CAIE IGCSE Biology 4.1.4 Structure of a Dna Molecule a Two Questions

Practise describing DNA structure and applying the A–T and C–G pairing rules.

Exam points

describe DNA as two strands forming a double helix held together by bonds between paired bases
apply A–T and C–G pairing to complete a complementary strand
use complementary-base proportions to calculate a missing base count or percentage

[Maximum number: 3]

A DNA molecule has two strands as shown in Fig. 6.1.

Fig. 6.1

(a)

Fill in the boxes on Fig. 6.1 to show the letter of the bases on strand 2 that will pair with the corresponding bases on strand 1.

[ 2 ]

(b)

State the name for the structure of a DNA molecule as shown in Fig. 6.1.

When molecules of DNA are used to classify species, only one of the two DNA strands is sequenced.

First the DNA sequence from one strand of a DNA molecule from each species is lined up against one strand from another species.

The bases of the DNA sequences from the same strand can then be compared with each other.
Fig. 6.2 shows a short section from the DNA sequences of eight plant species. There are ten differences between species A and species B. These differences are shown in Fig. 6.2.

Fig. 6.2

Species C: CTCATAGGGT GCAGGCCTAG CCCGTTGACG AATCACATTC CGATT
Species D: CTCATAGGGT GCAGGCCTAG CCCCTTGACG AATCCAATTC CGCTT
Species E: CTCATAGGGT GCAGGCCTAG CCCGTTGACG AATCCAATTC CGCTT
Species F: CTCCTAGGTT GCAGGCCTAG CCCTTTGAAG AATCACATTC CCCAA
Species G: CTCCTCGGGT GCAGGCATAG CCCTTTGACG AATCCCCTTC CGAAA
Species H: CTCCTAGGGT GCAGGCATAG CCCTTTGACG AATCCCCTTC CAAAAT

[ 1 ]

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CAIE IGCSE Biology 4.1.4 Structure of a Dna Molecule a Two Questions question 1

Question (a)

Question (b)