LECTURE 21 Flashcards
Before RNA pol ll can bind to the DNA strand,
Several transcription factors midst bind to DNA first
Transcription initiation complex is made of
RNA pol ll and transcription factors
Promoter sequence specifies the
Orientation of RNA pol binding (which strand is the template and the direction of transcription)
Three stages of transcription
1) initiation
2) elongation
3) termination
Initiation
DNA strands unwind, polymerase initiates RNA synthesis at the start point
Elongation
RNA pol catalyzes formation of phophsodiester bonds to produce the RNA polymer
- RNA pol moves down downstream making RNA transcript
Replication bubble is
A lot bigger and continues to grow
Transcription bubble
Stays the same size and closes up behind itself, does so with out additional enzymes
Termination
RNA transcript is released
- RNA pol detaches from DNA
Ways for termination in bacteria. And what do they require
1) intrinsic
2) extrinsic
- requires a terminator sequence
Terminator sequence
sequence in the newly transcribed RNA that signals the end.
Intrinsic
Stem-loop forms because of GC-rich inverted repeat, this displaces RNA form template
Extrinsic
Uses a no step-loop terminator sequence, and requires a protein with helices activity, called rho, which binds to the transcript and, travelling faster then RNA pol, reaches the DNA/RNA duplex and unwinds it, RNA pol falls off.
Termination of transcription in eukaryotes
RNA pol ll transcribes a sequence in DNA called polydentalation signal sequence
- signal sequence is recognized by proteins, cuts the mRNA. RNA pol keeps going
- RNA exonucleonase eventually removes the trailing RNA and he,ps knock the polymerase off.
RNA pol l is terminated when a protein binds to DNA downstream of coding region, it knocks it off.
In both eukaryotes and prokaryotes, a single gene can be transcribed
Simultaneously by several molecules of RNA pol.
- allows the cells to produce a lot of RNA in a short time.
In eukaryotes, RNA processing after transcription produces a mature
Messenger RNA (mature mRNA)
Post-transcriptional modifications
Are required to increase the stability of the transcript in the cytoplasm
In eukaryotes, there are THREE post-transcriptional modifications that occur to pre-mRNAs (in nucleus)
1) addition of a 5’ cap
2) addition of adenine nucleotides to 3’ end, making poly A-tail. The same enzyme that cut the RNA add the poly A-tail.
3) RNA splicing
Addition of 5’ cap
A modified form of a guanine is added to the 5’ end of the RNA polymer via an unusual 5’-5’ phosphate linkage.
addition of adenine nucleotides to 3’ end, making a
poly A-tail. The same enzyme that cut the RNA add the poly A-tail
5’ cap and poly-A tail important functions
1) facilitates the export of mRNA form nucleus
2) protected RNA form degration by exonucleases
3) helps ribosome attach to 5’ end of mRNA (in cytoplasm)
Introns
Large Portions of pre-mRNA are removed
Exons
Remaking sections after splicing are pasted together
RNA splicing
Introns are removed and exons are pasted together.
Almost all eukaryote genes have
Introns
RNA splicing is sometimes carried out by
Spliceosomes
Spliceoosomes consist of
Proteins and several small nuclear ribonucleoproteins (snRNPs) that recognize sites
Steps for RNA splicing reaction
1 ). SnRNPs and other proteins form a complex = spliceosome
2 ). SnRNA base-pairs with nucleotides at specific sites along the intron.
3 ). The spliceosome cuts the RNA, releasing the intron for rapid degradation, and legatees the exons together
Why do eukaryotes have introns?
1) some introns contain sequences that help regulate the expression of genes.
2) alternative splicing allows for an increase in diversity of proteins
Alternative splicing
After transcription to produce many pre-mRNA molecules, each one could be spliced in a different way in the same nucleus, to produce a mixture of mRNAs that have different exons.
This way, the same gene could make 2 different proteins with different functions
Is alternative splicing necessary
No
Translation
The process by which the genetic code in mRNA directs the synthesis of proteins form amino acids
TRNAs (transfer RNAs) are the adapter molecules that
Me diet the transfer of info from nuclei acids to proteins
A specific amino acids acid is bonded to tRNA via
- what’s the amino acid ?
Covalent bond
- Phe
tRNA contain multiple
Stems and loops due to complementary base pairing
Because of hydrogen bonds, tRNA molecules
twists and folds into an L shape.
The anticodon sequence in tRNA is
Complimentary and antiparallel to a codon in the mRNA
The base pairing in tRNA gives
Specificity to translation
Correct matching of tRNA and its associated amino acid is accomplished by
Aminoacytl-tRNA synthetases
Linking amino acids to tRNA is _______. The reaction is couples to the
Endergonic Hydrolysis of ATP
Proteins synthesis requires a steady supply of each of the 20 different charged tRNAs. How are they recharged ?
They get recharged by their own amino acetyl synthase after they transfer their amino acid to a growing peptide
Ribosomes - enzymes of translation
- facilitates the specific coupling of tRNA and mRNA codons during proteins synthesis
- catalyses formation of peptide bonds
- complexes made of proteins and rRNAs
Ribosomes are made of two major subunits
1) large subunits (+ rRNAs)
2) small ribosomal subunit (+rRNA)
S = Svedberg unit
Measures how fast something sediments (settles) in a liquid
Streptomycin
- binds to the small 16S rRNA is the 30S subunit of bacteria ribosome.
Interferes with translation in bacteria.
Why doesn’t streptomycin effect eukaryotes
Has limited effect on translation of eukaryotes.
High doses of streptomycin effects
Mitochondrial proteins synthesis
Ribosomal large subunits has three binding sites for tRNA molecules:
1) A site (aminoacyl-tRNA binding site)
2) P site (peptideyl-tRNA binding site)
3) E site (exit site)
A site (aminoacyl-tRNA binding site)
Aminoacytl tRNA enters here
P site (peptideyl-tRNA binding site)
Holds the tRNA attached to the growing peptide chain
E site (exit site)
Discharged tRNA leave the ribosomes form this site.
The small subunits binds to the
mRNA
Small subunits recognized the mRNA using its
rRNA to make initial complementary base pairing
Shine-dalgarno sequence is located on
mRNA
Shine-Dalgarno
This is recognized by the ribosome small subunit, using the 16S rRNA (S-D sequence is present in prokaryotes, not eukaryotes)
