Chapter 10 The Structure and Function of DNA

1. Chapter 10The Structure and Function of DNA CM Lamberty General Biology

2. Biology and Society • Flu Virus • 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.

4. DNA Structure and Replication • Molecular Biology • 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

5. DNA and RNA Structure • Nucleotides • Polynucleotides • Sugar-phosphate backbone • DNA: • 4 nucleotides • Thymine, T • Cytosine, C • Adenine, A • Guanine, G Uracil instead of thymine in RNA

8. Watson and Crick (and Franklin) • Determination of DNA double helix structure • From photo of X-ray image taken by Rosalind Franklin: diameter of helix is uniform, thickness suggested 2 polynucleotide strands • Combined info from x-ray photos and known data • Backbones allowed for the swivel in the interior • Base-pairs for support • originally AA or CC but that would cause bulges • Became apparent for double ring with single ring • Hydrogen bonding to link pairs • dasd

10. Rope Ladder Analogy • 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.

11. DNA Replication • DNA • Known to be a chemical in cells at end of 19th century • Has capacity to store genetic information • Can be copied and passed from generation to generation • DNA and RNA are nucleic acids • Consist of chemical unit called nucleotides • Nucleotides joined by sugar-phosphate backbone

13. DNA Replication • When a cell reproduces, a complete copy of the DNA must be passed on • Watson & Crick’s model suggested DNA replicates by a template mechanism

14. DNA Replication • DNA can be damaged by UV light • DNA polymerase: • Are enzymes • Make the covalent bond b/t nucleotides of a new DNA strand • Are invovled in repairing damaged DNA • DNA repliation in eukaryotes: • Begins at specific sties on a double helix • Proceeds in both directions

16. Flow of Genetic Information • DNA functions as the inherited directions for a cell or organism • How are the directions carried out?

17. How Genotype determines Phenotype • 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 synthesis of proteins in 2 stages: • Transcription, the transfer of genetic information from DNA into an RNA molecule • Translation, the transfer of information from RNA into a protein

18. Nucleus DNA Cytoplasm Figure 10.8-1

19. Nucleus DNA TRANSCRIPTION RNA Cytoplasm Figure 10.8-2

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

21. How Genotype determines Phenotype • The function of a gene is to dictate the production of polypeptide • A protein may consist of two or more different polypeptides

22. From Nucleotides to Amino Acids • Genetic information in DNA is • Transcribed into RNA then • Translated into polypeptides • What is the language of nucleic acids? • In DNA, it is the linear sequence of nucleotide bases • A typical gene consists of 1000s of nucleotides • A single DNA molecule may contains 1000s of genes • When DNA is transcribed, the result is an RNA • RNA is then translated into sequence of amino acids into a polypeptide • What are the rules for translating RNA message? • A codon is a triplet of bases which codes for one amino acid

23. The Genetic Code • Genetic code is the set of rules relating to nucleotide sequence to amino acid sequence • Is shared by all organisms • Of the 64 triplets • 61 code for amino acids • 3 are stop codons, indicating the end of the polypeptide

26. 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 • Initiation • The “start transcribing” signal is a nucleotide sequence called a promoter • The 1st phase of transcription is initiation in which • RNA polymerase attaches to the promoter • RNA synthesis begins

27. Transcription: from DNA to RNA • RNA Elongation • 2nd phase • RNA grows longer • RNA strand peels away form the DNA template • Termination • 3rd phase • RNA polymerases reaches a sequence of DNA bases called terminator • Polymerases detaches from RNA • The DNA strands rejoin

28. The Processing of Eukaryotic RNA • After transcription • Eukaryotic cells process RNA • Prokaryotic cells do not • RNA processing include • Adding a cap and tail • Removing introns • Splicing exons together to form messenger RNA (mRNA)

31. Translation: The Players • Translation is the conversion from the nucleic acid language to the protein language • Translation requires • mRNA • ATP • Enzymes • Ribosomes • Transfer RNA (rRNA)

32. Transfer RNA (tRNA) • Acts as a molecular interpreter • Carries amino acids • Matches amino acids with codons in mRNA using anticodons

33. Ribosomes • Organelles that • Coordinate the functions of mRNA and tRNA • Are made of two protein strands • Contain ribosomal RNA (rRNA) • A fully assembled ribosome holds tRNA and mRNA for using in translation

35. Translation: The Process • Divided into three phases • 1. Initiation • Brings together • mRNA • The first aa, Met, with its attached tRNA • 2 subunits of ribosome • mRNA has cap and tail to help it bind to ribosome • Occurs in two steps • mRNA moleucle binds to a small ribosomal subunti then an initiator tRNA binds to the start codon • Large ribosomal subunit binds, creating a funcitonal ribosome

37. Translation: The Process • Divided into three phases • 2. Elongation • Occurs in 3 steps • Codon recognation: • the anticodon of an incomoingtRNA pairs w/ mRNA codon at the A site of the ribosome • Peptide bond formation: • Polypeptide leaves the tRNA in the P site and attaches to the aa on the tRNA in the A site • The ribosome catalyzes the bond formation between 2 aa • Translocation • The P site tRNA leaves the ribosome • The tRNA carrying the polypeptide moves from the A to the P site

39. Translation: The Process • Divided into three phases • 3. Termination • Elongation continues until • Ribosomes reaches a stop codon • The completed polypeptide is freed • The ribosome splits into its subunits

40. 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 nucleotidepair, 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 • Insertions and deletions can: • Change the reading frame of the genetic message • Lead to disastrous effects • 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.

42. 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.

43. Bacteriophages • Bacteriophages, or phages, are viruses that attack bacteria. • 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

44. Plant Viruses • Viruses that infect plants can: • Stunt growth • Diminish plant yields • Spread throughout the entire plant • Viral plant diseases: • Have no cure • Are best prevented by producing plants that resist viral infection

46. Animal Viruses • Viruses that infect animals are: • Common causes of disease • May have RNA or DNA genomes • Some animal viruses steal a bit of host cell membrane as a protective envelope. • The reproductive cycle of an enveloped RNA virus can be broken into seven steps.

48. HIV, the AIDS Virus • HIV is a retrovirus, an RNA virus that reproduces by means of a DNA molecule. • Retroviruses use the enzyme reverse transcriptase to synthesize DNA on an RNA template. • HIV steals a bit of host cell membrane as a protective envelope. • The behavior of HIV nucleic acid in an infected cell can be broken into six steps. • AIDS (acquired immune deficiency syndrome) is: • Caused by HIV infection and • Treated with drugs that interfere with the reproduction of the virus