DNA The Genetic Material Section 12 1 Structure

DNA: The Genetic Material Section 12 -1: Structure of DNA

Learning Targets • I can… 1. Describe experiments that led to the understanding of the structure of DNA 2. Model the structure and function of DNA

The Discovery • 1928 – 1944 – Frederick Griffith and Oswald Avery determined that DNA is the genetic material while working with vaccines. • 1950 s – It was generally accepted that genes are made of DNA – James Watson and Francis Crick pieced together the structure of DNA James Watson & Francis Crick

DNA Structure • DNA is a double helix molecule. • What is a double helix? – Double Helix: two strands twisted around each other like a winding staircase • Think of a ladder that twists around.

DNA Structure • Each strand is made up of nucleotides. – Nucleotide: subunits that make up DNA • 3 Parts of a nucleotide: 1. Phosphate group 2. 5 -carbon sugar molecule 3. Nitrogen containing base • 5 -carbon sugar – called deoxyribose • “DNA” = deoxyribonucleic acid

DNA Structure • Sugar and phosphate groups are the same for all nucleotides – Nitrogen bases can change • 4 Possible Nitrogen Bases: 1. 2. 3. 4. Adenine (A) Guanine (G) Thymine (T) Cytosine (C)

DNA Structure 2 Types of nitrogen bases– Purines vs. Pyrimidines • Purines: made of two rings containing nitrogen and carbon atoms • Pyrimidines: made of a single ring of nitrogen and carbon atoms – Purines = Adenine and Guanine {A & G} – Pyrimidines = Thymine and Cytosine {T & C}

DNA Structure • Sugar and phosphate groups = “side rails” of DNA “ladder” • Paired nitrogen bases = “rungs” or steps of DNA “ladder” • Nitrogen bases face each other and are attracted by weak hydrogen bonds, which holds the double helix together

DNA Structure • Other scientists contributed to the discovery of DNA’s structure 1. Ervin Chargaff (1949) a. In every organism, the amount of adenine always equals the amount of thymine (A = T) b. In every organism, the amount of guanine always equals the amount of cytosine (G = C)

DNA Structure • Other Scientists cont. 2. Maurice Wilkins and Rosalind Franklin (1952) a. Used a method called x-ray diffraction to take pictures of DNA molecules. b. Showed that DNA resembled a tightly coiled helix composed of chains of nucleotides

DNA Structure: • Other Scientists cont. 3. Watson and Crick (1953) a. Used Chargaff, Wilkins, and Franklin’s observations b. Built a tin-and-wire model 2 Components: 1. Double Helix (“spiral staircase”) of nucleotides twisted around a central axis a. Supported Wilkins and Franklin 2. Pairing Between Bases a. Supported Chargaff

DNA Structure • Base-Pairing Rules – Adenine always pairs with Thymine • A – T or T – A – Guanine always pairs with Cytosine • G – C or C – G • The two strands of nucleotides making up a DNA molecule are complementary – The sequence of bases on one side determine the sequence on the other strand.

Practice Pairing Bases • #1)Below is one side of a strand of DNA. Based on the base-pairing rules, what would the other strand look like?

Practice Pairing Bases • Below is one side of a strand of DNA. Based on the base-pairing rules, what would the other strand look like?

Practice Pairing Bases Practice pairing nitrogen bases by completing the following strands. Find the complimentary bases. Example #2: TCGAACT Answer #2 AGCTTGA Example #3: CTTAGGCT Answer #3 GAATCCGA