chapter 9 part 2 Flashcards
how many ribosomes does the bacterial cell contain
20,000 - 25% of mass of cell
polyribosomes
groups of ribosomes all actively translating the same mRNA
- bacteria
benefit of polyribosomes
- speed, accuracy, and efficiency of polypeptide production
coupling of transcription and translation in bacteria
allows ribosomes to begin translating mRNAs that have not yet been completed
where are mRNAs produced in eukaryotes
the nucleus - processed to form mature mRNAs and then exported to cytoplasm for translation
polypeptide-producing genes in eukaryotes
monocistronic
polypeptide producing genes in prokaryotes
polycistronic
monocistronic mRNA
RNA that directs synthesis of single kind of polypeptide
polycistronic mRNA
lead to synthesis of several different proteins
what are groups of bacterial genes called
operons - share single promoter and produce polycistornic mRNAs
composition of polycistronic mRNAs
- start and stop codon
- Shine-Delgarno sequence (most, in bacterial and all but leaderless archaeal mRNAs)
- intercistronic spacer sequence
intercistronic spacer sequence
separates segments on polycistronic gene
- not translated
genetic code
correspondence between nucleotide sequences of mRNAs and the amino acid sequences of the resulting polypeptides
transfer RNAs
adaptor molecules that interpret and then act on information carried in mRNA
- have anticodon sequences complementary to mRNA codons
codons
groups of 3 consecutive nucleotides in an mRNA that each correspond to 1 amino acid
how many different codons in genetic code
64
64 codons but only 20 amino acids =
redundancy
synonymous codons
code for same amino acid
which amino acids have at least 2 codons?
all except methionine (start) and tryptophan
how many different tRNA genes in genomes
30-50 - but have 61 different codons
iso-accepting tRNAs
tRNA molecules with different anticodons that carry the same amino acid
how are most synonymous codons grouped
so that they differ only in the 3rd base
- both carry purine
- both carry pyrimidine
third-base wobble
creates flexible pairing at 3’ nucleotide of codon
what can bacteria be used for because the genetic code is universal?
producing important proteins from plants and animals
aminoacyl-tRNA synthetase (tRNA synthetase)
enzymes that catalyzes addition of correct amino acid to tRNAs
tRNA synthetase
large molecule that contacts several points on the tRNA in the recognition process
acceptor stem
in correct tRNA, fits into active site of enzyme
what provides energy for amino acid attachment
ATP
1957 - Sidney Brenner discovery
an overlapping code was not possible because it was too restrictive
1960 - Fraenkel-Conrat and colleagues discovery
single nucleotide changes led to single amino acid changes
where did proof of a triplet genetic code come from
1961- Crcik, Barnett, Brenner, and Watts-Tobin created mutations by insertion/deletion of single nucleotides in rII gene in T4 bacteriophage
what happens when an insertion/deletion occurs
change in reading frame of mRNA
reading frame
specific codon sequence as determined by the start codon
frameshift mutations
mutations that alter the reading frame and garble the sense of the translated message
1961 - Nirenberg and Matthai
- strings of synthetically created repeating nucleotides were translated in vitro and resulting polypeptide identified
- mRNA containing only oracles (poly(U)), resulted in polypeptides containing only phenylalanine (UUU = Phe)
Nirenberg and Leder
- used mini-RNAs just 3 nut long to resolve ambiguities of deciphering genetic code experiment
- tested all 64 possible codons
- identified 61 codon-amino acid association
- identified 3 stop codons
mini-RNA experiment
- mini-RNAs added to an in-vitro translation system
- each system contained all unlabeled amino acids, w/ one aa labeled w/ radioactive carbon (C14)
- isolated ribosome-tRNA-mRNA complexes and determined which mini-RNA was associated w/ each labeled amino acid
the production of functional proteins is not complete until…
polypeptides folded into their tertiary or quaternary structures
2 categories of post-translational events
- post-translational polypeptide processing - modifies polyp. into functional proteins by removal/chemical alteration of aa
- protein sorting - uses signal (leader) sequences to direct proteins to cellular destinations
types of post-translational processing
- removal of amino acids
- modification of amino acids
- cleavage of polypeptides
ex. of removal of amino acids from polypeptide
fMet not found in functional bacterial proteins, nor is methionivon always first amino acid in euk. proteins
- removed after translation
one of most common amino acid modifications
phosphorylation
phosphorylation
- kinases (enzymes) add phosphate groups to proteins
- can activate or inactivate a protein
functional groups that can be added to amino acids
- phosphates (kinase)
- methyl (methylase)
- hydroxyl (hydroxylase)
- acetyl (actylase)
carbohydrate side chains
can also be added to some proteins during modification of amino acids
why are some polypeptides cleaved
- separate functions
- aggregate to form functional protein
ex. of polypeptide cleavage
- insulin first produced as preproinsulin
- pre-amino segment at N-terminus cleaved to produce proinsulin
- proinsulin forms disulfide bonds and is cleaved again to produce insulin
insulin
functional protein consisting of A-chain and B-chain segments
signal sequences
15-20 amino acids at N-terminal end that directs proteins to cellular destinations
signal hypothesis
proposes that first 15-20 amino acids of many polypeptides contain an “address label”
Blobel
suggested that the signal sequence directs proteins to the ER and then Golgi, where they are sorted for specific destinations
what do proteins destined for the ER typically have
N-terminal signal sequences that direct forming polypeptide into ER lumen or membrane
rough ER proteins
polypeptides destined for secretion
ER and protein sortin
- polypeptides w/ signal sequence have N-terminal pushed into cisternal space through receptors on ER surface
- once inside, polypeptides have signal sequence removed, are glycosylated, and packaged into vesicle for transport to Golgi
- cotranslation of proteins into ER lumen or deposited within membrane itself as transmembrane proteins
how do some diseases result
when large amounts of mutant proteins accumulate
what are the diseases or incorrect protein folding called?
conformational diseases
- can be neurodegenerative or dementias
ex. of diseases of incorrect protein folding
- Alzheimer’s
- Parkinsons’
- Huntington’s
6th base
inosine
- purine
