Nucleotides, DNA structure and replicationEdexcel A-Level Biology A: Revision notes
Section 1
Mononucleotides
A mononucleotide is made of three parts joined together:
- a pentose sugar, deoxyribose in DNA or ribose in RNA
- a phosphate group
- a nitrogen-containing base
DNA has four bases: adenine (A), thymine (T), cytosine (C) and guanine (G). RNA has uracil (U) in place of thymine.
DNA: deoxyribose and thymine. RNA: ribose and uracil. Phosphate, adenine, cytosine and guanine are in both.
Section 2
Polynucleotides
Nucleotides join in condensation reactions between the phosphate group of one nucleotide and the sugar of the next. The bond formed is a phosphodiester bond, and water is released.
This builds a polynucleotide with a repeating sugar–phosphate backbone. The bases project to one side. DNA and RNA are both polynucleotides.
Section 3
DNA and RNA
DNA is a double helix made of two polynucleotide strands running in opposite directions (antiparallel). It is a very stable, long molecule that stores genetic information.
Complementary base pairing: A pairs with T by two hydrogen bonds, and C pairs with G by three hydrogen bonds. The strands are held together by many hydrogen bonds between the paired bases, which are individually weak but strong in total, and easy to separate when the strands need to be copied. In a double-stranded DNA molecule, A = T and C = G, so if 30% of bases are A, then 30% are T, and C and G share the remaining 40%, 20% each.
RNA is a single polynucleotide with ribose and uracil.
Hydrogen bonds join the bases of the two strands. Phosphodiester bonds join nucleotides along one strand.
Section 4
Semi-conservative replication
DNA replication copies the DNA before a cell divides.
- DNA helicase breaks the hydrogen bonds between the strands, unwinding the helix and exposing the bases.
- Each strand acts as a template. Free nucleotides pair with the exposed bases by complementary base pairing.
- DNA polymerase joins the nucleotides by phosphodiester bonds, forming the new strand.
Each new DNA molecule has one original strand and one new strand, so replication is semi-conservative. This copies the base sequence accurately, so genetic information passes to daughter cells.
Worked example: template ATGCCGTA gives new strand TACGGCAT.
Section 5
Meselson and Stahl's experiment
Meselson and Stahl tested three models: semi-conservative, conservative (one old molecule and one new molecule) and dispersive (strands each a mixture of old and new).
Bacteria were grown for many generations in ¹⁵N (heavy) medium, then moved to ¹⁴N (light) medium. DNA was extracted at each generation and centrifuged in caesium chloride, where it forms bands by density.
- Generation 1: one intermediate band, so all DNA was hybrid. This rejects the conservative model.
- Generation 2: one intermediate band and one light band. This rejects the dispersive model, which would give a single band.
The results support the semi-conservative model.
Always say what each model would predict, then say what was observed.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Nucleotides, DNA structure and replication
- A student extracts DNA and RNA from the same cells and analyses the nucleotides that each is made of.Describe how two DNA nucleotides are joined together in a polynucleotide.2 marks
- Two samples of double-stranded DNA of the same length, sample X and sample Y, are heated slowly until the two strands of each separate. In sample X, 20% of the bases are adenine. In sample Y, 35% of the bases are adenine.Explain why the strands of sample X separate at a higher temperature than those of sample Y.2 marks
- A section of one strand of a DNA molecule has the base sequence ATGCCGTA. This strand acts as a template during DNA replication in a dividing cell.Describe how a new strand is formed on this template during 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).