DNA and its Structure

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Vocabulary Nucleotide Deoxyribose Nitrogenous base Purine Pyrimidine Base-pairing rules Complementary Base pair Base Sequence

(Deoxyribonucleic acid) The Structure of DNA (Deoxyribonucleic acid) Where is DNA found? Eukaryotes: nucleus, chromosomes Prokaryotes: cytoplasm DNA is a polymer made up of many repeating single units (monomers) These monomers are called nucleotides Each nucleotide is made up of three things: 5-carbon sugar (deoxyribose) Phosphate group Nitrogenous base The “backbone” of DNA molecules is made of phosphate and sugar

Four Nitrogenous Bases Adenine (A) Guanine (G) Cytosine (C) Thymine (T) Chargaff’s Rule: Organisms should have 1:1 ratio purines and pyrimidines(base pair rule) A=T and G=C

The Double Helix 1950’s Rosalind Franklin used X-ray diffraction to study DNA Credit for the double helix was given to James Watson and Francis Crick Only Watson and Crick received the Nobel Prize in 1962.

The DNA molecule appears as a “twisted ladder” This shape is referred to as a double helix The phosphates and the sugars form the “handrails” of the helix with covalent bonds, and the hydrogen bonds are in the “steps” of the DNA helix.

Why is complementary base pairing important? Hydrogen bonds between the base pairs help hold the two strands together. 2) The complementary nature of DNA helps explain how DNA replicates before a cell divides.

Anti-parallel strands Video Anti-parallel strands We refer to DNA as being anti-parallel because the sides of the DNA are going in opposite directions. One side is travelling 5’ to 3’ 5 3 While the other side is travelling 3’ to 5’. 5 3

Bottom strand is opposite. It moves 3’ 5’ Looking within the sugars you can see there is an intrinsic difference between the two strands. Top strand is a 5’ carbon of each sugar is on the left and the 3’ carbon the right. Thus, it moves left to right 5’ to 3’ Bottom strand is opposite. It moves 3’ 5’

Nitrogenous Bases

DNA vs. RNA DNA RNA Sugar = deoxyribose Sugar = ribose DRAW DNA vs. RNA DNA RNA Sugar = deoxyribose Sugar = ribose Double-stranded molecule Single-stranded molecule Thymine bonds with adenine Uracil instead of thymine

Elbow Partner Group Work 1. Use the base-pairing rules to determine the base sequence that is complementary to the sequence below C-G-A-T-T-G 2. Distinguish between purines and pyrimidines. 3. A plant’s DNA has nucleotides that are 20% thymine. What percentage of guanine would be present?

Homework: 1. What piece of information did Rosalind Franklin have that helped Watson and Crick determine the double helix structure of DNA? 2. Name the three parts of a nucleotide. 3. Summarize the locations of covalent bonds and hydrogen bonds in a DNA molecule. 4. Describe why the two strands of the double helix are considered to be complementary. 5. State the base-paring rule in DNA. 6. How do the base-pairing rules relate to the structure of DNA?