transcription (in euk) Flashcards
(27 cards)
what is the first step of transcription?
initiation
what are the enzyme is used for RNA synthesis?
RNA polymerase II
how to begin transcription?
RNA polymerase must bind to a region of DNA near the beginning of a gene known as the promoter
where is the promoter sequence?
25-30 base pairs upstream from the transcription start point (is the nuclueotide where the RNA synthesis actually begins)
what does the promoter consist of?
TATA BOX
RNA polymerase II cannot recognise and bind to a promoter on its own.
- it requires proteins such as transcription factors to search for and attach to promoter DNA regions (known as TATA box) along the DNA molecules
what does the transcription factors do?
they recognise the TATA box in the promoter and bind to DNA –> facilitates the binding of RNA polymerase
what is it called after initiation is done?
- complete assembly of transcription assembly and RNA polymerase II bound to the promoter is called the transcription initiation complex
what does the formation of transcription initiation complex do?
it causes the DNA double helix to unwind at the initiation site
what is the template strand
one of the exposed strands of (-) DNA in the 3’ -> 5’ direction (is used as a template for transcription)
what is the other strand called?
it is called non-template strand
elongationnn
- a primer is not needed
- as the RNA polymerase moves along the DNA, it continues to unwind the double helix, exposing 10-20 DNA bases for base-pairing with ribonucleotides
what happens when RNA polymerase moves along the template strand of DNA?
it adds the corresponding free ribonucleotides to the 3’ end of the growing RNA chain
what direction is the RNA strand synthesised?
5’ to 3’ direction
they areeeee
selected according to complementary base pairing rule (G C T A in the DNA would signal the addition of C G A U respectively to the RNA transcript)
what bond are they joined together by?
they are joined together by phosphodiester bond
differences btwn polymerases
DNA polymerase has proof reading while RNA polymerase does not have proof reading mechanism
can it be transcribed simultaneously?
yes! a single gene in euk can be transcribed simultaneously by several molecules of RNA polymerase II
what happens after transcription?
the newly formed RNA chain falls away from the DNA template and the DNA strand rewind into a double helix
what happens at the termination of transcription?
it proceeds until the RNA polymerase II transcribes a sequence on the DNA called polyadenylation signal sequence (TTATTT) on the DNA –> (it codes for a polyadenylation signal AAUAAA in the pre-mRNA)
how does termination occur?
- proteins associated with the growing RNA chain then cleave the growing RNA transcript 10-35 nucleotides downstream from the AAUAAA signal (termination site) –> this terminates transcription, releasing the primary RNA transcript –> RNA polymerase II dissociates from the DNA and the DNA rewinds to form a double helix
what is the newly transcribed mRNA called?
pre-mRNA/primary mRNA transcript.immature mRNA (needs to be processed and undergo post-transcriptional modification)
how to alter mRNA
- capping and polyadenylation
- splicing
what is capping?
- additional of 7-methylguanosine (methylated GTP) (5’ cap) to the first base of the transcript
this happens during elongation
