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Chapter 9: Molecular Biology of Translation (Ribosome Functions (Ribosome…
Chapter 9: Molecular Biology of Translation
Amino Acids-building blocks of proteins (monomer units)
All Amino Acids have a Carboxyl and Amino Function Group
R Group: decides that amino acid it will be
Backbone of Amino Acid: Amino and Carboxyl Group
Polypeptide: Polymer of Amino Acids joined by peptide bonds
Peptide Bonds: covalent bond between amino and carboxyl functional groups in Amino Acid
Codon: nucleotide triplet of mRNA single AA
4 Levels of Protein Structure
Primary Structure: sequence of amino acids in polypeptides
Bond: Peptide (covalent)
Secondary Protein Structure: formation of a-helices and B-pleated sheets
Bond: hydrogen bonds between backbone of Amino Acids
Tertiary Structure: all bonds types between the R groups of the amino acids
Quaternary: interactions between polypeptides. Not all proteins have this level of structure.
Bond: All types
Ribosome Functions
Ribosomes composed of rRNA + proteins
2 Subunits: large and small
Function: assemble polypeptide
Bind mRNA, line up with complementary tRNA anticodon
Ribosomes translate in the 5'-3' direction
Begin with Start Codoe (AUG) and finish with stop codon
Ribosome Tasks
all cells have ribosomes
Bind mRNA and identify start codon
Facilitate complementary base pairing mRNA codon and tRNA anticodon
Catalyze peptide bond formation between amino acids in the growing polypeptide chain
Translation Initiation
Small ribosomal unit binds near 5' end of mRNA, identifies the start condone
Initiator tRNA (tRNA carrying the first amino acid) binds
always (met). codon AUG in mRNA
Large Subunits joins, forming intact ribosome * Initiation factors (proteins)- present to help facilitate process
GTP energy Source
Translation Initiation in Bacteria
Step 1. Formation of pre initiation complex
1.Small subunit (30S) + IF3
2. 30S/IF2 binds mRNA at Shine-Delgarno Sequence *
Shine-Dalgarno Sequence: Sequence on mTNA that base pairs with 30S rRNA and helps position the small subunit and the P site
Step 2: Formation of 30S Initiation Complex
: 1. Initiator tRNa binds to start codon (UG), this becomes the P site. 2. IF2 GTP help fMET bind 3. IF1 binds to prevent 50S from attaching
Step 3. Ribosome Assembly
1. 50S subunit forming intact ribosome 2.More GTP is used 3.IF1,IF2,IF3 dissociate
Formation of 70S initiation complex
*Overall Translation Initiation Complexes for Bacteria
: 1. Preinitiation Complex- 30s, IF3, mRNA.... 2. 30S Initiation Complex: 30S, IF3, mRNA+initiator tRNA, IF1, IF2
Translation Initiation in Eukaryotes
Formation of Preinitiation Complex: 1. 40S (small subunit) + eIF1, eIF1A, eIF3 = Preinitiation complex
Formation Initiation Complex: 2. Preinitiation Complex + Initiator tRNA, eIF5, mRNA = formation of initiation complex
Attachment and Messenger RNA scanning: 3. Scanning: initiation complex scans for start codon... Can be more than 1 AUG, look for Kozak Sequence
Ribosome assembly and translation initiation: 4. Large Subunit attaches forming 80S functional ribosome
Translation Elongation:
1
. Recruitment of Charged tRNAs to A site
2
. Formation of peptide bond between amino acids
3
. Translocation of Ribosome in the 5'-3' direction along mRNA
Polypeptide Elongation in Bacteria:
1
. Open A sire for charged tRNA recruitment ( Charged tRNAs bound to EF-Tu and GTP)
2
. Charged tRNA-codon pairing at A site ( tRNA enters A site)
Polypeptide Elongation in Bacteria:
3
. GTP hydrolysis ( tRNA pairs mRNA, hydrolysis of GTP releases EF-Tu/GDP from tRNA
4
. Peptide Bond Formation ( peptidyl transferase catalyzes peptide bond formation between the amino acid at the P site and the newly recruited amino acid in the A sire,
Polypeptide Elongation in Prokaryotes: **
5
. Translocation ( Thee ribosome slides to the next codon. Places the tRNA that was in the A site. into P sire. Uncharged tRNA moves to E sire and exits.
6**. A site opens for Charged RNA (next charged tRNA enters A site)
3. Termination of Translation:
Release Factors
recognize stop codons UAG,UGA,UAA
GTP provides energy that causes the polypeptide release from the P fire when the RF enters the A site
Release of the polypeptide causes the ribosome to disassemble.