Chapter 13 13-1 and 13-2 DNA History Structure and Replication The Experiment The disease causing kills the mice. When injected separately, neither the heat-killed disease-causing bacteria nor the live, harmless bacteria killed the mice. When the 2 types were injected together, the mice died. Griffith’s Results Some “factor” from the dead bacteria had transformed the harmless bacteria into disease-causing ones. The Conclusion From this, biologists inferred that genetic information could be transformed from one bacteria to another. 1944 The History of DNA Oswald Avery discovered that DNA is the nucleic acid that stores and transmits genetic information. The History of DNA 1950’s Erwin Chargaff The amount of adenine in DNA always equals the amount of thymine and the amount of cytosine always equals the amount of guanine. At this point, though, no one knew why A = T and C = G. The History of DNA 1952 Alfred Hershey and Martha Chase determined that DNA (not proteins) is the hereditary material of living organisms. The History of DNA 1950’s Rosalind Franklin crystallized DNA and was able to see the shape of DNA by taking an x-ray of the crystals. This technique is called x-ray diffraction. The History of DNA 1953 James Watson and Francis Crick Based on Franklin’s xrays, determined the exact shape of the DNA molecule. They described it as a double helix, in which two strands were wound around each other. (A twisted ladder) DNA: (Deoxyribonucleic Acid) DNA is an example of a nucleic acid. The basic units that make up nucleic acids are called nucleotides. What are nucleotides? DNA: (Deoxyribonucleic Acid) I. Nucleotides A. Nucleotides of DNA are made up of three materials: 1. a five-carbon sugar called deoxyribose 2. a phosphate group 3. A nitrogen base B. Example of one nucleotide: N.B. phosphate S If nucleotides are made up of the same three components, then what makes them different? DNA: (Deoxyribonucleic Acid) C. There are four types of nitrogen bases: 1. Adenine (A) 2. Thymine (T) 3. Guanine (G) 4. Cytosine (C) D. A and G are double-ring bases called purines. E. C and T are smaller, single-ring bases called pyrimidines. So there are four possible nucleotides, each containing one of these four bases. F. Nucleotides combine to form long chains, which combine to form one large molecule. G. The two chains are joined together by hydrogen bonds between nitrogen bases. II. DNA Structure A. Watson and Crick discovered hydrogen bonds can only form between certain base pairs. 1. A can only hydrogen bond with T 2. G can only hydrogen bond with C B. This principle, called base pairing, explained the findings of Chargaff years earlier. For every A there has to be a T and for every C there has to be a G. DNA Structure C. Double helix (a.k.a. twisted ladder) 1. It has 2 main sides or strands. (The sides are like the upright parts of a ladder.) 2. Hydrogen bonds between the nitrogen bases connect the two strands together. (The nitrogen bases form the rungs of the ladder) The Importance of DNA D. DNA is the molecule that makes up genes and determines the traits of all living things. E. Genes are made up of short pieces of DNA with a certain number of nitrogen bases. 1. The number and order of nitrogen bases is what leads to different traits in all living things. Video 13-2 DNA Replication The double helical structure of DNA also explains how DNA replicates or copies itself. Each strand of DNA has all the information needed to reconstruct the other half by the rules of base pairing. DNA Replication It takes about six hours to produce a complete copy of the genetic code. This process takes place before prophase of mitosis. It occurs during interphase. DNA Replication Every time a cell reproduces, DNA is copied. This is a process called replication. 1. The hydrogen bonds connecting the nitrogen bases are broken by enzymes (DNA helicase) and the 2 strands of the DNA molecule unwind. This allows the DNA to become “unzipped”. DNA Replication 2. The exposed strands contain the template of nucleotides that are to be copied to create an exact replica of the original DNA. Each strand serves as a template to make a new DNA molecule. nucleotide The DNA molecule unzips in both directions. DNA Replication 3. Loose nitrogen bases are present in the nucleus of the cell. The free nitrogen bases join with their complimentary nitrogen bases that are exposed on the open strand of DNA. new strand nucleotide DNA polymerase DNA Replication 4. This process continues until the entire DNA molecule has been unzipped and replicated. The enzyme DNA polymerase “proofreads” to make sure that there are no mistakes. DNA Replication 5. Replication is complete. Two new DNA molecules have formed. Each one is identical to the original and to each other. original strand new strand Two molecules of DNA BIG IDEA! When do we need to make a copy of DNA? When cells divide (so each cell will have a copy of the organism’s DNA to carry out life functions).