RNA Flashcards
Types of RNA
Messenger RNA (mRNA)
Made from DNA through transcription and then goes to ribosomes for translation.
Types of RNA
Transfer RNA (tRNA)
Associated with amino acids, which pairs with mRNA codons during translation.
Types of RNA
Ribosomal RNA (rRNA)
Component of ribosomes that is synthesized in the nucleolus.
Types of RNA
Small nuclear RNA (snRNA)
Component of spliceosomes.
Types of RNA
Small interfering RNA (siRNA)
Functions in RNA interference by marking a mRNA sequence for breakdown.
Types of RNA
Micro RNA (miRNA)
Functions in RNA interference by blocking a mRNA sequence.
Translation
Ribosomes
The functional unit of the translation process.
Prokaryotic and eukaryotic ribosomes differ in their size and sedimentation rate, measured in Svedberg units
By Svedberg units, prokaryotic ribosomes are 70S with 30S and 50S subunits while eukaryotes have 80S ribosomes with 40S and 60S subunits.
Translation
Ribosomal sites
Three sites:
E site (exit site), P site (peptidyl site), A site (aminoacyl site) oriented in the 5’ to 3’ direction.
Translation
Step 1. Initiation
In prokaryotes, the 30S ribosomal subunit attaches to the Shine-Dalgarno sequence.
In eukaryotes, the 40S subunit attaches to the 5’ cap of the mRNA transcript and looks for Kozak sequence and the start codon
Translation
Step 2. Elongation
In a stepwise process, tRNAs (attached to amino acids) in the A site will get attached to the growing chain in the P site.
The “empty” tRNA then exits through the E site.
Translation
Step 3. Termination
Translation stops from the signal of a stop codon.
The protein gets released from the ribosome.
Translation
Step 4. Post-translational modifications
Structures called chaperones help fold the protein into its final shape and add in other covalently bonded molecules if needed.
RNA enzymes
Helicase
Starts off transcription as it “unzips” and separates the strands of DNA. Breaks hydrogen bonds between base pairs.
RNA enzymes
RNA polymerase II
Binds to a promoter region and uses the antisense strand of the DNA as a template.
Then synthesizes RNA in the 5’ to 3’ direction, while moving along the DNA in the 3’ to 5’ direction.
RNA enzymes
RNAse H
Removes RNA primers during DNA replication.
RNA enzymes
Primase
Creates the RNA primer during DNA replication.
Operons
Lac operon
Classic example of an operon. Found in E. Coli, codes for proteins that transport lactose.
Operons
Lac operon regulation
By default, the lac repressor protein is bound to the operator site, meaning that expression is repressed.
Under conditions of low glucose and available lactose, it becomes active.
Operons
Lac operon induction
Allolactose binds to the repressor region and detaches the lac repressor protein.
This results in induction (e.g. initiation of expression).
Operons
Lac operon activation
cAMP can bind upstream to the promoter region to activate the catabolite activator protein.
Operons
Jacob-Monod model
Describes the structure of an operon.
Consists of an operator, promotor, and coding region.
Post-transcriptional Processing
Happens specifically in the nucleus of eukaryotic cells.
Additional processing after transcription to form mature RNA that moves on to translation.
Post-transcriptional Processing
Splicing
The process of cutting out noncoding regions called introns, which stay in the nucleus.
The coding regions are called exons.
Post-transcriptional Processing
Spliceosome
Complex of snRNPs and proteins that perform splicing.
Post-transcriptional Processing
Alternative splicing
A process that allows for different variations of introns and exons to be spliced from the same transcript sequence.
This allows multiple product variations, called protein isoforms, to come out of the same coding sequence.
Post-transcriptional Processing
Head and tail additions
A 5’ cap made of methylguanine and a poly-A tail made of A nucleotides are added onto the final RNA.
These function to protect the transcript from breaking down (e.g. from exonucleases).
Post-transcriptional Processing
Polycistronic genes
These are found in prokaryotic cells and have similar purpose to alternative splicing.
The same gene has multiple possible translation sites, allowing for variable gene products.
Central Dogma
States that DNA → RNA → Protein
DNA to RNA occurs through transcription, then RNA to protein occurs through translation.
Central Dogma
Location
In eukaryotes, transcription and post-transcriptional processing takes place in the nucleus.
Translation takes place in the cytoplasm on a ribosome.
Central Dogma
Sense and antisense
In DNA, the template strand is antisense and the coding strand is sense.
The transcribed mRNA is sense.
