DNA and the genome · GCSE Biology

DNA and genes

The genetic material in the nucleus of a cell is DNA. DNA is a polymer made of two strands that form a double helix.
DNA is found in structures called chromosomes.
A gene is a small section of DNA on a chromosome. Each gene codes for a particular sequence of amino acids, which makes a specific protein.

The genome of an organism is its entire genetic material. The whole human genome has now been studied.
This helps scientists search for genes linked to different diseases, understand and treat inherited disorders, and trace human migration patterns from the past.

DNA is made of repeating units called nucleotides. Each nucleotide has a sugar, a phosphate group and one of four bases: A, C, G or T.
The strands have alternating sugar and phosphate sections, with a base attached to each sugar.
The bases pair up across the two strands: A always pairs with T, and C always pairs with G. These are complementary pairs.

The order of bases in a gene determines the order of amino acids in a protein. Each amino acid is coded for by a sequence of three bases.
In the nucleus, the gene is copied into a molecule of mRNA, which moves to a ribosome in the cytoplasm.
Carrier molecules bring specific amino acids to the ribosome, which joins them in the order set by the mRNA.
When the chain is complete, it folds into a unique shape, which lets it do its job, for example as an enzyme, a hormone or a structural protein such as collagen.

Mutations are changes in DNA. They happen continuously.
Most mutations do not change the protein, or change it only slightly so its appearance or function is not affected.
A few mutations change the shape of a protein, so an enzyme's active site may no longer fit its substrate, or a structural protein may lose its strength.
Not all DNA codes for proteins: some non-coding parts switch genes on and off, so mutations there can affect how genes are expressed.

DNA: The polymer that carries genetic information, made of two strands in a double helix.
Double helix: The twisted-ladder shape of a DNA molecule.
Chromosome: A structure made of DNA that carries many genes.
Gene: A section of DNA that codes for a particular sequence of amino acids to make a protein.
Genome: The entire genetic material of an organism.
Nucleotide: The repeating unit of DNA: a sugar, a phosphate group and a base.
Complementary base pairs: The pairing of A with T and C with G across the two DNA strands.
Mutation: A change in the DNA.
mRNA: A copy of a gene that carries the code from the nucleus to a ribosome.
Ribosome: The organelle where amino acids are joined to make proteins.