• •

• • 1

Chapter 7

I. DNA Structure

DNA• DNA = Deoxyribonucleic acid• Provides instructions to the cell.• Trait = An inherited characteristic

(physical, behavioral, chemical)– Examples: height, weight, strength,

metabolism, personality….• DNA is the genetic material which

codes for traits.• Heredity = The passing of traits

from one generation to the next.• DNA controls heredity.

• •

• • 2

Discovery of DNA• For many years scientists

believed the genetic materialto be proteins.

• 1928 – Frederick Griffith– Studied a type of bacteria

which caused pneumonia inmammals.

– Transformation Harmlessbacteria changed into deadlybacteria.

– What caused thetransformation?

Discovery of DNA

• 1944 – Avery and McCarty– Repeated Griffith’s experiment with some changes.– Treated the heat-killed bacteria with enzymes which

destroyed lipids, proteins and carbohydrates transformation still occurred.

– Treated the heat killed bacteria with enzymes whichdestroyed RNA transformation still occurred.

– Treated the heat killed bacteria with enzymes whichdestroyed DNA Transformation did not occur!!

– Conclusion DNA is the transforming material!

• •

• • 3

Discovery of DNA• 1952 – Alfred Hershey

and Martha Chase• Bacteriophages –

Viruses which infectbacteria.

• 35S radioactively labelsthe protein coat of thevirus.

• 32P radioactively labelsthe DNA of the virus.

• Only 32P found inside ofthe bacteria.

• Proves conclusivelythat DNA is thegenetic material.

Discovery of the Structure of DNA

• 1950’s RosalindFranklin– Extracted DNA and

took pictures of itusing a beam of x-rays.

– The x-rays scatteredproducing a patternwhich provided cluesto the 3-D structureof DNA.

• •

• • 4

Discovery of the Structure of DNA

• Francis Crick – A British Physicist• James Watson – An American Biochemist• Both worked to together in Cambridge

England trying to discover the 3-D structure ofDNA.

• Watson visited Franklin’s colleague MauriceWilkins at King’s College in London andviewed Franklin’s photographs.

• Watson was familiar with interpreting thesephotos and recognized the helical shape ofthe DNA.

Discovery of the Structure of DNA• Watson and Crick built models of DNA and

matched it to the patterns in the x-rayphotograph.

James Watson(born 1928)

Francis Crick

(1916 - 2004)

• •

• • 5

The Structure of DNA• The monomer of DNA is a

nucleotide.• A nucleotide consists of

– Phosphate group– Deoxyribose (a sugar)– Nitogenous base

• There are two sides to anucleotide.– The phosphate group is

attached to the 5' carbon.– A hydroxyl group (-OH) is

attached to the 3' carbon.

The Structure of DNA• The 5’ side (phosphate) of

one nucleotide joinstogether with the 3’(Hydroxyl) side of anothernucleotide

• Nucleotides join together bydehydration synthesis.

• A polymer of nucleotidesmake up a molecule ofDNA

• •

• • 6

The Structure of DNA• There are two strands of

DNA in a DNA molecule.• The sides of the molecule

consist of alternating sugarand phosphate.– Called a sugar phosphate

backbone.• The center of the molecule

contains the nitrogenousbases.

The Structure of DNA

• Base Pairing - Thenitrogenous bases hydrogenbond to each other.

• There are two types ofnitrogenous bases.– Purines

• Adenine(A) and Guanine (G)– Pyrimidines

• Thymine (T) and Cytosine ( C )

• •

• • 7

The Structure of DNA

• Base Pairing Rules - APurine bonds to aPyrimidine– Adenine +Thymine (Two

hydrogen bonds)– Guanine +Cyotosine (Three

hydrogen bonds)

The Structure of DNA

• Anti-parallel - The sugarphosphate backbones ofDNA run in oppositedirections

• One side runs 5’ to 3’• The other side runs 3’ to 5’

• •

• • 8

The structure of DNA

• A DNA molecule isshaped like a doublehelix.

• The nitrogenous basesbind together on theinside of the the doublehelix following the basepairing rules.

Watson and Crick’s Discovery• Watson and Crick

discovered that DNA isshaped like a doublehelix

• Watson and Crickdiscovered that thenitrogenous bases are onthe inside of the doublehelix.

• Watson and Crickdiscovered that– Adenine binds to Thymine– Guanine binds to

Cytosine.

• •

• • 9

Genes and Chromosomes• Gene - A sequence of nitrogenous bases which

code for a trait.• A DNA molecule contains many genes.• Each gene has a unique sequence of bases.

Genes and Chromosomes• Each DNA molecule

makes up achromosome

• Chromosome = Astrand of DNA

• Each cell in our bodycontains 23 pairs ofchromosomes = 46 intotal.

• Each chromosomecontains many genes

• •

• • 10

Genes and Chromosomes• DNA is wrapped around

proteins called histones.• Histones enable DNA to fit

inside the nucleus.• The combination of DNA and a

histone is called anucleosome.

• DNA and the histones coil upto form a chromosome.

Histone

DNA

Chromosome

Nucleosome