Chapter 7 Flashcards
What do mRNAs code for?
proteins
RNAs that form the core of a ribosome
rRNAs
regulate gene expression
miRNAs
the process by which the information encoded in a DNA sequence is translated into a product that has some effect on a cell or organism
gene expression
start/stop sites of transcription
promoter / terminator
Is the promoter region transcribed? Terminator?
no / yes
RNA polymerase always moves in this direction
3’ ——> 5’
Initiation of transcription in bacteria:
- sigma factor binds to promoter
- RNA synthesis is initiated
- sigma subunit dissociates
- chain elongation continues until terminator sequence
- RNA polymerase halts and releases the DNA and RNA transcript
- polymerase reassociates with the sigma factor
accessory proteins in eukaryotes that assemble on the promoter where they position the RNA polymerase and pull apart the DNA double helix, allowing RNA polymerase to begin transcription
general transcription factors
How the transcription factors assemble:
- ) TFIID binds to TATA box causing a distortion in the DNA
- ) TFIID enables the binding of TFIIB
- ) more transcription factors and RNA polymerase II bind
- ) TFIIH uses ATP to pull apart DNA at the transcription start site
- ) TFIIH also phosphorylates RNA polymerase II
- ) RNA polymerase is released from complex
post-transcription professing
5’ cap
poly A tail
splicing
Is the poly A tail transcribed?
no
Purposes of modifications after transcription:
- marks transcript as mRNA
- facilitates exports to cytoplasm
- increases stability of mRNA
Where does post-transcription modifications occur?
nucleus
DMD gene (dystrophin)
- 78 introns
- occupies 2,000,000 base pairs on chromosome X
- mRNA is 14,00 nucleotides long
process of removing introns and stitching exons together
RNA splicing
When does splicing occur?
after capping
What structure does the intron form when it is cut out?
lariat structure
How does the lariat structure form?
- ) branch point adenine in the intron sequence attacks the 5’ splice site and cuts the sugar-phosphate backbone of the RNA
- ) the cut 5’ end of the intron becomes covalently linked to the 2’-OH group of the ribose of the A nucleotide
- ) the free 3’-OH end of the exon sequence reacts with the start of the next exon sequence, joining the two together
carries out RNA splicing
snRNAs
snRNAs are packaged with proteins to form?
snRNPs (U1, U2, U4, U5, U6)
the large assembly of RNA and protein molecules that carries out RNA splicing in the nucleus
spliceosome
form the core of the spliceosome
snRNPs
mediate the transport of RNA out of the nucleus
nuclear pore complexes
mRNA lifetime is longer in eukaryotes or prokaryotes?
eukaryotes
serves as an adaptor that “reads” a codon in mRNA and adds the correct amino acid to the growing polypeptide chain
tRNA
a set of three consecutive nucleotides that bind to the complementary codon in an mRNA molecule
anticodon
What does wobble mean?
some tRNAs require base-pairing only at the first two positions of the codon and can tolerate a mismatch at the third position
covalently couple each amino acid to it’s appropriate set of tRNA molecules
aminoacyl-tRNA synthetases
What is the large subunit of a ribosome composed of?
small proteins and rRNAs
function of large subunit? small subunit?
- large subunit— catalyzes peptide binds
* small subunit— matches mRNA codons to tRNAs
RNA molecules that posses catalytic activity
ribozymes
start and stop sites of translation
start codon (AUG) stop codon (UGA, UAG, UAA)
always carries that amino acid Met
initiator tRNA
mRNA that codes for several different proteins, each of which is translated from the same mRNA molecule (only in prokaryotes)
polycistronic
Do stop codons code for an amino acid?
no
What stops translation?
release factor enters A site, peptidyl transferase catalyzes the addition of water instead of an amino acid, polypeptide chain is released
many ribosomes working simultaneously on a single mRNA molecule
polyribosomes
specialized pathways that break down proteins
proteolysis
the enzyme that degrades proteins
professes
large proteins machines in eukaryotes that break down proteins
proteasomes
Where are proteasomes found?
cytosol and nucleus
How do proteasomes select which proteins should be degraded?
proteins marked with ubiquitin
Post-translational modifications:
- covalent modifications (phosphorylation)
- the binding of small-molecule cofactors
- association with other protein subunits
