Genes in chromosomal DNA

Chromosomes contain DNA, which carries genetic information. A gene is a length of DNA that codes for a protein. Genes are therefore parts of the DNA in chromosomes, rather than separate structures attached to them.

An allele is an alternative form of a gene. Different alleles of the same gene have differences in their DNA sequence.

Inheritance of sex in humans

Humans typically have two sex chromosomes. Females usually have two X chromosomes, written as $$\mathrm{XX}$$, while males usually have one X chromosome and one Y chromosome, written as $$\mathrm{XY}$$.

An egg contributes one X chromosome to the offspring. A sperm contributes either an X chromosome or a Y chromosome. When a sperm fertilises an egg, the two chromosomes form the offspring’s sex chromosome combination:

• An X-carrying sperm and an X-carrying egg produce $$\mathrm{XX}$$, typically female.
• A Y-carrying sperm and an X-carrying egg produce $$\mathrm{XY}$$, typically male.

Because the egg contributes an X chromosome in either case, which sperm fertilises the egg determines whether the offspring has an $$\mathrm{XX}$$ or an $$\mathrm{XY}$$ combination.

Gene base sequence and protein shape

A gene contains a sequence of bases. The order of these bases determines the order in which amino acids are joined to make a specific protein. Groups of bases specify which amino acids are used and where they are placed in the chain.

The amino acid chain folds into a three-dimensional shape. Its amino acid sequence affects how different parts of the chain interact and therefore how the chain folds. Different amino acid sequences can produce protein molecules with different shapes.

The base sequence of a gene therefore influences the protein’s structure by determining its amino acid sequence.

Protein synthesis using mRNA and ribosomes

A gene contains the instructions for making a protein, but the gene remains in the nucleus. A temporary copy of its instructions, called messenger RNA (mRNA), is made in the nucleus and moves into the cytoplasm.

In the cytoplasm, the mRNA passes through a ribosome. The ribosome reads the information in the mRNA and joins amino acids together to make a protein. Amino acids are the building blocks of proteins.

The instructions are carried in the sequence of bases in the mRNA. This sequence determines the order in which the ribosome assembles the amino acids, producing the protein specified by the gene.

Control of cell function through proteins

DNA contains instructions for making proteins. By controlling which proteins a cell produces, DNA influences what the cell can do and how it responds to its surroundings.

Different proteins carry out different roles:

• Enzymes catalyse chemical reactions. The enzymes present in a cell therefore affect which reactions can take place and how quickly they occur.
• Membrane carrier proteins move particular substances across cell membranes. They affect which substances can enter or leave the cell.
• Neurotransmitter receptors bind specific neurotransmitters, which are chemical signals released by nerve cells. Binding allows the cell to detect the signal and produce a response.

A cell’s activities and responses depend partly on which of these proteins it produces. For example, a cell needs the appropriate receptor to detect and respond to a particular neurotransmitter.

Selective gene expression in body cells

Most body cells in an organism contain the same genes, but a cell does not use all of its genes to make proteins. Instead, it makes the particular proteins it needs, while many of its genes remain unused.

A gene is expressed when the cell uses it to make its protein. An unexpressed gene is still present in the cell, but the cell is not using it to make that protein.

Different types of body cell express different sets of genes. They can therefore produce different sets of proteins, even though they contain largely the same genes.