Unlocking DNA's Mysteries: From Griffith to Watson & Crick

1. Chapter 10 SPI 3210.4.1Identify the structure and function of DNA. SPI 3210.4.2 Associate the process of DNA replication with its biological significance. SPI 3210.4.3 Recognize the interactions between DNA and RNA during protein synthesis. DNA, RNA, and Protein Synthesis

2. The History of DNA Fredrick Griffith(1928) working with Streptococcus pneumoniae conducted transformation experimentsof virulent & nonvirulent bacterial strains Oswald Avery(1949) showed that DNA, not protein or RNA, was responsible for the transformation seen in Griffith’s experiments Hershey & Chase (1952) used bacteriophages (viruses) to show that DNA, not protein, was the cell’s hereditary material Rosalind Franklin (1951) used x-rays to photograph DNAcrystals Erwin Chargaff (1950) determined that the amount of A=T and amount of C=G in DNA; called Chargaff’s Rule Watson & Crick (1953)discovered double helix shape of DNA (A pairs with T; C pairs with G) & built the 1st model

3. Griffith’s Experiments In 1928, Fredrick Griffithwas studying the bacteria (S. pneumoniae) that cause pneumonia. - Smooth strain (virulent)  Mouse dies - Rough strain (non-virulent)  Mouse lives - Heat-killed smooth strain  Mouse lives - Heath-killed smooth + rough strains  Mouse dies

4. Griffith’s Experiments Heat-killed, disease-causing bacteria (smooth colonies) Harmless bacteria (rough colonies) Harmless bacteria (rough colonies) Control(no growth) Heat-killed, disease-causing bacteria (smooth colonies) Disease-causing bacteria (smooth colonies) Dies of pneumonia Dies of pneumonia Lives Lives Live, disease-causingbacteria (smooth colonies)

5. Griffith’s Experiments Griffith called this process transformation:one type of bacteria turned into another ex. Rough strain turned into smooth strain when the two were mixed Through additional experiments, Oswald Avery concluded that DNA, not RNA or protein, is responsible for transformation in bacteria • Only bacteria with undestroyed DNA could transform other bacteria • Destroyed RNA and protein didn’t make a difference

6. Hershey-Chase Experiments • Hershey and Chase confirmed that DNA, not protein, is the hereditary material by conducting experiments with bacteriophages

7. Watson & Crick • Watson and Crick created a model of DNA by using Rosalind Franklin’s DNA diffraction x-rays • Watson and Crick are associated with the discovery of DNA as a double-helix (two strands wound around each other like a spiral staircase)

8. James Watson (1928-?) 89-years old Francis Crick (1916-2004) Rosalind Franklin (1920-1958)

9. DNA Structure

10. DNA Structure • A DNA nucleotideis made of three components: • 5-carbon deoxyribosesugar • Phosphate group • One of four nitrogenous bases: adenine (A), guanine (G), cytosine (C), or thymine (T).

11. DNA Structure The two categories of nitrogenous bases are purinesand pyrimidines Purines: Double-ring structure - Adenine(A) and Guanine (G) Pyrimidines: Single-ring structure - Cytosine (C) and Thymine (T)

12. DNA Structure • Bonds Hold DNA Together • Nucleotides along each DNA strand are linked by covalent bonds • Complementary nitrogenous bases are bonded by hydrogen bonds • Hydrogen bonding between the complementary base pairs, G-C and A-T, holds the two strands of a DNA molecule together

13. DNA Structure: Antiparallel Strands • One strand of DNA goes from 5’ to 3’ • The other strand is opposite in direction,going 3’ to 5’ • This will be important to remember when it comes to DNA replication

14. Chargaff’s Rule • Base-pairing rules of DNA are known as Chargaff’s Rule • Adenine (A) always pairs with thymine (T) • Guanine (G) always pairs with cytosine (C) • Therefore, in a DNA strand, the % of adenine = % of thymine; % of cytosine = % of guanine • Example: If a particular DNA molecule is 30% adenine, then it is 30% thymine, which equals 60%. This means that the remaining 40% of the molecule is 20% cytosine and 20% guanine.

15. DNA Replication • DNA has to be copied before a cell divides • DNA is copied during the S or synthesis phase of interphase • New cells will need identical DNA strands • Steps of DNA Replication • Replication begins with the separation of the DNA strands by helicases • To begin process, RNA primase must first add a small primer (sequence of RNA) • Then, DNA polymerases form new strands by adding complementary nucleotides to each of the original strands.

16. 3’ Parental DNA Molecule 5’ Replication Fork 3’ 5’ DNA Replication • Begins at origin of replication • DNA strands are opened (“unzipped”) by helicases • Two strands open, forming replication forks (Y-shaped region) • New strands grow at the forks • DNA polymerase adds complementary nucleotides to new strands

17. DNA Replication

18. https://www.youtube.com/watch?v=Cw8GAPuhAk4 DNA Replication • DNA polymerase can only add nucleotides to the 3’ endof the DNA • This causes the NEW strand to be built in a 5’ to 3’ direction • Leading strand (built toward replication fork)completed in one piece • Lagging strand(built moving away from the replication fork) is made in sections called Okazaki fragments • RNA primers are removed and replaced with DNA sequences • DNA ligase joins segments together

19. DNA Replication

20. Errors in DNA Replication • Changes in DNA are called mutations • DNA proofreading and repair prevent many replication errors • Unrepaired mutations that affect genes that control cell division can cause diseases such as cancer

22. RNA: Structure and Function RNA, like DNA, consists of long chains of nucleotides Three differences between DNA and RNA - The sugar is ribose - Single-stranded - Contains uracilinstead of thymine *Base pairings are A-U and C-G

23. DNA vs. RNA

24. RNA: Structure and Function • Types of RNA • Cells have three major types of RNA: • messenger RNA(mRNA): carries the genetic message from nucleus to cytoplasm • ribosomal RNA (rRNA): component of ribosomes • transfer RNA (tRNA): carries amino acids to add to a growing polypeptide (protein) chain

25. Messenger RNA (mRNA) • Single, uncoiled, straight strand of nucleic acid • Copies DNA’s instructions in nucleus & carries them to the ribosomes in cytoplasm where proteins can be made • mRNA’s base sequence is translated into the amino acid sequence of a protein • Three consecutive bases on mRNA called a codon (e.g. UAA, CGC, AGU) • Reusable

26. Ribosomal RNA (rRNA) • rRNA & protein make up the large and small subunits of ribosomes • Globular shape • Ribosomes are the sites of translation (the making of proteins)

27. Transfer RNA (tRNA) • Clover-leafshape (folds up) • Single stranded molecule with attachment site at one end for an amino acid • Opposite end has three nucleotide bases called the anticodon • Anticodon will be complementary to the codons of the mRNA

28. Nuclear membrane DNA Transcription Pre-mRNA RNA Processing mRNA Ribosome Translation Protein The Central Dogma of Biology DNA mRNA  Protein

29. Transcription Translation

30. TRANSCRIPTION DNA mRNA

31. Transcription • During transcription, DNA acts as a template for directing the synthesis of mRNA • Happens IN THE NUCLEUS (know this!) Transcription: the copying of the DNA into a complementary strand of RNA - Uses the enzyme RNA polymerase

32. Transcription: Step 1 • RNA polymerase, an enzyme,binds to a region of DNA known as the promoter • The promoteris a sequence of DNA that tells transcription where to begin

33. Transcription: Step 2 • Multiple enzymes unwind and separate the two DNA strands • The RNA polymerase will use oneof the DNA strands as a template from which to build a strand of RNA (the mRNA)

34. Transcription: Step 3 • Using one of the DNA strands as a template, RNA polymerase adds nucleotides that are complementary to the DNA strand • G with C, and U with A • This continues until RNA polymerase reaches a DNA sequence known as the terminator/termination signal • RNA polymerase falls off, and the newly-formed mRNA strand is released

35. Transcription Summary https://www.youtube.com/watch?v=5MfSYnItYvg&app=desktop

36. Eukaryotic RNA is Modified • Before leaving the nucleus, the mRNA is edited • This is called splicing • Splicing involves removing (cutting out) portions of the mRNA known as introns • The exons are fused together • One end of the mRNA also receives a poly-A tail, and the other end receives a 5’ cap (bases and chemical groups to prevent mRNA destruction) Remember it this way: the “exons” get to “exit” the nucleus and be translated into proteins! The introns stay “in” the nucleus.

37. TRANSLATION mRNA  Protein

38. Translation • Translation is the process of decoding the mRNA into a polypeptide chain (protein) The Genetic Code The genetic code is read in three letter segments called codons There are 64 different codon possibilities that code for only 20 amino acids -AUG is the start codon -there are 3 stop codons- UAA, UAG, UGA

40. Translation: Step 1 (Initiation) Start codon • mRNAtravels from the nucleus to the cytoplasm • mRNA attaches to one end of a ribosome; calledinitiation • tRNAs attach the correct amino acid floating in the cytoplasm to themselves • The tRNA anticodon “reads”& temporarily attaches to the mRNA codon in the ribosome (methionine is the start codon)

41. Translation: Step 2 (Elongation) • 5. Two amino acids at a time are linkedtogether by peptide bonds to make polypeptide chains (protein subunits); called elongation

42. Translation: Step 2 (Elongation), cont. Growing polypeptide chain

43. Translation: Step 3 (Termination) • 7. Ribosomes move along the mRNAstrand until they reach a stop codon (UAA, UGA, or UAG);calledtermination 8. tRNA’s break loose from amino acid, leave the ribosome, & return to cytoplasm to pick up another amino acid https://www.youtube.com/watch?v=8dsTvBaUMvw&app=desktop

44. End Product –The Protein! • The end product of protein synthesis is a primary structureof a protein • A sequence of amino acids bonded together by peptide bonds

45. End Product –The Protein! • The linear, primary structure will fold up into a functional protein • Once the protein is properly folded into a tertiary or quaternary structure, it can perform its job! • STRUCTURE = FUNCTION https://www.youtube.com/watch?v=gG7uCskUOrA&app=desktop

46. Example: How Mutations Affect Protein Structure and Function Sickle-Cell Anemia

47. Example: How Mutations Affect Protein Structure and Function Cystic Fibrosis • The thick build-up of mucus occurs in the lungs, which affects breathing and can cause recurring respiratory infections • Mucus build-up also occurs in the pancreas, which prevents the normal secretion of necessary digestive enzymes. This can lead to digestive problems and poor growth/development. (Many CF patients are prescribed pancreatic enzymes)