Chapter 7- RNA and The Genetic Code Flashcards
MRNA is synthesized in what direction?
5 to 3 and N to C
mRNA
Carries the information specifying the amino acid sequence of a protein to the ribosome.
MRNA in prokaryotes vs eukaryotes protein products
mRNA in prokaryotes are polycystronic meaning that one mRNA can result in different proteins.
In eukaryotes mRNA is monocystronic meaning that one mRNA only translates into one protein product.
tRNA
Responsible for converting the language of nucleic acids to the language of amino acids and peptides.
Includes a three nucleotide anti-codon. While inside the ribosome the tRNA anti-codon pairs with an appropriate codon and an mRNA molecule.
How many amino acids are in the eukaryotic proteins
20
Where in the cell is the tRNA found?
In the cytoplasm
What enzyme transfers activated amino acids to the end of the correct tRNA?
aminoacyl-tRNA synthetase
What end of the tRNA does aminoacyl tRNA synthetase transfer activated amino acids?
3’ end
rRNA
RNA helps catalyzes the formation of peptide bonds and is also important and spicing out its own entrance within the nucleus
What is the code on that is considered the start codon?
Methionine AUG
What are the three stop codons?
UAA UGA UAG
Which two amino acids are only coded for one codon?
Methionine and tryptophan
A mutation that only affects one of the nucleotides in a codon
Point mutation
Expressed mutation
When a point mutation affects the primary amino acid sequence of a protein
What are the two types of expressed mutations?
Missense mutation- A mutation where the error codes for another amino acid
Nonsense mutation- A mutation where the codon now in codes for a premature stop codon also known as a truncation mutation.
Frameshift mutation
When a nucleotide is added or deleted from the mRNA sequence
A mutation that causes a shift in the reading frame that usually results in changes in the amino acid sequence or premature truncation of the protein
DNA cannot leave the nucleus what does it do to overcome this?
DNA must use RNA to transmit genetic information.
MRNA is the only type of RNA that carries information from DNA directly.
Transcription results and what type of mRNA?
Single stranded
Template strand AKA
Antisense
Which DNA dependent RNA polymerase binds to the promoter region
RNA polymerase II
What is the name of the promoter region that RNA polymerase to bind to in eukaryotes?
TATA box
Named for its high concentration of thiamine and adenine bases.
Does RNA polymerase require a primer to bind?
No, unlike DNA polymerase three in DNA replication RNA polymerase two does not require a primer to start generating a transcript.
Where are the following RNA polymerase is located and what do they synthesize?
RNA polymerase I-
RNA polymerase II-
RNA polymerase III-

RNA polymerase I- located in the nucleus synthesizes rRNA
RNA polymerase II- located in the nucleus synthesizes hnRNA
RNA polymerase III- located in the nucleus synthesizes tRNA and some rRNA
Does RNA polymerase proofread?
No
What is the difference between the coding strand (sense) and the template strand?
Thymine nucleotides in turc coding/sense/template strand are replaced with Uracil nucleotides in the mRNA molecule.
Coding strand
AKA sense, template strand
Upstream versus downstream directions
Upstream
To the left
Toward the five prime end
Negative numbers
Downstream
To the right.
Towards the three prime end.
Positive numbers
Where does the Tata box normally fall
Around -25
Transcription continues until
It reaches either a termination sequence or stop signal.
What is the first result of transcription before it is converted into mRNA?
hnRNA after posttranscriptional modifications
What three processes transform hnRNA into mRNA?
1) Exons spliced out by splicesome
2) 5’ cap (7 methylguanate triphosphate) cap
3) Polyadenosyl tail added to 3’ end (time bomb)
Alternative splicing
Splicing together the hnRNA in different ways to produce multiple variants of proteins encoded by the same original gene. This allows for the genetic production of many more proteins from a limited number of genes.
How does mRNA exit the nucleus?
Through nuclear pores
When RNA is translated into proteins what direction is the mRNA read?
5’ to 3’
What are the three binding sites in the ribosome for a tRNA?
A site- aminoacyl
P site- peptidyl
E site- exit
The composition of prokaryotic versus eukaryotic ribosomes
Eukaryotic 40+60 = 80 S subunit.
Prokaryotic 30+50 = 70 S subunit.
Where does translation occur?
In the cytoplasm of the cell
What are the three stages of translation?
Initiation, elongation, termination.
What are the steps of initiation?
- The small ribosomal subunit binds to the mRNA in the 5’ untranslated region at the start codon methionine in the p site of the ribosome.
The large subunit and binds to the small subunit forming a complex.
The small subunits in the initiation phase binds to what specifically in prokaryotes versus eukaryotes?
Prokaryotes: Shine-Delgarno Sequence
Eukaryotes: 5’cap structure
What is the initial amino acid in prokaryotes versus eukaryotes
Eukaryotes: methionine
Prokaryotes: N-formylmethionine (fMet)
What are the steps of elongation?
Three step cycle that is repeated for each amino acid that is added to the protein after methionine.
Ribosomes move along the mRNA in the five prime to three prime direction synthesizing the Protien from N. to C terminus.
A- The a site holds the incoming amino acid that is being added to the growing chain.
P-The p site holds the tRNA that carries the growing polypeptide chain. It is also where the first amino acid methionine binds. Requires GTP to form peptide bonds.
E-The e site is where now inactivated or uncharged tRNA pauses before exiting the ribosome. It quickly unbinds from the mRNA and is ready to be recharged.
What enzyme uses GTP to form peptide bonds? What part of translation does this occur?
Peptidyl transferase
Elongation at the P site
Elongation Factors
Assist in recruiting aminoacyl tRNA and GTP for translation. Helps remove GDP once the energy has been used.
What are the steps of termination?
When any of the three stop codons move into the A site, a protein called release factor (RF) binds to the termination codon.
A water molecule is added to the polypeptide chain. This allows peptidyl transferase and termination factors to hydrolyze (cut) the completed polypeptide chain from the final tRNA.
The polypeptide chain is then released from the tRNA in the P site and the two ribosomal subunits dissociate.
What processes are included in post translational modifications?
1) Chaperones assist in the proper folding of proteins
2) Cleavage
3) Adding other biomolecules to the peptide via:
Phosphorylation (adding phosphate)
Carboxylation (adding carboxylic acid)
Glycosylation (adding oligosaccharides)
Prenylation (adding lipids)
Operon
Controls gene expression in prokaryotic cells.
Cluster of genes transcribed as a single mRNA.
Divided into operator, promoter and gene regulator.
Operator site
Upstream of the gene of interest (structural gene) non transcribable region of DNA capable of binding a repressor protein.
Promoter Site
Further upstream from gene of interest.
Provides a place RNA polymerase to bind.
Regulator gene
Furthest upstream.
Codes for protein known as the repressor.
What are the two types of operons?
Inducible and repressible
Inducible
Roadblock. OFF until TURNED ON.
Tightly binds to the operator system and acts as a roadblock.
RNA polymerase is unable to get from the promoter to the structural gene because the repressor is in the way.
To reverse: Inducer must bind to the repressor protein so that RNA polymerase can move down the gene.
Repressible System
Always ON they get TURNED OFF
Repressive is inactive until it binds to a corepressor. This complex then binds to the operator site to prove my further transcription.
Control of gene expression in eukaryotes
Transcription factors, gene amplification, enhancers
Transcription factor
Transcription activating proteins that search the dna for specific dna binding motifs.
Dna binding domain binds to specific nucleotide sequence in the promoter region or to a dna response element (sequence of dna in the enhancer region that only binds to specific transcription factors)
Enhancers
Response element grouped together are called enhancers. Controls one gene’s expression by multiple signals.
Can be up to 1000 base pairs away from the gene they regulate so they utilize the hairpin structure to make themselves closer in proximity.
Histone acetylation
Transcription factors recruit other coactivators like histone acetylases that are involved in chromatin remodeling.
Acetylation
Acetylation of histone proteins decrease the positive charge on lysine residues and weakens their interaction with dna resulting in a looser euchromatin confirmation easier for transcriptional machinery to access the DNA.
Deacetylation
By histone deacetylases that remove acrylic groups from histones that tighten dna to heterochromatic
DNA methylation
Involved with chromatin remodeling and regulation of gene expression.
Adds methyl groups to cytosine and adenine nucleotides soften linked with silencing of gene expression.
Heterochromatin heavily methylated.
