What does RNA polymerase produce?
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A complementary RNA to the template strand.
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What does RNA polymerase produce?
A complementary RNA to the template strand.
What is the location of DNA in prokaryotes?
DNA is found in the cytoplasm.
How many eukaryotic RNA polymerases are there?
Three.
What is the structure of tRNA?
tRNA is a single-stranded molecule that folds into a cloverleaf shape.
What type of reaction is involved in peptide bond formation?
Condensation reaction, which releases H2O.
What is the difference between mRNA sequence and coding strand sequence?
mRNA sequence uses U instead of T.
How are some genes transcribed from DNA?
Using one DNA strand as a template.
What is the role of eukaryotic release factor (eRF) in termination?
It binds the stop codon in the A site.
What is the enzyme responsible for eukaryotic transcription?
RNA polymerase.
What is the result of coordinate control of genes with related function?
It often produces polycistronic mRNA.
What is the role of transcription factors in eukaryotic transcription?
They help RNA polymerase bind to the promoter region of the DNA.
What type of RNA constitutes only a small portion of the total RNA in rapidly growing mammalian cells?
mRNA (messenger RNA).
What is the role of TATA Binding Protein (TBP) in transcription?
It binds directly to the TATA box and is part of TFIID.
What happens upon completion of the initiator complex in transcription?
It causes a conformational change, and TBP kinks DNA to help separate the strands.
What are the components of the basic eukaryotic gene structure?
Upstream and downstream regulatory sequences, 5’ and 3’ Untranslated Regions (UTR), exons, and introns.
What is the Core in eukaryotic gene structure?
The minimal sequence required to properly initiate transcription, including the transcription start site (or INR) and TATA box.
What is the function of exons in eukaryotic gene structure?
Exons encode RNA that will remain with the mature mRNA.
What is the template strand in DNA replication?
The strand used to make a complementary RNA copy.
How does Rho-independent termination stop transcription?
It relies on the formation of a secondary structure in the newly transcribed RNA to dislodge the RNA polymerase from the DNA template.
What is the convention for indicating the name of the amino acid codon that the tRNA anticodon recognizes?
By using a superscript.
In which direction does RNA polymerase read the Template Strand?
3' to 5'.
What is the 'Double sieve' proofreading?
A mechanism that results in 1 error in 40,000 reactions.
How many high-energy bonds are used from one ATP?
Two.
What is the activity of rRNA in the large ribosomal subunit?
Peptidyl transferase, which is a ribozyme.
What is the term used to describe the redundancy in the genetic code for amino acids?
Degeneracy or redundancy.
What is the function of regulatory sequences in a gene?
They bind protein factors that control and participate in the process of transcription.
What is the anticodon for methionyl-tRNA?
CAU.
What is the first step of translation initiation in bacteria?
Binding of a 30S ribosomal subunit to initiation factors IF1 and IF3.
What happens when a repressor is absent at the operator?
Transcription can occur (RNA polymerase can bind promoter).
What joins the complex after the 30S ribosomal subunit and initiation factors IF1 and IF3?
mRNA, the initiator N-formylmethionyl (fMet) tRNA, and IF2 (bound to GTP).
What is the target of Shiga Toxins in the ribosome?
The 28S rRNA in the 60S large subunit of the ribosome.
Where can the eukaryotic gene regulatory elements be located in relation to the composite core promoter?
10 to 50 kilobases either upstream or downstream.
What is the Promoter in eukaryotic gene structure?
A stretch of DNA upstream to gene coding sequence that contains binding sites for regulatory proteins such as transcription factors and RNA polymerase.
What is the preferred DNA sequence that each transcriptional activator (TSF) binds?
Consensus binding site.
What is the resulting structure of the RNA transcript?
Single-stranded and complementary to the template strand.
What is the function of the core polymerase in transcription?
To read and translocate down the DNA molecule and synthesize RNA.
What are the components of each gene?
A segment encoding the RNA to be transcribed and regulatory sequences binding protein factors.
What is an operator in DNA?
A negative regulatory element where repressor proteins bind and block the ability of RNA Polymerase to interact with the promoter.
What is the most abundant type of RNA in rapidly growing mammalian cells?
rRNA (ribosomal RNA).
How does the C terminus of a protein correspond to the mRNA sequence?
It corresponds to the 3’ end of mRNA.
What percentage of the total RNA in rapidly growing mammalian cells is tRNA?
15 percent.
What is the mRNA codon for methionine?
AUG.
What does transcription produce that is complementary to one strand of DNA?
RNA.
What is the structure of a protein-coding gene?
Intron-exon structure.
What happens to genes with 'better' binding sites for a transcription factor?
They will bind that transcription factor more readily than others when amounts are limiting.
What is the function of enhancer-promoter chromosomal looping?
It allows distal enhancer elements to physically interact with and activate gene promoters.
What do the proximal promoter elements include?
A variety of regulatory elements (RE).
What are the two mechanisms of transcription termination in bacterial cells?
Rho-independent termination and Rho-dependent termination.
Where can enhancers and silencers be located in relation to the transcription start site of the gene?
They can be located thousands of base pairs away or closer to the transcription start site.
What are the characteristics of enhancers?
Elevated DNA sequence conservation, open chromatin, transcription factor binding motifs, characteristic histone modifications, DNA hypomethylation, and bidirectional transcription to generate enhancer RNAs (eRNAs).
Why does the 5’ base of the tRNA anticodon undergo more movement?
Because it is not as spatially defined as the other two bases.
What does UTR stand for in molecular biology?
Untranslated Region.
How long is the region of hybrid DNA/RNA helix that forms transiently?
Approximately nine nucleotides in length.
In which direction is the RNA produced by RNA polymerase?
5' to 3'.
What is the genetic code?
The set of rules by which information encoded in genetic material (DNA or RNA sequences) is translated into proteins.
What are the active sites of ribosomes responsible for?
Catalyzing the formation of peptide bonds between amino acids during protein synthesis.
What is eukaryotic transcription?
The process of transcribing DNA into RNA in eukaryotic cells.
What is the upstream sequence in a gene?
DNA or RNA located towards the 5’ end from the reference point.
What is a gene?
A segment of DNA that contains all the information necessary for the synthesis of a polypeptide or RNA product.
know the RNA sequence from the template DNA strand.
Reverse transcription.
What do the bacterial toxins do to the 28S rRNA in the 60S subunit?
They clip an adenine residue from the 28S rRNA.
Why do transcription and translation occur simultaneously in prokaryotes?
Because prokaryotes do not have a nucleus to separate DNA from other cellular components.
What is the function of ribosomes in protein synthesis?
They are the site of protein synthesis.
What is the function of some proximal promoter elements?
Modulating transcription in response to cell signaling.
What is the Wobble Hypothesis?
It refers to nontraditional base-pairing between the 5′-nucleotide of the anticodon with the 3′-nucleotide of the codon.
How do transcriptional activators (TSFs) regulate transcription?
In response to environmental signals.
What is required for Rho-dependent termination in prokaryotic transcription?
The action of the rho protein.
What is the initiation codon for translation in both prokaryotic and eukaryotic cells?
Methionine (fMet for prokaryotes), usually encoded by AUG.
How many tRNA are found per cell to code for 20 amino acids?
30-34 tRNA.
What is a tRNA called once the amino acid is attached to it?
Aminoacyl tRNA.
What are the components of the prokaryote RNA Polymerase core polymerase?
One ω subunit, two α subunits, one β subunit, and one β’ subunit.
How are the names of amino acids commonly abbreviated?
In two common ways.
What happens when a new tRNA binds in the A site during elongation?
Peptidyl transferase makes a peptide bond.
What is the function of Basal/General Transcription Factors in relation to RNA polymerase?
They help position RNA polymerase properly at the start site.
What do Basal/General Transcription Factors help to create in the DNA?
A transcription bubble by separating DNA strands.
What are the various types of RNAs that genes encode?
mRNA, tRNA, rRNA.
What are consensus sites in transcription?
They are the binding sites of a given transcription factor, which may vary slightly on different genes.
How many nucleotides long is each codon?
Three nucleotides.
What are the two ends of a DNA strand called?
3' and 5'.
What happens when the RNA polymerase encounters a termination signal in prokaryotic transcription?
Transcription stops.
What do transcriptional activators (TSFs) bind in the regulatory regions of genes?
Enhancer elements.
What does the 'Start codon' (AUG) encode for in prokaryotes?
Formylmethionine (fMet).
How do enhancer sequences affect gene transcription?
Enhancer sequences can be positioned in both forward or reversed sequence orientations and still affect gene transcription.
What does the RNA polymerase holoenzyme bind to?
The promoter.
Where is the -35 Sequence located in relation to the transcriptional start site?
Centered about 35 bases upstream of the transcriptional start site (+1).
Which strand is displaced and not read by the polymerase?
The Coding Strand.
In what form is the tRNA said to be when it has an amino acid attached to it?
Charged form.
What are the names of the three eukaryotic RNA polymerases?
RNA polymerase I, RNA polymerase II, and RNA polymerase III.
What are the names of basal Transcription Factors (bTSF)?
TFIIB, TFIID, TFIIH
What is the enzyme responsible for prokaryotic transcription?
RNA polymerase.
Do both strands of DNA in a chromosome encode for genes?
Yes, both strands encode for genes.
What is the role of peptidyl transferase in termination?
It adds H2O to the C-terminus to release the protein in a reaction that hydrolyzes GTP.
Where is the downstream sequence located in a gene?
Towards the 3’ end from the reference point.
How are the structural genes of an operon transcribed?
As one long polycistronic mRNA.
What happens after IF1 and IF3 are released in translation initiation in bacteria?
A 50S subunit binds to the complex, and IF2 is released.
What are the components of a composite core promoter?
TATA box (TATA), initiator sequences (INRs), and downstream promoter elements (DPEs).
What type of proteins bind to silencers?
Repressors.
How does RNA polymerase add ribonucleotides to the RNA chain?
One-by-one, using an exposed DNA strand as a template.
What is the result of multiple ribosomes binding to a single mRNA molecule in prokaryotes?
Formation of polyribosomes, which dramatically increase the level of translation.
How many codons specify for stop codons?
Three codons (UAA, UAG, UGA).
What is the function of enhancer/silencer elements in gene expression?
They regulate the transcription of genes.
What is the process of 'charging' tRNAs also known as?
Activation of tRNAs.
How are initiation codons identified in prokaryotic and eukaryotic cells?
The signals that identify initiation codons are different in prokaryotic and eukaryotic cells.
What results in an open promoter complex and initiation of transcription?
Local DNA unwinding.
What is the term for an alanyl tRNA specific for alanine codons?
alanyl tRNA Ala.
Which protein brings the mRNA to the initiation complex in eukaryotes?
eIF4E.
Where is DNA confined in eukaryotes?
DNA is confined to the nucleus and mitochondria.
What is a characteristic of RNA polymerase in terms of primer requirement?
It does not require a primer.
What is the coding strand's role in transcription?
It is part of the gene but is not directly involved in transcription.
What is the function of transfer RNAs (tRNAs) during translation?
They serve as adapter molecules that couple the codons in mRNA with the amino acids they specify, aligning them for peptide bond formation.
In prokaryotes, where does transcription occur?
In the cytoplasm.
How does the N terminus of a protein correspond to the mRNA sequence?
It corresponds to the 5’ end of mRNA.
What is an operon?
A cluster of genes with related functions, regulated so that all the genes in the cluster are turned on and off together.
How much does the mRNA move through the ribosome during elongation?
Exactly 3 nucleotides.
What happens if an incorrectly paired product is released by an aminoacyl tRNA synthetase?
There is no mechanism during translation to detect the error, and the incorrect amino acid will be introduced into some protein.
What is the mechanism by which Diphtheria Toxin and Pseudomonas Exotoxin A inhibit translation?
They inhibit eukaryotic elongation factor - 2 (eEF2) through ADP-ribosylation.
What is the arrangement of multiple clustered enhancer modules interspersed with silencer and insulator elements called?
Eukaryotic gene regulatory elements.
What are the Proximal Regulatory Sequences in eukaryotic gene structure?
Approximately 250bp sequence upstream of the gene that contains primary regulatory elements.
What is the role of TFIIH in transcription?
It is the last to bind the initiation complex, has helicase activity to unwind DNA, and kinase activity to signal the beginning of transcription.
What is the function of tRNAs in protein synthesis?
They carry amino acids to the ribosome.
What does the 'Start codon' (AUG) encode for in eukaryotes?
Methionine.
How do enhancer-promoter interactions affect gene promoters?
They increase the binding of transcription factors, chromatin modifiers, and the Mediator complex to recruit RNA polymerase II (RNAPII).
What does the electron microscopy image show in prokaryotes?
Simultaneous transcription and translation.
What happens to the DNA behind the polymerase as it moves along the template?
It rewinds.
What is the initial step of prokaryotic transcription?
The binding of the holoenzyme to the untranscribed DNA promoter.
What are the two consensus sequences recognized by the holoenzyme σ factor in prokaryotic transcription?
What is the process of elongation in prokaryotes similar to in eukaryotes?
Elongation in prokaryotes is similar to elongation in eukaryotes.
What is the base pairing between tRNA anticodon loop and mRNA codon?
Complementary, antiparallel binding.
What causes the shift of two tRNAs to the E and P sites during elongation?
Conformational changes in the large ribosomal subunit.
What happens when a repressor is present at the operator?
Transcription is blocked (RNA polymerase cannot bind promoter).
Are Basal/General Transcription Factors regulated in response to the environment?
No, they are not regulated in response to the environment.
What is required for the translocation step during elongation?
Elongation factor - 2.
What will happen if an abnormal aminoacyl tRNA (ala - tRNA ser) encounters a serine codon?
Alanine will be incorporated into the protein.
How does the binding affinity of a transcription factor change based on DNA sequence?
The binding affinity is higher if the DNA sequence conforms well to the consensus.
What is the consequence of Shiga Toxins targeting the 28S rRNA?
It causes inhibition of elongation factor (EF1a)-dependent aminoacyl-tRNA binding and subsequent peptide elongation, resulting in cell death.
How many codons are there in total?
64 codons.
What is the role of TFIIB in transcription initiation?
It plays a critical role in the assembly of the RNA polymerase II preinitiation complex, stabilizes the TBP complex, and recruits RNA polymerase II and other transcription factors.
What enzyme transcribes DNA into RNA?
RNA polymerase.
What is the TATA box in eukaryotic gene structure?
A binding sequence for general transcription factors and RNA polymerase (preinitiation complex).
What is the function of introns in eukaryotic gene structure?
Introns encode RNA that will be spliced out and will not be part of the mature mRNA.
What is the coding strand in DNA replication?
The non-template strand that has the same sequence as the RNA transcript.
What do transcriptional activators (TSFs) facilitate the formation of?
Basal initiation complex.
What is the role of the σ subunit in transcription?
It is weakly bound to the core polymerase and is required for promoter recognition at the transcriptional start.
How many functionally different σ factors are there and what is their role?
There are several functionally different σ factors, each directing the core polymerase to different classes of promoters under different conditions, such as starvation or changing nutrient sources.
How many proteins are required for initiation in eukaryotes?
At least 11 proteins, designated eIFs (eukaryotic initiation factors).
What is the initiator methionyl tRNA bound to?
eIF2.
Where is the Pribnow Box located in relation to the transcriptional start site?
Centered at about 10 bases upstream of the transcriptional start site.
What is the starting codon for the genetic code?
The first AUG.
What is the equivalent of EF-Tu (G protein) in eukaryotes?
eEF1α.
What is the function of the 3’ UTR in eukaryotic mRNA?
Containing regulatory sequences that affect translation from the mRNA, important for mRNA stability, and encoding the transcriptional termination sequence.
What is the condition for reaching a stop codon in the genetic code?
An 'in-frame' stop codon.
What is another name for the coding strand?
Sense strand.
How many copies of the gene encoding the 45S rRNA precursor are clustered together on acrocentric chromosomes?
250 - 300 copies.
What is the process of converting genes into RNA called?
Transcription.
Where does eukaryotic transcription occur?
In the nucleus of eukaryotic cells.
What is the function of Aminoacyl - tRNA synthetases?
They have self-checking functions to prevent incorrectly paired aminoacyl tRNAs from forming.
Is a given RNA product derived from both strands of DNA?
No, it is only derived from one of the two strands.
What does a gene contain?
All the information necessary for the synthesis of a polypeptide or RNA product.
What happens during translation of the polycistronic mRNA?
Different start and stop codons lead to a number of distinct proteins being produced.
What is the function of enhancers in gene transcription?
Enhancers increase the rate of transcription from a gene.
What is the function of silencers in gene transcription?
Silencers decrease the rate of transcription from a gene.
What type of proteins bind to enhancers?
Activators.
What is the role of CTCF and cohesin in enhancer-promoter chromosomal looping?
They mediate the looping process.
What happens once newly synthesized RNA exits the RNA polymerase complex in prokaryotes?
Ribosomes bind and translation occurs.
What does the movement of the 5’ base of the tRNA anticodon allow?
It allows the formation of non-standard base-pairs.
How many tRNA species are needed to correspond to 61 codons according to the Wobble Hypothesis?
It is not necessary to have 61 tRNA species.
What is the function of the 5’ UTR in eukaryotic mRNA?
Controlling the translation of mRNA to protein and regulating the binding of ribosomes to mRNA for translation.
How does the rho protein cause transcription to stop in Rho-dependent termination?
It moves along the newly transcribed RNA, catching up with the RNA polymerase when it pauses at the termination site, and causing the polymerase to leave the DNA template.
How are tRNA molecules specific for alanine codons identified?
As tRNA Ala.
What happens to the σ factor once transcription is initiated?
The σ factor disassociates from the core polymerase.
How many enzymes are there in the family of Aminoacyl tRNA synthetases?
20 enzymes.
In which direction does the RNA polymerase move along the DNA template during elongation?
3’ to 5’.
What is the role of Aminoacyl tRNA synthetases in the accuracy of translation?
They are largely responsible for the accuracy of translation.
How are genes expressed in prokaryotes?
Genes are expressed polycistronic and clustered to form functional groups called operons.
What is the equivalent of EF-G in eukaryotes?
eEF2.
What is the requirement for mRNA processing in eukaryotes?
mRNA must be processed for nuclear export (mRNA has a 5’ methyl-cap and 3’ poly-A tail).
What does it mean for the genetic code to be 'non-overlapping'?
There is no punctuation and it starts at the first AUG.
How is gene expression regulated in eukaryotes?
Gene expression is regulated at many levels (epigenetic, transcriptional, nuclear shuttling, post-transcriptional, translational, and post-translational).
What is the function of Aminoacyl tRNA synthetases?
They create a high-energy bond between the correct amino acid and the tRNA.
Where does the 5’ UTR start and end?
It starts at the transcriptional start site and extends to 1bp before the AUG sequence of the gene.
What is the holoenzyme composed of?
The holoenzyme is composed of the core polymerase and the σ subunit.
How many nucleotides does the genetic code 'read' at a time?
3 nucleotides at a time.
How are genes expressed in eukaryotes?
Genes are expressed monocistronic.
What is the role of messenger RNA (mRNA) during translation?
It acts as a working copy of the gene with codons for each amino acid transcribed from DNA to mRNA.
Where does translation take place?
On ribosomes, complexes of rRNAs and proteins that coordinate interactions between mRNA, aminoacyl-tRNAs, enzymes, and protein factors for protein synthesis.
How many ribosomal proteins are combined with rRNAs to assemble ribosomal subunits in eukaryotic cells?
More than 80 ribosomal proteins.
What do some antibiotics target?
Bacterial ribosomes.
What is another name for the 5’ UTR?
Leader sequence.
What is involved in the recognition of the 5’ Cap and poly A tail in eukaryotic initiation?
eIFs.
What is the difference between the coding strand sequence and the RNA sequence?
Coding strand has T, RNA sequence has U instead of T.
How is gene expression regulated in prokaryotes?
Gene expression is regulated primarily at the transcriptional level.
Where are ribosomal subunits assembled in eukaryotic cells?
In the nucleolus.
What is the difference in the binding of mRNA to small ribosomal unit in eukaryotes and prokaryotes?
In eukaryotes, the 5' cap on mRNA binds eIFs and 40S subunit contains tRNA met, while in prokaryotes, the Shine-Delgarno sequence upstream of AUG binds complementary sequence in 16S rRNA.
How do eukaryotic mRNAs get recognized for translation initiation?
Eukaryotic mRNAs are recognized by the 7-methylguanosine cap at the 5′ terminus.
What is released as the RNA polymerase moves along the DNA template during elongation?
The σ subunit.
What is another name for the template strand?
Antisense strand.
What sequence does the 3’ UTR follow immediately after?
The stop codon of the ORF (Open Reading Frame).
What encodes a large 45S rRNA precursor in eukaryotic cells?
A single gene.
Where does transcription occur in eukaryotes?
Transcription occurs in the nucleus.
What is the role of aminoacyl-tRNA synthetases during translation?
They link amino acids to the correct transfer RNAs.
What are protein factors in translation responsible for?
They facilitate several steps in the translation process, including initiation, elongation, and termination of polypeptide chains.
What is the difference in the 1st Amino Acid used in protein synthesis initiation between eukaryotes and prokaryotes?
In eukaryotes, the 1st Amino Acid is Methionine, while in prokaryotes, it is Formyl-Methionine (f-Met).
What do ribosomes do after recognizing the 7-methylguanosine cap in eukaryotic mRNAs?
The ribosomes then scan downstream until they encounter the initiation codon.
What is the equivalent of EF-Ts GTP exchange factor in eukaryotes?
eEF1βγ.
What is the consequence of RNA polymerase lacking proofreading activity?
It results in a high rate of errors.
What is responsible for most of the catalytic activity of the ribosome?
rRNA.
Are there any exceptions to the universality of the genetic code?
Yes, a few exceptions, including in mitochondria.
What is the template for RNA formation during transcription?
Template strand.
How does Aminoacyl tRNA synthetases create a high-energy bond between the correct amino acid and the tRNA?
It uses 2 high-energy bonds from one ATP.
What type of tRNA is involved in eukaryotic initiation and what is it charged with?
Special initiator tRNA charged with methionine.
What does 'wobble' refer to in the genetic code?
The binding between tRNA and the mRNA codon.
What sources of energy are used in eukaryotic initiation?
Energy from ATP and GTP.
What happens to some nucleotides of the rRNA precursor in eukaryotic cells?
They are chemically modified (e.g. methylated).
What is meant by the genetic code being 'degenerate' or 'redundant'?
Most amino acids are encoded by more than one codon.
What is the difference in the initiation factors involved in protein synthesis initiation between eukaryotes and prokaryotes?
In eukaryotes, initiation factors (eIFs) have approximately 12 subunits, while in prokaryotes, initiation factors (IFs) have 3 subunits.
What has to be recycled in eukaryotic initiation, serving as a regulation point?
eIF-2-GTP.
What does it mean for the genetic code to be 'universal'?
It is conserved throughout evolution.
How does the RNA product relate to the template strand?
It is antiparallel and complementary to the template strand.
What are the sizes of ribosomes in eukaryotic cells?
80S with 40S and 60S subunits.
What is the difference in the composition of ribosomes involved in protein synthesis initiation between eukaryotes and prokaryotes?
In eukaryotes, ribosomes are composed of 40S + 60S, forming 80S, while in prokaryotes, ribosomes are composed of 30S + 50S, forming 70S.
What are the sizes of ribosomes in prokaryotic cells?
70S with 30S and 50S subunits.
How is the genetic code described in terms of specificity and ambiguity?
It is specific and unambiguous, with a given codon always encoding the same amino acid.
What do Basal/General Transcription Factors bind to?
The core promoter. TATA box
what help RNA polymerase get started?
two types of transcription factors.
the binding of TFIIB to TBP complex provides what?
a platform for PIC formation
TFIIB recruits what into the complex at the transcriptional start site?
RNA polymerase II and other transcriptional start site
TFIIH is also important for what?
nucleotide excision repair of DNA.
what does RNA pol I transcribe, what is the function?
pre-rRNA (28s, 18s, 5.8s rRNAs). ribosome components, protein synthesis.
what does RNA pol II transcribe?
messenger RNA, snRNAs, siRNAs, miRNAs
function of 1. mRNA, 2. snRNAs, 3. siRNAs, 4. miRNAs?
encodes protein.
RNA splicing,
chromatin-mediated repression and translation control.
translation control.
RNA Pol III transcribes what?
tRNAs, 5s rRNA, snRNA U6, 7s RNA
function of 1. tRNA 2. 5s rRNA 3. snRNA U6 4. 7S RNA
protein synthesis
ribosome component, protein synthesis
RNA splicing
signal recognition particale for insertion of polypeptides into the endoplasmic reticulum.
What precedes the initiation codons in bacterial mRNAs? what then happens?
A Shine-Dalgarno sequence that aligns the mRNA on the ribosome. they initiate translation at the 5’ end of an mRNA and at the internal initiation sites of polycistronic mRNAs.
what resets the ribosome so the A site is open?
the movement of mRNA (exactly 3 nucleotides) through the ribosome.
