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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 T ranslation: information transferred to protein Mutations: changes in DNA Changes in DNA produce changes in proteins.
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Genes as DNA: How Genes Encode Proteins Chapter 5
Central Points • Genesmade 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
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
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 bindsto promoter, DNA istemplateto produce mRNA • mRNA is a complementarycopy of DNA • Bases pair, except T, is replaced by U • End of the gene, marked termination sequence • mRNA processed before leaving nucleus
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) encodesfor 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
5.4 Turning Genes On and Off • Only 5–10% genes active • Gene regulation turns genes on and off • Promotercontrols expression • Also, cells receive signals • Enhancers increase protein production
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
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
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
Valine Histidine Leucine Threonine Proline Glutamic acid Glutamate 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