2.2.4Unit 2: Cells, Development, Biodiversity and Conservation
Nucleotides, DNA and RNA, Base Pairing — IAL Edexcel Linear Revision Notes & Topical Questions
5 min read 5 parts Lesson 16 of 26 Unit quiz
Curriculum Equation Key pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: pH + pOH = 14
Section 1 of 5Nucleotide Structure
Nucleotides are the monomers of the nucleic acids DNA and RNA.
Phosphate group
- A negatively charged group that forms part of the sugar–phosphate backbone and links one nucleotide to the next.
Pentose sugar
- A five-carbon sugar, which is deoxyribose in DNA and ribose in RNA.
Nitrogenous base
- The part that variesadenine, cytosine, guanine and thymine in DNA, with uracil replacing thymine in RNA.
Purines
- Adenine and guanine have a double ring structure and are therefore the larger bases.
Pyrimidines
- Cytosine, thymine and uracil have a single ring structure and are therefore smaller bases.
Phosphodiester bond
- A covalent bond formed by condensation between the phosphate of one nucleotide and the sugar of the next.
Section 2 of 5DNA and Base Pairing
1
DNA is a double helix of two polynucleotide strands running in opposite directions, which is described as antiparallel.
2
The bases on one strand pair with the bases on the other by complementary base pairing held together by hydrogen bonds.
3
Adenine pairs with thymine through two hydrogen bonds, and cytosine pairs with guanine through three hydrogen bonds.
4
A purine always pairs with a pyrimidine, so the width of the helix stays constant along its length.
5
The sequence of bases along a gene carries the genetic code, while the many hydrogen bonds make the molecule stable but still separable.
Section 3 of 5Comparing DNA and RNA
Comparing DNA and RNA
DNA
- Very long double-stranded molecule that stores genetic information in the nucleus and contains deoxyribose and thymine.
mRNA
- Single-stranded copy of a gene that carries the code from the nucleus to the ribosomes.
tRNA
- A folded single strand with an anticodon at one end and an amino acid attached at the other.
rRNA
- Combines with proteins to form ribosomes, where amino acids are joined to make polypeptides.
| Feature | DNA | RNA |
|---|---|---|
| Sugar | Deoxyribose | Ribose |
| Bases | A, T, C, G | A, U, C, G |
| Strands | Two (double helix) | One |
| Length | Very long | Short |
Section 4 of 5Worked Example
Question
- A DNA molecule has 30% adenine.
- Calculate the percentages of the other three bases, and the number of hydrogen bonds in a 100 base-pair section with these proportions.
- A pairs with T, so thymine is also 30%.
- A + T = 60%, so C + G = 40%, and as C = G, each is 20%.
- 100 base pairs contain 200 bases, so A = T = 60 bases and C = G = 40 bases.
- Each A–T pair has 2 hydrogen bonds and each C–G pair has 3, giving 60 A–T pairs and 40 C–G pairs.
- Hydrogen bonds = (60 × 2) + (40 × 3) = 120 + 120 = 240.
Answer T = 30%, C = 20%, G = 20%, and the section has 240 hydrogen bonds.
Section 5 of 5Exam Checklist & Tips
2.2.4 – Key Points for the Exam
- Nucleotide A monomer made of a phosphate group, a pentose sugar and a nitrogenous base.
- Polynucleotide A chain of nucleotides joined by phosphodiester bonds.
- Complementary base pairing Specific pairing of A with T (or U) and C with G.
- Antiparallel Running in opposite directions, as the two strands of DNA do.
Common Mistakes
- Pairing adenine with guanine, when it pairs with thymine in DNA and with uracil in RNA.
- Stating that hydrogen bonds join nucleotides in one strand, when phosphodiester bonds do this.
- Saying that RNA contains thymine; RNA has uracil instead.
- Confusing the number of hydrogen bondsA–T has two and C–G has three.
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- RNA: 5′ → 3′ 3′ → 5′.
- : AUG UAC .
- : UAA, UAG, UGA tRNA (Release Factor) .