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Genes as DNA: How Genes Encode Proteins Chapter 5 Central Points Genes made of DNA that encodes proteins Transcription: DNA copied into mRNA Translation: information transferred to protein Mutations: changes in DNA Changes in DNA produce changes in proteins 5.1 How Do Genes Control Traits? Individuals carry two copies of each gene One from each parent Different forms are alleles Genes contain information to produce proteins Proteins contribute to the observable traits or phenotype What Is a Protein? Provide structure Be enzymes Be chemical messengers Act as receptors Be carrier molecules Protein Subunits: Amino Acids 20 found in the body amino acids have different chemical groups All contain both a carboxyl group and an amino group Billions of combinations possible Chemical Structure of Amino Acids Essential Amino Acids Amino acids Fig. 5-3, p. 86 How Does DNA Carry Information? DNA carries four nucleotides: A, T, G, and C • Three nucleotide codon in messenger RNA (mRNA) specifies one amino acid Order of DNA bases determine the order of amino acids but not all DNA codes for proteins Gene to Protein Transcription: DNA mRNA Translation: mRNA Protein Animation: Overview of transcription 5.2 What Happens in Transcription? First step of information transfer Information in DNA sequence gene is copied into sequence of bases in mRNA RNA polymerase DNA to be transcribed Initiation 1 Promoter Terminator Elongation 2 mRNA transcript Termination 3 mRNA 4 RNA polymerase Completed pre-mRNA p. 88 Transcription RNA polymerase binds to promoter, DNA is template to produce mRNA mRNA is a complementary copy of DNA Bases pair, except T, is replaced by U End of the gene, marked termination sequence mRNA processed before leaving nucleus Animation: From DNA to proteins (gene transcription) 5.3 What Happens in Translation? Second step, processed mRNA to the ribosome Protein produced from information on mRNA Each mRNA codon codes for an AA Transfer RNA (tRNA) acts as an adaptor Transfer RNA Recognizes and binds to one amino acid Recognizes the mRNA codon for that amino acid At one end binds a specific amino acid Other end has a 3 nucleotide anticodon that pairs with mRNA codon for specific amino acid Ribosome mRNA tRNA Growing protein p. 88 Translation (1) Synthesis of protein from mRNA Occurs within ribosomes AUG (start codon) encodes for methionine Second AA is in position, an enzyme forms a peptide bond between the two AA tRNA for the first AA is released Translation (2) Ribosome to next codon and repeats adding AA to growing AA chain Stop codons (UAA, UAG, and UGA) do not code for AA and ribosome detaches from mRNA AA chain released, folds into a 3-D protein TRANSCRIPTION DNA tRNA mRNA tRNA rRNA Nucleus Cytoplasm mRNA Ribosomes TRANSLATION Protein p. 89 Animation: The 4 steps of translation Genetic Code for Amino Acids 5.4 Turning Genes On and Off Only 5–10% genes active Gene regulation turns genes on and off Promoter controls expression Also, cells receive signals Enhancers increase protein production Controlling Gene Expression 5.5 Mutations Changes in DNA Produce: • Nonfunctional protein • Partially functional protein • No protein Affect the timing and level of gene expression Some no change Mutagens Increase chance of mutation Mistakes during DNA replication By-product of normal cell functions Include: • Environmental factors • Radiation • Chemicals Animation: Mutations and translation Animation: From DNA to proteins (base substitution) 5.6 Cause of Genetic Disorders Change in DNA alters mRNA Single nucleotide change can alter codon and possibly amino acid Change in amino acid sequence causes changes in • 3-D structure of protein • Defective protein folding • Protein function Genetic Code Disorders from Altered 3-D Shape Cystic fibrosis Form of Alzheimer disease Mad cow disease Cruzfelt-Jacob disease (CJD) Sickle Cell Anemia Mutation in the hemoglobin gene Hemoglobin (HbA) is composed of two proteins: • Alpha globin • Beta globin Single nucleotide point mutation alters one of 146 AA, affects the beta globin Causes hemoglobin molecules to stick together Hemoglobin Molecule Red Blood Cells Point Mutation in Sickle Cell Anemia Valine Histidine Leucine Threonine Proline Glutamic Glutamate acid Valine Histidine Leucine Threonine Proline Valine Glutamate Fig. 5-9, p. 93 5.7 Other Single-Gene Defects Cystic fibrosis (CF) • Misfolded protein • Protein destroyed Huntington disease (HD) • Trinucleotide repeats • Multiple CAG repeats