Transcription + Translation Flashcards
What are tRNAs?
transfer RNAs are the intermediates between mRNA and amino acids
There should be 61 different tRNAs because of the genetic code, but why are there actually less?
because of the “Wobble Effect” where the pairing of the last position of the codon (3’ codon) is flexible and can still code for the same anticodon and amino acid
Explain the Wobble Hypothesis
the 3’ codon position of mRNA is flexible and can have a different base pair that tRNA can still recognize as coding for the same anticodon and amino acid
this makes up for the redundancy of the genetic code and allows there to be less different tRNAs than codons
How many subunits do ribosomes consist of? What are they?
2
large 50S subunit
small 30S subunit
When do the large and small subunits join together to form a functional ribosome?
when they are both attached to mRNA
What is the function of a ribosome?
to facilitate the coupling of tRNA anticodons with mRNA codons during protein synthesis
Where does mRNA bind to the ribosome?
in the small 30S subunit
What are the 3 tRNA binding sites on a ribosome?
E (Exit)
P (peptide bond formation)
A (aminoacyl-tRNA binding site)
What are the 3 tRNA binding sites on a ribosome?
E (Exit)
P (peptide bond formation)
A (aminoacyl-tRNA binding site)
Where do incoming tRNAs bind to the ribosome?
at the A site
Where is the growing peptide chain bound on the ribosome?
to a tRNA bound to the P site of the ribosome
WHere do empty tRNAs bind briefly before exiting the ribosome?
to the E site
What is the purpose of the decoding center of a ribosome?
it ensures only tRNA with the correct anticodon can enter A site
What catalyzes the formation of peptide bonds?
peptidyl transferase center
What are both the peptidyl transferase center and the decoding center made of?
rRNA
What are the 3 phases of translation?
initiation
elongation
termination
What is the main purpose of translation initiation in prokaryotes?
to place the initiator-tRNA in the P site of ribosome and establish the correct reading frame
How does the ribosome place the correct AUG start codon in the P site in prokaryotes?
the Shine-Dalgarno sequence in the 5’ UTR mRNA pairs with a sequence in the small ribosomal subunit aligns the initiator AUG codon in the P site
T or F: transcription and translation co-occur in prokaryotes
true
Where does the initiation complex form on the mRNA of prokaryotes?
near the 5’ end of mRNA that is still being transcribed
T or F: in eukaryotes, transcription and translation co-occur
false! they are spatially and temporally separated
Why does transcription happen separately from translation in eukaryotes?
because mRNA needs processing after transcription before it can be moved to the nucleus and recognized for translation
Explain translation initiation in eukaryotes?
initiation factors bind to the 5’ cap of mRNA
small ribosomal subunit and initiator tRNA form an initiation complex
complex moves 5’-3’ along mRNA while unwinding base-paired regions and the ribosome scans the mRNA for the exposed AUG codon
AUG codon aligns with initiator tRNA
large subunit binds to form complex ribosome
What 3 steps are involved in elongation?
codon recognition
peptide bond formation
translocation
What must happen in order for a tRNA to enter the ribosome?
an elongation factor binds to the tRNA outside of the ribosome
the tRNA is checked by the decoding center of the small subunit to ensure the correct anticodon is being brought in
the ribosome will change conformation and the EF will be released when the correct match is made
What happens when the tRNA bonds to the A site?
the aminoacyl from the tRNA at the P site is now very close to the aminoacyl-tRNA at the A site
methionine is transferred from the P site to the amino acid in the A site and a peptide bond forms
What happens after an amino acid is transferred from the P site to the A site and a peptide bond forms?
an EF enters A site to shift the tRNA in the A site to the P site and the tRNA from the P site into the E site as the ribosome moves one codon (3 nucleotides) towards the 3’ end of the mRNA
after the move, the EF leaves the ribosome and the A site is now open to accept another charged tRNA and cycle continues until stop codon is reached
How does termination occur?
elongation continues until a stop codon is reached
What happens when a stop codon is reached? aka describe termination
once a stop codon enters the A site, no more tRNA can enter
then release factors bind to the stop codon and hydrolyze the bond between the last amino acid and the tRNA to release the polypeptide
T or F: the genetic code is universal and all species use the same one
true
What is the purpose of the genetic code?
it allows sequences of nucleotides to be translated into sequences of amino acids
How do 4 nucleotides specify the 20 amino acids?
3 nucleotides make a codon in mRNA which specifies 1 amino acid
What is a codon?
a sequence of 3 nucleotides in mRNA which codes for an amino acid
How many different codons are possible? How many of these codons specify one of the 20 amino acids?
4^3 = 64 codons
61 code for one of 20 the amino acids
the other 3 are stop codons
Why is the genetic code redundant?
because there are 64 possible codons, of which 61 code for only one of 20 amino acids
Which direction is mRNA read in?
5’-3’ as a series of nonoverlapping codons
What ensures translation begins in the correct reading frame?
the start codon
Describe the structure of an RNA nucleotide
there are 4: Uracil Adenine Guanine Cytosine
describe the structure of an RNA polymer
a nucleic acid composed of a ribose sugar, nucleotide bases (A, U, G, C) and 1+ phosphate groups
sugar-phosphate backbone and RNA strands assembled by phosphodiester bonds between adjacent nucleotides
What is the difference between informational (aka coding) RNA and functional RNA?
informational: coding RNA that is translated into protein (aka mRNA)
functional: RNA that do not code for proteins and have other functional roles in the cell (ex. tRNA, rRNA, etc.)
What is mRNA and its function?
messenger RNA
coding RNA that is made from protein producing genes and undergoes translation
What is rRNA and its function?
ribosomal RNA combines with proteins from the small and large ribosomal subunits to make a ribosome for translation
What is tRNA and its function?
transfer RNA carries amino acids from the cytoplasm to the ribosome where they can bind to mRNA by complementary base pairing and elongate polypeptides
What is telomerase RNA and its function?
functional RNA that provides a template for synthesis of repetitive DNA telomere sequences
What is snRNA and its function?
small nuclear RNA joins with proteins to form spliceosome subunits which remove introns from pre-mRNA during mRNA processing in eukaryotes