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Chapter 10

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  1. 0 Chapter 10 The Structure and Function of DNA Laura Coronado Bio 10 Chapter 10

  2. Biology and Society: Tracking a Killer • The influenza virus is one of the deadliest pathogens in the world. • Each year in the United States, over 20,000 people die from influenza infection. • In the flu of 1918–1919, about 40 million people died worldwide. • Vaccines against the flu are the best way to protect public health. • Because flu viruses mutate quickly, new vaccines must be created every year. Laura Coronado Bio 10 Chapter 10

  3. 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. Laura Coronado Bio 10 Chapter 10

  4. 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 Laura Coronado Bio 10 Chapter 10 Figure 10.1

  5. Discovery of the Double Helix • James Watson and Francis Crick determined that DNA is a double helix. • Rosalind Franklin collected the X-ray crystallography data. • Watson and Crick used X-ray crystallography data to reveal the basic shape of DNA. Laura Coronado Bio 10 Chapter 10


  6. X-ray image of DNA Rosalind Franklin Laura Coronado Bio 10 Chapter 10 Figure 10.3b

  7. DNA Structure • 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. • 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. • DNA bases pair in a complementary fashion: • Adenine (A) pairs with thymine (T) • Cytosine (C) pairs with guanine (G) Laura Coronado Bio 10 Chapter 10

  8. Twist Laura Coronado Bio 10 Chapter 10 Figure 10.4

  9. Hydrogen bond (a) Ribbon model (c) Computer model (b) Atomic model Laura Coronado Bio 10 Chapter 10 Figure 10.5

  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. • DNA replication in eukaryotes: • Begins at specific sites on a double helix • Proceeds in both directions Laura Coronado Bio 10 Chapter 10

  11. Parental (old) DNA molecule Daughter (new) strand Daughter DNA molecules (double helices) Laura Coronado Bio 10 Chapter 10 Figure 10.6

  12. DNA Polymerases • Are enzymes • Make the covalent bonds between the nucleotides of a new DNA strand • Are involved in repairing damaged DNA • DNA can be damaged by ultraviolet light. Laura Coronado Bio 10 Chapter 10

  13. Origin of replication Origin of replication Parental strands Origin of replication Parental strand Daughter strand Bubble Two daughter DNA molecules Laura Coronado Bio 10 Chapter 10 Figure 10.7

  14. THE FLOW OF GENETIC INFORMATION FROM DNA TO RNA TO PROTEIN • DNA functions as the inherited directions for a cell or organism. • An organism’s genotype is its genetic makeup, the sequence of nucleotide bases in DNA. • The phenotype is the organism’s physical traits, which arise from the actions of a wide variety of proteins. • 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 • Transcription and translation are how genes control: The structures & the activities of cells Laura Coronado Bio 10 Chapter 10

  15. Nucleus DNA TRANSCRIPTION RNA TRANSLATION Protein Cytoplasm Laura Coronado Bio 10 Chapter 10 Figure 10.8-3

  16. Overview - The Big Picture • The function of a gene is to dictate the production of a polypeptide. • In DNA, it is the linear sequence of nucleotide bases. • A typical gene consists of thousands of nucleotides. • A single DNA molecule may contain thousands of genes. • When DNA is transcribed, the result is an RNA molecule. • RNA is then translated into a sequence of amino acids in a polypeptide. • A protein may consist of two or more different polypeptides. Laura Coronado Bio 10 Chapter 10

  17. The RNA message is translated into a polypeptide by using a codon is a triplet of bases, which codes for one amino acid. • The genetic code is: • The set of rules relating nucleotide sequence to amino acid sequence • Shared by all organisms • There are 64 codons: • 61 code for amino acids • 3 are stop codons, indicating the end of a polypeptide Laura Coronado Bio 10 Chapter 10

  18. Gene 1 DNA molecule Gene 2 Gene 3 DNA strand TRANSCRIPTION RNA TRANSLATION Codon Polypeptide Amino acid Laura Coronado Bio 10 Chapter 10 Figure 10.10

  19. 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) Laura Coronado Bio 10 Chapter 10 Figure 10.11

  20. 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. • The first phase of transcription is initiation, in which: • RNA polymerase attaches to the promoter • RNA synthesis begins Laura Coronado Bio 10 Chapter 10

  21. RNA Elongation • 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 Laura Coronado Bio 10 Chapter 10

  22. RNA polymerase DNA of gene Promoter DNA Terminator DNA Initiation RNA Area shown in part (a) at left Elongation RNA nucleotides RNA polymerase Termination Growing RNA Completed RNA Newly made RNA Direction of transcription Template strand of DNA RNA polymerase (a) A close-up view of transcription (b) Transcription of a gene Laura Coronado Bio 10 Chapter 10 Figure 10.13

  23. Laura Coronado Bio 10 Chapter 10 Figure 10.12

  24. The Processing of Eukaryotic RNA • After transcription: • Eukaryotic cells process RNA • Prokaryotic cells do not • RNA processing includes: • Adding a cap and tail • Removing introns • Splicing exons together to form messenger RNA (mRNA) Laura Coronado Bio 10 Chapter 10

  25. Intron Exon Intron Exon Exon DNA Transcription Addition of cap and tail Cap RNA transcript with cap and tail Tail Introns removed Exons spliced together mRNA Coding sequence Nucleus Cytoplasm Laura Coronado Bio 10 Chapter 10 Figure 10.14

  26. Translation: The Players • Translation is the conversion from the nucleic acid language to the protein language. • Translation requires: • Messenger RNA (mRNA) • ATP • Enzymes • Ribosomes • Transfer RNA (tRNA) Laura Coronado Bio 10 Chapter 10

  27. Transfer RNA (tRNA) • Transfer RNA (tRNA): • Acts as a molecular interpreter • Carries amino acids • Matches amino acids with codons in mRNA using anticodons Laura Coronado Bio 10 Chapter 10

  28. Amino acid attachment site Hydrogen bond RNA polynucleotide chain Anticodon tRNA (simplified representation) tRNA polynucleotide (ribbon model) Laura Coronado Bio 10 Chapter 10 Figure 10.15

  29. Ribosomes • Ribosomes are organelles that: • Coordinate the functions of mRNA and tRNA • Are made of two protein subunits • Contain ribosomal RNA (rRNA) • A fully assembled ribosome holds tRNA and mRNA for use in translation. Laura Coronado Bio 10 Chapter 10

  30. Next amino acid to be added to polypeptide tRNA binding sites Growing polypeptide P site A site tRNA Ribosome Large subunit mRNA mRNA binding site Small subunit Codons (b) The “players” of translation (a) A simplified diagram of a ribosome Laura Coronado Bio 10 Chapter 10 Figure 10.16

  31. Translation: The Process • Translation is divided into three phases: • Initiation • Elongation • Termination Laura Coronado Bio 10 Chapter 10

  32. 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. Laura Coronado Bio 10 Chapter 10

  33. Cap Start of genetic message End Tail Laura Coronado Bio 10 Chapter 10 Figure 10.17

  34. Met Large ribosomal subunit Initiator tRNA P site A site mRNA Small ribosomal subunit Start codon Laura Coronado Bio 10 Chapter 10 Figure 10.18

  35. 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 • Step 3, translocation: • The P site tRNA leaves the ribosome • The tRNA carrying the polypeptide moves from the A to the P site Laura Coronado Bio 10 Chapter 10

  36. Termination • Elongation continues until: • The ribosome reaches a stop codon • The completed polypeptide is freed • The ribosome splits into its subunits Laura Coronado Bio 10 Chapter 10

  37. Amino acid Polypeptide P site mRNA Anticodon A site Codons Codon recognition ELONGATION Stop codon Peptide bond formation New peptide bond mRNA movement Translocation Laura Coronado Bio 10 Chapter 10 Figure 10.19-4

  38. Review: DNA RNA Protein • In a cell, genetic information flows from DNA to RNA in the nucleus and RNA to protein in the cytoplasm. Laura Coronado Bio 10 Chapter 10

  39. 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 Laura Coronado Bio 10 Chapter 10 Figure 10.20-6

  40. As it is made, a polypeptide: • Coils and folds • Assumes a three-dimensional shape, its tertiary structure • Several polypeptides may come together, forming a protein with quaternary structure. Laura Coronado Bio 10 Chapter 10

  41. 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 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 Laura Coronado Bio 10 Chapter 10

  42. Insertions and deletions can: Change the reading frame of the genetic message Lead to disastrous effects Have beneficial effects Laura Coronado Bio 10 Chapter 10

  43. Normal hemoglobin DNA Mutant hemoglobin DNA mRNA mRNA Sickle-cell hemoglobin Normal hemoglobin Laura Coronado Bio 10 Chapter 10 Figure 10.21

  44. Mutagens • Mutations may result from: • Errors in DNA replication • Physical or chemical agents called mutagens • Although mutations are often harmful, they are the source of genetic diversity, which is necessary for evolution by natural selection. Laura Coronado Bio 10 Chapter 10

  45. mRNA and protein from a normal gene (a) Base substitution Deleted (b) Nucleotide deletion Inserted (c) Nucleotide insertion Laura Coronado Bio 10 Chapter 10 Figure 10.22

  46. VIRUSES AND OTHER NONCELLULAR INFECTIOUS AGENTS • Viruses exhibit some, but not all, characteristics of living organisms. Viruses: • Possess genetic material in the form of nucleic acids • Are not cellular and cannot reproduce on their own. • Bacteriophages, or phages, are viruses that attack bacteria. DNA Protein coat Laura Coronado Bio 10 Chapter 10

  47. Head Bacteriophage (200 nm tall) Tail Tail fiber DNA of virus Bacterial cell Colorized TEM Laura Coronado Bio 10 Chapter 10 Figure 10.25

  48. Bacteriophages Reproduction • Phages have two reproductive cycles. (1) In the lytic cycle: • Many copies of the phage are made within the bacterial cell, and then • The bacterium lyses (breaks open) (2) In the lysogenic cycle: • The phage DNA inserts into the bacterial chromosome and • The bacterium reproduces normally, copying the phage at each cell division Laura Coronado Bio 10 Chapter 10

  49. Phage Phage attaches to cell. Phage DNA Bacterial chromosome (DNA) Cell lyses, releasing phages. Phage injects DNA Many cell divisions Occasionally a prophage may leave the bacterial chromosome. LYTIC CYCLE LYSOGENIC CYCLE Phage DNA circularizes. Phages assemble Lysogenic bacterium reproduces normally, replicating the prophage at each cell division. Prophage OR Phage DNA is inserted into the bacterial chromosome. New phage DNA and proteins are synthesized. Laura Coronado Bio 10 Chapter 10 Figure 10.26-2

  50. Plant Viruses • Viruses that infect plants can: • Stunt growth • Diminish plant yields • Spread throughout the entire plant • Viral plant diseases: • Have no cure • Are prevented by producing plants that resist viral infection, controversial Laura Coronado Bio 10 Chapter 10