DNA STRUCTURE

Why Do We Study DNA? We study DNA for many reasons: It’s central importance to all life on Earth Medical benefits such as cures for diseases Better food crops About better food crops, this area is controversial. There is a Dr. Charles Arntzen who is working on bioengineering foods with vaccines in them. People in poor countries could be immunized against diseases just by eating a banana, for instance.

Chromosomes and DNA Our genes are on our chromosomes. Chromosomes are made up of a chemical called DNA. {Ask students where the chromosomes are in this picture. Or ask them where the DNA is. Remind them that the mitochondria also have DNA.}

Watson & Crick Built their work off of x-ray crystallography performed by Rosalind Franklin Created an accurate model of DNA structure Awarded Nobel prize in 1967

The Shape of the Molecule DNA is a very long polymer The basic shape is like a twisted ladder or zipper This is called a double helix {Show students a model of the double helix. Explain what a spiral is and a helix is.}

DNA Structure DNA is often called the blueprint of life. In simple terms, DNA contains the instructions for making proteins within the cell. A molecule of DNA is made up of millions of tiny subunits called Nucleotides Each nucleotide consists of: Phosphate group Pentose sugar Nitrogenous base

Nucleotide Structure Phosphate Nitrogenous Base Pentose Sugar

The phosphate and sugar form the backbone of the DNA molecule, whereas the bases form the “rungs” There are four types of nitrogenous bases.

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.

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

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

Hydrogen Bonds The bases attract each other because of hydrogen bonds Hydrogen bonds are weak but there are millions and millions of them in a single molecule of DNA The bonds between cytosine and guanine are shown to the right with dotted lines C N O

Hydrogen Bonds cont. C N O When making hydrogen bonds, cytosine always pairs up with guanine Adenine always pairs up with thymine (as shown to the right)

Nitrogenous Bases cont. Each base will only bond with one other specific base. Adenine (A) Thymine (T) Cytosine (C) Guanine (G) Form a base pair Form a base pair

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.}

One Strand of DNA cont. The backbone of the molecule is alternating phosphates and deoxyribose sugar The teeth are nitrogenous bases phosphate deoxyribose {Point to the 3-D mode, if you have one, to show the parts as you discuss them.} bases

Two Stranded DNA Remember, DNA has two strands that fit together something like a zipper Complimentary base paring between the nitrogenous bases hold the strands together {Point to the 3-D model to show the parts as you discuss them.}

Chargraff’s Rule: Adenine and Thymine always join together A T Cytosine and Guanine always join together C G

Chargraff cont. Erwin Chargraff Determined that the number of Thymine in a molecule of DNA has to equal the number of Adenine due to strict base pairing rules Number of Cytosine is equal to the number of Guanine

Base Pairing To crack the genetic code found in DNA we need to look at the sequence of bases. The bases are arranged in triplets called codons A G G - C T C - A A G - T C C - T A G T C C - G A G - T T C - A G G - A T C

Central Dogma DNA  RNA  Protein A gene is a section of DNA that codes for a protein Each unique gene has a unique sequence of bases This unique sequence of bases will code for the production of a unique protein It is these proteins and combination of proteins that give us a unique phenotype (an organism’s physical appearance)

Central Dogma cont. DNA is stored inside the nucleus of eukaryotic cells Because it cannot leave the nucleus a messenger molecule, RNA, has to be made in order to send the genetic blueprint to the ribosomes in the cytoplasm that do the building Proteins do the work of the genetic blueprints

DNA Gene Protein Trait

Central Dogma cont. Transcription = the process of using DNA to make complementary RNA RNA uses the nitrogen base Uracil in place of Thymine to pair with Adenine Translation = the process of using the RNA to make proteins

Your Task Draw a flow chart to show how to get from: