The Structure of DNAThe Structure of DNA

What is DNA?What is DNA?• DNADNA (or (or DDeoxyriboeoxyribonnucleic ucleic

AAcid) cid) is often called “The blueprint of life”.

• DNADNA, like Proteins, , like Proteins, Carbohydrates, and Fats, Carbohydrates, and Fats, are biochemical are biochemical polymers. The monomer polymers. The monomer is a “nucleotide”is a “nucleotide”

• DNADNA contains all of the instructions to make each organism the DNADNA is contained in.

The Discovery of the structure of DNA

• The structure of DNA was discovered in 1952 by Rosalind Franklin (who showed DNA was a helix using X-rays) and Jim Watson and Francis Crick who built models.• They realized that the structure told us much about how DNA functioned as the genetic material.

Why do we study DNADNA?• DNA is in ALL living cells, it is

centrally important to ALL life on Earth.

• It works the same way in all of these organisms. If we understand the principles of function in one organism we can often extrapolate those functions to other organisms.

• Understanding how DNA works has, and will continue to have, profound medical implications and tell us much about the history of life on this planet.

DNA, Genes, & ChromosomesDNA, Genes, & Chromosomes• Chromosomes are long strands

of DNA (each of ours are on average 200 million nucleotides long).

• Individual instructions on the DNA are called genes.

• Each gene has instructions to build 1 protein.

• Our 20000 genes are contained on our 46 chromosomes (E. coli has 2000 genes and only 1 chromosome) and are passed on when organisms reproduce.

The Shape of the MoleculeThe Shape of the Molecule• DNA is a very long polymer often

consisting of millions of nucleotides.

• Its basic shape is that of a twisted ladder or a twisted zipper.

• The poles of the ladder are the sugar-phosphate backbones and the rungs are the bases

• Each pole of the ladder is arranged in the shape of a helix and because the two poles are intertwined with each other it is called a double helix.

One Strand of DNAOne Strand of DNA• The backbone of the

molecule is made of alternating molecules of phosphatephosphate and deoxyribose sugardeoxyribose sugar.

• The “rungs of the ladder” are called nitrogenous basesnitrogenous bases.

phosphate

deoxyribose

bases

NucleotidesNucleotides

C C

COPhosphate

O

CC

O -P OO

O

•One deoxyribose, together with its phosphate and base, make a nucleotide.•Nucleotides are the monomer of DNA

Nitrogenous base

Deoxyribose

One Strand of DNAOne Strand of DNA

• One strand of DNA is a polymer of nucleotides.

• One strand of DNA can have many millions of nucleotides within that polymer.

A nucleotide monomer.

Four nitrogenous basesFour nitrogenous bases

• Cytosine - C• Thymine - T• Adenine - A • Guanine - G

DNA has four different bases:

Two Kinds of Bases in DNA

• PyrimidinesPyrimidines are are single ring single ring (Hexagon) bases(Hexagon) bases..

• PurinesPurines are double are double (Hexagon and (Hexagon and pentagon) ring pentagon) ring bases.bases.

C

C

C

C

N

N

OO

N

C

C

C

C

N

N

N

N

N

C

Thymine and Cytosine are pyrimidinesThymine and Cytosine are pyrimidines

• Thymine and cytosine each have one Thymine and cytosine each have one ring of carbon and nitrogen atoms.ring of carbon and nitrogen atoms.

C

C

C

C

N

N

OO

N

cytosinecytosine

C

C

C

C

N

N

OO

OO

thyminethymine

C

Adenine and Guanine are purinesAdenine and Guanine are purines

• AdenineAdenine and and GuanineGuanine each have two each have two rings of carbon and nitrogen atoms.rings of carbon and nitrogen atoms.

C

C

C

C

N

N

N

AdenineAdenine N

N

C

C

C

C

C

N

NOO

N

GuanineGuanine N

N

C

Nucleotide sequenceNucleotide sequence• The order (or

sequence) of the nucleotides on the DNA is important.

• The sequence of bases is a code for the instructions to make an organism.

• Each organisms DNA has a unique sequence.

Base Pairing and bondingBase Pairing and bonding• The bases attract each

other because they have shapes that fit with each other.

• Hydrogen bonds link the bases together (like a magnet).

• H bonds are a special type of weak ionic bond.

• They are formed when hydrogen is “sandwiched” between two electronegative atoms (e.g. N or O)

Hydrogen BondsHydrogen Bonds.• When making

hydrogen bonds, cytosine always pairs up with guanine

• Adenine always pairs up with thymine

• These are known as Chargraff’s Rule… After the man who discovered them.

DNA’s incredible NumbersDNA’s incredible Numbers• Each human cell has about

6 feet of DNA (enough for 3 billion bases).

• The average human has about 100 trillion cells.

• The average human has enough DNA to go from the earth to the sun and back about 500 times.

• DNA has a diameter of only 0.000000002 meters.

The earth is 150 billion mor 93 million miles from the sun.