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Genes as DNA: How Genes Encode Proteins

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

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  1. Genes as DNA: How Genes Encode Proteins Chapter 5

  2. 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

  3. 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

  4. What Is a Protein? • Provide structure • Be enzymes • Be chemical messengers • Act as receptors • Be carrier molecules

  5. 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

  6. Chemical Structure of Amino Acids

  7. Essential Amino Acids

  8. Amino acids Fig. 5-3, p. 86

  9. 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

  10. Gene to Protein • Transcription: DNA  mRNA • Translation: mRNA Protein

  11. Animation: Overview of transcription

  12. 5.2 What Happens in Transcription? • First step of information transfer • Information in DNA sequence gene is copied into sequence of bases in mRNA

  13. 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

  14. 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

  15. Animation: From DNA to proteins (gene transcription)

  16. 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

  17. 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

  18. Ribosome mRNA tRNA Growing protein p. 88

  19. 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

  20. 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

  21. TRANSCRIPTION DNA tRNA mRNA tRNA rRNA Nucleus Cytoplasm mRNA Ribosomes TRANSLATION Protein p. 89

  22. Animation: The 4 steps of translation

  23. Genetic Code for Amino Acids

  24. 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

  25. Controlling Gene Expression

  26. 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

  27. Mutagens • Increase chance of mutation • Mistakes during DNA replication • By-product of normal cell functions • Include: • Environmental factors • Radiation • Chemicals

  28. Animation: Mutations and translation

  29. Animation: From DNA to proteins (base substitution)

  30. 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

  31. Genetic Code

  32. Disorders from Altered 3-D Shape • Cystic fibrosis • Form of Alzheimer disease • Mad cow disease • Cruzfelt-Jacob disease (CJD)

  33. 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

  34. Hemoglobin Molecule

  35. Red Blood Cells

  36. Point Mutation in Sickle Cell Anemia

  37. Valine Histidine Leucine Threonine Proline Glutamic acid Glutamate Valine Histidine Leucine Threonine Proline Valine Glutamate Fig. 5-9, p. 93

  38. 5.7 Other Single-Gene Defects • Cystic fibrosis (CF) • Misfolded protein • Protein destroyed • Huntington disease (HD) • Trinucleotide repeats • Multiple CAG repeats

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