Transcript Chapter 14
Chapter 14 From DNA to Protein Byssus • Marine mussel manufactures the ultimate underwater adhesive, a protein called byssus DNA is like a book of instructions in each cell • The instructions are written in the alphabet of A,T,G,C. But merely knowing the letter does not tell us how the genes work • DNA consist of two strands of nucleotides twisted together in a double helix. – In replication, the two strands unwind to serve as templates for assembly of new complenetary strands Continue… • Each gene is a linear stretch of DNA nucleotides that codes for the assembly of amino acids into a polypeptide Three things to move • Chromosomes are made of DNA • Segments of DNA code for a protein • Protein in turn, relates to a trait (eye color, enzymes, hormones..) The path from genes to proteins has two steps: • Transcription: molecules of RNA are produced on the DNA templates in the nucleus • Translation: RNA molecules shipped from the nucleus to the cytoplasm are used as templates for polypeptide assembly Transcription Translation DNA RNA proteins Transcription Translation How is RNA Transcribed from DNA? • Three classes of RNA • 1- Messenger RNA (mRNA) carries the blueprint for protein assembly to the ribosome (Goes in the nucleus and translates the material from DNA to RNA) • 2- Transfer RNA (tRNA) brings the correct amino acid to the ribosome and pairs up with an mRNA code for that amino acid Continue… • 3- Ribosomal RNA (rRNA) combines with proteins to form ribosome upon which polypeptide are assembled DNA VS RNA • Double Stranded • A, T, G, C- Nitrogen Bases • Sugar: Deoxyribose • Purpose: hereditary • Single Stranded • A, U, G, C- Nitrogen Base (notice that thymine changes to uracil • Sugar: Ribose • Purpose: Protein Synthesis Transcription Difference from Replication • Only one region of one DNA strand is used as a template • RNA polymerase is used instead of DNA polymerase Continue… • Transcription begins when RNA polymerase binds to a promoter region (a base sequence at the start of a gene) and then moves along to the end of a gene Finishing Touches on mRNA Transcripts • New formed mRNA is an unfinished molecule, not yet ready for use • mRNA transcripts are modified before leaving the nucleus – The 5’ end is capped with a special nucleotide that may serve as a “start” signal for translation – Noncoding portions (introns) are snipped out, and actual coding regions (exons) are spliced together to produce the mature transcript What is a gene code? • Both DNA and its RNA transcript are linear sequences of nucleotides carrying the hereditary code Continue… • Every three bases (a triplet) specifies an amino acid to be included into a growing polypeptide chain; the complete set of triplets or is called the genetic code – Each base triplet in RNA is called codon – The genetic code consists of sixty-one triplets that specify amino acids and three that serve as the stop protein synthesis – AUG: Starts protein sythesis – UAA, UAG, UGA: Stop protein synthesis Codon Chart • DNA Strand: T G C A T C A G A • RNA Strand: A C G U A G U C U • Hyperlink\animationstranscription.htm Structure and Function of tRNA and rRNA • Each kind of tRNA has an anticodon that is complementary to an mRNA codon; each tRNA also carries one specific amino acid • After the mRNA arrives in the cytoplasm, anticodon on a tRNA bonds to the codon on the mRNA, and thus a correct amino is brought into place Continue… • The first bases of the anticodon must pair up with the codon by the usual rules base pairing (A with U and G with C), but there is some latitude in the pairing of the third base (called the wobble effect) • A ribosome has two subunits (each composed of rRNA and proteins) that perform together only during translation Stages of Translation • Initation – a complex forms in this sequence: initiator tRNA + small ribosomal subunit + mRNA + large ribosomal subunit • Elongation: Start codon on mRNA defines the reading frame; a series of tRNA deliver amino acids in sequence by codon-anticodon matching; peptide bond joins each amino acids to the next in sequence Continue… • Termination: a stop codon is reached and the polypeptide chain is released into the cytoplasm or enters the cytomembrane system for further processing What happens to the new polypeptides? • Three steps just outlined can be repeated many times on the same mRNA at the same time • Some polypeptide joins the cytoplasm’s pool of free proteins; other enter the rough ER of the cytomembrane system • LOOK DRAWINGS (REPLICATION, TRANSCRIPTION, AND TRANSLATION) Do mutations affect protein synthesis? • Gene mutation is a change in one to several bases in the nucleotide sequence of DNA, which can result in a change in the protein synthesized Mutations • Mutations (“GENE MISTAKES”) can results from base-pair substitutions, insertions (frameshift mutations), deletion • Results when DNA regions (called transposable elements) move form one location to another in the same DNA molecule of different one Causes of Gene Mutation • Mutations are rare, chance events but each gene has a characteristics mutations rate • Mutations can be caused by mutagens such as ultraviolet radiation, ionizing radiation (gamma rays and X-rays) and chemical such as alkylating agents, which act as carcinogens The proof is in the protein • If a mutation arises in a somatic cells, it will affect only the owner of the at cell and will not be passed on to offspring • If mutations arises in a gamete, it may be passed on and thus enter the evolutionary arena • Mutations may prove to be harmful, benefical, or neutral in its effects