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Download Chapter 10B: Gene Expression
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Chapter 10B: Gene Expression 1. Overview of Gene Expression 2. Transcription 3. The Genetic Code 4. Translation 1. Overview of Gene Expression Overview of Gene Expression DNA (genetic info) transcription (in nucleus) RNA (copy) translation (via ribosomes in cytoplasm) Protein Gene Expression The expression of a gene into an actual protein occurs by 2 basic processes: 1) Transcription of a gene into RNA • RNA is a nucleic acid very similar to DNA (RNA uses “U” instead of “T”) • this is essentially creating a “photocopy” of the gene • occurs in the nucleus 2) Translation of the RNA transcript into protein • accomplished by ribosomes, in the cytoplasm Comparison of DNA & RNA RNA DNA • sugar = Ribose • sugar = Deoxyribose • single-stranded • double-stranded • A, C, G & U (uracil) • A, C, G & T (thymine) 2. Transcription Transcription Transcription of gene to make an RNA copy is much like DNA replication except for the following: RNA nucleotides RNA polymerase A T C C A A U T U A G G T Direction of transcription Newly made RNA C • produces complementary RNA C G A G C A U C C A G T T • catalyzed by RNA polymerase T C • only 1 strand is used as a template T A A • involves only 1 gene Template Strand of DNA • RNA is released, DNA “zips” back up! *results in a copy of one DNA strand* The Process of Transcription Functions of RNA made by Transcription * 1) mRNA* • copy of gene 2) tRNA • delivers AAs to ribosomes 3) rRNA *provides genetic info for translation • part of ribosomes 3. The Genetic Code How are Genes related to DNA? Genes are segments of DNA that code for a particular protein (or RNA molecule) • the human genome contains ~3 billion base pairs (bps) and ~25,000 genes • almost all genes encode proteins • when we talk about “genes” we will focus on those that express proteins ( the “end products” for a small percentage of genes are special types of RNA molecules) What does DNA actually code for? In other words, “How do genes encode proteins”? • recall that proteins are linear polymers made of the 20 different amino acids ***genes need simply to encode the identity of each amino acid in a given protein*** • i.e., genes must be capable of encoding 20 different amino acids and their order in a protein • although DNA contains only 4 “letters” (i.e., nucleotides), this is more than sufficient… The Genetic Code Each amino acid in a protein is specified by 3 nucleotide sequences called codons • each of the 20 amino acids is coded for by a unique set of codons: e.g. ATG = methionine (start codon) GGN = glycine CAA or CAG = glutamine • there are 64 possible “codon” triplets (4 x 4 x 4) • more than enough to encode 20 amino acids and the signal to “stop” or end the protein (TGA, TAA or TAG) Table of the Genetic Code *always starts w/AUG (met) If the DNA sequence is: CATGCCTGGGCAATAG The RNA copy is: CAUGCCUGGGCAAUAG (transcription) The protein sequence is: *Met-Pro-Gly-Gln-“stop” (translation) From DNA to RNA to Protein The Effects of Mutation MUTATION: any change in DNA sequence Normal gene A U G A A G U U U G G C G C A mRNA Met Protein Lys Phe Gly Ala Base substitution A U G A A G U U U A G C G C A Met Lys Phe Ser Ala U Missing Base deletion A U G A A G U U G G C G C A U Met Lys Leu Ala His • deletions, insertions cause a change in reading frame (frameshift) Sickle Cell Anemia is due to a Base Substitution Normal hemoglobin DNA C T Mutant hemoglobin DNA T mRNA C A T G U A mRNA G A A Normal hemoglobin Glu Sickle-cell hemoglobin Val 1 nucleotide changes 1 amino acid resulting in a misfolded hemoglobin protein (clump together in RBCs > sickle shape) 4. Translation Ribosomes translate mRNA to Protein • in the cytoplasm • with the help of tRNAs tRNA Structure & Function * …the amino acid* attached to the tRNA is then added to the growing polypeptide mRNA codons will “base pair” with a complementary anticodon in a tRNA… * Translation (aka “protein synthesis”) The building of a polypeptide, 1 amino acid at a time, by ribosomes using info in mRNA: • ribosomes bind directly to mRNA, “read” codon by codon • ribosomes always start at AUG (methionine) • translation also involves tRNAs, each of which is attached to 1 of the 20 amino acids (AAs) • ribosomes match the right tRNA (via the anticodon) with the right codon in the mRNA, then add its AA to the growing protein Translation by multiple Ribosomes • the same mRNA can be translated many, many times • a given mRNA can be translated by many ribosomes at the same time mRNA General Steps of Translation 1) ribosome begins translation at AUG of mRNA 2) ribosome binds 2 tRNA-AAs, 2 codons at a time • i.e., tRNAs with anti-codons complementary to the mRNA codons 3) ribosome then catalyzes peptide bond formation between the amino acids attached to each tRNA 4) ribosome shifts 3 nucleotides (1 codon) on mRNA and repeats the process 5) this continues until the ribosome reaches a “stop” codon which causes translation to end Key Terms for Chapter 10B • mRNA, tRNA, rRNA • transcription, RNA polymerase • codon, anti-codon • genetic code • mutation: substitution, deletion, insertion • reading frame, frameshift • translation, ribosome Relevant Review Questions: 1, 3-5, 7, 9