DNA and mRNA are both nucleic acids with different... | Biology (9-1) 0970 | Paper 3 Mock 01 | Theory (Core)

Question 1 Report

DNA and mRNA are both nucleic acids with different jobs. Fig. 21.1 shows a gene being copied and the copy leaving the nucleus. Table 21.1 compares the two molecules, with some boxes blank.

Feature	DNA	mRNA
Number of strands	Two	...............
Where it is found or used	Stays in the nucleus	...............

(a) Complete Table 21.1 by giving the number of strands in mRNA and where mRNA is used [3]
(b) State which molecule stores the genetic code permanently [1]
(c) Describe how the code is copied from DNA into mRNA [3]
(d) Explain why the mRNA copy, and not the DNA itself, travels to the ribosome [2]

Answer

Feature	DNA	mRNA
Number of strands	Two	One
Where it is found or used	Stays in the nucleus	Used in the cytoplasm, at a ribosome

Answer Details

This compares DNA and mRNA. The key idea is that DNA is the permanent double-stranded store kept safe in the nucleus, while mRNA is a single-stranded, disposable working copy that carries the code out to the ribosome.

(a) Completed table: mRNA has one strand [1], and it is used in the cytoplasm [1], at a ribosome [1].
Feature DNA mRNA
Number of strands Two One
Where it is found or used Stays in the nucleus Used in the cytoplasm, at a ribosome
(b) DNA stores the genetic code permanently [1].
(c) The DNA strands separate/unzip over the gene [1]; free bases (nucleotides) pair with the exposed template strand by complementary base pairing [1]; this forms a single mRNA strand that is a copy of the gene [1].
(d) The DNA must stay safe in the nucleus because it is the master copy that must not be lost or damaged [1]; so a mobile mRNA copy carries the code out to the cytoplasm/ribosome instead [1].