Regulation of transcription and translation (A-level only) Flashcards
Function of transcriptional factors
Transcription factors are proteins that control gene expression by stimulating or inhibiting the transcription of target genes.
Transcription factors are produced in the cytoplasm and move to the nucleus.
In the nucleus, transcription factors bind to a specific region of DNA to stimulate or inhibit the gene.
Activators
Transcription factors that stimulate gene expression are called activators.
Activators promote the transcription of the genes by interacting with an enzyme called RNA polymerase and allowing it to bind to DNA.
Repressors
Transcription factors that inhibit gene expression are called repressors.
Repressors prevent the transcription of genes by stopping RNA polymerase from binding to DNA.
Peptide hormones
Peptide hormones bind to the cell surface membrane and trigger a secondary messenger response.
The secondary messenger will lead to the activation or inhibition of transcription of some genes.
Lipid-soluble steriod hormones
Lipid-soluble steroid hormones can pass through the phospholipid membrane.
Steroid hormones interact directly with DNA to promote or inhibit gene expression.
E.g. Oestrogen.
Oestrogen is a lipid-soluble steroid hormone that can enter the cell and directly interact with DNA to initiate gene transcription. The steps involved are:
Enter the cell
Bind to transcription factors
Bind to DNA
Enter the cell
Oestrogen enters the cytoplasm of the cell through the cell surface membrane.
Oestrogen is lipid-soluble so it can pass through the phospholipid bilayer.
Bind to transcription factors
Oestrogen binds to receptors on transcription factors in the cytoplasm.
Binding of oestrogen causes the transcription factors to change shape.
The transcription factors form a receptor-hormone complex that can now enter the nucleus.
Bind to DNA
The receptor-hormone complex binds to the promoter region of the DNA.
Binding to DNA activates transcription.
This stimulates protein synthesis.
Chromatin
DNA in the nucleus combines with proteins called histones.
The combination of DNA and histones is called chromatin.
A chemical layer surrounds the chromatin.
This is called the epigenome.
Epigenome
The epigenome interacts with the chromatin and changes its structure.
The epigenome can cause the chromatin to become either:
More condensed.
This prevents transcription factors from binding to DNA so transcription is inhibited.
Less condensed.
This allows easier access to transcription factors, promoting transcription.
Epigenetic markers
Chromatin becomes more or less condensed when epigenetic markers are attached or removed to the DNA or histone proteins.
Epigenetic markers are groups (e.g. methyl groups) that do not alter the base sequence but influence chromatin structure.
E.g. Methylation of DNA makes chromatin more condensed.
Increased methylation
Methyl groups bind to a CpG site on DNA.
CpG sites are areas in DNA where cytosine and guanine are together in the base sequence.
Methyl groups cause the chromatin to be more condensed.
When chromatin is more condensed transcription factors can’t reach the DNA.
Methylation inhibits transcription.
Decreased acetylation
Acetyl groups (CH3CO) are removed from histone proteins.
Removal of acetyl groups increases the positive charge on histone proteins.
This increases the attraction to phosphate groups on DNA.
Decreased acetylation causes the chromatin to condense.
When chromatin is more condensed transcription factors can’t reach the DNA.
Inheritance
The action of epigenetic markers results in changes in the chromatin structure.
Epigenetic markers can be inherited by offspring.
Inheritance of epigenetic control means that environmental factors (e.g. methylation) experienced by an individual can influence the gene expression of their offspring.
E.g. Starvation of human adults can influence the gene expression in their offspring.
