A gene is a length of DNA, associated with some function, that may be inherited. In eukaryotes (nucleated cells), a gene usually consists of:
- A transcribed segment, which is translated into RNA for some effect
- The transcribed segment contains introns and exons; exons are expressed whereas introns are in between exons and are incised by splicing mechanics after translation has occured
- Each end of the transcribed segment contains a 5' and 3' untranslated region. This may allow the cell to recognise sequences of genetic code after they have been transcribed into RNA.
- Non-transcribed parts, which include:
- Regulatory segment(s), which are lengths of DNA allowing the cell to control which genes are expressed.
- Start and stop segments which flank the transcribed segment, allowing transcription enzymes to bind
The classical gene codes for a protein; the genetic code is transcribed into a strand of RNA by RNA polymerase. This RNA, known as pre-messenger RNA, undergoes splicing where the introns are removed. Once completed, the messenger RNA is transported to the ribosomes in the cytoplasm for translation into protein.
Other genes code for RNA products that do not require translation into protein. This includes the RNA that forms the ribosome (ribosomal RNA), or micro-RNAs which are important in the post-transcription control of gene expression.
There are about 25,000 genes in the human genome.
Location of regulatory segments
Gene regulatory segments need not be located immediately adjacent to the transcribed segment. Regulatory segments can exert an influence over thousands of base pairs, and different regulatory segments can also interact to enhance (or further suppress) the transcription of a gene.