1. 8.2 Structure of DNA KEY CONCEPT (K.C.) DNA structure is the same in all organisms. MAIN IDEA S (M.I.) 1. 2. 3. continue

2. 8.2 Structure of DNA Review VOCABULARY Nucleotide Small units, or monomers, that make up DNA. Each has a phosphate group, a sugar, and a nitrogen-containing base. Double Helix Shape that DNA creates when 2 strands of DNA wind around each other like a twisted ladder. Model that compares the structure of a DNA molecule. Base Pairing Rules Rule that describes how nucleotides form bonds in DNA; adenine (A) always pairs with thymine (T), while guanine (G) always pairs with cytosine (C). Covalent Bond Chemical bond formed when two atoms share one or more pairs of electrons Hydrogen Bond Attraction between a slightly positive hydrogen atom and a slightly negative atom. New VOCABULARY

3. 8.2 Structure of DNA Main Idea (M.I.) DNA is composed of four types of nucleotides. DNA is made up of a long chain of nucleotides. Each nucleotide has three parts. –a phosphate group –a deoxyribose sugar –a nitrogen-containing base phosphate group deoxyribose (sugar) nitrogen-containing base

4. 8.2 Structure of DNA The nitrogen containing bases are the only difference in the four nucleotides.

5. 8.2 Structure of DNA Main Idea (M.I.) Watson and Crick developed an accurate model of DNA’s three-dimensional structure. Watson and Crick’s discovery built on the work of Rosalind Franklin and Erwin Chargaff. Franklin studied DNA using x-rays; the x-ray images suggested that DNA was a double helix of even width. For a long time scientists hypothesized that DNA in all organisms was exactly the same. Chargaff found that the same 4 bases are found in the DNA of all organisms. It is the proportion of the 4 bases that differs from organism to organism. Chargaff’s rules stated that A=T and C=G.

6. 8.2 Structure of DNA Working with these findings of other scientists, Watson and Crick realized that DNA is a double helix that is made up of a sugar- phosphate backbone on the outside, with bases on the inside.

7. 8.2 Structure of DNA T A C G Main Idea (M.I.) Nucleotides always pair in the same way. DNA nucleotides of a single strand are joined together by covalent bonds. –These connect the sugar of one nucleotide to the phosphate of the next nucleotide. –The sugars and phosphates alternate to form the sides of the double helix. –The double helix is held strongly together by the many hydrogen bonds between the bases in the middle. –This is how the DNA structure is maintained.

8. 8.2 Structure of DNA The base-pairing rules show how nucleotides always pair up in DNA. A pairs with T : Apples grown on Trees C pairs with G: Cars need Gas Because a pyrimidine (single ring) pairs with a purine (double ring), the helix has a uniform width. T A C G

9. 8.2 Structure of DNA The backbone is connected by covalent bonds. hydrogen bond covalent bond The bases are connected by hydrogen bonds. A and T form 2 hydrogen bonds. C and G form 3 hydrogen bonds.