DNA: Structure and Replication

Two Types of Nucleic Acids Nucleic Acids: carry the genetic instructions for all life Nucleic Acid Stands forType of Sugar # of strands Function DNA Deoxyribonucleic acid deoxyribose2 “double helix” Stores genetic material RNA Ribonucleic acid ribose 1 singleTransfers genetic material Slide # 2

Discovery of Nucleic Acid : Friedrich Miescher discovered nuclein –Noticed that nuclein had a lot of phosphate 2. He found nuclein in every type of cell he studied. Slide # 3 Miescher’s lab

Griffith’s Experiment : Frederick Griffith a. Showed that bacteria could be transformed b. transformation: process in which bacteria can take up the genes of another bacteria and express those genes Slide # 4

Avery’s Experiment Slide # 5 Avery showed that DNA is the transforming agent! Avery continued Griffith’s experiment!

Hershey and Chase Experiment : Hershey & Chase used radioactive markers on viruses a. showed that virus only injects nucleic acid into bacteria b. bacteria take up nucleic acid and can express the new genes c. demonstrated that DNA is the genetic material of the cell Slide # 6

Adenine Guanine Cytosine Thymine Phosphate group Deoxyribose Go to Section: Components of DNA Nucleotide has 3 parts: 1. A phosphate group 2. A sugar (deoxyribose) 3. Nitrogen base (4 different bases) –Adenine “A” –Thymine “T” –Cytosine “C” –Guanine “G” Slide # 7 DNA is made up small, repeating units (monomers) called nucleotides.

1.1940’s: Erwin Chargaff discovered that the % of base “A” was almost equal to the % of base “T” & the percent of base “C” was nearly equal to the % of base “G”– didn’t know why 2.Also discovered that composition of DNA (% of A, T, C, & G) varies from species to species 3.Both discoveries gave support that DNA carries the genetic code! Chargaff: Discovered Ratios of Nitrogen Bases Chargaff Slide # 8

Percentage of Bases in Four Organisms Slide # 9 Source of DNAATGC Streptococcus Yeast Herring Human Streptococcus Yeast Herring Human Go to Section:

Rosalind Franklin: The Famous Picture Taker : Rosalind Franklin: an expert in x-ray crystallography. 2.She took the x-ray picture of DNA -- shows DNA is a double helix : Franklin died (37 years old) Slide # 10

James Watson & Francis Crick: Puzzle Solvers : Discovered the structure of DNA 2.Described DNA as a double helix (twisted ladder) a. Sides of ladder are made up of sugar & phosphate groups b. Steps of ladder are made up of nitrogen base pairs (A-T & C-G) -- Applied Chargaff’s rules c. Base pairs (steps) are held together by weak hydrogen bonds d. Sequence (order) of nitrogen bases determines the genetic instructions / “genetic code” of organism. Slide # 11 James Watson Francis Crick

Knowing the Structure Tells how DNA Replicates 1.DNA Replication: makes 2 identical DNA strands by copying the original model 2.Each new strand contains one old (parent) strand & one new (daughter) strand 3.DNA replication occurs during cell division – inside the nucleus Slide # 12 Parent strand: gray Daughter strand: red

Steps in DNA Replication 1.DNA unwinds (DNA Helicase) 2.DNA polymerase breaks hydrogen bonds that hold nitrogen bases together 3.DNA ligase hydrogen bond new nitrogen bases onto DNA parent strand Chargaff’s rules applied: A-T & C-G Nucleotides added in 5’ to 3’ direction 4.Sugar-phosphate groups bond to nitrogen bases to complete daughter strand 5.Result: 2 identical DNA strands each contains 1 parent strand & 1 daughter strand Slide # 13

Steps in DNA Replication Parent strand Daughter strand Slide # 14 Strands are antiparallel

When Does Replication Occur? 1. The cell replicates (makes a copy) its DNA right before cell division. 2. When two new cells are produced in cell division, each new cell needs its own copy of the DNA (identical copy!) 3. After DNA replication, the cell will divide in half (cell division) and give each new cell a copy of the DNA Slide # 15