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DNA Structure

DNA Structure. Frederick Griffith . In 1928, Frederick Griffith wanted to learn how certain types of bacteria produce pneumonia Griffith injected mice with disease-causing bacteria, the mice died of pneumonia Griffith injected mice with harmless bacteria, the mice didn’t get sick

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DNA Structure

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  1. DNA Structure

  2. Frederick Griffith • In 1928, Frederick Griffith wanted to learn how certain types of bacteria produce pneumonia • Griffith injected mice with disease-causing bacteria, the mice died of pneumonia • Griffith injected mice with harmless bacteria, the mice didn’t get sick • Griffith thought that the disease-causing bacteria might produce a poison

  3. Frederick Griffith • Griffith mixed heat-killed disease-causing bacteria with harmless bacteria and injected the mixture into mice • To his amazement, the mice developed bacteria and many died • He found that the lungs of those mice who died were not filled with harmless bacteria, but of disease-causing bacteria

  4. Frederick Griffith • The heat-killed bacteria had passed their disease-causing ability to the harmless strain • Griffith coined this process transformation because one strain of bacteria had been changed into another • Griffith hypothesized that when the two types of bacteria were mixed, some factor was transformed from the heat-killed cells into the live harmless bacteria

  5. Frederick Griffith • Griffith discovered that the ability to cause disease was inherited by the transformed bacteria’s offspring, the transforming factor was a gene

  6. Oswald Avery • In 1944, a group of scientists repeated Griffith’s work • Avery and his colleagues made an extract from the heat killed bacteria and treated the extract with enzymes that destroyed proteins, carbohydrates, lipids, and other molecules • Since transformation still occurred, these molecules were not responsible for the transformation

  7. Oswald Avery • Avery and other scientists repeated the experiment, this time using enzymes that targeted DNA • When they destroyed the nucleic acid DNA, transformation did not occur • Avery and other scientists discovered that the nucleic acid DNA stores and transmits the genetic information from one generation of an organism to the next

  8. The Hershey-Chase Experiment • In 1952, American scientists Alfred Hershey and Martha Chase collaborated in studying viruses • One type of virus that infects bacteria is known as a bacteriophage • Bacteriophages are composed of a DNA or RNA core and a protein coat • When a bacteriophage enters a bacterium, the virus attaches to the surface of the cell and injects its genetic information into it

  9. The Hershey-Chase Experiment • Hershey and Chase reasoned that if they could determine which part of the virus entered the infected cell, they would learn whether genes were made of protein or DNA • Viruses were grown in cultures containing radioactive isotopes of phosphorus-32 (32P) and sulfur-35 (32S) • Proteins contain almost no phosphorus and DNA contains no sulfur

  10. The Hershey-Chase Experiment • The radioactive substances were used as markers • If 32S was found in the bacteria, it would mean that the viruses’ bacteria had been injected into the bacteria • If 32P was found in the bacteria, then it was the DNA that had been injected

  11. The Hershey-Chase Experiment • Marked viruses were mixed with bacteria, after a few minutes viruses injected their genetic material • Viruses were separated were from bacteria, bacteria was tested for radioactivity • Nearly all the radioactivity in the bacteria was from phosphorus, the marker found in DNA • Hershey and Chase demonstrated that DNA, not protein, functions as the phage’s genetic material

  12. The Components and Structure of DNA • DNA is made up of nucleotides • Nucleotides are made up of three basic components: a 5-carbon sugar called deoxyribose, a phosphate group, and a nitrogenous base

  13. The Components and Structure of DNA • There are four kinds of nitrogenous bases in DNA • Adenine and Guanine are two nitrogenous bases that belong in a group of compounds known as purines • Cytosine and Thymine are known as pyrimidines • Purines have two rings in their structures, whereas pyrimidines have one ring

  14. Adenine, guanine, cytosine, and thymine are four kinds of _________ bases in DNA. Nitrogenous

  15. The Components and Structure of DNA • The phosphate of one nucleotide is attached to the sugar of the next nucleotide in line • The result is a “backbone” of alternating phosphates and sugars

  16. The Components and Structure of DNA • Erwin Chargaff discovered that the percentages of guanine and cytosine bases are almost equal in any sample of DNA • The same thing is true for the other two nucleotides, adenine and thymine • The observation that [A]=[T] and [G]=[C] became known as Chargaff’s rules

  17. The Components and Structure of DNA • According to Chargaff’s rules, the percentages of ______are equal to thymine and the percentages of ______ are equal to guanine in the DNA molecule. adenine cytosine

  18. Describe the components and structure of a DNA nucleotide. • DNA is made of nucleotides. Each nucleotide has three parts: a 5-carbon sugar, a phosphate group, and a nitrogenous base • The four nitrogenous bases are adenine and guanine (purines), cytosine and thymine (pyrimidines).

  19. The Components and Structure of DNA • Francis Crick and James Watson were trying to understand the structure of DNA by building 3 dimensional models of the molecule • Watson was shown a copy of Rosalind Franklin’s X-ray pattern of DNA

  20. The Components and Structure of DNA • Using clues from Franklin’s pattern, Watson and Crick had built a structural model that explained the puzzle of how DNA could carry information, and how it could be copied • Watson and Crick’s model of DNA was a double helix in which two strands were wound around each other

  21. The Components and Structure of DNA • Explain how Chargaff’s rules helped Watson and Crick model DNA. • The two stands of DNA are held together by hydrogen bonds between certain bases-A and T, and G and C-which explained Chargaff’s rules.

  22. The Double Helix • The two strands are held together by hydrogen bonds between the nitrogenous bases, which are paired in the interior of the double helix • Hydrogen bonds can form only between certain base pairs-adenine and thymine, and guanine and cytosine. • Base pairing principle that bonds DNA can form only between adenine and thymine and between cytosine and guanine

  23. DNA and Chromosomes • Eukaryotic chromosomes contain both DNA and proteins, packed together to form chromatin • Eukaryotic chromosomes contain DNA wrapped around proteins called histones (globular proteins that DNA tightly coils around to form chromosomes)

  24. DNA Replication • The parent molecule has two complimentary strands of DNA • The first step in replication is separation of the two DNA strands • Each parent strand now serves as a template that determines the order of nucleotides along a new complementary strand • Each parent stand produces two new complimentary strands following the rules of base pairing

  25. DNA Replication • What is the complimentary strand of bases for a strand with the bases TACGTT? • ATGCAA

  26. DNA Replication

  27. DNA Replication • The replication of a DNA molecule begins at special sites called origins of replication • Proteins recognize a stretch of DNA having a specific sequence of nucleotides • These proteins attach to the DNA and separate the two strands and open up a replication “bubble”

  28. DNA Replication • Once the two strands of the double helix have separated, two replication forks (a Y-shaped where the new strands of the DNA are elongating) form at the end of a replication “bubble” • As each new strand forms, new bases are added following the rules of base pairing

  29. DNA Replication • Elongation of new DNA at the replication fork is catalyzed by enzymes called DNA polymerases (enzymes that catalyzes the elongation of new DNA at a replication fork by the addition of nucleotides to the existing chain)

  30. DNA Replication separates • The DNA molecule _______, or unzips, into two strands ↓ Each strand of the DNA molecule serves as a _______, or model, to produce the new strands ↓ Two new ___________ strands are produced, following the rules of _________ template complimentary Base pairing

  31. DNA Replication

  32. DNA Replication • The two strands of DNA are anti-parallel (their sugar phosphate backbones run in opposite directions) • In the double helix, the two sugar-phosphate backbones are upside down relative to each other

  33. DNA Replication • The 5’ to 3’ direction of one strand runs counter to the 5’ to 3’ of the other

  34. DNA Replication • DNA polymerases only add nucleotides to the free 3’ end of a growing DNA strand, never to the 5’ end • A new DNA strand can only elongate in the 5’ to the 3’ direction

  35. DNA Replication • DNA polymerase adds nucleotides to a complimentary strand as the replication fork progresses • The DNA strand made by this mechanism is called the leading strand

  36. DNA Replication • To elongate the other new strand of DNA, DNA polymerase works along the other template strand in the direction away from the replication fork • DNA synthesized in this direction is called the lagging strand

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