Chapter 22 Flashcards
what kinds of RNA are involved in translation
all three!
how are the different kinds of RNA used in translation
mRNA is read; tRNA brings in amino acids; rRNA makes the ribosomes
what serves as the site of translation
ribosomses
what does it mean that the genetic code is degenerate or redundant
a single amino acid can be specified by more than one triplet
what does it mean for the genetic code to be non-overlapping
the reading frame advances 3 bases at a time and the frame does not change
codon on mRNA
triplet of nucleotides that serves as a coding unit for amino acids, start signal, or stop signal
nearly universal genetic code
except for small exceptions in mitochondria and bacteria
start codon and what does it code for
AUG; methionine
Stop codons and what they code for
UAA, UAG, UGA; don’t code for anything
aminoacyl-tRNA synthetases
attach amino acids to tRNAs
what are ribosomes composed of
rRNA and protein
function of rRNA in ribosomes
reads the mRNA
function of protein in ribosomes
facilitate protein synthesis
3 basic steps of translation
initiation, elongation, termination
initiation
3 kinds of RNA come together and first amino acid is placed
elongation
successive addition of amino acids by a 3 step process. starts as soon as 2nd tRNA binds to 2nd codon
direction of protein synthesis
amino acids are added starting at N-terminus and proceeding towards C-terminus
3 differences of prokaryotic ribosomes
smaller in size, contain fewer proteins, smaller RNA molecule
characteristics of all ribosomes
large and small subunits that bind only on the mRNA
what are the 4 important ribosome binding sites
mRNA binding site, A site, P site, E site
mRNA binding site
in between large and small subunits; place where mRNA binds
A site
(aminoacyl); binds newly arrived tRNA
P site
(peptide); site of growing polypeptide chain
E site
(exit); site where the empty tRNA is moved prior to release
structure of transfer RNA
3 major loops, 4 base paired regions, anticodon triplet, 3’ terminal sequence (CCA)
aminocylation
attachment of correct amino acid to CCA sequence
when is tRNA in charged form
when amino acid is activated
anticodon triplet
recognizes the appropriate codon on the mRNA and base pairs with codon on mRNA
wobble hypothesis
often the 3rd base on the codon can pair outside the normal rules
Inosine
tRNA’s unusual base that can pair with U,C, or A; reason for “wobble”
wobble hypothesis seen with codons for…
proline, serine, phenylaline
messenger RNA contains
start codon, stop codon, and untranslated areas
start codon
starting point for protein synthesis; establishes reading frame
stop codon
instructs ribosome to terminate protein syn
untranslated areas
5’ UTR and 3’ UTR (everything outside of start and stop codons);
essential in protein syn and seen in euk and prok
coding sequence or open reading frame
everything in between start and stop codons
N-Formylmethionine (tRNA^fMET)
modified amino acid that initiates polypeptide syn in proks and establishes directionality
how does N-Formylmethionin work
formyl group binds to amino group and blocks it from peptide bonding so only carboxyl group can bond; this establishes directionality of protein syn N to C
3 steps of initiation of translation in prok
- initiation factor binds to 30s subunit with GTP
- mRNA with 1st tRNA (fMET) binds to 30s subunit
- 50s subunit joins 30s initiation complex by hydrolysis of GTP
Shinge-Delgarno site on mRNA
positions the 30s subunit in prok
what is the purpose of these steps in this order
makes it easier to get mRNA inside ribosomes
what happens after teh 50s and 30s subunits join
initiation factors are released and the ribosome is ready
initiation of translation in euks
a lot more initiation factors and tRNA carries tRNA (not formulated)
3 steps of polypeptide chain elongation in proks
- binding of aminoacyl tRNA to ribosome
- peptide bond formation
- translocation-mRNA advanced to next codon
peptide bond formation caused by
peptidyl transferase activity of the 23s rRNA molecule
termination of translation
stop codon triggers release of the polypeptide; accompanied by GTP hydrolysis, the ribosomal complex comes apart and mRNA and ribosomes are available for reuse
how is the stop codon recoginized
not recognized by tRNA; recognized by proteins that mimic the appearance of tRNA called release factors
Polyribosome (polysomes)
cluster of 2 or more ribosomes simultaneously translating a single mRNA
Molecular chaperones
facilitate protein folding inside cells, rescue or destroy improperly folded proteins, and prevent polypeptide from interacting they’re in proper conformation
silent mutation
doesn’t change amino acid sequence
missense mutation
alters single amino acid (worst is to change hydrophilic and hydrophobic ones)
frameshift mutation
alter the reading frame (insertions or deletions)
indels
insertions or deletions
nonsense mutations
premature stop codon (substitutions, insertions, deletions)
3 major destinations for proteins
cytosol, other organelles, endomembrane system
importance of protein targeting and sorting
proteins must be in the right spot for their function
protein targeting to cytosol
protein is synthesized in cytosol and released
protein targeting to other organelles
protein is syn in cytosol and released then taken in by posttranslational import
what is necessary for the protein to have for posttranslational import
targeting signal
cotranslational import
mRNA associates with free ribosomes which then associate with ER; ribosomes attach to ER shortly after protein syn begins and as protein is syn, it crossed the membrane
protein targeting to endomembrane systerm
cotranslational import
