Chapter 7 I. DNA Structure DNA DNA = Deoxyribonucleic acid Provides instructions to the cell. Trait = An inherited characteristic (physical, behavioral, chemical) Examples: height, weight, strength, metabolism, personality. DNA is the genetic material which codes for traits. Heredity = The passing of traits from one generation to the next. DNA controls heredity. 1
Discovery of DNA For many years scientists believed the genetic material to be proteins. 1928 Frederick Griffith Studied a type of bacteria which caused pneumonia in mammals. Transformation Harmless bacteria changed into deadly bacteria. What caused the transformation? Discovery of DNA 1944 Avery and McCarty Repeated Griffith s experiment with some changes. Treated the heat-killed bacteria with enzymes which destroyed lipids, proteins and carbohydrates transformation still occurred. Treated the heat killed bacteria with enzymes which destroyed RNA transformation still occurred. Treated the heat killed bacteria with enzymes which destroyed DNA Transformation did not occur!! Conclusion DNA is the transforming material! 2
Discovery of DNA 1952 Alfred Hershey and Martha Chase Bacteriophages Viruses which infect bacteria. 35S radioactively labels the protein coat of the virus. 32P radioactively labels the DNA of the virus. Only 32P found inside of the bacteria. Proves conclusively that DNA is the genetic material. Discovery of the Structure of DNA 1950 s Rosalind Franklin Extracted DNA and took pictures of it using a beam of xrays. The x-rays scattered producing a pattern which provided clues to the 3-D structure of DNA. 3
Discovery of the Structure of DNA Francis Crick A British Physicist James Watson An American Biochemist Both worked to together in Cambridge England trying to discover the 3-D structure of DNA. Watson visited Franklin s colleague Maurice Wilkins at King s College in London and viewed Franklin s photographs. Watson was familiar with interpreting these photos and recognized the helical shape of the DNA. Discovery of the Structure of DNA Watson and Crick built models of DNA and matched it to the patterns in the x-ray photograph. James Watson (born 1928) Francis Crick (1916-2004) 4
The monomer of DNA is a nucleotide. A nucleotide consists of Phosphate group Deoxyribose (a sugar) Nitogenous base There are two sides to a nucleotide. The phosphate group is attached to the 5' carbon. A hydroxyl group (-OH) is attached to the 3' carbon. The 5 side (phosphate) of one nucleotide joins together with the 3 (Hydroxyl) side of another nucleotide Nucleotides join together by dehydration synthesis. A polymer of nucleotides make up a molecule of DNA 5
There are two strands of DNA in a DNA molecule. The sides of the molecule consist of alternating sugar and phosphate. Called a sugar phosphate backbone. The center of the molecule contains the nitrogenous bases. Base Pairing - The nitrogenous bases hydrogen bond to each other. There are two types of nitrogenous bases. Purines Adenine(A) and Guanine (G) Pyrimidines Thymine (T) and Cytosine ( C ) 6
Base Pairing Rules - A Purine bonds to a Pyrimidine Adenine +Thymine (Two hydrogen bonds) Guanine +Cyotosine (Three hydrogen bonds) Anti-parallel - The sugar phosphate backbones of DNA run in opposite directions One side runs 5 to 3 The other side runs 3 to 5 7
The structure of DNA A DNA molecule is shaped like a double helix. The nitrogenous bases bind together on the inside of the the double helix following the base pairing rules. Watson and Crick s Discovery Watson and Crick discovered that DNA is shaped like a double helix Watson and Crick discovered that the nitrogenous bases are on the inside of the double helix. Watson and Crick discovered that Adenine binds to Thymine Guanine binds to Cytosine. 8
Genes and Chromosomes Gene - A sequence of nitrogenous bases which code for a trait. A DNA molecule contains many genes. Each gene has a unique sequence of bases. Genes and Chromosomes Each DNA molecule makes up a chromosome Chromosome = A strand of DNA Each cell in our body contains 23 pairs of chromosomes = 46 in total. Each chromosome contains many genes 9
Chromosome Genes and Chromosomes DNA is wrapped around proteins called histones. Histones enable DNA to fit inside the nucleus. DNA The combination of DNA and a histone is called a nucleosome. Histone DNA and the histones coil up to form a chromosome. Nucleosome 10