Introduction to genes
Genes are sections of DNA, They're found on chromosomes.
Genes code for proteins (polypeptides) - they contain the instructions to make them.
Proteins are made from amino acids.
Different proteins have a different number and order of amino acids.
It's the order of nucleotide bases in a gene that determine the order of amino acids in a protein.
Each amino acid is coded for by a sequence of three bases (called a triplet) in a gene.
Different sequences of bases code for different amino acids.
Not all DNA codes for proteins...
Genes in eukaryotic DNA contain sections that don't code for amino acids.
These sections of DNA are called introns. The bits that do code for amino acids are called extrons.
Introns are removed from DNA during protein synthesis, Their purpose is still unknown.
Eukaryotic DNA also contains regions of multiple repeats outside of genes.
These are DNA sequences that repeat over and over.
These areas don't code for amino acids either.
Nature and development of organisms
Enzymes speed up most of our metabolic pathways - these determine how we grow and develop.
All enzymes are proteins, which are built using the instructions contained within genes. The order of bases in the gene decides the order of amino acids in the protein and so what type of protein is made.
So, our genes help to determine our nature, development and phenotype.
A gene can exist in more that one form - these forms are called alleles.
The order of bases in each allele is slightly different, so they code for slightly different versions of the same characteristic.
Pairs of matching chromosomes are called homologous pairs.
In these, both chromosomes are the same size and have the same genes, although they could have different alleles.
Alleles coding for the same characteristic will be found at the same position (locus) on each chromosome in a homologous pair.
Mutations are changes in the base sequence of an organisms DNA.
So, mutations can produce new alleles of genes.
If the sequence of bases in a gene changes, a non-functional or different protein could be produced.
If there's a mutation in a gene that codes for an enzyme, then that enzyme may not fold up properly. This may produce an active site that's the wrong shape and so a non-functional enzyme.
Good luck on your exams!