Protein Synthesis
Making proteins from genes One gene-one enzyme
hypothesis (Beadle and Tatum) Later revised to the one
gene-one polypeptide hypothesis
Transcription synthesis of RNA under
the direction of DNA (mRNA)
Translation synthesis of a polypeptide
under the direction of mRNA
DNARNAProtein ‘U’ (uracil) replaces ‘T’ in
RNA The genetic instructions
for a polypeptide chain are ‘written’ in the DNA as a series of 3-nucleotide ‘words’ Triplet code/codons
More than one triplet for each Amino Acid
The Genetic Code
Initiation of Transcription Transcription factors
attach to TATA box Contains sequences of
only adenine and thymine
RNA polymerase attaches to begin transcription
Elongation and Termination Elongation
RNA polymerase continues unwinding DNA and adding nucleotides to the 3’ end
Termination RNA polymerase reaches
terminator sequence Polyadenylation signal--
AAUAAA
Transcription Helpers RNA polymerase: pries DNA apart and hooks RNA
nucleotides together from the DNA code Promoter region on DNA: where RNA polymerase
attaches and where initiation of RNA begins Terminator region: sequence that signals the end of
transcription Transcription unit: stretch of DNA transcribed into an
RNA molecule
mRNA modification 5’ cap: modified guanine molecule
Protection recognition site for ribosomes
3’ tail: poly (A) tail (up to 250 adenine nucleotides) Protection Recognition transport
RNA Splicing Exons (expressed
sequences) kept Introns (intervening
sequences) spliced out Splicing done by snRNPs
which bind together to form a large compound known as a spliceosome
Made of snRNA RNA that acts as an
enzyme= ribozyme
Basics of Translation mRNA moved
through ribosome “code” read in
triplets tRNA transfers
correct amino acid to the polypeptide chain
tRNA Has an anticodon (nucleotide
triplet) Codes for a specific amino
acid
Picking the right Amino Acid Aminoacyl-tRNA synthetase
Joins a specific amino acid to its tRNA molecule
Wobble Relaxation in the base-
pairing rules Third nucleotide in a codon
can be different in many cases and still code for the same amino acid
Ribosomes rRNA (ribosomal RNA)
site of mRNA codon & tRNA anticodon coupling
P site holds the tRNA carrying the
growing polypeptide chain
A site holds the tRNA carrying the
next amino acid to be added to the chain
E site discharged tRNA’s leave the
ribosome
Initiation of Translation Union of mRNA, tRNA, and small ribosomal
subunit Start codon = AUG (codes for methionine)
Large subunit then attaches
Elongation Codon recognition
Anticodon of tRNA binds with the complementary mRNA codon in the A site
Peptide bond formation Large subunit forms a
peptide bond between the amino acid in the A site and the growing polypeptide
Translocation tRNA in A site moves to P
site… the one in the P site moves to the E site
Termination When a stop codon (UAG, UAA, or UGA) is
reached, the A site accepts a “release factor”
Release factor cuts the polypeptide free from the tRNA molecule
Ribosome dissociates
Polyribosomes
Translation of mRNA by many ribosomes many copies of a
polypeptide very quickly
Where does it happen? Protein always begins in the cytosol
Will continue there unless it receives a signal peptide telling it to bind to the ER
SRP’s (signal-recognition particles) Binds to a ribosome and brings it to the membrane of
the ER Attaches to a receptor molecule
Roles of RNA
MutationsPoint mutation- The change
of just one nucleotide Base-pair substitution
One nucleotide substituted for another
Sometimes a “silent mutation” if it still codes for the same amino acid
Missense mutation- still codes for an amino acid, but not the right one
Nonsense mutation- changes to a stop codon and terminates translation
Frame-shift Mutations Insertion
Nucleotide is added Deletion
Nucleotide is lost Causes the codons from that
point on to be incorrectly grouped
Mutagen- anything that causes a mutation (chemicals, X-rays, carcinogens, etc.)
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