The Structure of DNA All life on earth uses a chemical called DNA to carry its genetic code or blueprint. In this lesson we be examining the structure of this unique molecule. {Point out the alligator’s eyes in the first picture.} By the way, can you make out what this is? *************************************************************** [The goal of this presentation is to introduce high school biology students to the chemical structure of DNA. It is meant to be presented in the classroom while accompanying the teacher’s lecture, under the control of the teacher.]

DNA DNA is often called the blueprint of life. In simple terms, DNA contains the instructions for making proteins within the cell. Why is DNA called the blueprint of life?

The Shape of the Molecule The basic shape is like a twisted ladder. This is called a double helix. It has two strands twisted together. {Show students a model of the double helix. Explain what a spiral is and a helix is.}

ONE STRAND OF DNA The backbone of the molecule is alternating phosphate and deoxyribose (sugar) parts. The “rungs” are nitrogenous bases. phosphate deoxyribose {Point to the 3-D mode, if you have one, to show the parts as you discuss them.} bases

NUCLEOTIDES One deoxyribose together with its phosphate and base make a nucleotide. O -P O O C O {Ask students where they have seen a similar molecule before in this class. Answer: ATP . Emphasize that nucleotides are the basic building blocks or units of a DNA molecule and that a single molecule has many millions of nucleotides.} Phosphate C Deoxy ribose Nitrogenous base C C Deoxyribose O

ONE STRAND OF DNA One strand of DNA is a polymer of nucleotides. One strand of DNA has many millions of nucleotides. {Point to the 3-D mode, if you have one, to show the parts as you discuss them.}

Four nitrogenous bases DNA has four different bases: Cytosine C Thymine T Adenine A Guanine G These four bases are abbreviated by using their respective first letters.

Two Kinds of Bases in DNA AnGels have wings. AnGels are pure – a way to remember which bases are purines!!!! Pyrimidines CUT Pyrimidines are single ring bases. Thymine Cytosine Purines are double ring bases. Adenine Guanine C N O cytosine C N O thymine C N Adenine C N O Guanine

Hydrogen Bonds C N O The two strands of DNA stick to each other because of hydrogen bonds. Hydrogen bond

Base Pairing Adenine and Thymine always join together A T Cytosine and Guanine always join together C G

DISCOVERY OF DNA STRUCTURE Erwin Chargaff showed the amounts of the four bases on DNA ( A,T,C,G) In a body or somatic cell: A = 30.3% T = 30.3% G = 19.5% C = 19.9% copyright cmassengale

copyright cmassengale CHARGAFF’S RULE Adenine must pair with Thymine Guanine must pair with Cytosine The bases are held together by weak hydrogen bonds T A G C copyright cmassengale

copyright cmassengale DNA STRUCTURE Rosalind Franklin took diffraction x-ray photographs of DNA crystals In the 1950’s, Watson & Crick built the first model of DNA using Franklin’s x-rays copyright cmassengale

copyright cmassengale ROSALIND FRANKLIN copyright cmassengale

copyright cmassengale REPLICATION FACTS DNA has to be copied before a cell divides DNA is copied during the S or synthesis phase of interphase New cells will need identical DNA strands copyright cmassengale

copyright cmassengale DNA REPLICATION Begins at Origins of Replication Two strands open forming Replication Forks (Y-shaped region) New strands grow at the forks Replication Fork Parental DNA Molecule 3’ 5’ copyright cmassengale

copyright cmassengale DNA REPLICATION As the 2 DNA strands open at the origin, Replication Bubbles form Prokaryotes (bacteria) have a single bubble Eukaryotic chromosomes have MANY bubbles copyright cmassengale

copyright cmassengale DNA REPLICATION Enzyme Helicase unwinds and separates the 2 DNA strands by breaking the weak hydrogen bonds copyright cmassengale

copyright cmassengale DNA REPLICATION Before new DNA strands can form, there must be RNA primers present to start the addition of new nucleotides Primase is the enzyme that synthesizes the RNA Primer DNA polymerase can then add the new nucleotides copyright cmassengale

Steps of DNA replication