ZIMSEC A Level Biology DNA and Molecular Genetics questions

Question 1

In what ways do polyribosomes make protein synthesis efficient?

Accepted Answer

Several copies of the same protein are made simultaneously from a single mRNA molecule.

Question 2

The diagram illustrates complementary base pairing in a segment of DNA, where the small rectangles represent pyrimidines and large rectangles represent purines. Which rectangle correctly identifies guanine?

Accepted Answer

A

Question 3

The following diagram shows a single DNA strand. Which is the direction of the strand as indicated by the arrow and the type of bond represented at X?

Accepted Answer

direction of strand 3¹ – 5¹, bond at X phosphoester

Question 4

Bacteria were grown for several generations in a medium containing ¹⁵N. Some of the resulting bacteria were then grown in ¹⁴N for one generation. Which test tube shows the position of first generation DNA in the density gradient?

Accepted Answer

D

Question 5

In Meselson and Stahl's experiment, bacterial cells were first grown in medium containing the heavy nitrogen isotope ¹⁵N, then transferred to medium containing the lighter isotope ¹⁴N. After each generation the DNA was centrifuged and separated into bands of differing density, showing that DNA molecules become progressively lighter as replication proceeds in ¹⁴N medium, with each new DNA molecule retaining exactly one original strand. What process of DNA replication does this experiment demonstrate?

Accepted Answer

semi-conservative replication

Question 6

In Meselson and Stahl's experiment, cells were grown in medium containing the heavy isotope ¹⁵N, then transferred to medium containing the light isotope ¹⁴N and allowed to undergo semi-conservative DNA replication. After the DNA had replicated twice in ¹⁴N medium (the 2nd generation), what proportion of the DNA molecules were hybrid (¹⁵N/¹⁴N) compared with fully light (¹⁴N/¹⁴N)?

Accepted Answer

Half of the DNA was hybrid and half was fully light.

DNA and Molecular Genetics Questions and Answers

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In what ways do polyribosomes make protein synthesis efficient?

The diagram illustrates complementary base pairing in a segment of DNA, where the small rectangles represent pyrimidines and large rectangles represent purines. Which rectangle correctly identifies guanine?

The following diagram shows a single DNA strand. Which is the direction of the strand as indicated by the arrow and the type of bond represented at X?

Bacteria were grown for several generations in a medium containing ¹⁵N. Some of the resulting bacteria were then grown in ¹⁴N for one generation. Which test tube shows the position of first generation DNA in the density gradient?

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