Transcription & RNA Processing Flashcards
3 steps of transcription
initiation, elongation, termination
define promoter
sequence of double stranded DNA where RNA polymerase binds
when does initiation occur
when RNA polymerase binds to the promoter
what helps RNA polymerase bind to the promoter in eukaryotes?
transcription factors
transcription factors + RNA polymerase + promoter = ?
pre-initiation complex
How much of your genome is replicated right before your cells divide?
all of it
How much of your genome is converted to protein (transcribed, then translated)?
<2%
How much of your genome is converted to RNA (transcribed)?
80%
What are transcription factors?
proteins that bind to the promoter first and allow RNA polymerase to bind
What sort of sequence does a promoter usually contain? Why is this helpful?
T-A rich sequences called TATA boxes; easier to seaparate because they have fewer hydrogen bonds
What happens during elongation?
RNA polymerase starts moving along DNA strand and synthesizes the pre-mRNA transcript
Where do transcription factors go during elongation?
stay at the promoter
Why do transcription factors stay at the promoter? What does this allow?
allows another RNA polymerase to bind at the promoter and start transcribing another pre-mRNA transcript; this makes transcription faster and more efficient
How fast does RNA polymerase add nucleotides?
10-100 nucleotides / sec
What is the “sense” strand?
strand of DNA that is the same sequence as the pre-mRNA transcript
What is the “antisense” or “template” strand?
the strand of DNA that serves as the template for RNA polymerase
Which strand is “read” by RNA polymerase?
“antisense” strand
all “sense” strands are…
5’ —> 3’
What happens during elongation?
RNA polymerase untwists DNA and synthesizes mRNA by connecting free ribonucleotides to elongating strand
Does RNA polymerase have proofreading activity?
no
Does RNA polymerase need a primer to start synthesizing?
no
What happens during termination?
RNA polymerase, DNA and RNA transcript dissociate
How does RNA polymerase know when to stop in bacteria?
stops at “termination sequence” by itself
How does RNA polymerase know when to stop in eukaryotes?
proteins/enzymes are needed to recognize DNA sequences and make RNA stop in the right place
Does RNA polymerase need topoisomerase?
no, the double stranded DNA quickly rewinds into the double helix after being read by RNA polymerase
When a single pre-mRNA molecule is made in one of our cells, how much information does it contain?
the genetic code for one allele from one chromosome
What nucleotide does RNA use that is different from DNA?
uracil instead of thymidine
what do most RNA transcripts code for?
some for protein, others for rRNA, tRNA, some mysterious
transcripts that encode protein
pre-mRNA
average human pre-mRNA transcript length
27,000 nucleotides
average human protein length
400 amino acids (requires only 1200 nucleotides)
exons
blocks of pre-mRNA transcripts or “primary transcripts” that are kept and encode for proteins and some regulatory sequences
introns
intervening sequences that get cut out of pre-mRNA
what are “UTRs”
untranslated regions at the 5’ and 3’ ends that are part of exons and are not translated into proteins
purpose of the 5’UTR and 3’UTR sequences
regulate the lifespan of the mRNA, translation rate, and where in the cytoplasm the mRNA is placed
what is splicing and what does it result in?
process of removing introns and ligating exons together to make mature mRNA (messenger RNA)
what enzyme performs splicing and what is its structure?
spliceosome; made of several “small nuclear RNAs” or “snRNAs”
how does the spliceosome accurately remove introns?
recognizes the “splice donor” and “splice acceptor” sites at the 5’ and 3’ ends of each intron
what consequence might mutating the DNA sequences encoding the splice donor or splice acceptor have?
prevents splicing
how might we make different splice forms of the same gene? what is this process called?
by removing exons; called alternative splicing
what does alternative splicing allow?
allows a single gene to code for several mRNAs, and thus make several different proteins
after splicing, what two modifications are made to the ends of mRNA?
5’ cap and poly-A tail
what is the 5’ cap? how is it accomplished? purpose?
a modified guanine nucleotide added to the 5’ end by capping enzymes; needed for stabilization and ribosome binding
what is the poly-A tail? purpose?
50-250 adenine nucleotides are added to the 3’ end; needed for stabilization and ribosome binding
how is the poly-A tail added to the transcript?
ribonuclease recognizes the AAUAAA (polyadenylation signal) in the primary transcript and cleaves 11-30 nucleotides 3’ to that sequence; then poly(A) polymerase can add As
how long does it take to make a single 10,000 nucleotide primary transcript?
2 minutes
