lecture 16 Flashcards
(21 cards)
what are the features of aa?
how are they joined?
alpha carbon, NH3 group, carboxyl group
-peptide bond between carboxyl group of one + NH3 group of another
how many aa are non polar, polar, basic and acidic?
np: 10–can form h bonds
p: 5–can participate in forming H bonds
acidic: 2
basic: 3
where and how does polypeptide assembly occur?
what is the N and C terminal?
in ribosome
-ribosomes bind mRNA and allow for mRNA codon pairing with tRNA anticodon, which determines what aa is added to the poly peptide
- in 5—> 3 direction
N is at 5’ end of mRNA
C is at 3’ end of mRNA
what 2 segments of mRNA that do not undergo translation?
5 UTR –helps initiate translation
3 UTR—–termination
what are the 4 levels of polypeptide structure?
primary— sequence, via peptide bonds
secondary— a helices and b sheet, via H bonds
tertiary–3d shape of polypeptide
quaternary— shape produced by combinations of polypeptides (2 or more)
what are the 3 essential tasks that ribosomes do in all domains of life?
what type of enzyme is the ribosome?
bind mRNA and identify start codon
allow base pairing of mRNA codons + tRNA anticodons
catalyze peptide bond formation between aa-
is a ribozyme, performed via rRNA
what are the 3 regions that the large and small subunits of the ribosome make up?
what does each one specifically have?
P site: holds tRNA where polypeptide is attached
A site: binds a new tRNA carrying the next aa to be added to the polypeptide
E site: where tRNA exit after their aa has been added to the polypeptide
—-small subunit has a place for initial binding of mRNA, scans mRNA to find start then binds
—large has peptide transferase activity – catalyze peptide bonds
what is the structure of the eukaryotic ribosome?
prokaryotic?
80s ribsome
60s subunit: 28s, 5.8s, 5s rRNA—49 proteins
40s: 18s rRNA–33 proteins
prokaryotic: 70s
50s: 23s, 5s– 31 ribosomal proteins
30s: 16s–21 ribosomal proteins
what is aminoacyl tRNA snythetase?
enzyme recognizes both aa and tRNA, 20 of them so specific for each aa
-catalyzes bond between carboxyl of aa and 3’ OH of Adenine
how is translation initiated in prokaryotes?
1) 30s small subunit + IF3 binds–> looks for start codon
2) once found, preinitiation complex forms (16srRNA, 30s, mRNA) —>then binds to Shine Dalgarno sequence (purine rich) upstream of the start codon
3) initiator tRNA + 50s large subunit binds to start codon, remains in P site–> IF is released
how does translation initiation in eukaryotes work?
1) preintiation complex 40s small subunit + eIF forms
2) initiation complex forms when small subunit binds 5’ cap—>then looks for start codon
3) start codon is found, 60s subunit is recruited + initiator tRNA binds–> eIFs are released
what does it mean that prokaryotes are polycistronic and eukaryotes are monocistronic?
polycistronic: multiple Shino dalgarno sequences–> many translatin initiation sites, so multiple peptides
monocistronic: 5’ cap is the only ribosome entry site, so one peptide
how does translation elongation work, step 1 ,2, 3?
1) entry of second tRNA into the A site helped by Ef (elongation factors)
- fMet is already in the P site
2) peptide bond is formed between Met + new aa–> met is released from P site ( uncharged) and the dipeptide is in A site
3) the dipeptide is moved into the P site & the uncharged tRNA from the P site was moved to the E site to leave–> new peptide is ready to come in the A site
how does translation termination work?
1) stop codon enter the A site of the ribosome ( they have no tRNA)
2) since stop codons have no tRNA, RF bind the stop codon in the A site
3) RF allow for the polypeptide to be released from tRNA in the P site
4) tRNA and mRNA separate from ribosome AND ribosome disassembles into small & large subunit
what allows for bacteria to have coupled transcription and translation?
why are both of these processes not coupled in. eukaryotes?
no nucleus, so happen in the same general area
-transcription is in the nucleus , where the transcript is processed
-translation is in the cytoplasm after mature mRNA is capped
what are features of the genetic code?
no overlaps
almost universal-except in ciliates & in mitochondria
triplet code
degenerate— more than one codon can specifiy a specific amino acid
3 stop codons + 1 start codon
what are isoaccepting tRNAs?
tRNA with different anti codon sequences that carry the same codon
this is why there are 64 codons that only code for 20 aa
what is codon bias and 3rd base wobble theory?
it shows the frequency of synonymous codons
-flexible base paring on the 3’ end of codon and 5’ end of anticodon
the production of functional proteins is not complete until?
what is signal hypothesis?
polypeptides are folder into their tertiary or quatanary structure
removal of 1 or more aa from the polypeptide is the most common form of postransltional polypeptide processing –> ex is fMet, is not functional!
—15-20 aa at the N terminus tells the polypeptide where to be transported, usually to the ER
how did Crick and Brenner prove that the genetic code is triplet?
–used proflavin to create FCO mutations in rII via inserting or deleting base pairs
used insertions
how did Nirenberg, Matthaei and Khorana assign codons to aa?
made artificial mRNA using U to decipher the genetic code
devised in virto
