Translation
mRNA read by cellular components and specific primary AA sequence assembled
translation requires
proteins, tRNA, synthetase enzymes, rRNA
proteins involved in translation
release factors, ribosomal proteins
cellular components of translation
- intiation, elongation, and release factor proteins
- trna
- synthetase enzymes: facilitate matching of trna and aas
- rrna
ribosomal proteins (make up site of translation, ribosomal sub units
tRNA function
tranfer amino acids from cytoplasmically situated amino acids to growing polypeptide chain in ribosome
why are there different types of tRNA
- each trna translates a specific mrna codon into a specific amino acid
what is the structure of trna
- single stranded, 70-90 nucleotides long
- lots of streaks of complimentary nucleotide bases within one strand
-trna forms h bonds with itself - 4 double helical segments, 3 characteristic loops, clover shape in 2d
- trna folds upon itself into L shape in 3d
explain anticodon, codon pairing
trna anticodons (3’ to 5’) are complimentary base pairs to mrna codons (5’ to 3’)
- 3’ end of anticodon pairs with 5’ end of codon
How does a trna recruit an amino acid
3’ end has protuding amino acid site consisting of a single strand of 3 nucleotides: CCA the adenine/termnial A is where the aa attaches
What is it called when an aa attaches to the a terminal of a trna molecule? What carries out this process?
trna molecule activation
aminoacyl trna snythetases
How do aminoacyl tRNA synthetases operate?
- each aminoacyl tRNA synthetase recognizes a specific anticodon and specific region of the amino acid attachment site
- there are 20 aminoacyl tRNA synthetases, one per aa
- each tRNA synthetase catalyzes the covalent bond between tRNA and aa using ATP from ATP hydrolysis
- creates charged/aminoacyl tRNA which is released from the enzyme and can now deliver the amino acid to the growing polypeptide chain
what is an aug codon and why is it significant
- codes for aa methionine, signals protein translation machinery to begin translating at that location
How many tRNA molecules are there?
- 45 tRNA molecules, meaning some may be able to bind to more than one codon, as there are 64 codons
What is a wobble?
- trna anticodons ability to bind to more than one codon as there is a flexibility between the third codon and complementary anticodon base pairings
-helps explain redundancy of genetic code
process of initiation in eukaryotic translation
translation initiation complex forms towards 5’ cap of mrna and reads mrna until aug start codon is encountered
How does initiation of translation differ in prokaryotes?
- translation initiation complex binds at one or more ribosome binding sites called Shine-Dalgarno sequences a few base pairs upstream of the aug start codon
- polycistronic mrna sequences in prokaryotes allow translation to occur along multiple regions simultaneously, having open reading frames for different proteins translated along a single mrna strand
- prokaryotes have related genes grouped together along a single mrna strand which are transcribed as one unit from the promoter
what components must assemble for translation to begin?
- two ribosomal subunits
- mrna
- charged tRNA methionine
- initiation factors that assemble initiation complex
Steps of formation of initiation complex
- initiation factors bind to 5’ cap of mrna
- small ribosomal subunit recruited
- initiation factors bind to charged trna methionine
- partially assembled initiation complex moves in 5’ to 3’ direction until aug codon is encountered
- large ribosomal sub unit then binds, using energy of GTP hydrolysis
- next charged trna molecule can join the ribosome
7.initation factors released once ribosomal translational complex forms
describe how the polypeptide sequence is synthesized
- first aa, methionine, located in P (peptydal) site, all subsequent trnas will bind to a (aminoacyl) site (before being added to polypeptide sequence
-incoming charged trna bound to gtp elongation factor
-after correct codon anticodon pairing, gtp hydrolyzed and aa released from elongation factor - conformational change induced by ribosomal rna once charged trna binds causing peptidyl tranferase reaction
- peptide bond forms (condensation reaction)
- polypeptide sequence transferred onto trna in a site
- gtp elongation factors and peptide bond that formed between aas move deacylated trna from p site to e site.
- new aminoacyl trna enters a site and allows for release of trna in e site
how is translation terminated?
- gtp bound release factors bind to a site and synthesize hydrolysis reaction between terminal aa and trna in p site
- gtp hydrolysis causes dissociation of translation complex
Beadle and tatum vs Srb and Horowitz
B and T: establish relationship between gene and enzymes (1940s):
neurospora can grow on minimal medium and therefor must have genes that code for enzymes which synthesize simple supplements into aas and vitamins needed to grow.
S and H: genetic screen of radiation treated neurospora to determine if there were specific genes coding for each of the three enzymes involved in the cascade of arginine synthesis
Summarize Srb and Horowitz findings
- mutated neurospora on no supplement medium: no growth = gene mutations in essential enzymes
- ornithine or citrulline supplemented medium: no growth
- 3 mutations of arg1, arg2, and arg 3 for enzyme 1,2,3 identified
Findings after Beadle, Tatum, Srb, Horowitz
- a gene dictates structures of all proteins in general: “one gene one polypeptide”
- human genome sequenced: 20,000-25,000 protein coding genes, whereas a larger proteome, means one gene can code for more than one protein (post transcription and post translation mods): “one gene one …”
What is the role of GTP in translation
GTP dependant factors synthesize THE polypeptide chain
