Genomes and gene products
The genetic information encoded in the genome contains the blueprint for the potential development and activity of an organism, but the implementation of this information depends on the functions of the gene products, including proteins. Proteins perform a wide variety of biological functions, from catalysis of biochemical reactions, transport of nutrients or recognition and transmission of signals to structural and mechanical roles within the cell.
Protein expression is a multi-step process, beginning with transcription of DNA in the nucleus. In prokaryotic cells, the first product is messenger RNA, while in eukaryotic cells, the primary transcript then undergoes post-transcriptional modification, including splicing of introns (noncoding parts of the gene) to produce the final mRNA. The mRNA is then used as input for translation. In translation, the mRNA is decoded to produce a specific sequence of amino acids, the protein sequence, according to the rules specified by the trinucleotide genetic code. The protein amino acid sequence, also known as the primary structure, is held together by covalent or peptide bonds, which are made during the process of protein biosynthesis or translation. Protein sequences range in size from tens to several thousand residues.
To be able to perform their biological function, proteins must then fold into one or more specific spatial conformations, driven by a number of non-covalent interactions such as hydrogen bonding, ionic interactions, Van Der Waals forces, and hydrophobic packing. This 3D structure, or tertiary structure, is formed when smaller secondary structure elements, such as alpha-helices and beta-sheets, are folded into one or more self-stabilizing domains. A protein may undergo reversible structural changes in performing its biological function. The alternative structures of the same protein are referred to as different conformations, and transitions between them are called conformational changes.
Single protein molecules often form larger assemblies. This quaternary structure is stabilized by the same non-covalent interactions and disulfide bonds as the tertiary structure. Very large aggregates can be formed from protein subunits: for example, many thousand actin molecules assemble into a microfilament.
The DNA sequence of an organism can be modified by sudden and spontaneous changes in the cell. Mutations are caused by radiation, viruses, transposons and mutagenic chemicals, as well as errors that occur during meiosis or DNA replication. They can result in several different types of changes: no effect, alter the expression of a gene, or alter the protein sequence/structure, preventing the gene from functioning properly or completely.