Regulation of Transcription and Translation

Transcription factors control the transcription of target genesTranscription is when a gene is copied from DNA into mRNA. The enzyme responsible for synthesising mRNA from DNA is called RNA polymerase. All the cells in an organism carry the same genes (DNA) but the structure and function of different cells varies. This is because not all genes in a cell are expressed. Because different genes are expressed, different proteins are made and these proteins modify the cell - they determine the cell structure and control cell processes. The transcription of genes is controlled by proteins molecules called transcription factors: In eukaryotes, transcription factors move from the cytoplasm to the nucleus. In the nucleus they bind to specific DNA sites near the start of their target genes - the genes they control the expression of. They control expression by controlling the rate of transcription. Some transcription factors, called activators, stimulate or increase the rate of transcription - e.g. they help RNA polymerase bind to the start of the gene and activate transcription. Other transcription factors, called repressors, inhibit or decrease the rate of transcription - e.g. they bind to the start of the target gene, preventing RNA polymerase from binding, stopping transcription.

Oestrogen can initiate the transcription of target genesThe expression of genes can also be affected by other molecules in the cell, e.g. oestrogen: Oestrogen is a steroid hormone that can affect transcription by binding to a transcription factor called an oestrogen receptor, forming an oestrogen-oestrogen receptor complex. The complex moves from the cytoplasm into the nucleus where it binds to specific DNA sites near the start of the target gene. The complex can act as an activator of transcription, e.g. helping RNA polymerase bind to the start of the target gene.

RNA interference (RNAi) can inhibit the translation of mRNAIn eukaryotes, gene expression is also affected by RNA interference (RNAi)RNAi is where small, double stranded RNA molecules stop mRNA translating target genes into proteins. A similar process to RNAi can also occur in prokaryotes. The molecules involved in RNAi are called siRNA (small interfering RNA) and miRNA (microRNA). EXAMPLE siRNA (and miRNA in plants) Once mRNA has been transcribed, it leaves the nucleus for the cytoplasm. In the cytoplasm, double-stranded siRNA associates with several proteins and unwinds. A single strand then binds to the target mRNA. The base sequence of the siRNA is complementary to the base sequence in sections of the target mRNA. The proteins associated with the siRNA cut the mRNA into fragments - so it can no longer be translated. The fragments then move into a processing body, which degrades them. A similar process happens with miRNA in plants. EXAMPLEmiRNA in mammals In mammals, the miRNA isn't usually fully complementary to the target mRNA. This makes it less specific than siRNA and so it may target more than one mRNA molecule. Like siRNA, it associates with proteins and binds to target mRNA in the cytoplasm. Instead of the proteins associated with miRNA cutting mRNA into fragments, the miRNA-protein complex physically blocks the translation of the target mRNA. The mRNA is then moved into a processing body where it can either be stored or degraded. When it's stored, it can be returned and translated at another time.