The Structure of DNA James Watson and Francis Crick (1953) Using the work of Franklin, Wilkens, and Chargaff they discovered the structure of DNA. This discovery won them the Nobel Prize in Medicine/Physiology (along with Wilkens) in 1962.

The Structure of DNA Watson and Crick’s Model of DNA DNA has a double helix shape. DNA is a nucleic acid. A Nucleic acid is made up of a long chain of individual nucleotides.

The Structure of DNA

Watson and Crick’s Model of DNA A Nucleotide has 3 parts (1) A 5 carbon sugar (deoxyribose) (2) A phosphate group (3) A nitrogenous base

OH O CH 2 Sugar H H H A Nucleotide Adenosine Mono Phosphate (AMP) OH NH 2 N N N N Base P O OH HO O Phosphate 2’3’ 4’ 5’ 1’ Nucleotide Nucleoside H+H+ -

The Structure of DNA Watson and Crick’s Model of DNA There are 4 different nitrogenous bases in DNA Adenine & Guanine are called purines because they have a double ring structure. Thymine & Cytosine are called pyrimidines because they have a single ring structure.

Pyrimidines NH 2 O N N NH N Guanine N N Adenine N N NH 2 N O N O N Cytosine Uracil (RNA) CH 3 N O N O NH N O N O Thymine (DNA) Purines

The Structure of DNA Base Pairing Adenine (A) always bonds with Thymine (T) and vice versa. Cytosine (C) always bonds with Guanine (G) and vice versa. The reason only A & T and G & C can bond together is because weak hydrogen bonds can form between these pairs and not any others.

Base Pairing Adenine And Cytosine N O H N O N N H Cytosine N N N N H N H - + Adenine

Base Pairing Guanine And Thymine CH 3 N O N O N H + - Thymine H O N N N N N H H Guanine + + -

CH 3 N O N O N H + - Thymine N N N N H N H - + Adenine Base Pairing Adenine And Thymine

N O H N O N N H Cytosine H O N N N N N H H Guanine Base Pairing Guanine And Cytosine

The Structure of DNA Base Pairing The force created by the weak hydrogen bonds between the base pairs is what holds the two strands of the DNA molecule together. Due to base pairing, one strand of the DNA molecule can be used to reconstruct the other strand. Therefore, the strands are said to be complementary.

SUGAR-PHOSPHATE BACKBONE H P O HO O O CH 2 HOH P O O HO O O CH 2 H P O OH HO O O CH 2 NH 2 N N N N O O N NH N N N O NH 2 N B A S E S DNADNA O H P O HO O O CH 2 HO O H2NH2N N HN N N H H P HO O O CH 2 O O N O H2NH2N N H H2OH2O HOH P O HO O O CH 2 CH 3 O O HN N H2OH2O

The Structure of DNA DNA Replication The process by which DNA is duplicated before a cell divides is called DNA replication. This ensures that each new cell will have a complete set of DNA molecules.

The Structure of DNA DNA Replication DNA copying process A molecule of DNA is unzipped by enzymes at the weak hydrogen bonds. Each DNA strand serves as a template for new bases to attach to. This results in two DNA molecules that are identical to each other and the original.

The Structure of DNA

RNA (ribonucleic acid) RNA is the nucleic acid that acts as a messenger between DNA and the ribosomes. It carries out the process by which proteins are made from amino acids.

RNA (ribonucleic acid) There are 3 types of RNA (1) Messenger RNA (mRNA) which differs from DNA in three main ways. The 5 carbon sugar is ribose, DNA has deoxyribose. It is single stranded, DNA is double stranded. It has the nitrogenous base uracil instead of thymine (which DNA has).)

RNA (ribonucleic acid)

There are 3 types of RNA (continued) (2) Transfer RNA (tRNA) (3) Ribosomal RNA (rRNA)