control of gene expression Flashcards
what needs to happen for transcription to begin?
for transcription to begin the gene needs to be switched on by specific molecules that move from the cytoplasm into the nucleus. these molecules are called transcriptional factors.
describe the process of the regulation of transcription
- in eukaryotes, transcription factors move from the cytoplasm to the nucleus.
- in the nucleus, each transcriptional factor has a site that binds to a specific base sequence of DNA. this is known as the promoter region.
- binding of the transcriptional factor to the promoter region allows the attachment of RNA polymerase to the DNA and transcription begins.
- mRNA is produced and the genetic code it carries is then translated into a polypeptide.
- when a gene is not being expressed (is switched off), the site on the transcriptional factor that binds to DNA is blocked by an inhibitor molecule.
- this inhibitor molecule prevents the transcriptional factor binding to DNA and so prevents transcription and polypeptide synthesis.
what are two types of transcription factors?
- activators : stimulate or increase the rate of transcription (help RNA polymerase bind to the start of the target gene and activate transcription)
- repressors : inhibit or decrease the rate of transcription (bind to the start of the target gene, preventing RNA polymerase from binding, stopping transcription)
what is oestrogen? where is it transported?
oestrogen is a lipid-soluble, steroid hormone. like other hormones it is transported in the blood stream and can reach almost any cell in the body.
why can oestrogen only bind to certain cells?
only some cells will have transcriptional factors that contain receptor sites for oestrogen. oestrogen will only bind to and activate transcription factors in these specific cells.
what is the binding of oestrogen to it’s receptor site similar to?
the binding of oestrogen to its receptor site is similar to that between a substrate and an enzyme (they have a complementary shape)
what can hormones like oestrogen do?
hormones like oestrogen can ‘switch on’ or activate a gene and therefore start transcription by combining with a receptor site on the transcriptional factor. this releases the inhibitor molecule.
describe the effect of oestrogen on gene transcription
- oestrogen, like all steroid hormones, is a lipid, so crosses the cell membrane by lipid diffusion and enters the cytoplasm.
- oestrogen binds to the receptor site on the transcription factor (oestrogen receptor), changing its shape and forming an oestrogen-oestrogen receptor complex.
- this results in the inhibitor molecule no longer fitting into the transcription factor, so it detaches, exposing the DNA binding site.
- the active transcription factor diffuses into the nucleus through a nuclear pore.
- in the nucleus the transcription factor binds to a specific base sequence on a DNA promoter region near the start of the target region upstream of RNA polymerase.
- this binding stimulates RNA polymerase to transcribe genes and so stimulates protein synthesis (acts as an activator of transcription).
what is RNAi, what molecules are involved and where are they found?
RNAi is where small double stranded RNA molecules stop mRNA from target genes being translated into proteins. the molecules involved in RNAi are called siRNA (small interfering RNA) and miRNA (microRNA), and are found in the cytoplasm.
what is RNAi key to? why may this be important?
they are key to a process called RNA interference which is the inhibition of gene expression at the translation stage. this might be important in a cell as if mRNA concentration is regulated it will limit the amount of protein translated.
describe the effect of RNAi on gene expression
- 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 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, so protein synthesis stops. the fragments then move into a processing body, which contains tools to degrade them.
what can RNA with specific sequences be used for?
RNA with specific sequences can be synthesised and used to silence specific genes making them valuable tools in both research and medicine:
- it could be used to identify the role of genes in a biological pathway. some RNAi that blocks a particular gene could be added to cells. by observing the effects (or lack of them) we could determine what the role of the blocked gene is.
- as some diseases are caused by genes, it may be possible to use RNAi to block these genes and so prevent the disease.
