transcription Flashcards
where does transcription start?
at the promoter of the gene
what is required for initiation of transcription?
- RNA polymerase
- basal transcription factors, which is a group of proteins
what is the promoter of a gene?
it is a set of sequences of nucleotides
where is the promoter of the gene located?
on the DNA strand itself
what does the promoter contain?
the TATA box, a short sequence of T & A nucleotides
where is the TATA box located?
it is located about 30 base pairs from the transcription start site
what are the processes involved in the formation of a transcription initiation complex?
- the TATA-Binding Protein, a subunit of basal transcription factors, recognizes & binds to the TATA box
- other basal transcription factors & RNA polymerase are then recruited to bind around the TATA box to form a transcription initiation complex (TIC)
what unwinds & separates the double stranded DNA?
1 basal transcription factor
are both strands of DNA used as the template for RNA polymerase for transcription?
no, only 1 exposed DNA strand containing the promoter is used
when does unwinding & separation of the double helix occur?
when the transcription initiation complex is formed
does the initiation of transcription require energy?
yes, and the energy is provided by ATP
what is required for the formation of mRNA?
free ribonucleotides
what happens once RNA polymerase starts transcribing the DNA template?
it would be able to continue separating the DNA helix by disrupting the hydrogen bonds between complementary base pairs
what is the 1st step in transcription elongation?
- RNA polymerase separate the double stranded DNA by disrupting the hydrogen bonds between complementary base pairs
- free RNA nucleotides base pair with the template strand via complementary base pairing
what is the 2nd step in transcription elongation?
- RNA polymerase synthesizes the growing mRNA by catalyzing the formation of phosphodiester bonds between RNA nucleotides
- RNA polymerase moves along the DNA template strand in the 3’ to 5’ direction, as mRNA is synthesized in the 5’to 3’ direction
does RNA polymerase catalyze the formation of hydrogen bonds between RNA nucleotides & their complementary DNA nucleotides on the template?
no, RNA polymerase does not catalyze the formation of hydrogen bonds between RNA nucleotides & their complementary DNA nucleotides on the template strand, as the hydrogen bonds are synthesized automatically
what is the 3rd step in transcription elongation?
as the RNA polymerase continues to move along the template strand in the 3’ to 5’ direction, the DNA strands behind it rewinds to reform the double helix
how is transcription terminated in eukaryotes?
- RNA polymerase transcribes the polyadenylation signal sequence, a sequence on the DNA which codes for a polyadenylation signal (AAUAA) in the pre-mRNA
- afterwards, about 10-30 nucleotides from the polyadenylation signal, specific proteins bind to the growing pre-mRNA & cut the pre-mRNA free from the RNA polymerase, terminating transcription
how is transcription terminated in prokaryotes?
the transcribed terminator sequence on the RNA serves as a termination signal to stop transcription
what is the terminator sequence made up of?
it is rich in guanines & cytosines, followed by a repeating sequence of adenines
what happens when RNA polymerase transcribes the terminator sequence?
- a hairpin loop forms in the guanine rich portion of the mRNA transcribed
- the formation of the hairpin causes the RNA polymerase to pause, placing it directly over the A-U pairs, straining & causing the molecule to separate from the DNA, releasing the RNA polymerase
why do many molecules of RNA polymerase simultaneously transcribe the same gene?
to speed up the rate of transcription of certain genes
