterial cell is the prokaryotic organism with circular double-stranded DNA as the genetic material. The animal cells are the eukaryotes which have linear double-stranded DNA. The major difference between these two cells is that the bacterial cell DNA possess only one site known as ori site where the replication starts. In the case of the animal cell, there are many sites where the replication starts and gets completed. 

In 1961, M.Nirenberg and H.Khorana elucidated the nature of genetic codes by using cell free systems to translate polyuracil RNA sequence. They deduced UUU type codon of amino acid phenylalanine.

The correct answer is C.

• DNA ligase is an enzyme used to facilitate the joining of the DNA strands together by catalyzing the formation of the phosphodiester bond. Okazaki fragments are the short sequences of DNA  nucleotides which are synthesized discontinuously and later linked together. DNA ligase plays a role in repairing these short sequences.
  
• DNA polymerase enzyme helps in DNA dependent DNA synthesis (or replication). It forms two identical DNA strands from a single DNA molecule.
  
• Primase catalyzes the synthesis of a short RNA segment called primer complementary to a ssDNA (single-stranded DNA).
  
• Helicase enzyme helps in the unwinding of the DNA structure while replication process.
  

Each triplet codon codes for a specific amino acid. There are 64 triplet codons that code for 20 amino acids. Example: AUG codes for methionine, UGG codes for tryptophan, CCC for proline, and so on.

In molecular biology, DNA ligase is a specific type of enzyme that facilitates the joining of DNA strands together by catalyzing the formation of a phosphodiester bond. It plays a role in repairing single-strand breaks in duplex DNA in living organisms. Single-strand breaks are repaired by DNA ligase using the complementary strand of the double helix as a template, with DNA ligase creating the final phosphodiester bond to fully repair the DNA.

DNA polymerase enzyme helps in DNA dependent DNA synthesis by addition of deoxyribonucleotides, building blocks of DNA. This enzyme uses DNA as the template and synthesizes DNA. This process is also known as replication.