17.1 Chromosomes, genes and proteins
- Syllabus
- 0610–2026–2027
- Topic
- 17.1
- Level
- —
Learning objectives
17.1.1Chromosomes are made of DNA, which• State that chromosomes are made of DNA, which contains genetic information in the form of genes17.1.2Gene as a length of DNA that codes• Define a gene as a length of DNA that codes for a protein17.1.3Allele as an alternative form• Define an allele as an alternative form of a gene17.1.4Inheritance of sex in humans• Describe the inheritance of sex in humans with reference to X and Y chromosomes17.1.5Sequence of bases in a gene determines• State that the sequence of bases in a gene determines the sequence of amino acids used to make a specific protein (knowledge of the details of nucleotide structure is not required)17.1.6Different sequences of amino acids• Explain that different sequences of amino acids give different shapes to protein molecules17.1.7DNA controls cell function• Explain that DNA controls cell function by controlling the production of proteins, including enzymes, membrane carriers and receptors for neurotransmitters17.1.8Protein is made• Explain how a protein is made, limited to: • the gene coding for the protein remains in the nucleus • messenger RNA (mRNA) is a copy of a gene • mRNA molecules are made in the nucleus and move to the cytoplasm • the mRNA passes through ribosomes • the ribosome assembles amino acids into protein molecules • the specific sequence of amino acids is determined by the sequence of bases in the mRNA (knowledge of the details of transcription or translation is not required)17.1.9Most body cells in an organism contain• Explain that most body cells in an organism contain the same genes, but many genes in a particular cell are not expressed because the cell only makes the specific proteins it needs17.1.10Haploid nucleus as a nucleus• Describe a haploid nucleus as a nucleus containing a single set of chromosomes17.1.11Diploid nucleus as a nucleus• Describe a diploid nucleus as a nucleus containing two sets of chromosomes17.1.12Diploid chromosome pairs• State that in a diploid cell, there is a pair of each type of chromosome and in a human diploid cell there are 23 pairs
Place genes within chromosomes and DNA
A chromosome is a long, thread-like structure made of DNA. The DNA carries genetic information in sections called genes: cell → nucleus → chromosome → DNA → gene.
A chromosome therefore contains many genes at particular positions. A gene is part of a DNA molecule; it is not a separate substance attached to the chromosome.
DNA is the material, a chromosome is one organised DNA structure, and genes are information-carrying lengths within that DNA.
Define a gene precisely
A gene is a length of DNA that codes for a protein.
Its sequence provides the information needed to place amino acids in the order for that protein. Different proteins can have different roles, such as enzymes or membrane carriers.
A gene does not directly code for a whole characteristic. It codes for a protein, and the protein can contribute to the characteristic.
Distinguish an allele from a gene
An allele is an alternative form of a gene.
The gene names the inherited instruction—for example, a gene affecting a particular protein. Alleles are different versions of that same instruction and may have different DNA base sequences.
Two alleles are not two unrelated genes. They are alternative forms at the same gene position on corresponding chromosomes.
Explain inheritance of human sex chromosomes
Human females usually have XX sex chromosomes and males usually have XY. Every egg carries X, while sperm carry either X or Y.
| sperm X | sperm Y | |
|---|---|---|
| egg X | XX (female) | XY (male) |
| egg X | XX (female) | XY (male) |
Fertilisation therefore gives an expected 1:1 ratio: a sperm carrying X produces XX, while a sperm carrying Y produces XY. The sperm from the father determines which combination forms.
The 1:1 ratio is a probability across many fertilisations, not a guarantee that every family has equal numbers of girls and boys.
Link a gene's base sequence to amino-acid sequence
The sequence of bases in a gene determines the sequence of amino acids used to make a specific protein.
gene base sequence → information copied into mRNA → amino acids assembled in a particular order → specific protein
It is the order of bases—not merely the number of bases—that carries the information. Detailed nucleotide structure is outside this syllabus objective.
Connect amino-acid sequence to protein shape
Different sequences of amino acids cause protein molecules to fold into different shapes. The amino-acid order affects the attractions and bonds within the chain, so it determines the final three-dimensional shape.
Shape controls function. For example, an enzyme needs an active site with a complementary shape to its substrate, and a receptor needs a binding site that fits its signalling molecule.
A changed amino-acid sequence may change a protein's shape and function; it does not simply make the same protein shorter or longer.
Explain how DNA controls cell function through proteins
DNA controls cell function by controlling which proteins the cell produces. A gene's base sequence specifies an amino-acid sequence; that sequence gives a protein its shape, and the shape enables its function.
| Protein type | How it affects cell function |
|---|---|
| enzyme | catalyses a particular reaction |
| membrane carrier | moves a particular substance across a membrane |
| neurotransmitter receptor | binds a particular neurotransmitter and allows a response |
DNA controls these functions through protein production; the DNA molecule does not itself catalyse reactions or carry substances across the cell membrane.
Follow the syllabus-limited steps of protein synthesis
A cell makes a protein by copying a gene's information into messenger RNA (mRNA), then using a ribosome to assemble amino acids in the specified order.
- The gene coding for the protein remains in the nucleus.
- An mRNA copy of the gene is made in the nucleus.
- The mRNA moves into the cytoplasm and passes through a ribosome.
- The ribosome assembles amino acids into a protein.
- The sequence of bases in mRNA determines the specific amino-acid sequence.
mRNA carries the copied information; the ribosome is the assembly site. The DNA stays protected in the nucleus while its information is used in the cytoplasm.
At this level, the detailed molecular stages called transcription and translation are not required.
Explain selective gene expression in specialised cells
Most body cells in an organism contain the same genes, but they do not use all of them. A particular cell expresses only the genes needed to make the proteins for its function; many other genes are not expressed.
A neurone and a phagocyte contain the same set of genes, but they make different combinations of proteins. This produces different structures and specialised functions.
Specialised cells usually differ because different genes are switched on or off—not because each cell type has a completely different set of genes.
Recognise a haploid nucleus
A haploid nucleus contains a single set of chromosomes. The symbol n represents the haploid number.
Human gametes are haploid: an egg or sperm nucleus contains 23 chromosomes, one chromosome of each type.
Haploid means one complete set, not one chromosome in total and not one chromosome pair.
Recognise a diploid nucleus
A diploid nucleus contains two sets of chromosomes. The symbol 2n represents the diploid number.
One set was inherited from the mother and one from the father. A zygote and most body cells are diploid.
Diploid means two complete sets, not simply any nucleus containing two chromosomes.
Relate chromosome pairs to diploid number
In a diploid cell, there is a pair of each type of chromosome: one member of each pair came from each parent. The two chromosomes in a pair carry the same types of genes at corresponding positions, though their alleles may differ.
A human diploid cell has 23 pairs, so it has 46 chromosomes in total. A human haploid gamete has one chromosome from each pair, so it has 23 chromosomes.
‘23 pairs’ means 46 individual chromosomes. It is different from the haploid number of 23 individual chromosomes.