DNA: STRUCTURE

WHAT IS DNA?

§DNA is the molecule of life§ Found in the nucleus of a cell§Contains genetic code

§DNA = deoxyribose nucleic acid§Contains deoxyribose (a 5-carbon sugar) §A nucleic acid (made up of nucleotides)

BRIEF HISTORY OF DNA STRUCTUREJames Watson & Francis Crick

(1953)§ Determined the shape of DNA§ Discovered that the shape of DNA

was a double helix

§ Watson & Crick relied on the works of other scientists to develop their DNA model

§ Rosalind Franklin & Maurice Wilkins’s x-ray of DNA crystals

THE DOUBLE HELIX

§ The sides of the double helix are composed of sugar molecules & phosphates§ aka the “sugar phosphate backbone”

§ Nitrogen bases make up the rungs (steps) of the ladder

§ The two strands are held together at the base by hydrogen bonds

DNA NUCLEOTIDESEach nucleotide consists of three parts:

§ 5 carbon sugar§ Phosphate group § Nitrogenous base

Four possible nitrogen bases:§ Adenine (A)§ Guanine (G)§ Cytosine (C)§ Thymine (T)

These 4 different bases allow for genetic

diversity

COMPLIMENTARY BASES§ DNA strands are connected by the

nitrogen base§ Adenine can only pair with thymine

(and vice versa)§ Guanine can only pair with cytosine

(and vice versa)

§ Base sequence: the order of nitrogenous bases on a chain of DNA

§ 2 reasons complimentary base pairing is important in DNA structure and function:1. The hydrogen bonds between

the base pairs help to hold the two strands together

2. The complimentary nature helps to explain how DNA replicates itself before a cell divides

PURINES VS. PYRIMIDINESThe molecular structure of the 4 bases fall under two categories:§ Purines: double ring structures

à Adenine & guanine

§ Pyrimidines: single ring structuresà Thymine & cytosine

§ Purines always bond with pyrimidines

CHARGAFF’S RULEErwin Chargaff (1949)§ Observed that the percentage of adenine equals the percentage of thymine

& the percentage of cytosine equals the percentage of guanine§ Key to understanding the structure of DNA

Example: If a DNA molecule contains 28% cytosine, we can figure out how much guanine, thymine, and adenine are present in the molecule.

Cytosine 28%According to

Chargaff’s rule, how much guanine is present?

Guanine 28%

How much of the DNA have we

accounted for so far?

28% + 28% = 56% This tells us that 44% of

the molecule must be made up of adenine and

thymine.

Adenine 22%

Thymine 22%

§ Hydrogen bonds are the weakest type of bond

§ DNA comes apart during DNA replication

DNA Paper Model§ In small groups, students will

construct a paper model of a DNA molecule

§ Students will be able to better understand the base-pairing rule and overall structure of DNA