Chromosomes, Genes and Proteins Notes
Cambridge IGCSE Biology: Revision notes
Key facts
- Chromosomes are made of DNA and carry genes.
- A gene is a length of DNA that codes for a protein; an allele is an alternative form of a gene.
- The base sequence of a gene sets the amino acid sequence, which sets the protein’s shape and function.
- A gene stays in the nucleus; an mRNA copy carries the code to a ribosome, where the protein is made.
- Cells have the same genes, but each cell expresses only the genes for the proteins it needs.
- Females are XX; males are XY.
Chromosomes, genes and alleles
A chromosome is a structure of DNA, a gene is a length of DNA coding for one protein, and an allele is a version of a gene.
Chromosomes are made of DNA, which carries genetic information as genes. Different alleles of one gene code for slightly different versions of the same protein.
In humans, sex is inherited through the X and Y chromosomes: females are XX and males are XY.
- 1
Chromosome
a structure made of DNA
- 2
Gene
a length of DNA that codes for a protein
- 3
Allele
an alternative form of that gene
What is an allele?
From base sequence to protein
The order of bases in a gene sets the order of amino acids, which sets the protein’s shape and so its job.
The sequence of bases in a gene determines the sequence of amino acids in the protein. Different genes have different base sequences, so their proteins fold into different shapes.
Shape decides function, for example the shape of an enzyme’s active site or an antibody’s binding site.
- 1
Base sequence
order of bases in the gene
- 2
Amino acid sequence
order of amino acids in the protein
- 3
Protein shape
the sequence folds into a specific shape
- 4
Protein function
the shape decides what the protein does
What directly determines the shape of a protein?
Making a protein
A copy of the gene, mRNA, leaves the nucleus and is read at a ribosome, which assembles the amino acids in order.
The gene stays in the nucleus. mRNA is made there as a copy of the gene and moves into the cytoplasm, where it passes through a ribosome. The ribosome assembles amino acids into a protein, in the order set by the mRNA bases.
- 1
Gene in nucleus
stays in the nucleus
- 2
mRNA copy made
a copy of the gene is made in the nucleus
- 3
To cytoplasm
mRNA moves out of the nucleus
- 4
Ribosome
mRNA passes through a ribosome
- 5
Protein
amino acids assembled in the order set by the mRNA
Where are amino acids assembled into a protein?
Why cells differ
Most body cells hold the same genes, but each expresses only the genes for the proteins it needs.
DNA controls what a cell does by controlling which proteins it makes, including enzymes, membrane carriers and receptors for neurotransmitters. Only some genes are expressed (switched on) in any one cell.
So a muscle cell and a nerve cell with identical DNA can look and behave completely differently.
| Red blood cell | White blood cell | |
|---|---|---|
| Genes in nucleus | Same complete set of genes | Same complete set of genes |
| Gene expressed | Expresses the gene for haemoglobin | Expresses genes for antibodies |
Red blood cell
- Genes in nucleus:
- Same complete set of genes
- Gene expressed:
- Expresses the gene for haemoglobin
White blood cell
- Genes in nucleus:
- Same complete set of genes
- Gene expressed:
- Expresses genes for antibodies
Why can a muscle cell and a nerve cell with identical DNA behave differently?
Try an exam question
Describe how a gene is used to make a protein in a cell.
[4 marks]
- [1]A copy of the gene (mRNA) is made in the nucleus.
- [1]The mRNA moves out of the nucleus into the cytoplasm.
- [1]The mRNA passes through a ribosome.
- [1]The ribosome assembles amino acids into a protein in the order set by the mRNA base sequence.
That's the notes covered.
Carry on to the next subtopic.