Ch. 23: RNA and Protein Synthesis (Transcription) Flashcards
What is the central dogma?
Replication–>transcription–>translation–>proteins
Transcription
1st step when sequence of bases in gene (DNA) converted to complementary sequence of RNA
Types of RNA
ribosomal RNA (rRNA) messenger RNA (mRNA) transfer RNA (tRNA)
Messenger RNA function
Carries genetic information contained in gene
Transfer RNA and Ribosomal RNA function
Convert info on sequence of bases on mRNA to corresponding amino acids–>proteins
Translation
mRNA “read” by tRNAs, ribosomes (made of rRNAs and ribosomal proteins)
Describe the structure of RNA
Single stranded polynucleotide (AUGC)
Ribosomal RNA function
Major structural component of ribosome with complementary sequence to mRNA (with whom it interacts)
3 Prokaryote ribosome molecules
5S, 16S, 23S rRNA (S=sedimentation size)
4 Eukaryotic ribosome molecules
5S, 5.8S, 18S, 28S
What structures are specific for tRNA?
secondary and tertiary structures (4-leaf clover containing extensive double stranded regions)
Loops connected by double-stranded stems
What do tRNA carry?
Covalently attached amino acid (3’ end) and complementary base sequence to mRNA recognition end (anticodon) that interacts with mRNA (5’ end)
Standard tRNA features
- 5’P terminus always base paired (stability)
- 3’OH terminus always 4-base single stranded with sequence XCCA-3’OH (acceptor stem that attaches at adenine)
- Contains many “modified” bases: dihydrouridine (DHU); ribosylthymine (rT), pseudouridine, and inosine I
- 3 single-stranded loops (anti-codon loop (opposite acceptor stem)
- 4 double stranded stems contain many GU pairs
- Extra arm
Start codon
AUG
Stop codons
UAG, UAA, UGA
What is mRNA
photocopy of gene by having sequence of complementary to one strand and identical to other. Carries info stored in DNA (in nucleus) to cytoplasm where ribosome can make it a protein
Prokaryotic mRNA are ____genic/cistronic
polycistronic (polygenic) - carry info to make many proteins from 1 mRNA
True/False: transcription (DNA to mRNA) and translation (mRNA to protein) are coupled
True (not separated in nucleus and cytoplasm like eukaryotes)
Eukaryotic mRNA are _____genic/cistronic
Mono: carry only 1 protein (but each gene can have millions of nucleotides because of exons/introns)
How can the mRNA that is transcribed (primary transcript) be processed? (Post-transcriptional modification)
- Introns spliced out, exons joined together (carried out by small nuclear ribonuclearproteins snRNPs)
- 5’ end has 7 methyl-guanine cap
- 3’ end has poly A tail
- Alternative splicing (produce mRNA with different set of exons) and RNA editing where bases modified/changed in original transcript
- Transported to cytoplasm where translation occurs (on free/ER ribosomes)
What are the basic features of the synthesis of RNA?
- Precursors (AGCU) and ribonucleotide so -OH on 2’ AND 3’ carbon
- 3’OH creates phosphodiester bond with adjacent 5’P (catalyzed by RNA polymerase)
- Sequence of bases determined by base sequence of sense DNA anti-sense template strand. RNA complementary to anti-sense strand
- Grows in 5’-3’ direction
- Anti-parallel strands
- No primer needed (RNA polymerase is good to go)
- 5’ end of growing RNA has triphosphate (removed in tRNA, rRNA, and eukary mRNA)
What are distinct features of transcription?
- Initiates from more sites than replication (goal is to make many copies, not just 1 like replication)
- More RNA polymerase than DNA polymerase
- RNA polymerase slower than DNA polymerase
- Lower fidelity than DNA polymerase (allowable since makes so many copies, so shmeh)
Describe the 5 subunits of a prokaryotic RNA polymerase
E.coli has 5 subunits: 2 alpha, 2 beta, 1 sigma (complete enzyme=holoenzyme)
Which subunit of E. coli RNA polymerase is used for DNA binding?
beta’ subunit
Which subunit of E. coli RNA polymerase is used for rNTP substrates and interacting with sigma?
beta subunit
What is the purpose of the sigma subunit of the prokaryotic RNA polymerase?
recognize promoter sequence on DNA (dissociates soon after)
What is the prokary RNA polymerase called after the sigma subunit dissociates?
core enzyme (complete enzyme=holoenzyme)
Which classes of RNA are synthesized by prokaryotic RNA polymerase?
ALL (mRNA, tRNA, rRNA)
What’s the first step of prokaryotic transcription?
Promoter recognition (sigma binds to promoter region on DNA) -35 and Pribnow box (-10) regions interact with RNA polymerase to initiate transcription
What happens after RNA polymerase binds to promoter?
conformational change so segment of DNA unwound and RNA polymerase positioned at start site
The mRNA sequence is homologous to which DNA strand?
The DNA sense strand
What are the inhibitors of transcription initiation?
Rifamycin B, rifampin, actinomycin D
MOA of rifamycin B
Binds to beta subunit of RNA polymerase, blocking binding of incoming rNTP at initiation site (effective only against prokary)
MOA of rifampin
prevents translocation of RNA polymerase along DNA (effective only against prokary)
MOA of actinomycin D
bind to DNA (effective against pro/eukaryote) - good for cancer
Which RNA transcription inhibitors are used for prokaryotes? eukaryotes?
rifamycin/rifampin only for prokary (good for gram + bacteria and TB)
Actinomycin good for pro and eukary (good for childhood neoplasms and carcinoma)
Where does termination of RNA synthesis occur?
Specific base sequence with:
- Inverted repeat base sequence with middle non-repeating segment ABCDEF–XYZ-F’E’D’C’B’A’ where A and A’ are complementary
- High GC content
- High AT content
