Exam 4: Transcription & RNA processing Flashcards
(44 cards)
Transcription
synthesis of RNA molecules that are complementary in sequence to a DNA template
RNA is different from DNA because
it contains ribose, it uses uracil to pair with adenine, it is single stranded
RNA polymerase
unwinds template DNA & creates RNA in 5’ to 3’ direction
Initiate RNA synthesis without a primer & more prone to error than DNA polymerase
RNA polymerase I
synthesizes ribosomal RNA (rRNA), which is a component of ribosome required for protein synthesis
RNA polymerase III
synthesizes tRNA - then chemically modified to make mature tRNA
RNA polymerase II
transcribes mRNA
Promoter
start of transcription
Terminator
end point of transcription
General transcription factors
essential to allow initiation of transcription of genes by Pol II - help Pol II recognize and bind to promotors
Basal transcription complex - initiating transcription only at low rate
TFIID (transcription factor for Pol II)
general transcription factor
complex of several proteins, contains TBP - binds to TATA box
TATA box
DNA sequence area in promotor region - always found in genes transcribed by Pol II
Acts as a binding site for general transcription factor, needed for binding of Pol II
TBP (TATA binding protein)
part of TFIID; binds to TATA box
Distorts DNA and directs other components of general transcription complex & Pol II to promoter - formation of transcriptional initiation complex
High rates of transcriptional initiation require
additional factors bound to other DNA sequences - GC-rich sequences (bind transcription factor SP-1) & CAAT box (binds transcription factor NF1) & enhancer elements (located further upstream of the promoter)
Actively transcribed regions of genome are more
Relaxed than inactive regions
Death cap mushroom (Amanita phalloides)
extremely poisonous fungus
contains toxin alpha-amanitin - inhibitor of Poll II (blocks synthesis of mRNA)
Results in massive liver failure - mRNA degraded during metabolism
No antidote
Rifampicin
antibiotic, inhibitor of RNA polymerase found in bacteria
Eukaryotic Pol II is unaffected - selectively kill bacteria (antibiotic against Mycobacterium tuberculosis)
Capping
RNA processing modification at 5’ end of pre-mRNA
“Cap” structure allows cell to distinguish mRNA molecules
Important for further processing & export
Splicing
in order to form mature mRNA the introns must be removed from the pre-mRNA & exons join together
spliceosomes
catalyze pre-mRNA splicing
recognize boundaries between exons and introns
differential (alternate) splicing
a pre-mRNA molecule may contain multiple exons, different subsets of which may be spliced together generating different mRNA molecules
Gives rise to tissue-specific isoforms of enzymes and other proteins
Polyadenylation
3’ end of RNA specified by DNA signal - polyadenylation signal
transcribed into RNA & recognized by specific protein factors - cleaves RNA molecule and adds “tail” of around 200 adenine nucleotides - poly-A tail
Ribonucleases
degrade mRNA molecule from both ends - first shortening of poly-A tail, which triggers removal of 5’ cap
beta-thalassemia
results from reduced synthesis of beta-chain of hemoglobin
reduces amount of hemoglobin that can be formed = profound anemia
Phenylketonuria (PKU)
inability to convert phenylalanine to tyrosine
due to mutation in phenylalanine hydroxylase gene - single base change in 5’ splice donor site of one particular intron
Incorrectly spliced mRNA and truncated protein that lacks one exon