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Biology IB HL - 7.2 Transcription Part 2
This deck covers key concepts related to transcription in eukaryotes, including post-transcriptional modifications, the role of various proteins in transcriptional regulation, and the mechanisms of splicing.
Can only 1 gene transcript be generated at a time?
Many RNA polymerase enzymes can transcribe a DNA sequence sequentially, producing a large number of transcripts
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Key Terms
Term
Definition
Can only 1 gene transcript be generated at a time?
Many RNA polymerase enzymes can transcribe a DNA sequence sequentially, producing a large number of transcripts
What must be done in eukaryotes after transcription?
In eukaryotes, post-transcriptional modification of the RNA sequence is necessary to form mature mRNA
What 3 post-transcriptional events must occur?
capping
polyadenylation
splicing
What does capping involve?
Capping involves the addition of a methyl group to the 5’-end of the transcribed RNA
What is the role of the methyl cap?
The methylated cap provides protection against degradation by exonucleases
It also allows the transcript to be recognised by the cell’s translational ...
What does polyadenylation involve?
Polyadenylation describes the addition of a long chain of adenine nucleotides (a poly-A tail) to the 3’-end of the transcript
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| Term | Definition |
|---|---|
Can only 1 gene transcript be generated at a time? | Many RNA polymerase enzymes can transcribe a DNA sequence sequentially, producing a large number of transcripts |
What must be done in eukaryotes after transcription? | In eukaryotes, post-transcriptional modification of the RNA sequence is necessary to form mature mRNA |
What 3 post-transcriptional events must occur? | capping
polyadenylation
splicing |
What does capping involve? | Capping involves the addition of a methyl group to the 5’-end of the transcribed RNA |
What is the role of the methyl cap? | The methylated cap provides protection against degradation by exonucleases
It also allows the transcript to be recognised by the cell’s translational machinery (e.g. nuclear export proteins and ribosome) |
What does polyadenylation involve? | Polyadenylation describes the addition of a long chain of adenine nucleotides (a poly-A tail) to the 3’-end of the transcript |
What is the purpose of the poly a tail? | The poly-A tail improves the stability of the RNA transcript and facilitates its export from the nucleus |
What is the purpose of splicing? | Within eukaryotic genes are non-coding sequences called introns, which must be removed prior to forming mature mRNA |
What is done once the introns have been removed? splicing | The coding regions are called exons and these are fused together when introns are removed to form a continuous sequence |
What is the difference between introns and exons? | Introns are intruding sequences whereas exons are expressing sequences |
What is alternative splicing? | Splicing can also result in the removal of exons – a process known as alternative splicing |
What is the purpose of alternative splicing? | The selective removal of specific exons will result in the formation of different polypeptides from a single gene sequence |
Give an example of when alternative splicing is used? | For example, a particular protein may be membrane-bound or cytosolic depending on the presence of an anchoring motif |
What regulates transcriptional activity? | Transcriptional activity is regulated by two groups of proteins that mediate binding of RNA polymerase to the promoter |
What are the two groups of proteins that regulate transcriptional activity? | transcription factors and regulatory proteins |
How do transcription factors and RNA polymerase interact? | Transcription factors form a complex with RNA polymerase at the promoter |
How do transcription factors regulate gene expression? | RNA polymerase cannot initiate transcription without these factors and hence their levels regulate gene expression |
What is the role of regulatory proteins? | Regulatory proteins bind to DNA sequences outside of the promoter and interact with the transcription factors |
What are the 2 types of regulatory proteins? | activator and repressor proteins |
What is the role of activator proteins? | Activator proteins bind to enhancer sites and increase the rate of transcription (by mediating complex formation) |
What is the role of repressor proteins? | Repressor proteins bind to silencer sequences and decrease the rate of transcription (by preventing complex formation) |
Is the presence of transcription factors and regulatory proteins the same in every cell? | The presence of certain transcription factors or regulatory proteins may be tissue-specific |
What may also affect transcriptional activity? | Additionally, chemical signals (e.g. hormones) can moderate protein levels and hence mediate a change in gene expression |
What are control elements? | The DNA sequences that regulatory proteins bind to are called control elements |
Where are control elements located? | Some control elements are located close to the promoter (proximal elements) while others are more distant (distal elements) |