7.3: Translation

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  • Created by: sofiedb
  • Created on: 19-03-18 18:34
Translation
final step in conversion of genetic information to proteins - once a mature mRNA has been synthesised, the translation process can start the matching of amino acids to codons
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Genetic code discovery
Nirenberg & Matthaei cracked the genetic code in 1961
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Bioinformatics
concerns translation & analysis of genomic sequences
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Translation start
when mature mRNA binds to a small ribosomal subunit at mRNA binding site
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mRNA initiation codon
AUG - can be linked to initiator tRNA which always carries methionine (contains sulfur) - all proteins start with this amino acids
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Stages of translation
initiation, elongation, termination
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Initiation
mRNA, small ribosomal subunit, initiator tRNA & large ribosomal subunit assemble to form translation complex - when tRNA molecule binds to corresponding amino acid it forms an aminoacyl tRNA complex
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tRNA binding sites on ribosome
Exit (E) site, aminoacyl-tRNA (A) site, peptidyl-tRNA (P) site
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Exit (E) site
tRNA moves here after transferring its amino acid to growing polypeptide chain, ready to be released from ribosome
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Aminoacyl-tRNA (A) site
where the incoming tRNA with its attached amino acid binds
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Peptidyl-tRNA (P) site
where tRNA from A site moves to after forming peptide bond with growing polypeptide chain i.e. where tRNA holding the growing polypeptide chain is found
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Elongation step 1
new tRNA carrying an amino acid comes in & binds to A site
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Elongation step 2
new amino acid joined to existing polypeptide chain which is linked to last amino acid by peptide bond
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Elongation step 3
tRNA that was bound to polypeptide chain is ready to be recycled as bond with growing amino acid chain is broken
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Elongation step 4
Ribosome translocates tRNA from A site to P site shifting 'empty' tRNA to E site (can leave ribosome & be recycled)
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Elongation step 5
whole process repeats itself until ribosome reaches termination codons (UAG, UAA, UGA)
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Termination
once termination codon is reached, a release factor binds & causes the disassembly of the components of the translation complex - can be reused for another translation complex
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Ribosomes
proteins & ribosomal RNA molecules - have a small & large subunit with 3 binding sites for tRNA molecules
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tRNA
single-stranded RNA molecule that folds on itself to form a typical cloverleaf-shaped structure with double stranded regions & 3 hair pin loops
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Hair pin loops
one of loops contains a sequence called the anticodon - can decode & bind to mRNA codon
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Codons & amino acids - tRNA
each tRNA has corresponding amino acid attached to 3' end of CCA (5' to 3'), when tRNA recognises & binds to corresponding codon on ribosome the tRNA transfers appropriate amino acid to end of growing polypeptide
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t-RNA activating enzymes
aka. aminoacyl-tRNA synthetase - 20 different aminoacyl-tRNA synthesases corresponding to 20 different amino acids - each enzyme specific for an amino acid & corresponding tRNA molecule that has matching anticodon
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Hydrolysis of ATP molecule
required by enzyme - ATP+H2O -> AMP & PP (pyrophosphate)
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Enzyme attachment
enzyme attaches to 3' end of tRNA - once tRNA has attached amino acid, it is called a charged tRNA molecule & is used in initiation/elongation stages
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Bound ribosomes
proteins that function inside the ER, Golgi apparatus, lysosomes & plasma membrane that will be exported outside the cell are synthesised on the bound ribosomes attached to the ER - once mRNA attaches to ribosome, translation starts immediately
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Beginning of polypeptide
contains signal sequence which is recognised by signal recognition particle (SRP)
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signal recognition particle
SRP binds to SRP recognition protein on endoplasmic reticulum allowing polypeptide to enter rER as it grows in length - when translation ends & translation complex dissembles, the whole polypeptide is taken into ER
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location of post-translational modifications & actual sorting
lumen of ER
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Free (unbound/cytoplasmic) ribosomes
proteins translated on free ribosomes will function in mitochondria/chloroplasts/cytoplasm/nucleus of cell
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Eukaryotes translation initiation
once mRNA moves into cytoplasm from nucleus, ribosomes attach to it & translation begins
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Bacteria translation initiation
no separation between areas of cell where transcription & translation take place (no nuclear membrane) - once mRNA is produced (before molecule has been synthesised), small & large ribosomal subunits attach to beginning of mRNA & translation begins
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Bound ribosomes
synthesise proteins mainly meant for secretion or for use in lysosomes
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Free ribosomes
produce proteins for use mainly within the cell
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Polysome
structure formed when multiple ribosomes attach themselves to the same mRNA to allow production of polypeptides & hence proteins at faster rate - can occur in prokaryotes & in eukaryotes (bound/free ribosomes)
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Primary structure
sequence (order & identity) & number of amino acids in polypeptide - maintained by peptide bonds between subunits
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Secondary structure
formed when polypeptide chain folds back on itself into alpha helices or beta pleated sheets - stabilised by H-bonds between -NH groups (from peptide bonds) & -C--O groups on another peptide bond further along the same chain
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Tertiary structure
further folding of polypeptide, stabilised by interactions between R groups (single chain tertiary protein example is lysozyme)
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Possible interactions between R groups
ionic, covalent (e.g. disulphide bridges), H-bonds, non-specific hydrophobic interactions between non-polar amino acids which will fold polypeptide chain so that hydrophilic amino acids are located on outside of protein
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Quaternary structure
exists in proteins with more than one polypeptide chain where subunits are joined together by strong bonds similar to tertiary structure (often have non-protein molecular unit, co-factor or prosthetic group that is tightly attached to polypeptides)
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Example quaternary structure
haemoglobin - 2 alpha chains, 2 beta chains, 4 heme groups (heme is iron containing prosthetic group that helps protein transport oxygen)
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Conjugated protein
protein with prosthetic group attached
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Anticodons & codons on tRNA/mRNA
anti-codons on tRNA are complementary to codons on mRNA
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Key components of translation
1) mRNA has a sequence of codons that specifies the amino acid sequence of the polypeptide, 2) tRNA molecules carry the amino acid corresponding to their codon, 3) tRNA molecules have an anticodon of 3 bases that binds to complementary codon on mRNA
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Ribosomes
act as binding site for mRNA & tRNA, catalyse peptide bonds of polypeptide
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Process of translation: initiation steps
1) mRNA binds to small subunit of ribosome, 2) small subunit moves along mRNA molecule in 5' to 3' direction until it reaches start codon (AUG), 3) molecule of tRNA (methionine attached) complementary to start codon (UAC) binds to P site on ribosome
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Process of translation: initiation steps
4) large subunit of ribosome binds to tRNA & small subunit
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Process of translation: elongation steps
5) second tRNA (with amino acid attached) complementary to second codon on mRNA then binds to A site on ribosome, 6) amino acid carried by tRNA in P site is transferred to amino acid in A site due to ribosome catalysing new peptide bond -condensation
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Process of translation: elongation steps
growing polypeptide increases in length, 7) ribosome moves one codon along mRNA (in 5' - 3' direction) - tRNA in P site is moved to E site & released, tRNA in A site is moved into P site
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Process of translation: elongation steps
8) another tRNA binds, complementary to A site, 9) steps 6/7/8 are repeated until stop codon is reached
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Process of translation: termination steps
10) when stop codon is reached, translation is stopped - release factor attaches to A site, polypeptide chain, ribosome complex dissembles ready for reuse translating another mRNA molecule
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Translation in prokaryotes
ribosomes can be adjacent to the chromosomes whereas in eukaryotes the mRNA needs to be relocated from the nucleus to the cytoplasm (through the nuclear membrane)
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Translation in eukaryotes
the mRNA is modified e.g. spliced (processing) after transcription before translation (does not occur in prokaryotes)
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Polysome
structure that consists of multiple ribosomes attached to a single mRNA - multiple ribosomes translating mRNA simultaneously enables the cell to quickly create many copies of the required polypeptide
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Polysomes prokaryotes
chromosome may have numerous polysomes directly attached to it
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Polysomes eukaryotes
polysomes occur separately in the cytoplasm or on the ER - mRNA needs to exit the nucleus in order to be translated
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Nirenberg & Matthaei cracked the genetic code in 1961

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Bioinformatics

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Translation start

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Card 5

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mRNA initiation codon

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