The Structure of DNA What is DNA DNA

The Structure of DNA

What is DNA? • DNA (or Deoxyribonucleic Acid) is often called “The blueprint of life”. • DNA, like Proteins, Carbohydrates, and Fats, are biochemical polymers. The monomer is a “nucleotide” • DNA contains all of the instructions to make each organism the DNA 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 DNA? DNA • 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, & 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 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 DNA • The backbone of the molecule is made of alternating molecules of phosphate and deoxyribose sugar • The “rungs of the ladder” are called nitrogenous bases phosphate deoxyribose bases

Nucleotides • One deoxyribose, together with its phosphate and base, make a nucleotide. • Nucleotides are the monomer of DNA O O -P O Nitrogenous base O O C Phosphate C C C O C Deoxyribose

One 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 bases DNA has four different bases: C • Thymine - T • Adenine - A • Guanine - G • Cytosine -

Two Kinds of Bases in DNA • Pyrimidines are single ring (Hexagon) bases. N N O C C C N • Purines are double (Hexagon and pentagon) ring bases. N C C C N N C

Thymine and Cytosine are pyrimidines • Thymine and cytosine each have one ring of carbon and nitrogen atoms. N O C C N C thymine N O C C N C cytosine

Adenine and Guanine are purines • Adenine and Guanine each have two rings of carbon and nitrogen atoms. N N C C C N Adenine O N C N C C C N Guanine C N N C

Nucleotide 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 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 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 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. 00002 meters. The earth is 150 billion m or 93 million miles from the sun.