Nucleotides and nucleic acids：練習問題

Which of the following statements correctly distinguishes between the structures and functions of purines and pyrimidines in nucleic acids?

In the structure of a DNA molecule, the ratio of purines to pyrimidines is always \(1:1\). If a sample of double-stranded DNA contains \(18\%\) Guanine, what is the percentage of Adenine in the sample?

A sample of double-stranded DNA was analysed and found to contain 1400 base pairs. If 30% of the nitrogenous bases in this sample are Cytosine, how many Hydrogen bonds are present between the base pairs in this specific DNA molecule?

Which of the following describes the correct sequence of events during the transcription phase of protein synthesis?

Bacteria were grown for many generations in a medium containing only heavy isotope nitrogen (\(^{15}\text{N}\)) until all their DNA was fully labelled with \(^{15}\text{N}\). The bacteria were then transferred to a medium containing only light isotope nitrogen (\(^{14}\text{N}\)) and allowed to divide synchronously.

DNA was extracted from samples taken after one generation and after three generations, and then analysed using density gradient centrifugation.

What percentage of the DNA molecules extracted after three generations consists entirely of light DNA (containing only \(^{14}\text{N}\))?

A researcher is investigating a novel synthetic polynucleotide. The researcher found that the molecule undergoes semi-conservative replication in a manner similar to DNA but uses a different set of nitrogenous bases.

(a) State the precise nature of a phosphodiester bond and explain how its formation allows the synthesis of a stable polynucleotide chain. (3 marks)

(b) During replication, a mutation occurred where a purine base was substituted with a pyrimidine base. Explain the structural consequences of this specific mutation on the double-helix stability, referring to the dimensions of the DNA molecule. (2 marks)

(c) The genetic code of this synthetic organism is non-overlapping and degenerate. If a polypeptide produced by this organism contains 120 amino acids, calculate the minimum number of nucleotides in the coding region of the mRNA, including a start and stop codon, assuming each codon consists of 4 bases instead of 3. Show your working. (2 marks)