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DNA: the Molecule of Heredity

DNA: the Molecule of Heredity. What is DNA?. Deoxyribonucleic acid DNA determines an organism’s traits DNA achieves control by producing proteins Remember: proteins give us structural building material and allow function (enzymes) DNA is the information for life.

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DNA: the Molecule of Heredity

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  1. DNA: the Molecule of Heredity

  2. What is DNA? • Deoxyribonucleic acid • DNA determines an organism’s traits • DNA achieves control by producing proteins • Remember: proteins give us structural building material and allow function (enzymes) • DNA is the information for life

  3. Time Line – Early History of Genetics Key Players1840’s MendelEarly 1900’s T.H. Morgan1928 Griffith1944 Avery1952 Hersey and Chase1947 ChargaffEarly 1950’s Franklin and WilkinsEarly 1950’s Pauling1953 Watson and Crick Place these names and dates on your timeline under “Scientists”

  4. DNA is the genetic material • Early 1900’s, the identification of the molecules of inheritance loomed as a major challenge to biologists • T. H. Morgan’s group showed that genes are located on chromosomes, the two components of chromosomes—DNA and protein—became candidates for the genetic material

  5. Early History of Genetics • The discovery of the genetic role of DNA began with research by Frederick Griffith in 1928 • He worked with two strains of a bacterium, one pathogenic and one harmless. He did transformation experiments • Basically he found that harmless bacteria became deadly when they took in DNA from dead pathogenic bacteria

  6. Fig. 16-2 Mixture of heat-killed S cells and living R cells EXPERIMENT Living R cells (control) Living S cells (control) Heat-killed S cells (control) Avery - 1944 He separated the components of the bacteria and found only the DNA extract caused mice to die RESULTS Mouse dies Mouse healthy Mouse healthy Mouse dies Living S cells

  7. Evidence That Viral DNA Can Program Cells • More evidence for DNA as the genetic material came from studies of viruses that infect bacteria • Such viruses, called bacteriophages (or phages), are widely used in molecular genetics research

  8. Fig. 16-4-3 Hersey and Chase EXPERIMENT Empty protein shell Radioactivity (phage protein) in liquid Radioactive protein Phage Bacterial cell DNA Batch 1: radioactive sulfur (35S) Phage DNA Centrifuge Pellet (bacterial cells and contents) Radioactive DNA Batch 2: radioactive phosphorus (32P) Centrifuge Radioactivity (phage DNA) in pellet Pellet

  9. Next Steps… What is DNA made of? • It was known that DNA is a polymer of nucleotides, each consisting of a nitrogenous base, a sugar, and a phosphate group • In 1950, Erwin Chargaff reported that DNA composition varies from one species to the next, however that the nitrogen based are found in predictable ratios: A = T and C = G

  10. Finding the Structure of DNA • After most biologists became convinced that DNA was the genetic material, the challenge was to determine how its structure accounts for its role • Maurice Wilkins and Rosalind Franklin were using a technique called X-ray crystallography to study molecular structure • Franklin produced a pictureof the DNA molecule using this technique

  11. The Discovery of DNA • Watson and Crick – 1953 • Double Helix – long twisted zipper Segment with James Watson

  12. Structure of DNA • DNA is a long molecule • Composed of nucleotides • Simple sugar – deoxyribose • Phosphate group • Nitrogen base – Adenine - Guanine - Cytosine - Thymine

  13. DNA Structure Cont. • Animation on how DNA is packaged into the nucleus

  14. S – A ..…T – S | | PP | | S – G ..…C – S | | PP | | S – C ..…G – S | | PP A pairs with T C pairs with G Weak Hydrogen Bond

  15. DNA Instructions for life • The sequence of nitrogen bases forms the genetic instructions for an organism A-T-T-G-A-C is different than T-T-C-A-A-G They code for different proteins and therefore structure and function of an organism

  16. How can we use DNA? • Nucleotide sequences can be used to determine evolutionary relationships • Organisms that are closely related have similar DNA • Ex. Gorilla and Chimp – very similar Gorilla and Rose Bush – very different • It can be used to determine if two people are related • DNA can be used to compare DNA from a crime scene to DNA from a suspect

  17. Complementary Strands • If one side of the DNA molecule consisted of the following nucleotide bases, what would the other side be? • ATC CTG GAT TAT GAC CAT ATG

  18. DNA Replication

  19. DNA Replication • You have learned that cells divide through the process of mitosis and meiosis • In order to do this, each cell has to make a copy of its DNA • DNA is copied through the process of DNA Replication • What might happen DNA replication did not occur prior to cell division?

  20. How DNA Replicates • Remember: DNA is composed of two strands • A pairs with T • C pairs with G • So if you know the order of bases on one side, you know the order on the other side (the complementary strand) • During replication, each strand serves as a pattern

  21. Fig. 16-9-1 A T C G T A A T C G (a) Parent molecule *What has to happen first in to make a copy of the DNA?

  22. Fig. 16-9-2 A T T A C G G C A T A T T A T A C C G G (b) Separation of strands (a) Parent molecule What type of molecule might help the two sides of the DNA molecule separate?

  23. Fig. 16-9-3 A T A T A T A T C G C G C G C G A T A T A A T T T A T A T T A A C C G C G C G G (c) “Daughter” DNA molecules, each consisting of one parental strand and one new strand (b) Separation of strands (a) Parent molecule What types of molecules might be used to add nucleotides and bind the sides together?

  24. Enzymes involved in DNA Replication • Helicase – unwinds the DNA strand to begin replication(it’s like unzipping a zipper) • DNA Polymerase – adds nucleotides, one at a time to the open DNA strand (in humans up to 50 nucleotides per second) • Ligase - joins the sugar-phosphate backbones of the newly formed strand.(it’s like gluing the sides together)

  25. Steps of DNA Replication • Step 1 – An enzyme breaks the H+ bonds between the nitrogen bases that holds the two strands together (un-zipping the molecule) • Step 2 – Free floating nucleotides in the cell bond to the complementary bases on each of the original strands • Step 3 – An enzyme secures the two strands together, forming two new chains

  26. DNA Replication Cont. • DNA replication results in the formation of two identical strands from the one original DNA molecule. What do you think the word “semiconservative” means?

  27. DNA Replication is Semiconservative • Watson and Crick’s semiconservative model of replication predicts that when a double helix replicates, each daughter molecule will have one old strand (derived or “conserved” from the parent molecule) and one newly made strand

  28. DNA replication animation Animation 2 HHMI animation

  29. Compare the two new strands of DNA. Are they the same or different? Why? A T A T A T A T C G C G C G C G A T A T A A T T T A T A T T A A C C G C G C G G (c) “Daughter” DNA molecules, each consisting of one parental strand and one new strand (b) Separation of strands (a) Parent molecule

  30. From DNA to Protein

  31. From DNA to Protein • The sequence of nucleotides in DNA contains information that produces proteins • Proteins • Structures • Enzymes • By controlling protein production, DNA controls cells

  32. Different from DNA in 3 ways RNA – single strand Sugar in RNA is ribose (DNA = deoxyribose) RNA has uricil (U) instead of thymine RNA

  33. The cell works like a factory • DNA provides “workers” with instructions for making proteins • “workers: bring over the parts (amino acids) to the assembly line • Workers = RNA

  34. 3 Types of RNA • Messenger RNA (mRNA) • brings the info from the nucleus to the factory floor (cytoplasm) • Ribosomal RNA (rRNA) – ribosomes are made of rRNA • Clamp onto mRNA and use its info to assemble amino acids • Transfer RNA (tRNA) – “supplier” • Transports amino acids to the ribosomes where they are assembled into proteins

  35. RNA Transcription • Read steps in figure 11.6 (pg 296) • Explain how it is different from DNA replication • Animation of Transcription • HHMI animation

  36. The role of tRNA • For proteins to be built, the 20 different amino acids dissolved in the cytoplasm must be brought to the ribosomes • This is the role of tRNA

  37. tRNA • Composed of about 80 nucleotides • Each tRNA only recognizes only one amino acid • The amino acid bonds to the tRNA • Located on the base of the tRNA molecule are three nitrogen bases, called an anticodon, that pair up with an mRNA codon during translation

  38. tRNA • Basically, the tRNA molecule transfers the information for making proteins to the correct codon on the mRNA. • If the mRNA has the codon for that particular amino acid, the tRNA binds, if it does not, the tRNA doesn’t bind and the amino acid that the tRNA is carrying is not made.

  39. Amino Acids to Proteins • Proteins are made in the Ribosomes • Proteins are made of Amino Acids • As multiple tRNA molecules attach to the mRNA, an enzyme joins the two amino acids by forming a peptide bond.

  40. Translation of DNA to Protein • Translation Animation • HHMI animation

  41. A code is needed to convert the language of mRNA into the language of proteins  amino acids There are 20 different amino acid mRNA only has 4 bases (AUCG) The Genetic Code

  42. Scientist found that a group of 3 nucleotides codes for 1 amino acid Each set of 3 nucleotides that code for an amino acid is called a codon The Genetic Code Cont.

  43. The Genetic Code Cont. • Some codons don’t code for amino acids, they are instructions for assembling proteins • Stop codon = UAA • Start codon = AUG

  44. Genetic Code Cont. • All organisms use the same genetic code for assembling proteins • UAC = tyrosine in humans, birch trees, and bacteria

  45. Genetic Code Cont. Try these:

  46. Critical Thinking Questions • How specific are the tRNA molecules? • How does energy play a role in all this hustle and bustle? • How does translation begin and end? • What happens to the mRNA strands?

  47. Compare and contrast Transcription and Translation

  48. Compare and Contrast Replication and Protein Synthesis

  49. Read the Help Wanted ad below. Based on your notes, tell me “who” is qualified to fill each position. Your choices are DNA, tRNA, and mRNA.Help Wanted! • Positions Available in the genetics industry. Hundreds of entry-level openings for tireless workers. No previous experience necessary. Must be able to transcribe code in a nuclear environment. The ability to work in close association with ribosomes is a must. • Accuracy and Speed vital for this job in the field of translation. Applicants must demonstrate skills in transporting and positioning amino acids. Salary commensurate with experience. • Executive Position available. Must be able to maintain genetic continuity through replication and control cellular activity by regulation of enzyme production. Limited number of openings. All benefits. • Supervisor of production of proteins—all shifts. Must be able to follow exact directions from double-stranded template. Travel from nucleus to the cytoplasm is additional job benefit.

  50. Central Dogma of Genetics

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