DNA Structure

Discovering the structure of DNA DNA = Deoxyribose nucleic acid Made out of sugars (deoxyribose), phosphates and nitrogen bases

DNA Structure DNA consists of two molecules that are arranged into a ladder-like structure called a Double Helix. In 1950’s, James Watson & Francis Crick determined model of DNA A molecule of DNA is made up of millions of tiny subunits called Nucleotides. Each nucleotide consists of: 1. Phosphate group 2. 5-carbon (Pentose) sugar 3. Nitrogenous base

Discovering the structure of DNA Structure was discovered in 1953 by James Watson and Francis Crick

Nucleotides Phosphate Nitrogenous Base Sugar

copyright cmassengale DNA Nucleotide O=P-O O Phosphate Group N Nitrogenous base (A, G, C, or T) O CH2 O C1 C4 C3 C2 5 Sugar (deoxyribose) copyright cmassengale 6 6

Nucleotides The phosphate and sugar form the backbone of the DNA molecule, whereas the bases form the “rungs”. There are four types of nitrogenous bases.

Nucleotides A Adenine T Thymine C Cytosine G Guanine

Nucleotides Each base will only bond with one other specific base. Adenine (A) Thymine (T) Cytosine (C) Guanine (G) Form a base pair. Form a base pair.

DNA Structure Because of this complementary base pairing, the order of the bases in one strand determines the order of the bases in the other strand.

Chargaff’s Rule A == T C == G T == A G == C Erwin Chargaff – Scientist: Discovered base-pairing rules The bases form the “rungs” on the DNA ladder by complementary pairing A == T C == G T == A G == C Sugar-phosphate backbone

Discovering the structure of DNA Rosalind Franklin’s DNA image “Chargoff’s rule” A = T & C = G

A C T G G A T C

copyright cmassengale Nitrogenous Bases Double ring PURINES Adenine (A) Guanine (G) Single ring PYRIMIDINES Thymine (T) Cytosine (C) A or G T or C copyright cmassengale 14 14

Four Nitrogen Bases of DNA Pyrimidines: Nitrogeneous bases That have a SINGLE Ring of Carbon and Nitrogen Atoms. Ex. Cytosine and Thyamine Purines: That have a Double Ex. Adenine and Guanine

copyright cmassengale Base-Pairings Purines only pair with Pyrimidines Three hydrogen bonds required to bond Guanine & Cytosine C G 3 H-bonds copyright cmassengale 16 16

How Chemical Bonds hold DNA together Covalent bonds- between sugar & phosphates of 2 nucleotides Hydrogen bonds- between complementary nitrogenous bases www.molecularstation.com/dna/dna-structure/

DNA Structure To crack the genetic code found in DNA we need to look at the sequence of bases. The bases are arranged in triplets called codons. A G G - C T C - A A G - T C C - T A G T C C - G A G - T T C - A G G - A T C

DNA Structure A gene is a section of DNA that codes for a protein. Each unique gene has a unique sequence of bases. This unique sequence of bases will code for the production of a unique protein. It is these proteins and combination of proteins that give us a unique phenotype.

DNA Gene Protein Trait