Unit 7: Prokaryotic Transcription Flashcards
TRUE OR FALSE: DNA is the genetic material of most organisms other than some viruses or viroids that have RNA
TRUE
How do viruses that RNA as genetic material complete transcription?
their genetic material must first be transcribed to DNA first before it can be transcribed
TRUE OR FALSE: RNA is translated into protein and this is unidirectional
TRUE: central dogma theory
What does it mean that translation and transcription is coupled in prokaryotes?
This means that as the mRNA is being transcribed, ribosomes can bind to the newly synthesized messenger RNA and start translation of the mRNA into protein even before the Messenger RNA has been fully transcribed.
Why does coupled translation and transcription not occur in eukaryotes?
In prokaryotes, there is not distinct nucleus hence they can have coupled transcription and translation. In eukaryotes transcription occurs in the nucleus and the mRNA needs to come out into the cytoplasm where the ribosomes can bind and initiate translation.
What is meant by a polyribosome or polisome?
This happens when several ribosomes bind simultaneously to a single emerging RNA molecule and translate it into a protein.
Transcription occurs in what direction? It reads off a template strand that runs in what direction?
Transcription is always in the 5’-3’ direction (non-template strand) and the template it is reading off should be the 3’-5’ strand called the template strand.
The non-template strand is also called what? Why?
the coding strand b/c it will have the same sequence as the mRNA and caries the code for protein sequence. It runs in the 5’-3’
What is an RNA polymerase? What is the function?
an enzyme that synthesizes the RNA using a DNA template (formally described as DNA-dependent RNA polymerase).
Function: to copy one strand of duplex DNA into RNA.
The RNA sequence is complementary to which DNA strand? and identical to which DNA strand?
complementary to template strand
identical to coding strand
Promoter
a region of DNA where RNA polymerase binds to initiate transcription.
Start Point
the position on DNA corresponding to the first base incorporated into RNA
Terminator
a sequence of DNA that causes RNA polymerase to terminate transcription
Transcription Unit
A sequence of DNA transcribed into a single RNA, starting at the promoter and ending at the terminator. it may include more than one gene or multiple genes
When is transcription initiated?
when the RNA polymerase binds to the promoter region.
Sequences prior to the start point of transcription is described as what? Where does numbering of the start point begin?
upstream of transcription (sequences that are in the opposite direction from expression)
numbering begins with the start point -1 and increases as your go further upstream
Sequences after the start point (w/in the transcribed sequence) are described as what? Where does numbering of start point begin?
Downstream - sequences proceeded farther in the direction of expression
numbering begins at +1
What is a primary transcript?
the initial transcript that comes off the ribosome (original unmodified RNA produce corresponding to a transcription unit)
What is the maturation process?
a process in which sequences at the ends of primary transcripts are cleaved off (processed) by endonucleases
-Ribosomal RNA and transfer RNA in both prokaryotes and eukaryotes undergo this process which render them stable.
TRUE OR FALSE: Transcription occurs by base pairing
TRUE: just like replication and translation, transcription also occurs by base pairing in a “bubble” of unpaired DNA
How is a transcription bubble made and In the transcription bubble, which stand is used to direct synthesis of RNA?
RNA polymerase binds to promoter and separates the two strands of DNA to make the transcription bubble.
A template strand is used to direct synthesis of the complementary sequence of RNA
The transcription bubble is about 12-14 basepairs, how long is the RNA-DNA hybrid w/in the bubble?
8 to 9 base pairs
What are the three stages of the transcription reaction?
1) Initiation - RNA poly binds to promoter site on the DNA as a closed complex (closed means DNA stays double stranded). The RNA poly then opens up the DNA helix, in the promoter region, which also includes the start site to form the transcription bubble (which is an open complex and begins synthesis of complementary RNA strand)
2) Elongation - polymerase synthesizes RNA: Involves the RNA strand by the addition of new nucleotides to the 3’ end of the growing chain. As a result there is a RNA-DNA hybrid inside the bubble. Behind the bubble the DNA reforms the double strand helix.
3) Termination - when transcription stops and RNA poly and RNA are released. The polymerase recognizes certain sequences called terminator sequences that signal transcription to stop further addition of nucleotides to the RNA chain. No more phosphodiester bonds formed and the bubble collapses.
Why is the initiation stage sometimes extended for a short period of time before the elongation stage?
because there may be several abortive attempts where short RNA transcripts of about 10 nucleotides are synthesized as the polymerase stays bound to the promoter. The short nucleotides are released and synthesis is restarted until the polymerase can finally extend the chain and clear the promoter.