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Biology II—Ch 17 Notes—From Genes to Proteins -one gene produces one polypeptide chain -one gene codes for one polypeptide chain -polypeptide chains may combine to form 1 protein -Hemoglobin made from 2 kinds of polypeptides—thus 2 genes code for this protein -ok to say one gene codes for one protein -DNA contains instructions for making proteins, RNA actually Makes the proteins -3 main structural differences between DNA and RNA 1. DNA has 2 strands; RNA has 1 strand 2. DNA has the sugar deoxyribose; RNA has the sugar ribose 3. DNA has the nitrogen bases A,T,G,C; RNA replaces T with Uracil -2 main steps to get from DNA to a protein 1. Transcription: synthesis of RNA under direction of DNA -carries genetic info from nucleus to the cytoplasm 2. Translation: actual synthesis of a protein (or polypeptide) that occurs under the direction of mRNA -Translation (cont) -3 types of RNA work together 1. mRNA carries instructions from nucleus to cytoplasm -codon: group of 3 nucleotides in mRNA that code for one of 20 amino acids -each codon codes for a specific amino acid -may be more than one codon for one amino acid 2. tRNA brings amino acids from the cytoplasm to the ribosome to be assembled into a protein -anticodon: group of 3 nucleotides complimentary to the codon—ensures that amino acids are added in the correct order 3. rRNA found in ribosomes -ribosomes consist of 2 subunits--small and large -constructed of proteins and rRNA -large and small subunits join to form a functional unit only when they attach to an mRNA molecule -about 60% of the weight of each ribosome is rRNA -ribosomes bring mRNA and tRNA together -ribosomes have a binding site for mRNA and 3 binding sites for tRNA -P site: holds tRNA carrying the growing polypeptide chain -A site: holds tRNA carrying the next amino acid -E site: discharges tRNAs after they deliver the amino acid -act like a vise and holds the tRNA and mRNA close together and positions the new amino acid for addition to the growing protein -Transcription (cont) -mRNA first synthesized needs to be modified before it can leave the nucleus -RNA splicing -primary RNA transcript is about 8000 nucleotides long -only takes about 1200 nucleotides to code for an average size protein of about 400 amino acids -most genes and their RNA transcripts have long noncoding stretches of nucleotides—regions not translated -most of these noncoding sections are interspersed between coding segments of a gene -Introns: noncoding segments of nucleic acid that lie between coding regions -Exons: coding regions that are eventually expressed -both introns and exons are originally transcribed -but, introns are cut out and exons are spliced together to form an mRNA molecule with a continuous coding sequence -this leaves the nucleus and enters the cytoplasm Mutations -Mutation—any change in the DNA sequence -Point Mutations—chemical changes in just one or a few base pairs In a single gene -3 main types 1. Substitution: change in a single base pair -may have no affect if resulting codon codes for the same amino acid 2. Insertion: addition of a single base pair -affects every amino acid after the mutation -The dog bit the cat -The dob gbi tth eca t 3. Deletion: removal of a base pair -affects every amino acid after the mutaion -The dog bit the cat -The dob itt hec at -Mutagens: any agent that interacts with the DNA and causes a Mutation -high energy radiation: UV light, X-rays, nuclear radiation -chemicals: formaldehyde, benzenes, asbestos -high temperatures