Week 6_Transcription in prokaryotes and eukaryotes Flashcards
prokaryotes have ___ type of RNA polymerase
1
RNA polymerase enzyme has ___ distinct sections
2
what are the 2 distinct sections of RNA polymerase?
sigma factor
core enzyme
what is sigma factor?
the part of RNA polymerase that promotes transcriptional initiation
what is the core enzyme?
what is it made up of?
the part of RNA polymerase that synthesizes the RNA
made up of 2 alpha and 2 beta subunits
what is the holoenzyme?
core enzyme + sigma factor
how does the sigma factor (SF) initiate transcription in prokaryotes?
(1) SF binds to the promoter regions of genes (DNA sequence recognition)
(2) SF separates the coding and template strands
(3) enables the core enzyme to begin transcription
describe elongation and termination for prokaryote transcription
(1) once transcription begins, the SF dissociates from the core enzyme
(2) the core enzyme completes transcription without the SF
(3) the SF will then associate with another core enzyme and initiate transcription of another gene
in prokaryotes, transcription and translation are ___, meaning they happen…
coupled
they happen at the same time and in the same place within the cell
why are transcription and translation coupled in prokaryotes?
no nucleus to separate genes and ribosomes –> the mRNA is translated as soon as it’s made
in prokaryotes, many mRNAs are ___
polycistronic
what is a polycistronic mRNA?
contains the coding region of more than 1 gene
where do polycistronic mRNAs come from?
gene clusters called operons
what do operons contain?
sets of genes that work together in a biological function
(genes that need to be expressed at the same time)
genes in an operon are transcribed ___ and share…
transcribed together
share a single promoter region and termination region
what is an example of an operon in prokaryotes?
lac operon
what is a lac operon?
cluster of 3 genes involved in the transport and metabolism of lactose (a carbohydrate used for energy)
what is the result of RNA polymerase transcribing a polycistronic mRNA containing the lac operon?
translated into 3 different proteins
lacZ, lacY, lacA
what does the lac operon polycistronic mRNA contain?
1 promoter region
3 coding regions (lacZ, lacY, lacA)
1 termination region
is transcription/translation coupled in eukaryotes?
no
where does transcription occur in eukaryotes?
nucleus
where does translation occur in eukaryotes?
cytoplasm
there are ___ RNA polymerases with different roles in eukaryotes
they are:
3
RNA Pol I
RNA Pol II
RNA Pol III
what does RNA Pol I do?
transcribes ribosomal RNA genes (rRNAs)
what does RNA Pol II do?
transcribes mRNA genes
what does RNA Pol III do?
transcribes transfer RNA genes (tRNA) and rRNA genes
do eukaryotic RNA polymerases have sigma factor?
no;
they have transcription factors instead
how is transcription initiated in eukaryotes?
proteins called transcription factors attach to the promoter region and attract RNA polymerase
in eukaryotes an mRNA is called a ___ until it is modified in ___ important ways
pre-mRNA
3
what are the 3 important ways eukaryotic pre-mRNAs are modified to become mRNAs?
(1) 7-methy-guanosine is added to the 5’ end of the mRNA (5’-cap)
- protects the mRNA from degradation on the 5’ end
(2) addition of a poly-A-tail (~200 adenines added to the 3’ end of the mRNA) (added by the enzyme poly-A polymerase).
- protects the mRNA from degradation on the 3’ end
(3) mRNA is spliced
the ___ of a eukaryotic protein coding gene contains…
coding region
contains both coding and non-coding sequences
what are exons?
DNA sequences that code for part of the protein
what are introns?
“intervening” DNA sequences that do not code for part of the protein
___ and ___ are both included in the pre-mRNA
introns and exons
what is splicing?
introns are cut out of the mRNA and exons are spliced together
what are the requirements for a mature mRNA?
(1) 5’ cap
(2) 3’ PolyA tail
(3) has been spliced
all 3 mRNA modifications occur in the ___
nucleus
splicing is done by an enzyme called the ___
spliceosome
