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.
Abnormal methylation
Epigenetic changes can cause diseases (e.g. cancer).
Methyl groups are epigenetic markers that bind to DNA to inhibit transcription.
Methyl groups are important in regulating processes like cell division.
If methylation is not regulated properly, this can affect the regulation of these important processes.
This is called abnormal methylation.
Increased methylation
If methylation is increased too much, it can decrease the gene expression of tumour suppressor genes more than normal.
Tumour suppressor genes prevent cell division from taking place.
If the genes are underexpressed, the cells divide uncontrollably and tumours are produced.
Decreased methylation
If methylation is decreased too much, it can increase the gene expression of proto-oncogenes more than normal.
Proto-oncogenes promote cell division.
If the genes are overexpressed, the cells divide uncontrollably and tumours are produced.
Epigenetic change
Epigenetic changes that cause disease (e.g methylation leading to cancer) are temporary and can be reversed.
E.g. The action of the methyl transferase enzyme can be inhibited.
Methyl transferase adds methyl groups (a type of epigenetic marker) to DNA to alter gene expression.
If methylation is the cause of disease, inhibiting the enzyme can help treat the disease.
Drugs and therapies
The ability to reverse epigenetic change is important for designing new drugs and therapies.
Researchers are investigating how drugs can reverse the silencing or overexpression of genes.
Drugs that can reverse epigenetics can help return gene expression to its normal level.
Future of epigenetics
The understanding of epigenetics is increasing.
It is now understood that epigenetic change could play a role in conditions like allergies and autism.
In the future, using drugs that reverse epigenetic change could be used to treat many different conditions.
Translation
Translation is a process involved in synthesising proteins using the genetic code.
After transcription, mRNA moves from the nucleus to the cytoplasm.
Translation reads the sequence of bases on mRNA and joins the corresponding amino acids together to produce a protein.
RNAi
RNAi is a small molecule of double-stranded RNA.
RNAi interferes with mRNA by binding to the mRNA molecule and breaking it down.
Interfering with mRNA prevents it from being translated into protein.
RNAi influences gene expression in this way.
siRNA
siRNA is a type of RNAi that is complementary to the mRNA sequence it inhibits.
siRNA targets a specific sequence of mRNA.
After siRNA has bound to mRNA, the mRNA is broken down into smaller fragments.
The fragments of mRNA are degraded.
miRNA
miRNA is not fully complementary to the mRNA sequence.
miRNA can target multiple sequences of mRNA.
After miRNA has bound to mRNA, the mRNA is either degraded or stored for future use.
