Introducing DNA Structure of DNA l DNA stands

Introducing: DNA

Structure of DNA l DNA stands for Deoxyribonucleic Acid l The primary function of DNA is to store and transmit the genetic information that tells cells WHICH proteins to make and WHEN to make them. l Each segment of DNA that codes for a particular protein is called a GENE.

Your Turn ! • Put the following terms in order from largest to smallest: Gene § Protein § DNA §

Structure of DNA l DNA is made up of repeating subunits called nucleotides. NUCLEOTIDE

Parts of a Nucleotide: A sugar molecule called deoxyribose 2. A phosphate group 3. A nitrogen base 1. Nitrogen Base Phosphate Sugar-deoxyribose

Your Turn ! • How many nucleotides are shown in this DNA segment?

Your Turn ! • Compare and contrast the DNA nucleotides.

Parts of a Nucleotide: l The deoxyribose sugar and the phosphate group are the same in ALL DNA nucleotides. l However, the nitrogen base can be 1 of 4 different bases.

Nitrogen Bases Found in Nucleotides: 1. Adenine (A) 2. Guanine (G) 3. Cytosine (C) 4. Thymine (T)

Things to notice about the Nitrogen bases… l Adenine and guanine have 2 rings of carbon and nitrogen atoms l Bases with 2 rings are called PURINES. l Thymine and cytosine have only 1 ring of carbon and nitrogen atoms l Bases with 1 ring are called PYRIMIDINES.

The Shape of DNA Molecules… l In 1953 a model of DNA was proposed by James Watson and Francis Crick. l Their model was based on research conducted by Rosalind Franklin in 1952. l The model: DNA is composed of 2 nucleotide chains that wrap around each other creating a shape similar to a spiral staircase l This shape is called a DOUBLE HELIX.

Your Turn ! • Why is DNA considered a double helix?

The Shape of DNA Molecules…

How the bases “stick” together… l The individual nucleotides are connected by COVALENT BONDS between the deoxyribose sugar and the phosphate molecules. l This creates the “backbone” to which the nitrogen bases attach.

How the bases “stick” together… l The nitrogen bases can then form HYDROGEN BONDS creating the “rungs” to the ladder.

Your Turn ! • Covalent bonds are relatively strong bonds, and hydrogen bonds can be easily broken. • What does that mean about the direction in which DNA can be separated into two strands?

Base Pairing Which bases can bond together? l DNA nucleotides pair into what are called COMPLIMENTARY BASE PAIRS. l There are 2 base-pairing rules: Cytosine bonds with Guanine 2. Adenine bonds with Thymine 1.

Your Turn ! • What bases would pair with the following bases: Adenine - ? § Guanine - ? § Cytosine - ? § Thymine - ? §

Base Pairing…

DNA in Our Bodies: l Found in the nucleus. l The DNA is coiled around proteins. We call DNA in this state chromosomes. l Humans have 23 pairs of chromosomes

Your Turn ! • Put the following terms in order from largest to smallest: Protein § Gene § Chromosome § DNA §

DNA Replication: Before a cell divides, it duplicates its DNA in a copying process called DNA replication. l Steps of DNA replication: l The enzyme helicase separates (“unzips”) the DNA molecule into 2 strands 2. Two new complimentary strands are built by the enzyme DNA polymerase following the base pairing rules 1.

Your Turn ! • What is meant by a complimentary strand? • What is the complimentary DNA strand for the segment below?

DNA Replication: l Each strand of the double helix of DNA serves as a template, or model, for the new strand. l The principle enzyme involved in DNA replication is called DNA polymerase. l It also “proofreads” the DNA to minimize mistakes

Your Turn ! • After DNA replication how many complete strands of DNA do you end up with? • Draw the following DNA segment after replication and indicate which part is the old strand which is new strand.

DNA Replication: