DNA History, Structure, and Replication

DNA History Important People: 1928 Frederick Griffith 1928 Frederick Griffith 1944 Oswald Avery 1944 Oswald Avery 1952 Hershey-Chase 1952 Hershey-Chase Erwin Chargaff Erwin Chargaff Early 1950’s Rosalind Franklin and Maurice Wilkins Early 1950’s Rosalind Franklin and Maurice Wilkins 1954 James Watson and Francis Crick 1954 James Watson and Francis Crick

Griffith Discovers that a factor in heat-killed, disease-causing bacteria can “transform” harmless bacteria into ones that can cause disease

Avery Determines that genes are composed of DNA

Chargaff’s Base Pairing Rule Discovers the percentage of Guanine is about the same as Cytosine in any DNA sample. The same with Adenine and Thymine. As a result, develops base pairing rule: A with T and C with G A with T and C with G

Chargaff’s Rule

Rosalind Franklin Takes pictures of chromosomes using x-ray crystallography (assisted by Maurice Wilkins) x-ray crystallography (assisted by Maurice Wilkins) Maurice WilkinsRosalind Franklin Franklin’s x-ray crystallography photograph

Watson and Crick 1 st to discover and model the shape of DNA as a double helix.

1962 Nobel Prize Winners Unfortunately, Rosalind Franklin died of breast cancer before the Nobel prize was awarded. The Nobel prize is not awarded post- humously. Watson Wilkins Crick

DNA NAME Deoxyribonucleic Acid TYPE OF SUGAR Deoxyribose # OF STRANDS 2 strands that twist (Double Helix) FUNCTION Store genetic information

Structure of a DNA Molecule DNA is made of small repeating units (monomers) called nucleotides. Each nucleotide has 3 parts:sugar 1. Phosphate group 2. Sugar 3. Nitrogen base S P NB

There are 4 different nitrogen bases: Adenine “A”Cytosine “C” Thymine “T” Guanine “G” Thymine “T” Guanine “G” Remember Chargaff’s Rule: A pairs with T C pairs with G

Double Helix These nucleotides are arranged in the shape of a double helix (a two-sided, twisted ladder.) Who was responsible for the first double helix model? How many nucleotides do you see? Notice Chargaff’s Base Pairing Rule: A-T and C-G

More Structure Facts: Sugars and phosphates make up the sides of the ladder Strong covalent bonds connect the sugars and phosphates The nitrogen bases make the steps of the ladder Weak hydrogen bonds connect the bases The order of the bases determines what proteins are made (we’ll talk more about this later!)

DNA Replication Why Does DNA make a copy of itself? Purpose: Before a cell divides, it must make sure that each new cell receives its own DNA copy. Before a cell divides, it must make sure that each new cell receives its own DNA copy. DNA replicates (makes an exact copy) to make sure this happens. DNA replicates (makes an exact copy) to make sure this happens.

Where Does Replication Occur? …in the nucleus DNA is found in the nucleus; thus, all DNA activities take place there.

Steps of DNA Replication 1.DNA polymerase (an enzyme) causes DNA to unwind and unzip. (The weak hydrogen bonds are broken.) DNA Polymerase

Extra nucleotides attach to the exposed bases on each side of the DNA strand. (A pairs with T, C pairs with G.)

Results When DNA is finished replicating, there are two identical DNA molecules. Each molecule contains one strand from the original, and one new strand.

When Does DNA Replication Happen? …Right before the cell divides. Remember: When two new cells are made, each cell needs its own identical copy of the DNA

Structure of Chromosomes Humans have 46 chromosomes in each cell. Each chromosome contains a section of the DNA. histones.

Structure of Chromosomes Chromosomes are made of condensed DNA and proteins called histones.

DNA wraps tightly around each histone and condenses to ultimately form the chromosome.

Chromosome Structure a. One chromosome (unduplicated) b. One duplicated chromosome c. A duplicated chromosome consists of two identical sister chromatids, attached at the center by the centromere.