Lec 4 Outline - Gene Expression - A01-2024 PDF

Expression of genetic information (Part 2) Relevant reading: Morris text, 4th ed., Chapter 5  Tertiary structure of a protein is its three- TERTIARY dimensional shape, usually made of several STRUCTURE secondary structure elements. determined by the s...

Expression of genetic information (Part 2) Relevant reading: Morris text, 4th ed., Chapter 5  Tertiary structure of a protein is its three- TERTIARY dimensional shape, usually made of several STRUCTURE secondary structure elements. determined by the spatial distribution of hydrophilic and hydrophobic R groups along the molecule as well as by chemical bonds and interactions that form between the R groups.  QUATERNARY STRUCTURE Many proteins are composed of multiple polypetides that come together as subunits of a functional, large protein. Subunits assembled together create the quaternary structure.  α helix β sheet Primary structure à Shape Shape = Function  SHAPE = FUNCTION Chaperones are proteins that bind newly made proteins and help them fold http://fastbleep.com/biology-notes/31/172/980  Protein function can be altered by the environment of the protein. Protein chaperones can help protect against denaturation.  α helix β sheet Shape = Function Primary structure à Shape How is the sequence of amino acids specified? TRANSLATION COMPONENTS FOR TRANSLATION  Ribosomes Ribosomes are made up of proteins and ribosomal RNAs. The ribosome moves down the mRNA from 5′ to 3′ and reads individual codons to incorporate the appropriate amino acids.  Three Functional Sites of the Ribosome There are three functional sites within the ribosome. The A site accepts the aminoacyl tRNA. The P site is where peptide bond formation occurs. The E site is where the tRNA exits the ribosome. -3 nucleotide reads along the mRNA are called codons -Translation begins at the start codon  INITIATION IN PROKARYOTES Polycistronic mRNA The initiation complex can be formed at multiple internal sequences where a Shine–Dalgarno sequence is found. Elongation and termination are similar to processes seen in eukaryotes. Initiation in INITIATION IN eukaryotes begins at EUKARYOTES the 5' cap, and the first AUG is the start codon. Monocistronic mRNA AUG codon Stop codon 5' cap 3' Peptide  Each tRNA has the nucleotide TRANSFER RNA CCA at its 3’ end, and the 3’ hydroxyl of the A is the attachment site for the amino acid. tRNA Synthetases  Codon– Anticodon Interactions  Most codons specify an amino acid according to a genetic code. Many amino acids are specified by more than one codon  Process of Translation The process of translation has three stages: 1. initiation 2. elongation 3. termination Initiation in Eukaryotes -bind to and begin at the 5’ cap -next, a tRNA complementary to the next codon binds to the A site.  Elongation: Peptide Bond Formation Note: Both the RNA and protein components of the large subunit are required for peptide bond formation. Ribosome Movement Elongation  TERMINATION When a stop codon is encountered, a release factor enters the A site. The polypeptide is released from the tRNA in the P site, and the ribosome disassociates.  PROTEIN FAMILIES Many proteins are similar enough in sequence and structure that they can be grouped together in a protein family Many members of a protein family share small regions of 3-D structure = folding domains A folding domain in a protein folds in a similar way independent of the rest of the protein.  PROTEIN FOLDING DOMAINS Folding domains can be present in different combinations to provide for different functions among different proteins. VISUAL SYNTHESIS 1: Gene Expression (Part 1) VISUAL SYNTHESIS 1: Gene Expression (Part 2) VISUAL SYNTHESIS 1: Gene Expression (Part 3) VISUAL SYNTHESIS 1: Gene Expression (Part 4) VISUAL SYNTHESIS 1: Gene Expression  Protein Sorting All mRNA is bound in the cytosol by the ribosome. Translation begins in the cytosol. The first few amino acids incorporated into the polypeptide dictate where it will be transported within the cell after translation is complete. Chapter 5 Active Lecture Slide 53 From Module 2… Proteins destined for the endomembrane system Proteins synthesized by ribosomes on the RER end up within the lumen of the endomembrane system, embedded in its membrane, or secreted out of the cell.  Translation at the RER Some ribosomes move from the cytosol to the RER membrane to translate proteins that are destined for the secretory pathway – Signal recognition particle (SRP) mediated targeting and translocation FIG. 5.23 Interaction of a signal sequence, signal-recognition particle (SRP), and SRP receptor.  Translation at the RER – Polypeptide first translated on free cytosolic ribosomes – SRP then recognizes the N-terminal signal sequence – Translation stalls and resumes once ribosome docks at RER translocon – Polypeptide translated into ER lumen Cap-dependent and cap-independent translation  Cap-dependent translation of most host proteins in the cell Ribosomes and translation initiation factors start scanning from the cap to find the AUG, where protein http://www.proprofs.com/flashcards/upload/q7224748.jpg synthesis begins  Poliovirus protein chops up the eIF- 4G protein needed for this scanning from the cap =No translation of capped mRNAs =No more host cell proteins http://www.proprofs.com/flashcards/upload/q7224748.jpg How about the Poliovirus mRNAs? How are they translated then? They are not capped! They use an internal ribosome entry site (IRES).