DNA structure and replicationEdexcel A-Level Biology B: Revision notes
Section 1
The DNA nucleotide
DNA is a polymer of nucleotides. Each nucleotide has:
- a deoxyribose sugar (a pentose)
- a phosphate group
- a nitrogenous base
The bases fall into two groups. Purines, adenine (A) and guanine (G), have a double ring. Pyrimidines, cytosine (C) and thymine (T), have a single ring.
Section 2
The double helix
Nucleotides join by condensation reactions forming phosphodiester bonds between the sugar of one nucleotide and the phosphate of the next. This gives a sugar-phosphate backbone. DNA has two backbones forming two antiparallel polynucleotide strands.
Bases on opposite strands pair by hydrogen bonds: A with T (two bonds) and G with C (three bonds). Each pair is one purine and one pyrimidine, so the helix has a constant width. The two strands twist into a double helix.
Worked example: if 28% of the bases are A, then T is 28%, so G + C = 44% and G = 22%.
Hydrogen bonds hold the two strands together; phosphodiester bonds join nucleotides in one strand. Do not mix them up.
Section 3
Genes and information
A gene is a sequence of bases on a DNA molecule that codes for a sequence of amino acids in a polypeptide. The order of bases is the stored information. DNA is suited to this role because the strong sugar-phosphate backbone makes it stable, the molecule is long so it can store many genes, and the base pairs are protected inside the helix.
Section 4
Semi-conservative replication
Before cell division DNA is copied by semi-conservative replication: each new molecule has one original strand and one new strand.
- DNA helicase breaks the hydrogen bonds between bases and unwinds the helix.
- Each original strand is a template.
- DNA polymerase joins free nucleotides to the template by complementary base pairing, forming phosphodiester bonds in the new strand.
- DNA ligase joins fragments of the new strand, sealing the backbone.
Because the template determines the new sequence, the two molecules are identical to the original.
Polymerase builds the new strand; it does not unwind the helix. That is helicase.
Section 5
Evidence for semi-conservative replication
Bacteria grown in ¹⁵N (heavy) were moved to ¹⁴N (light) medium and their DNA separated by density.
- Generation 1: all DNA is intermediate (one ¹⁵N strand, one ¹⁴N strand). This rules out conservative replication, which would give heavy and light bands.
- Generation 2: half intermediate, half light, because each intermediate molecule makes one intermediate and one light molecule.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on DNA structure and replication
- A scientist extracts double-stranded DNA from a bacterium and analyses its base composition. She finds that 28% of the bases are adenine.Explain why the percentage of adenine in double-stranded DNA is equal to the percentage of thymine.2 marks
- Bacteria were grown for many generations in a medium containing the heavy isotope ¹⁵N, so all the nitrogen in their DNA was ¹⁵N. They were then transferred to a medium containing only the lighter isotope ¹⁴N and allowed to divide. DNA from each generation was extracted and separated by density to show whether each molecule was heavy (both strands ¹⁵N), intermediate (one strand ¹⁵N and one ¹⁴N) or light (both strands ¹⁴N).Explain why the result after one division in the ¹⁴N medium supports semi-conservative replication and not conservative replication.2 marks
- A cell is preparing to divide. Before division, every DNA molecule in the nucleus must be copied so that each daughter cell receives a complete set of genetic information.Describe the roles of DNA helicase, DNA polymerase and DNA ligase in DNA replication.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).