Inheritance and ecology · GCSE Biology
DNA, genes and chromosomes
Define DNA, gene and chromosome for GCSE Biology: double helix, a gene coding for a protein, 23 pairs in humans, and why alleles are versions of a gene rather than “the gene for brown”.
DNA is the polymer. A gene is a length of DNA that codes for a protein. Chromosomes are DNA structures in the nucleus. Humans: 46 chromosomes, 23 pairs. Alleles are different versions of the same gene.
The important bits
What you need to know
- 1
DNA is a polymer of nucleotides, made of two strands twisted in a double helix. It is found in the nucleus of eukaryotic cells (and as a loop plus plasmids in bacteria).
- 2
A gene is a section of DNA that codes for a sequence of amino acids, which folds into a protein (enzyme, antibody, structural protein, hormone receptor).
- 3
A chromosome is a long DNA molecule packaged with proteins. Human body cells have 23 pairs (46). Gametes have 23 unpaired chromosomes.
- 4
The genome is all the genetic material of an organism. The Human Genome Project mapped human DNA; uses include identifying genes linked to disease and tracing migration — Triple discussion.
- 5
Alleles are different forms of the same gene, for example B and b. Genotype is the alleles present (Bb). Phenotype is the characteristic expressed (brown eyes, if brown is dominant).
- 6
Dominant alleles are expressed if at least one copy is present. Recessive alleles are expressed only if two copies are present (homozygous recessive).
- 7
Homozygous: two identical alleles (BB or bb). Heterozygous: two different alleles (Bb). Do not call a heterozygote “the dominant gene only”.
- 8
On Higher/Triple, complementary base pairing (A–T, C–G) and the idea that three bases code for one amino acid may appear. Combined still needs gene → protein as the core.
Quotations worth analysing
Short evidence. Real method.
“A gene is a section of DNA that codes for a protein.”
This is the highest-value sentence on the page. “A gene is a characteristic” is wrong; the phenotype is the characteristic.
“Alleles are different versions of the same gene.”
Brown and blue are phenotypes caused by alleles of a gene, not two different genes in the sloppy sense students write.
“Human body cells have 23 pairs of chromosomes.”
46 in a diploid cell, 23 in a gamete. Mitosis keeps 46; meiosis halves to 23.
Go deeper
How do I keep DNA, gene, chromosome and allele in a hierarchy?
Nucleus contains chromosomes. Each chromosome is one DNA molecule (plus proteins). Along that DNA sit genes, like recipes. Each gene can exist as different alleles — slightly different DNA sequences, so slightly different proteins, so different phenotypes. A cell has two alleles of each gene (one from each parent), on the two chromosomes of a pair (except X and Y in males for some genes). If you only remember “DNA is genetic material”, you will lose separate marks for gene and chromosome. Practise a nested sentence: “A chromosome is a structure of DNA in the nucleus; a gene is a section of that DNA coding for a protein; alleles are versions of that gene.” Genome is the whole library, not one book.
Go deeper
What does “codes for a protein” actually mean at GCSE?
The order of bases in the gene determines the order of amino acids. The protein’s shape, including an enzyme’s active site, depends on that order. A mutation that changes a base can change an amino acid, change the shape, and change the phenotype — or do nothing if the protein still works. That is why cystic fibrosis can be a faulty membrane-protein gene: the protein does not fold or work, so chloride transport fails. You do not need transcription and translation detail on most Combined papers; you do need gene → amino acid sequence → protein → characteristic. “DNA makes characteristics” skips the protein and loses a mark. Ribosomes assemble the protein in the cytoplasm; the gene stays in the nucleus (eukaryotes).
Go deeper
How should I talk about sex chromosomes without overclaiming?
Females XX, males XY. The Y chromosome carries genes that trigger male development. Eggs all carry X; sperm carry X or Y, so sex is decided at fertilisation by which sperm arrives — 50% as a Punnett square of XX × XY. Do not write that the mother determines sex, or that X means girl as if a male had no X. Males are XY, so they have an X. Sex-linked disorders (haemophilia, red-green colour blindness) sit on the X and appear more in males because they have only one X; mention this only if your specification includes it. For everyone: 22 pairs of autosomes plus one pair of sex chromosomes in a typical body cell.
See the idea in action
A student writes “the gene for brown eyes”. Better: “an allele of a gene that codes for a pigment protein; the brown-eye allele is dominant, so genotypes BB and Bb give a brown phenotype, and only bb is blue if blue is recessive.” DNA in the nucleus is organised as 23 pairs of chromosomes. Each eye-colour gene is a section of DNA on a chromosome. The protein is made at ribosomes. That chain — DNA, gene, allele, protein, phenotype — is the answer, not a single woolly “gene”.
Exam technique
Turn knowledge into marks
Define gene as a DNA section coding for a protein. Define allele as a version of a gene. Quote 23 pairs. Keep genotype (alleles) separate from phenotype (feature). Never say DNA is a protein.
Common mistakes
Do not give these marks away
- 01
Calling a gene a characteristic, or saying alleles are different genes.
- 02
Writing that humans have 23 chromosomes in body cells, or 46 in gametes.
- 03
Saying DNA is made of amino acids, or that genes are made of protein.
What is a gene?
AA protein that digests starch
BA section of DNA that codes for a protein
CA type of carbohydrate in the cell wall
DThe same thing as a chromosome in bacteria only
Show the answer
A section of DNA that codes for a protein. Genes are lengths of DNA. Chromosomes are structures that carry many genes. Enzymes are proteins coded for by genes, not genes themselves.
Quick questions
If this is the bit you searched
What is the difference between a gene and an allele?
A gene is a section of DNA that codes for a protein. Alleles are different versions of that gene, such as B and b.
How many chromosomes are in a human body cell?
46, arranged as 23 pairs. Gametes have 23 chromosomes.
What is the difference between genotype and phenotype?
Genotype is the alleles an organism has (for example Bb). Phenotype is the characteristic that is expressed (for example brown eyes).
Where is DNA found in a human cell?
In the nucleus, organised as chromosomes. Mitochondria also contain a small amount of DNA, but GCSE questions mean nuclear DNA unless they say otherwise.