Protein Synthesis! From DNA to Protein! (Transcription & Translation)! I. An Overview A. Certain sequences of nucleotides in DNA [called genes], can be expressed/used as a code to determine the sequence of amino acids in a single polypeptide. B. The DNA code/template is first transcribed into an RNA sequence called mrna (messenger RNA) and then the code (now in the language of RNA) is translated from the mrna template into a polypeptide sequence. C. RNA - like DNA - is a nucleic acid 1. RNA differs from DNA: a. the 5' carbon sugar is ribose not? b. uracil takes the place of thymine [A-U] *dbl bond
Overview of Transcription and Translation p. 329 II. Transcription A. Definition: The synthesis of mrna using a specific strand of DNA as a template 1. takes place in the nucleus of eukaryotes B. Rxn is catalyzed by RNA Polymerase which: 1. separates (via denaturing) the DNA dbl helix and links RNA nucleotides as the enzyme moves along the DNA, one nucleotide at a time. * only one strand of the dbl helix acts as the RNA template - and the DNA re-natures after each full turn is transcribed 2. adds RNA nucleotides to the 3' end of the mrna strand; thus, mrna molecules are synthesized in the 5' to 3' direction. [ 60 bases/sec. ]
C. Transcription Unit: a nucleotide sequence on the template strand of DNA that is transcribed into a single RNA molecule p. 332 captions important III. Steps of Transcription A. Polymerase Binding and Initiation 1. RNA polymerases bind to promoter regions 2. Eukaryotic promoter region consists of (in order from left to right): a. TATA box, a nucleotide sequence including a TATA sequence that act as a binding site for DNA-binding proteins called transcription factors that help initiate transcription. b. polymerase binding site c. initiation site, where transcription begins p. 333
B. Termination of Transcription 1. transcription proceeds until individual RNA polymerases reach a termination sequence at the end of the transcription unit a. this sequence causes RNA polymerase to stop transcription and release the elongated RNA molecule b. in eukaryotes, the most common terminator sequence is AATAAA 2. in eukaryotes (only) mrna molecules must be processed before leaving the nucleus to be functional in the translation process a. within individual genes expressible (coding) sequences of DNA are separated by non-coding sequences b. coding sequences = exons c. non-coding sequences = introns d. mrna sequences reflecting introns are spliced out of the molecule leaving only exons Fig. 17.10 pg.335 * recall, only in eukaryotes and happens in the nucleus This Video Does Not Include Post Transcription Processing
IV. Translation A. During translation, proteins are synthesized B. Translation refers to a change in chemical language: mrna base sequence (the template) into amino acid sequence. C. Transfer RNA (trna) is the interpreter and is responsible for: 1. physically transferring amino acids from the cytoplasm to the organelle called ribosome (60% rrna + 40% protein) 2. translating the nucleotide sequence of mrna into a sequence of amino acids 3 nucleotides at a time = codon D. Each trna bonds with only one of the 20 a.a. 1. one end of the trna (a.a. attachment site) bonds to its specific a.a 2. the other end of the molecule (the anticodon) bonds to the mrna codon * based on the universal code p. 338 Fig. 17.14
What Is The Universal Code? Universal Code: where groups of 3 mrna nucleotides are translated by trnas into amino acids p. 330 E. Ribosomes 1. consists of two subunits (large and small) 2. ribosomes coordinate the bonding/pairing of the trna anticodons to the mrna using two binding and one non-binding site: a. P Site - holds the trna carrying the growing polypeptide chain b. A Site - holds the trna carrying the next amino acid to be added c. E Site (non-binding) empty trna leaves the ribosome from this location pg. 339
F. Polypeptide Elongation Steps (from the beginning) 1. trna w/the a.a. Methionine (Met) binds to the first P-Site and the AUG codon (start codon) of the mrna 2. trna carrying the next a.a. (as dictated by the next codon) binds to the A-Site 3. the Met then binds to the a.a. associated w/the trna in the A-Site 4. the trna that carried the Met, and is in the P-Site, moves into the E-Site leaving the ribosomal/mrna complex 5. via movement of the ribosome, the trna w/the growing polypeptide, in the A-Site now finds itself in the P-Site as the ribosome moves down the mrna. 6. additional details: a. each reaction requires one GTP molecule for energy b. summary: codon recognition; peptide bond formation; translocation G. Polypeptide Termination 1. final stage of translation occurs when the A-site reaches one of three special "stop" codons: UAA, UAG, or UGA (see the universal code) 2. a protein called a release factor binds to the termination codon in the A-site, and the ribosome catalyzes the addition of a water molecule instead of another amino acid to the growing polypeptide - the trna in the P site releases the polypeptide, translocates to the E site and exits the ribosome
pg. 341 Fig. 17.19 pg. 342 The End! Protein Synthesis is life the rest is just details