Home / A level Biology 3.1 Mode of action of enzymes – Exam style question – Paper 2

A level Biology 3.1 Mode of action of enzymes – Exam style question – Paper 2

A level Biology Paper 2 - All Chapters
Question

The five bases found in nucleic acids are described as nitrogenous organic compounds. There are two types of base.

Fig. 3.1 shows the structure of the five bases.

(a) (i) State the name of the type of base that includes U, T and C.

(ii) State why it is not correct to say that all nucleic acids have five bases.

(b) Fig. 3.2 shows a stage in the replication of DNA. The circled part is enlarged in Fig. 3.3 to show the elongation of the DNA strand that is being synthesised.


Describe the sequence of events that occurs at the stage shown in Fig. 3.3 to extend the synthesised strand.

(c) The two strands in a molecule of DNA are described as antiparallel.

With reference to Fig. 3.2 and Fig. 3.3, state what is meant by antiparallel, and explain how the antiparallel arrangement of the strands determines how new strands are synthesised.

▶️ Answer/Explanation
Solution

(a)(i) Pyrimidine

Explanation: The bases Uracil (U), Thymine (T), and Cytosine (C) are all pyrimidine bases, which are characterized by a single-ring structure. This distinguishes them from purine bases (Adenine and Guanine) which have a double-ring structure. Pyrimidines are smaller molecules that pair with purines in nucleic acids.

(a)(ii) Each nucleic acid has only 4 types of bases (DNA: A,T,C,G; RNA: A,U,C,G)

Explanation: While there are five bases in total across all nucleic acids, no single nucleic acid molecule contains all five. DNA contains A, T, C, and G, while RNA contains A, U, C, and G. Uracil replaces Thymine in RNA, so the complete set of five bases is never present in any single nucleic acid molecule.

(b) The sequence of events in DNA strand extension:

  1. A free nucleotide with three phosphate groups (triphosphate) binds to the template strand.
  2. The base on the nucleotide (e.g., thymine) pairs with its complementary base on the template strand (adenine) through hydrogen bonding.
  3. DNA polymerase catalyzes the formation of a phosphodiester bond between the 3′ hydroxyl group of the growing strand and the phosphate group of the incoming nucleotide.
  4. Two phosphates (pyrophosphate) are released in the process.
  5. The nucleotide is added to the 3′ end of the growing strand, extending it in the 5′ to 3′ direction.

Explanation: DNA polymerase can only add nucleotides to the 3′ end of the growing strand, so synthesis always proceeds in the 5′ to 3′ direction. The enzyme proofreads each addition to ensure correct base pairing. Energy for the bond formation comes from the hydrolysis of the nucleotide triphosphate.

(c) Antiparallel nature of DNA strands:

1. Definition: The two DNA strands run in opposite directions – one strand runs 5′ to 3′ while its complement runs 3′ to 5′.

2. Impact on replication:

  • The leading strand is synthesized continuously in the 5′ to 3′ direction toward the replication fork.
  • The lagging strand is synthesized discontinuously away from the fork in short Okazaki fragments.
  • DNA polymerase can only synthesize in the 5′ to 3′ direction, so it must work backward on the lagging strand.
  • DNA ligase later joins the Okazaki fragments on the lagging strand.

Explanation: The antiparallel arrangement is crucial because it allows for complementary base pairing while accommodating the 5′ to 3′ synthesis constraint of DNA polymerase. This arrangement ensures that both strands can be accurately replicated despite their opposite orientations, with one strand being synthesized continuously and the other in fragments.

More A level Biology Paper 2 Question.................
Scroll to Top