A7. Regulation of Transcription and Translation Flashcards
Controlling transcription
When a gene is copied from DNA into messenger RNA (mRNA). The enzyme responsible for synthesising mRNA from DNA is called ____ ______________.
All the cells in an organism carry the same _____(DNA) but the structure and function of different cells varies. This is because not all the genes in a cell are _____________(transcribed and used to make a protein). Because different genes are expressed, different __________are made and these proteins modify the cell-they determine the cell structure and control cell processes (including the expression of more genes, which produce more proteins). The transcription of genes is controlled by protein molecules called ______________ _________.
When a gene is copied from DNA into messenger RNA (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 the genes in a cell are expressed (transcribed and used to make a protein). Because different genes are expressed, different proteins are made and these proteins modify the cell-they determine the cell structure and control cell processes (including the expression of more genes, which produce more proteins). The transcription of genes is controlled by protein molecules called transcription factors.
The role of transcription factors
In eukaryotes what happens?
What happens after this happens?
Brief note on what transcription factors do?
What are the two types of transcription factor and what do they do?
- In eukaryotes, transcription factors move from the cytoplasm to the nucleus.
- In the nucleus they bind to specific DNA sites called promoters, which are found near the start of their target genes the genes they control the expression of.
- Transcription factors control expression by controlling the rate of transcription.
Two types of transcription factor:
Activators
- stimulate or increase the rate of transcription
- e.g. they help RNA polymerase bind to the start of the target gene and activate transcription.
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.
Figure 1 shows activators and repressors at work.
Oestrogen
What can oestrogen affect?
3 steps?
Oestrogen affects the expression of genes
1. 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
2. The complex moves from the cytoplasm into the nucleus where it binds to specific DNA sites near the start of the target gene.
3.The complex can act as an activator of transcription, e.g. helping RNA polymerase bind to the start of the target gene
Figure 2: The formation of an oestrogen-oestrogen receptor complex
Figure 3: An oestrogen- oestrogen receptor complex activating transcription.
Tip: Not all cell types have ___________ ___________- so not all cells are affected by oestrogen.
Tip: Not all cell types have oestrogen receptors - so not all cells are affected by oestrogen.
Tip: In some cells, the oestrogen-oestrogen receptor complex can act as a ___________of transcription instead of an ___________. It depends on the type of cell and the _______ _____.
Tip: In some cells, the
oestrogen-oestrogen receptor complex can act as a repressor of transcription instead of an activator. It depends on the type of cell and the target gene.
RNAi
What happens in eukaryotes?
What is it?
What happens?
In eukaryotes, gene expression is also affected by RNA interference (RNAi). RNAi is where small, double-stranded RNA molecules stop mRNA from target genes being translated 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). Here’s how RNAi works:
Tip: RNAi molecules are small lengths of ___-_________ ____(they don’t code for proteins).
Tip: RNAi molecules are small lengths of non-coding RNA (they don’t code for proteins).
siRNA (and miRNA in plants)
(4 steps)
How is it similar to miRNA in plants? (3)
- Once mRNA has been transcribed, it leaves the nucleus for the cytoplasm (see Figure 4).
- In the cytoplasm, double-stranded siRNA associates with several proteins and unwinds. One of the resulting single strands of siRNA is selected and the other strand is degraded (broken down) - see Figure 5
- The single strand of siRNA then binds to the target mRNA. The base sequence of the siRNA is complementary to the base sequence in sections of the target mRNA (see Figure 6)
- 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 contains ‘tools’ to degrade them (see Figure 7)
- A similar process happens with miRNA in plants.
- Like siRNA, the base sequence of plant miRNA is complementary to its target mRNA sequence and so binding results in the cutting up and degradation of the mRNA.
- However, its production in the cell is similar to that of mammalian miRNA
Tip: Unlike mRNA and tRNA, siRNA is ________-_________
Tip: Unlike mRNA and tRNA, siRNA is double-stranded
Tip: Double-stranded siRNA is unwound into ___ _______-_________ ______ molecules by an __________
Tip: Double-stranded siRNA is unwound into two single-stranded siRNA molecules by an enzyme
Tip: siRNA is actually about __-__nucleotides long
Tip: siRNA is actually about 20-25 nucleotides long
Tip: siRNA has a potential use in treating genetic disorders, for example stopping a known harmful gene from being ___________. siRNA molecules with a ____ ___________ ______________to the mRNA from that gene could be inserted into the affected cells-they will bind to the _____and so block ___________of that _______.
Tip: siRNA has a potential use in treating genetic disorders, for example stopping a known harmful gene from being expressed. siRNA molecules with a base sequence complementary to the mRNA from that gene could be inserted into the affected cells-they will bind to the mRNA and so block translation of that protein.
miRNA in mammals (5 steps)
- 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.
- When miRNA is first transcribed, it exists as a long, folded strand. It is processed into a double stand, and then into two single strands, by enzymes in the cytoplasm.
- Like siRNA, one strand 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.
Figure 8: miRNA affects gene expression by blocking the translation of mRNA.
Tip: miRNA is processed like this: (picture note)
