Protein Synthesis Notes PDF

Chapter 2 (Part 2) Cell Structure: Plasma Membrane and Protein Synthesis A Typical Cell Plasma Membrane Structure Selective barrier Phospholipid bilayer: Phosphate heads contact water Fatty acids: Hydrophobic center restricts the movement of water, water-soluble molecules, and i...

Chapter 2 (Part 2) Cell Structure: Plasma Membrane and Protein Synthesis A Typical Cell Plasma Membrane Structure Selective barrier Phospholipid bilayer: Phosphate heads contact water Fatty acids: Hydrophobic center restricts the movement of water, water-soluble molecules, and ions Proteins and phospholipids constantly move laterally; “fluid- mosaic model” of membrane structure Proteins allow many polar substances to pass selectively through membrane Plasma Membrane Structure Membrane Proteins: Integral proteins – most are transmembrane proteins Peripheral proteins Functions: Structural support “Self” markers for the immune system Transport of substances Receptors for extracellular signals Enzymatic reactions to control cell processes Glycocalyx: glycolipids & glycoproteins Cholesterol  DNA in the Nucleus Chromatin: DNA in the nucleus is packaged with proteins called histones Histones: positively charged and interact with negatively charged DNA to cause spooling Euchromatin: active in transcription & looser chemical changes in histones allow molecules access to the DNA. Heterochromatin: inactive regions, highly condensed Genetic Code & Protein Synthesis Nucleus contains DNA, which directs synthesis of all proteins in the body Gene: DNA sequence that codes for the specific amino acid sequence of a polypeptide chain OR is transcribed into RNA One molecule of DNA contains multiple genes Genome: total genetic information coded in the DNA of a typical cell Human genome ~25,000 genes Genetic Expression: Production of a protein or RNA from a specific gene Genetic code: DNA triplets encode specific amino acids Basic Steps of Protein Synthesis DNA is transcribed in the nucleus to mRNA (“Transcription”) mRNA moves from the nucleus to the cytoplasm mRNA is translated to protein by a ribosome (rRNA) & tRNA (“Translation”)  Protein Synthesis Genetic code is used by all living cells Triplet code: a sequence of 3 nucleotides codes for a specific amino acid DNA triplet is complementary to a 3-base sequence in mRNA “codon” Template strand Genetic Code Code for which of the 20 amino acids is specified by each possible DNA triplet (codon) Four bases: 43 = 64 codons Each codon codes for one amino acid Example: CCC → proline One amino acid can be coded for by more than one codon Example: CCC → proline CCG → proline One initiation codon (AUG) AUG → methionine Three termination codons Protein Synthesis: Transcription Transcription: DNA-directed RNA synthesis Messenger RNA (mRNA): specifies the amino acid sequence of a protein. Carries this message from the DNA to the site of protein synthesis in the cytoplasm Protein Synthesis: Transcription Initiation: RNA Polymerase binds to a promoter region of a gene Promoter: area of DNA that indicates where to begin transcription of a gene Transcription factors can bind to the promoter region and activate or repress transcription This initiates the separation of the two strands of DNA (hydrogen bonds between DNA base pairs break) Allows ribonucleotides to bind to DNA template strand Protein Synthesis: Transcription Elongation: RNA Polymerase moves along the template strand, joining RNA nucleotides complementary to DNA template Termination: At the stop signal at the end of the gene, RNA Polymerase releases the precursor RNA transcript & detaches from DNA Pre-mRNA modification Post-transcriptional processing Splicing: Primary RNA transcript (pre-mRNA) must be modified by a spliceosome in the nucleus before moving to the cytoplasm as mature mRNA Introns (portions of the gene that do not actually code for proteins) must be spliced out Exons are joined together into continuous sequence Alternative splicing  RNA Interference Some RNA molecules (that don’t code for proteins) may prevent some mRNA molecules from being translated Prevents tRNA from bringing amino acids to mRNA, so translation is prevented, the gene is silenced → Regulation of protein synthesis: The expression of at least 30% of genes is regulated in this way Post-transcriptional Processing: Protein Synthesis: Translation Translation: Polypeptide synthesis Mature mRNA binds to a ribosome in the cytoplasm to complete translation Ribosome: complex particle composed of 2 subunits of ribosomal RNA (rRNA) + proteins Protein factories that align the mRNA with tRNAs. Enzymes Ribosome catalyze peptide bonds between amino acids Free & fixed Transfer RNA (tRNA): carry amino acids to the ribosome based on interactions with the mRNA codons Binds a specific amino acid and contains an anticodon to base pair with the complementary mRNA codon Ribosome One mRNA-binding site Three tRNA-binding sites: A site holds tRNA with the next amino acid to be added to the polypeptide chain P site holds tRNA with the last amino acid added to the polypeptide chain E site: for empty tRNA that exits Protein Synthesis: Translation 1. Initiation: Initiation factors help assemble the mRNA, small ribosomal subunit (30S), large ribosomal subunit (50S), & first tRNA Initiator tRNA (with methionine) binds to mRNA start codon on the small ribosomal subunit Large ribosomal subunit binds and Met-tRNA is in P site Protein Synthesis: Translation 2. Elongation: tRNA with next amino acid binds to open A site Amino acid detaches at P site and is joined to next amino acid by enzyme in the ribosome, peptidyl transferase Ribosome moves to the next codon, empty tRNA moves to E site and exits A new tRNA comes into the A site & cycle repeats 3. Termination: Ribosome reaches the stop codon Completed polypeptide is released Posttranslational Modifications Chaperone proteins help polypeptide chain fold into correct tertiary structure following translation (also help form quaternary structure) Proper folding and cross-links are required for a functional protein Potential modifications: Methionine removal Cleavage into smaller proteins Addition of carbohydrate or lipid groups to a.a. side chains Protect protein from degradation or help direct protein to a particular location Destination of Proteins 1. Cytosol 2. Targeted to certain organelles (Mitochondria, peroxisome, nucleus) 3. Rough ER → Golgi complex → Secretory vesicles to ECF; or PM proteins; or lysosomes Determined by the leader sequence/signal sequence If there is no leader sequence, proteins remain in the cytosol In some cases, the leader sequence causes the ribosome to attach to the rough ER to finish translation Destination of Proteins Leader sequences direct proteins being translated to their final locations Role of Rough ER in Protein Synthesis Proteome Proteome: the specific proteins expressed in a given cell at a particular time Determines the structure and function of the cell at that time Controlled by gene regulation: cells use various mechanisms to block or make promoters more accessible to RNA polymerase More proteins than genes (100,000 proteins): After transcription, mRNA can be spliced different ways Polypeptide chains can associate in different combinations Protein modification can occur by adding a lipid or carbohydrate Posttranslational modification: methylation, phosphorylation, cutting into smaller units Major Regulation of Protein Synthesis Regulation of transcription at the level of RNA polymerase binding Transcription factors can promote or repress the initiation process Regulation of translation during initiation Various factors can affect the efficiency of initiation Regulation of Protein Synthesis The major steps required to convert the genetic code of DNA into a functional protein. Protein Synthesis Video