Molecular Basis of Inheritance Test - 41

Transcription is when RNA is made from DNA. During transcription, RNA polymerase makes a copy of a gene from the DNA to mRNA as needed. This process is similar in eukaryotes and prokaryotes. One difference, however, is that eukaryotic RNA polymerase associates with mRNA-processing enzymes during transcription so that processing can proceed quickly after the start of transcription.

The primary nucleobases are cytosine (DNA and RNA), guanine (DNA and RNA), adenine (DNA and RNA), thymine (DNA) and uracil (RNA), abbreviated as C, G, A, T and U respectively. Because A, G, C, and T appear in the DNA, these molecules are called DNA-bases. A, G, C and U are called as RNA-bases. Uracil and thymine are identical except that uracil lacks the 5' methyl group. Adenine and guanine belong to the double-ringed class of molecules called purines. Cytosine, thymine, and uracil are all pyrimidines. In normal spiral DNA the bases form pairs between the two strands. A with T and C with G. Purines pair with pyrimidines mainly for dimensional reasons. only this combination fits the constant width geometry of the DNA spiral. The A-T and C-G pairings are required to match the hydrogen bonds between the amine and carbonyl groups on the complementary bases. So, if a mRNA sequence of N$$_2$$ base is 5' AUG GUG CUC AAA 3', the correct sequence of anticodons are UAC, CAC, GAG and UUU. Thus, option C is correct and other options are wrong.

• The genetic code is the set of rules by which information encoded within genetic material, i.e., the DNA or mRNA sequences, is translated into proteins by living cells. 
• The genetic code is highly similar among all organisms and can be expressed in a simple table with 64 entries. Hence, it is universal.
• There is no ambiguity in the genetic code. A given codon always codes for a particular amino acid wherever it is present.
  
• The coding units or codons for amino acids comprise three-letter words, 4 x 4 x 4 = 64 codons to specify 20 proteinous amino acids.
  

According to the Chargaff's rule, DNA from any cell of all organisms have pyrimidine and purine bases in 1:1 ratio. Hence, the amount of guanine is equal to cytosine and the amount of adenine is equal to thymine. This is because, in a dsDNA, adenine pairs with thymine and guanine pairs with cytosine. But in the given sample DNA, the mole ratio of purines and pyrimidines does not satisfy the rule. Hence, the given DNA is single stranded. Option C is correct.

During transcription, RNA polymerase makes a copy of a gene from the DNA to mRNA as needed. During transcription, the DNA site at which RNA polymerase binds is called as a promoter. Transcription commences with one or more sigma factor protein binds to the RNA polymerase holoenzyme, allowing it to bind to  promoter DNA. RNA polymerase creates a transcription bubble, which separates the two strands of the DNA helix. This is done by breaking the hydrogen bonds between complementary DNA nucleotides.

A sequence of three nucleotides in messenger RNA makes a codon for an amino acid. 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. A codon is defined by the initial nucleotide from which translation starts.

Correct Option: A

Explanation:

• Translation is initiated with a chain initiation codon and terminates with a stop codon.
• The initiation codon is the first codon of a messenger RNA translated by a ribosome.
• AUG codes for methionine in eukaryotes.
•  AUG is an initiation codon as translation begins from this codon. 

The DNA polymerases are enzymes that create DNA molecules by assembling nucleotides, the building blocks of DNA. These enzymes are essential to DNA replication and usually work in pairs to create two identical DNA strands from a single original DNA molecule. During this process, DNA polymerase reads the existing DNA strands to create two new strands that match the existing ones. Every time a cell divides, DNA polymerase is required to help duplicate the cells DNA, so that a copy of the original DNA molecule can be passed to each of the daughter cells. In this way, genetic information is transmitted from generation to generation.

DNA replication requires DNA polymerase and ligase. A molecule of a DNA polymerase binds to one strand of the DNA and begins moving along it in the 3' to 5' direction, using it as a template for assembling a leading strand of nucleotides and reforming a double helix. Because DNA synthesis can only occur 5' to 3', a molecule of a second type of DNA polymerase binds to the other template strand as the double helix opens. This molecule must synthesize discontinuous segments of polynucleotides (called as Okazaki fragments). Another enzyme, DNA ligase then stitches these together into the lagging strand.