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Chapter 10

Chapter 10. 0. The Structure and Function of DNA. 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

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Chapter 10

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  1. Chapter 10 0 The Structure and Function of DNA

  2. 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.

  3. Phosphate group Nitrogenous base Nitrogenous base (can be A, G, C, or T) Sugar Nucleotide Thymine (T) DNA double helix Phosphate group Sugar (deoxyribose) DNA nucleotide Polynucleotide Sugar-phosphate backbone Figure 10.1

  4. 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.

  5. James Watson (left) and Francis Crick Figure 10.3a

  6. X-ray image of DNA Rosalind Franklin Figure 10.3b

  7. 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.

  8. DNA bases pair in a complementary fashion: • Adenine (A) pairs with thymine (T) • Cytosine (C) pairs with guanine (G)

  9. (a) Ribbon model Figure 10.5a

  10. 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.

  11. Parental (old) DNA molecule Daughter (new) strand Daughter DNA molecules (double helices) Figure 10.6

  12. 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

  13. 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

  14. Nucleus DNA TRANSCRIPTION RNA TRANSLATION Protein Cytoplasm Figure 10.8-3

  15. When DNA is transcribed, the result is an RNA molecule. • RNA is then translated into a sequence of amino acids in a polypeptide (protein).

  16. 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.

  17. Second base of RNA codon Phenylalanine (Phe) Cysteine (Cys) Tyrosine (Tyr) Serine (Ser) Stop Stop Leucine (Leu) Stop Tryptophan (Trp) Histidine (His) Arginine (Arg) Proline (Pro) Leucine (Leu) Glutamine (Gln) First base of RNA codon Third base of RNA codon Asparagine (Asn) Serine (Ser) Isoleucine (Ile) Threonine (Thr) Arginine (Arg) Lysine (Lys) Met or start Aspartic acid (Asp) Alanine (Ala) Glycine (Gly) Valine (Val) Glutamic acid (Glu) Figure 10.11

  18. 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.

  19. 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

  20. RNA nucleotides RNA polymerase Newly made RNA Direction of transcription Template strand of DNA (a) A close-up view of transcription Figure 10.13a

  21. 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)

  22. Amino acid attachment site Hydrogen bond RNA polynucleotide chain Anticodon tRNA (simplified representation) tRNA polynucleotide (ribbon model) Figure 10.15

  23. Next amino acid to be added to polypeptide Growing polypeptide tRNA mRNA Codons (b) The “players” of translation Figure 10.16b

  24. Translation: The Process • Translation is divided into three phases: • Initiation • Elongation • Termination

  25. 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.

  26. 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

  27. 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

  28. Amino acid Polypeptide P site mRNA Anticodon A site Codons Codon recognition ELONGATION Stop codon Peptide bond formation New peptide bond mRNA movement Translocation Figure 10.19-4

  29. Review: DNA RNA Protein • In a cell, genetic information flows from DNA to RNA in the nucleus and RNA to protein in the cytoplasm.

  30. RNA polymerase Transcription Polypeptide Nucleus DNA Stop codon mRNA Intron RNA processing Cap Termination Tail mRNA Intron Anticodon Amino acid Ribosomal subunits Codon tRNA Enzyme Elongation ATP Initiation of translation Amino acid attachment Figure 10.20-6

  31. 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

  32. 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

  33. Normal hemoglobin DNA Mutant hemoglobin DNA mRNA mRNA Sickle-cell hemoglobin Normal hemoglobin Figure 10.21

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