Promoter & Suppressor Sequences
Promoter Sequences
- Only some DNA sequences code for the production of polypeptides, these are called coding sequences
- Noncoding sequences produce functional RNA molecules like transfer RNA (tRNA) or are involved in the regulation of gene expression such as enhancers or promoters
- The promoter is a noncoding sequence located near a gene whereas enhancers are noncoding regions of DNA that are usually found further away from a gene
- The promoter and enhancer regions are not transcribed
- Transcription factors are a type of protein molecule that bind to the promoter or enhancer sequences to help initiate transcription
- This helps RNA polymerase to attach to the promoter and result in an increase in the rate of transcription
- These transcription factors regulate transcription and ensure that only the required genes are expressed in the correct cells, at the correct time and to the appropriate level depending on the specific needs of the cell
- This is the most common way for cells to control gene expression
A Promoter Region and the Bonding of a Transcription Factor Diagram
A transcription factor binding to the promoter region of a gene which allows RNA polymerase to bind and for transcription to occur
Suppressor Sequences
- Some factors inhibit gene expression by binding to DNA and blocking transcription
- These are called suppressors
- The most widely documented are tumor-suppressor genes
- These code for proteins that help keep cell division under control
Tumor suppressor genes
- Cancers demonstrate how important it is that cell division is precisely controlled, as cancers arise due to uncontrolled mitosis
- Cancerous cells divide repeatedly and uncontrollably, forming a tumor (an irregular mass of cells)
- Tumor suppressor genes are normal genes that code for proteins that regulate the cell cycle
- The proteins encoded for by tumor suppressor genes carry out the following functions:
- DNA repair
- Slowing the cell cycle by ensuring checks are made
- Signalling apoptosis (cell death) when the cell is faulty
- These proteins ensure that cells do not replicate if they contain mutated DNA or are faulty as these characteristics can lead to tumor formation
- Tumors develop if tumor suppressor genes are mutated or silenced
- A mutation can be any type that results in a non-functional protein
- Silencing can occur through epigenetic changes
- Hypermethylation of DNA (over-addition of methyl groups to cytosine nucleotides) causes transcription inhibiting proteins to bind the DNA
- If this occurs around tumor suppressor genes, this could result in tumor development as the necessary regulatory proteins coded for by tumor suppressor genes will not be produced
- Hypermethylation of DNA (over-addition of methyl groups to cytosine nucleotides) causes transcription inhibiting proteins to bind the DNA