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Chapter 10 The Structure and Function of DNA PowerPoint® Lectures for Campbell Essential Biology, Fourth Edition – Eric Simon, Jane Reece, and Jean Dickey Campbell Essential Biology with Physiology, Third Edition – Eric Simon, Jane Reece, and Jean Dickey Lectures by Chris C. Romero, updated by Edward J. Zalisko © 2010 Pearson Education, Inc. DNA: STRUCTURE AND REPLICATION • DNA: – Was known to be a chemical in cells by the end of the nineteenth century – Has the capacity to store genetic information – Can be copied and passed from generation to generation • DNA and RNA are nucleic acids. – They consist of chemical units called nucleotides. – The nucleotides are joined by a sugar-phosphate backbone. © 2010 Pearson Education, Inc. Phosphate group Nitrogenous base Sugar Nucleotide DNA double helix Nitrogenous base (can be A, G, C, or T) Thymine (T) Phosphate group Sugar (deoxyribose) DNA nucleotide Polynucleotide Sugar-phosphate backbone Figure 10.1 • The four nucleotides found in DNA differ in their nitrogenous bases. These bases are: – Thymine (T) – Cytosine (C) – Adenine (A) – Guanine (G) • RNA has uracil (U) in place of thymine. • James Watson and Francis Crick determined that DNA is a double helix. © 2010 Pearson Education, Inc. James Watson (left) and Francis Crick Figure 10.3a X-ray image of DNA Rosalind Franklin Figure 10.3b • The model of DNA is like a rope ladder twisted into a spiral. – The ropes at the sides represent the sugar-phosphate backbones. – Each wooden rung represents a pair of bases connected by hydrogen bonds. © 2010 Pearson Education, Inc. • DNA bases pair in a complementary fashion: – Adenine (A) pairs with thymine (T) – Cytosine (C) pairs with guanine (G) © 2010 Pearson Education, Inc. (a) Ribbon model Figure 10.5a DNA Replication • When a cell reproduces, a complete copy of the DNA must pass from one generation to the next. • Watson and Crick’s model for DNA suggested that DNA replicates by a template mechanism. © 2010 Pearson Education, Inc. Parental (old) DNA molecule Daughter (new) strand Daughter DNA molecules (double helices) Figure 10.6 • DNA can be damaged by ultraviolet light. • DNA polymerases: – Are enzymes – Make the covalent bonds between the nucleotides of a new DNA strand – Are involved in repairing damaged DNA © 2010 Pearson Education, Inc. • DNA specifies the synthesis of proteins in two stages: – Transcription, the transfer of genetic information from DNA into an RNA molecule – Translation, the transfer of information from RNA into a protein © 2010 Pearson Education, Inc. Nucleus DNA TRANSCRIPTION RNA TRANSLATION Protein Cytoplasm Figure 10.8-3 • When DNA is transcribed, the result is an RNA molecule. • RNA is then translated into a sequence of amino acids in a polypeptide (protein). © 2010 Pearson Education, Inc. • What are the rules for translating the RNA message into a polypeptide? • A codon is a triplet of bases, which codes for one amino acid. © 2010 Pearson Education, Inc. Second base of RNA codon First base of RNA codon Leucine (Leu) Leucine (Leu) Isoleucine (Ile) Serine (Ser) Stop Stop Proline (Pro) Threonine (Thr) Met or start Valine (Val) Tyrosine (Tyr) Alanine (Ala) Histidine (His) Glutamine (Gln) Cysteine (Cys) Stop Tryptophan (Trp) Arginine (Arg) Asparagine (Asn) Serine (Ser) Lysine (Lys) Arginine (Arg) Aspartic acid (Asp) Glutamic acid (Glu) Third base of RNA codon Phenylalanine (Phe) Glycine (Gly) Figure 10.11 Transcription: From DNA to RNA • Transcription: – Makes RNA from a DNA template – Uses a process that resembles DNA replication – Substitutes uracil (U) for thymine (T) • RNA nucleotides are linked by RNA polymerase. • The “start transcribing” signal is a nucleotide sequence called a promoter. © 2010 Pearson Education, Inc. Initiation of Transcription • The first phase of transcription is initiation, in which: – RNA polymerase attaches to the promoter – RNA synthesis begins • During the second phase of transcription, called elongation: – The RNA grows longer – The RNA strand peels away from the DNA template • During the third phase of transcription, called termination: – RNA polymerase reaches a sequence of DNA bases called a terminator – Polymerase detaches from the RNA – The DNA strands rejoin © 2010 Pearson Education, Inc. RNA nucleotides RNA polymerase Newly made RNA Direction of transcription Template strand of DNA (a) A close-up view of transcription Figure 10.13a Translation: The Players • Translation is the conversion from the nucleic acid language to the protein language. • Transfer RNA (tRNA): – Acts as a molecular interpreter – Carries amino acids – Matches amino acids with codons in mRNA using anticodons • Ribosomes are organelles that: – Coordinate the functions of mRNA and tRNA – Are made of two protein subunits – Contain ribosomal RNA (rRNA) © 2010 Pearson Education, Inc. Amino acid attachment site Hydrogen bond RNA polynucleotide chain Anticodon tRNA polynucleotide (ribbon model) tRNA (simplified representation) Figure 10.15 Next amino acid to be added to polypeptide Growing polypeptide tRNA mRNA Codons (b) The “players” of translation Figure 10.16b Translation: The Process • Translation is divided into three phases: – Initiation – Elongation – Termination © 2010 Pearson Education, Inc. Initiation • Initiation brings together: – mRNA – The first amino acid, Met, with its attached tRNA – Two subunits of the ribosome • The mRNA molecule has a cap and tail that help it bind to the ribosome. • Initiation occurs in two steps: – First, an mRNA molecule binds to a small ribosomal subunit, then an initiator tRNA binds to the start codon. – Second, a large ribosomal subunit binds, creating a functional ribosome. © 2010 Pearson Education, Inc. Elongation • Elongation occurs in three steps. – Step 1, codon recognition: – the anticodon of an incoming tRNA pairs with the mRNA codon at the A site of the ribosome. – Step 2, peptide bond formation: – The polypeptide leaves the tRNA in the P site and attaches to the amino acid on the tRNA in the A site – The ribosome catalyzes the bond formation between the two amino acids © 2010 Pearson Education, Inc. – Step 3, translocation: – The P site tRNA leaves the ribosome – The tRNA carrying the polypeptide moves from the A to the P site • Elongation continues until: – The ribosome reaches a stop codon – The completed polypeptide is freed – The ribosome splits into its subunits © 2010 Pearson Education, Inc. Amino acid Polypeptide P site mRNA Anticodon A site Codons Codon recognition ELONGATION Stop codon New peptide bond Peptide bond formation mRNA movement Translocation Figure 10.19-4 Review: DNA RNA Protein • In a cell, genetic information flows from DNA to RNA in the nucleus and RNA to protein in the cytoplasm. © 2010 Pearson Education, Inc. Transcription RNA polymerase Polypeptide Nucleus DNA mRNA Stop codon Intron RNA processing Cap Tail Termination mRNA Intron Anticodon Ribosomal Codon subunits Amino acid tRNA ATP Enzyme Amino acid attachment Initiation of translation Elongation Figure 10.20-6 Mutations • A mutation is any change in the nucleotide sequence of DNA. • Mutations can change the amino acids in a protein. • Mutations can involve: – Large regions of a chromosome – Just a single nucleotide pair, as occurs in sickle cell anemia • Mutations may result from: – Errors in DNA replication – Physical or chemical agents called mutagens © 2010 Pearson Education, Inc. Types of Mutations • Mutations within a gene can occur as a result of: – Base substitution, the replacement of one base by another – Nucleotide deletion, the loss of a nucleotide – Nucleotide insertion, the addition of a nucleotide © 2010 Pearson Education, Inc. Normal hemoglobin DNA Mutant hemoglobin DNA mRNA mRNA Normal hemoglobin Sickle-cell hemoglobin Figure 10.21