DNA (Deoxyribonucleic Acid) is the molecule that carries genetic information in all living organisms.
DNA is a POLYMER made of repeating units called NUCLEOTIDES.
Each nucleotide contains:
A deoxyribose SUGAR.
A PHOSPHATE group.
One of four BASES: Adenine (A), Thymine (T), Cytosine (C) or Guanine (G).
Two strands of nucleotides wind around each other to form the famous DOUBLE HELIX — like a twisted ladder.
The strands are held together by hydrogen bonds between complementary BASE PAIRS:
A always pairs with T (adenine — thymine).
C always pairs with G (cytosine — guanine).
This base pairing rule (A-T, C-G) is fundamental — it allows DNA to be replicated accurately before cell division.
Genes and Chromosomes
A GENE is a specific sequence of DNA bases that codes for the production of a specific PROTEIN.
The sequence of bases in a gene determines the sequence of AMINO ACIDS in a protein — different base sequences produce different proteins with different functions.
Genes are located on CHROMOSOMES — long, tightly coiled DNA molecules.
Humans have 46 chromosomes arranged as 23 PAIRS. One chromosome in each pair came from the mother; one from the father.
Each chromosome contains thousands of genes — there are approximately 20,000–25,000 protein-coding genes in the human genome.
ALLELES are different versions of the same gene, caused by slightly different base sequences at the same position on homologous chromosomes.
The Human Genome
The GENOME is the complete set of genetic information in an organism — every gene in every chromosome.
The HUMAN GENOME PROJECT was an international scientific collaboration that:
Ran from 1990 to 2003.
Mapped and sequenced all approximately 3 BILLION base pairs of the human genome.
Identified the location and sequence of all human genes.
Why it matters:
Identifying genes linked to INHERITED DISEASES — e.g. BRCA1/2 (breast cancer risk), cystic fibrosis gene.
Developing PERSONALISED MEDICINE — treatments tailored to an individual's genetic make-up.
Understanding HUMAN EVOLUTION and our relationship to other species.
Forensic science — DNA profiling to identify individuals.
Understanding gene function in health and disease.
⚠️ Common Mistake
A GENE is not the same as a CHROMOSOME. A chromosome contains THOUSANDS of genes. The GENOME is the COMPLETE set of ALL genetic information — all genes on all chromosomes. Base pairing: A pairs with T only. C pairs with G only. Never A-G or C-T.
📌 Key Note
DNA = double helix. A-T and C-G base pairs. Gene = section of DNA coding for a protein. Chromosome = contains thousands of genes. Genome = complete genetic information of an organism. Human genome: ~20,000 genes, ~3 billion base pairs.
🎯 Matching Activity — DNA and Genome Vocabulary
Match each term to its correct definition. — drag the symbols on the right to match the component names on the left.
Nucleotide
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Gene
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Chromosome
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Allele
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Genome
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Base pairing
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A different version of the same gene — slightly different base sequence
A long coiled DNA molecule — contains thousands of genes
A specific sequence of DNA bases that codes for a particular protein
A always pairs with T; C always pairs with G — holds DNA strands together
The monomer unit of DNA — contains a sugar, phosphate and one base
The complete set of genetic information in an organism
🎯 Test Yourself
Question 1 of 3
1. In DNA, which bases pair together?
2. What is a gene?
3. What was the Human Genome Project?
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