How is the structure of DNA suited to storing genetic information and copying it accurately?
Describe the structure of DNA and explain the semi-conservative mechanism of DNA replication
A focused answer to the H2 Biology Molecular Genetics outcome on DNA. The double helix and complementary base pairing, the antiparallel strands, and the semi-conservative mechanism of replication using helicase, DNA polymerase and ligase.
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What this dot point is asking
SEAB wants you to describe the molecular structure of DNA (the double helix, nucleotides, complementary base pairing, antiparallel strands) and to explain how DNA is copied by the semi-conservative mechanism, naming the key enzymes. This is the foundation of all molecular genetics that follows.
The answer
The structure of DNA
DNA is a polymer of nucleotides. Each nucleotide is made of a deoxyribose sugar, a phosphate group, and one of four nitrogenous bases: adenine (A), thymine (T), guanine (G) or cytosine (C). Nucleotides join through phosphodiester bonds between the sugar of one and the phosphate of the next, forming a sugar-phosphate backbone.
Two such strands wind around each other into a double helix. The bases face inwards and pair by hydrogen bonding: A pairs with T (two hydrogen bonds) and G pairs with C (three hydrogen bonds). This is complementary base pairing. The two strands run in opposite directions; they are antiparallel.
Why the structure suits its role
The base sequence is a stable store of information; complementary pairing lets each strand template an exact copy; and the many hydrogen bonds make the molecule stable yet separable when needed.
Semi-conservative replication
Replication is semi-conservative: each new molecule has one original strand and one new strand.
- Helicase unwinds the helix and breaks the hydrogen bonds, separating the two strands.
- Each exposed strand acts as a template.
- DNA polymerase joins free nucleotides to the template by complementary base pairing, forming phosphodiester bonds along the new strand.
- DNA ligase joins the fragments synthesised on the strand that is built in pieces.
The result is two identical double helices, each conserving one original strand.
Examples in context
Example 1. Proofreading by DNA polymerase. DNA polymerase can check each newly added base and remove a mismatched one before continuing. This proofreading keeps the error rate extremely low, which is why the genetic information is copied so faithfully from generation to generation.
Example 2. The basis of PCR. The polymerase chain reaction copies DNA in a tube by repeatedly separating the strands with heat and using a heat-stable DNA polymerase to template new strands. The technique works precisely because DNA replication is template-directed and complementary, the same principles described here.
Try this
Q1. State the base that pairs with cytosine and the number of hydrogen bonds between them. [1 mark]
- Cue. Cytosine pairs with guanine by three hydrogen bonds.
Q2. Explain the role of DNA ligase in replication. [1 mark]
- Cue. It joins the short fragments of new DNA together by forming phosphodiester bonds, producing a continuous strand.
Q3. A length of DNA has 600 base pairs and 360 of them are A-T pairs. Calculate the number of cytosine bases in this length. [2 marks]
- Cue. 600 - 360 = 240 G-C pairs; each pair has one cytosine, so there are 240 cytosine bases.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original5 marksDescribe the structure of a DNA molecule and explain three features of this structure that suit it to its role in storing and transmitting genetic information.Show worked answer →
Examiners want the structure first, then three features each linked to a role.
A DNA molecule is two polynucleotide strands wound into a double helix. Each nucleotide has a deoxyribose sugar, a phosphate, and one of four bases (adenine, thymine, guanine, cytosine). The strands are held together by hydrogen bonds between complementary base pairs: adenine with thymine (two bonds) and guanine with cytosine (three bonds). The two strands run antiparallel.
Feature one: complementary base pairing means each strand acts as a template for an exact copy, which suits accurate replication and transmission.
Feature two: the sequence of bases is a stable code that stores information, and because the bases are protected on the inside of the helix the information is chemically protected.
Feature three: the many hydrogen bonds collectively make the molecule stable, while individually being weak enough to be separated for replication and transcription.
Markers reward an accurate description of the double helix with antiparallel strands and correct base pairing, and three clearly linked structure-to-function points.
Original4 marksExplain why DNA replication is described as semi-conservative and outline the roles of helicase and DNA polymerase in the process.Show worked answer →
The answer needs the meaning of semi-conservative and the two named enzymes.
Replication is semi-conservative because each new DNA molecule consists of one original (parental) strand and one newly synthesised strand. The original double helix is conserved by half in each daughter molecule.
Helicase unwinds the double helix and breaks the hydrogen bonds between the base pairs, separating the two strands to expose them as templates.
DNA polymerase catalyses the formation of the new strand by joining free DNA nucleotides to the exposed template, adding each nucleotide according to complementary base pairing and forming phosphodiester bonds along the new strand.
Markers reward the definition of semi-conservative (one old, one new strand), the unwinding role of helicase, and the template-directed synthesis catalysed by DNA polymerase.
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