RNA Structure & Transcriptase 😄 Flashcards
Types of RNA
Ribosomal RNA or rRNA
- ” rampant”
- most common
- associated with several proteins as a component of the ribosomes
- prokaryotes have 3 distinct size species
- eukaryotes have 4 distinct size species
Transfer RNA or tRNA
- “tiny” because it is the smallest RNA
- adaptor molecule that carries a specific amino acid to the site of protein synthesis
- contains many unusual bases and extensive intra-chain base pairing
- each of the 20 amino acid has at least 1 tRNA
Messenger mRNA or mRNA
- carries genetic information from the nuclear DNA to the cytosol, where it is used as the template for protein synthesis
- in eukaryotic cells, modification include: a long sequence of adenine nucleotides on the 3’end (poly-A tail), a “cap” on the 5’-end consisting of a molecule of 7-methylguanosine attached backward (5’-5’) through a triphosphate linkage
Small nuclear RNA or snRNA
- a subset of RNAs significant involved in mRNA processing and gene regulation
Polymers of nucleotides, but differ from DNA by containing:
- ribose instead of deoxyribose
- Uracol instead of thymine
RNA
4-subunit enzyme that synthesizes RNA
Possesses 5’ –> 3’ polymerase activity
Requires the ff:
- Sigma factor
- Rho factor
RNA polymerase
Recognizes the nucleotide sequence (promoter region) at the beginning of the length of the DNA to be transcribed
Sigma factor
Required for termination of transcription of some genes
Rho factor
Steps in Prokaryotic DNA transcription
Step 1: INITIATION
- RNA polymerase holoenzyme binds to the promoter region
- this sequence contains characteristic consensus nucleotide sequences that are highly conserved: Pribnow box, -35 sequence
Step 2: ELONGATION
- RNA polymerase copying one strand of the DNA double helix, pairing Cs with Gs and As with Us
- substrates are ribonucleoside triphosphates
- RNA polymerase does not require a primer and has no known exonuclease or endonuclease activity
Step 3: TERMINATION
- may be accomplished by RNA polymerase alone or may require p factor:
( p factor binds to a C-rich region near the 3’aend of the newly synthesized RNA and migrates along the 5’–>3’ direction until the termination site is reached; p independently termination required a stable hairpin loop turn and a palindrome sequence)
Stretch of six nucleotides (5’-TATAAT-3’) centered about 8 to 10 nucleotides to the left of the transcription start site
Pribnow box
Second consensus nucleotide sequence (5’-TTGACA-3’) about 35 bases to the left of the transcription start site
-35 Sequence
There are 3 distinct classes of RNA polymerase in the nucleus of eukaryotic cells
RNA polymerase I for large rRNAs in the nucleolus
RNA polymerase II for mRNAs
RNA polymerase III for tRNAs and some other small rRNAs in the nucleoplasm
TATA or Hogness box, CAAT box and GC box
Serve as binding sites for proteins called general transcription factors, which in turn interact with each other and with RNA polymerase II
Promoter sequences
DNA sequences that increase the rate of initiation of transcription by binding to specific transcription factors called activators
Enhancers
Linear copy of the transcriptional unit, the segment of DNA between specific initiation and termination sequences
Primary transcript
In both prokaryotic and eukaryotic cells, they are synthesized from long precursor molecules called PREERIBOSOMAL RNAs
These precursors are cleaved and trimmed by ribonucleases, producing the 3 largest rRNAs
rRNAs
Also made from longer precursor molecules
These must have an intervening sequence (intron) removed, and the 5’-and 3’-ends of the molecule are trimmed by ribonuclease
A 3’-CCA sequence is added and bases at specific positions are modified, producing unusual bases
tRNAs
Prokaryotic mRNA is generally identical to its primary transcript, whereas eukaryotic mRNA is extensively modified
A 7-methyl-guanosine “cap” is attached to the 5’-terminal end of the mRNA
A long poly-A tail (not transcribed from DNA) is attached to the 3’aend of most mRNAs
mRNAs
