Regulation of transcription and translation - AL only (3.8.2.2)
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Transcription of genes occurs when transcription factors move from the cytoplasm into the nucleus through the nuclear pore and bind with DNA.
The transcriptional factor is complementary to a specific section of DNA. Transcription of the gene begins when the transcription factor binds to the DNA.
If a gene has been switched off, the transcription factor can still be activated, but cannot bind with the DNA.
Transcription and translation cannot occur if the gene has been switched off, even if the transcription factor has been activated.
Oestrogen is a steroid hormone which can move through the cell membrane. Oestrogen binds with a receptor on the transcriptional factor, changing its shape.
The transcription factor moves into the nucleus through the nuclear pore before binding with the complementary section of DNA. Transcription of the gene can then occur.
Epigenetics refers to changes in gene expression that do not alter the DNA sequence. Instead, epigenetic changes can activate or silence genes.
The following are mechanisms that regulate gene expression:
- DNA methylation: addition of methyl groups () to DNA, usually at cytosine bases. This reduces transcription, so the gene is more likely to be silenced.
- Histone Modification: addition or removal of chemical groups such as acetyl or methyl groups to histone proteins. Acetylation loosens DNA around histones and results in gene activation. Deacetylation and methylation cause DNA to get more tightly wrapped around histones and results in gene silencing.
Some epigenetic changes can be maintained through mitosis, allowing daughter cells to
retain the same patterns of gene expression as the parent cell. This helps maintain their specialised structure and function.
Some epigenetic changes are reversible, allowing gene expression to be dynamically regulated in response to developmental or environmental signals.
Oestrogen is a steroid hormone which can move through the cell membrane. Oestrogen binds with a receptor on the transcriptional factor, changing its shape.
The transcription factor moves into the nucleus through the nuclear pore before binding with the complementary section of DNA. Transcription of the gene can then occur.
Cancer cells usually have decreased DNA methylation, leading to increased gene activation.
Certain cancer cells can also have abnormal methylation of promoter regions of genes, causing genes which are normally activated to be switched off.
Eukaryotes and prokaryotes have other methods of regulating gene expression.
After transcription, mRNA can be inhibited, preventing translation:
- Double stranded RNA is broken up by an enzyme, producing small sections of double stranded RNA, called siRNA (small interfering RNA).
- The two strands can separate, and one strand can bind with an enzyme.
- The siRNA in the enzyme can bind with the complementary section of mRNA. This causes the mRNA to be broken down, preventing translation and protein production.
