Transcription Flashcards
What is the pathways of information transfer in cells?
- DNA synthesis (DNA replication)
- RNA synthesis (transcription)
- protein synthesis (translation)
what two regions does each gene have?
regulatory and coding regions
what are regulatory regions?
nucleotide sequences which can increase or decrease gene expression
what is a promoter?
a sequence of DNA where proteins bind to initiate transcription (part of the regulatory region)
What are the three coding regions called?
- transcription start site
- exons
- termination region
Where are the coding regions usually found?
- the transcription start site in usually next to the promoter sequence
- exons are between the start site and termination region
- the termination region is found at the end
What are introns?
noncoding sections of an RNA transcript that aresplicedout before the RNA molecule is translated into a protein
where does transcription take place?
in the nucleus
what is the process of producing RNA and proteins from a gene called?
gene expression
what is the main RNA class involved in protein synthesis?
- mRNA
What four elements are required for transcription to take place?
-a starting DNA template
-a transcription unit
(promoter, RNA coding sequence, terminator)
- the machinery for transcription
- rNTPs
what are the two stands of DNA called in transcription?
- coding strand
- template strand
from which strand of DNA are genes transcribed?
the template strand
in which direction does translation take place?
5’ to 3’
what is RNA polymerase?
an enzyme that synthesises RNA using a DNA template
what do the promoter, RNA coding region and terminator represent together?
a transcription unit
What are the three sequential stages of transcription?
- initiation
- elongation
- termination
What happens during the initiation stage of transcription?
the transcription machinery recognises and binds to the promoter to start synthesis from DNA (no primer needed)
What happens during the elongation stage of transcription?
the RNA polymerase moves along the template during elongation and synthesises RNA, adding nucleotides at the 3’
What happens during the termination stage of transcription?
the enzyme dissociates at the end of the transcription unit and releases RNA
What must happen to chromatin structure for transcription to occur?
Chromatin structure must be modified by regulatory proteins binding near the promoter to make the DNA accessible
Where is the information about the frequency and level of transcription located?
In the promoter
What enzyme is responsible for transcription, and what additional proteins assist?
RNA polymerase (multisubunit, multifunctional) is the enzyme, assisted by accessory proteins, transcription factors, and the mediator complex.
Why does eukaryotic RNA polymerase need many accessory proteins?
becuase it can’t recognise the DNA sequence on it’s own
What role do regulatory proteins play in transcription?
They bind to DNA to modify chromatin structure
What makes up the basal transcription apparatus in eukaryotes?
RNA polymerase II + general transcription factors + mediator
What are the three types of RNA polymerase to exist in eukaryotes?
RNA polymerase I, II, and III
Why are different types of RNA polymerase needed?
because different polymerases bind to different promoters and transcribe different genes
what does RNA polymerase II do?
transcribes genes encoding proteins
What are the components of RNA polymerase II promoters?
The core promoter and the regulatory promoter
What is the TATA box, and how does it function in transcription initiation?
The TATA box is a sequence in the core promoter recognized by TFIID, a transcription factor, via the TATA-binding protein (TBP). Binding leads to DNA bending and unwinding
What role does the regulatory promoter play in transcription?
It binds transcription factors (TFs) and regulates the speed of transcription via the mediator complex.
What is added to the 5’ cap during mRNA processing?
A guanine nucleotide is added and methylated (7-methylguanosine) via 5’-5’ binding
What additional modification may occur to the 1st and 2nd nucleotides in the 5’ cap?
Methylation of the sugar in the 1st and 2nd nucleotide at the 2’ position
When is the 5’ cap added to the mRNA?
Immediately after the initiation of transcription, only in RNA pol II transcripts
What are the functions of the 5’ cap?
- Increases mRNA stability
- Regulates intron removal
- Is important for translation.
What is the poly(A) tail and its role in mRNA?
- A chain of 50-250 adenine nucleotides added to the 3’ end of mRNA.
- Added after cleavage of extra material, guided by:
- AAUAAA sequence: 11-30nt upstream of cleavage site.
- U/G-rich sequence: downstream of cleavage site.
What are the functions of the poly(A) tail
- Improves mRNA stability.
- Regulates gene expression.
- Aids in ribosome attachment for translation.
What are the key consensus sequences required for splicing?
- 5’ splice site: GU at the beginning of an intron.
- 3’ splice site: AG at the end of an intron.
- Branch point: A nucleotide (A) located 18-40nt upstream of the 3’ splice site.
What mediates the splicing process in eukaryotes?
Splicing is mediated by the spliceosome, which consists of:
- 5 RNA molecules.
- 300 proteins.
What are the two steps of splicing?
Cleavage at the 5’ splice site:
- The 5’ end of the intron is cut.
- The cut end attaches to the branch point (A) to form a lariat structure.
Cleavage at the 3’ splice site and joining of exons:
- The 3’ end of the intron is cut, releasing the intron as a lariat.
- The two exons are joined together to form the mature mRNA.
What is alternative splicing?
the same DNA molecule can be spliced in different ways to generate different mRNA products (hence, proteins)
