B31: DNA and the Genome
DNA structure, genes, chromosomes and the human genome
Key Definitions
DNA (deoxyribonucleic acid): The chemical that carries genetic information. It is a polymer made up of two strands coiled into a double helix, found in the nucleus of animal and plant cells.
Genome: The entire genetic material of an organism — all the DNA, including all the genes and non-coding regions.
Gene: A short section of DNA that codes for a specific protein (and therefore a specific characteristic).
Allele: A different version of the same gene. Alleles produce different versions of the same characteristic.
Chromosome: A long, coiled molecule of DNA found in the nucleus. Human body cells contain 46 chromosomes (23 pairs); gametes contain 23 chromosomes.
DNA Structure
DNA has a double helix structure — two strands twisted around each other like a spiral staircase. Each strand is made up of repeating units called nucleotides.
Each nucleotide consists of:
- A sugar (deoxyribose)
- A phosphate group
- A base (one of four types)
Complementary base pairing:
Adenine (A) always pairs with Thymine (T)
Cytosine (C) always pairs with Guanine (G)
Adenine (A) always pairs with Thymine (T)
Cytosine (C) always pairs with Guanine (G)
The two strands are held together by weak hydrogen bonds between the complementary base pairs. The sugar and phosphate form the “backbone” of each strand.
DNA is a polymer — a long molecule made up of repeating nucleotide units. Use this term in your answers for higher marks.
Chromosomes
| Cell Type | Number of Chromosomes | Description |
|---|---|---|
| Body cell (somatic) | 46 (23 pairs) | Diploid — two of each chromosome, one from each parent |
| Gamete (sperm/egg) | 23 | Haploid — one of each chromosome |
One pair of chromosomes are the sex chromosomes: XX in females, XY in males. The remaining 22 pairs are called autosomes.
Example 1: Chromosome Numbers
A skin cell has 46 chromosomes (23 from the mother, 23 from the father). When a sperm fertilises an egg, the 23 chromosomes from each gamete combine to give the zygote 46 chromosomes — restoring the diploid number. This is why offspring inherit characteristics from both parents.
Genes and Alleles
| Term | Definition | Example |
|---|---|---|
| Gene | A short section of DNA coding for a specific protein | The gene for eye colour |
| Allele | A different version of the same gene | Brown eye allele vs blue eye allele |
| Chromosome | A long, coiled molecule of DNA carrying many genes | Humans have 46 in body cells |
| Genome | The entire genetic material of an organism | All 3 billion base pairs of human DNA |
Example 2: Genes and Alleles
The gene for eye colour has different alleles. One allele codes for brown eye pigment, another codes for blue. A person inherits one allele from each parent — the combination of alleles determines their eye colour. Since chromosomes come in pairs, you have two alleles for each gene (one on each chromosome of the pair).
The Human Genome Project
The Human Genome Project was an international effort to sequence the entire human genome. It was completed in 2003.
| Benefit | Explanation |
|---|---|
| Medical benefits | Identifying genes linked to diseases enables better diagnosis, targeted treatments and personalised medicine |
| Understanding evolution | Comparing genomes of different species shows how closely related they are, providing evidence for common ancestry |
| Tracing human migration | DNA analysis reveals how human populations have moved across the Earth over thousands of years |
| Predicting disease risk | Individuals can be screened for genes that increase risk of certain conditions (e.g. certain cancers) |
| New medicines | Understanding genetic diseases helps develop targeted drugs and gene therapies |
Example 3: Using the Genome in Medicine
By sequencing the human genome, scientists identified the BRCA1 and BRCA2 genes. Mutations in these genes significantly increase the risk of breast and ovarian cancer. People with a family history can now be tested for these mutations and offered earlier screening or preventive treatment if they carry the faulty allele. This is an example of personalised medicine made possible by the Human Genome Project.
Worked Examples
Example 4: Complementary Base Pairing
If one strand of DNA has the base sequence A-T-C-G-G-A, the complementary strand will be T-A-G-C-C-T.
Reasoning: A pairs with T, T pairs with A, C pairs with G, G pairs with C, G pairs with C, A pairs with T.
Example 5: Calculating Chromosome Numbers
A human body cell has 46 chromosomes. How many chromosomes are in a human egg cell?
Solution: Egg cells are gametes (haploid), so they contain half the number of body cells.
46 ÷ 2 = 23 chromosomes
Comparison: Key Terms
| Term | Scale | What It Is | Analogy |
|---|---|---|---|
| DNA | Molecule | The chemical carrying genetic information in a double helix | The instruction manual |
| Chromosome | Structure | A long, coiled DNA molecule carrying many genes | A chapter in the manual |
| Gene | Section of DNA | A short section coding for one specific protein | A single instruction |
| Allele | Version of a gene | A different version of the same gene | A different option for that instruction |
| Genome | Entire set | All the genetic material in an organism | The complete manual |
Practice Questions
Q1: Foundation Name the four bases found in DNA and state which bases pair together.
Q2: Foundation Describe the structure of DNA.
Q3: Foundation How many chromosomes are found in a human body cell, and how many in a gamete? Explain the difference.
Q4: Higher A strand of DNA has the base sequence C-G-A-T-T-C. Write the complementary strand sequence.
Q5: Higher Give one medical and one evolutionary benefit of sequencing the human genome.
Q6: Higher Explain the difference between a gene and an allele, using an example.
Answers
- The four bases are adenine (A), thymine (T), cytosine (C), and guanine (G). A pairs with T, and C pairs with G.
- DNA is a polymer made of two strands of nucleotides coiled into a double helix. Each nucleotide contains a sugar, a phosphate, and a base. The strands are linked by complementary base pairing (A–T, C–G) via hydrogen bonds.
- A human body cell has 46 chromosomes (23 pairs — diploid). A gamete has 23 chromosomes (haploid). Gametes have half the number so that when fertilisation occurs, the zygote receives 23 from each parent, restoring the diploid number of 46.
- C pairs with G, G pairs with C, A pairs with T, T pairs with A, T pairs with A, C pairs with G. Complementary strand: G-C-T-A-A-G.
- Medical: Doctors can identify genes associated with diseases such as cancer, enabling earlier diagnosis and the development of personalised/targeted treatments. Evolutionary: Scientists can compare human DNA with that of other species to determine how closely related they are and trace human evolutionary history.
- A gene is a short section of DNA that codes for a specific protein/characteristic. An allele is a different version of that gene. For example, the gene for eye colour has multiple alleles — one coding for brown eyes and another coding for blue eyes.
Exam Tips
- Don’t confuse gene and allele — a gene is the section of DNA; an allele is a version of that gene
- Remember the base pairing rule: A–T and C–G. A common mistake is pairing A with C
- Body cells = 46 chromosomes (diploid). Gametes = 23 chromosomes (haploid). This comes up frequently
- When asked about the Human Genome Project, give both medical and evolutionary benefits for full marks
- DNA is a polymer — use this word in your answers for higher marks
- Remember: DNA → Gene → Protein → Characteristic. DNA does NOT code for characteristics directly
š¢ Maths Skills
Mathematical Skills
Probability in genetics: each allele from a parent has a 50% chance of being passed on. In a monohybrid cross (e.g. Ff Ć Ff), the probability of offspring being homozygous recessive (ff) is 1/4 = 0.25 = 25%. Probabilities can be expressed as fractions, decimals or percentages.
Example: If two carriers (Ff) have 4 children, the expected number with cystic fibrosis is 4 Ć 0.25 = 1. However, this is a prediction, not a certainty ā each pregnancy is an independent event.
ā ļø Common Misconceptions
Watch Out!
Students often think DNA is only found in the nucleus. Wrong: DNA is only in the nucleus Correct: Mitochondria and chloroplasts also contain small amounts of DNA
Students often think the genome is just the genes. Wrong: The genome is just the genes Correct: The genome includes all the DNA ā genes AND non-coding regions
āļø 6-Mark Question
Extended Answer
6 marks: Explain what the genome is and discuss the benefits of the Human Genome Project.
The genome is the entire genetic material of an organism ā that is, all the DNA, including all the genes and the non-coding regions between them. The Human Genome Project was an international effort to sequence the entire human genome, completed in 2003. Benefits include: identifying genes linked to diseases enables better diagnosis and personalised medicine; comparing genomes of different species shows evolutionary relationships and common ancestry; tracing human migration patterns over thousands of years; screening individuals for genes that increase disease risk (e.g. BRCA genes); and developing new targeted drugs and gene therapies.
Mark scheme: 1 mark for genome definition, 1 mark for non-coding DNA included, 1 mark for HGP description, up to 3 marks for valid benefits explained
š AO3: Analyse & Evaluate
Analysis and Evaluation
A scientist compares the genomes of Species A and Species B and finds they share 98% of their DNA. Species C shares only 75% with both.
What does this data suggest about the evolutionary relationships between these three species? Evaluate the limitations of using genome data alone to classify organisms.
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