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Transcript
DNA: Structure and Replication Slide # 2 Two Types of Nucleic Acids Nucleic Acids: carry the genetic instructions for all life Nucleic Acid Stands for Type of Sugar DNA Deoxyribonucleic deoxyribose acid RNA Ribonucleic acid ribose # of Function strands 2 “double helix” Stores genetic material 1 single Transfers genetic material Slide # 3 Discovery of Nucleic Acid 1.1869: Friedrich Miescher discovered nuclein – Noticed that nuclein had a lot of phosphate 2. He found nuclein in every type of cell he studied. Miescher’s lab Slide # 4 Griffith’s Experiment 1. 1928: 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 # 5 Avery’s Experiment Avery continued Griffith’s experiment! Avery showed that DNA is the transforming agent! Slide # 6 Hershey and Chase Experiment 1. 1952: 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 # 7 Components of DNA DNA is made up small, repeating units (monomers) called nucleotides. 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” Adenine Guanine Cytosine Thymine Phosphate group Go to Section: Deoxyribose Slide # 8 Chargaff: Discovered Ratios of Nitrogen Bases 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 Slide # 9 Percentage of Bases in Four Organisms Go to Section: Source of DNA A T G C Streptococcus 29.8 31.6 20.5 18.0 Yeast 31.3 32.9 18.7 17.1 Herring 27.8 27.5 22.2 22.6 Human 30.9 29.4 19.9 19.8 Slide # 10 Rosalind Franklin: The Famous Picture Taker 1.1952: Rosalind Franklin: an expert in x-ray crystallography. 2.She took the x-ray picture of DNA -- shows DNA is a double helix. 3.1958: Franklin died (37 years old) James Watson & Francis Crick: Puzzle Solvers Slide # 11 1. 1953: 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. James Watson Francis Crick Slide # 12 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 Parent strand: gray Daughter strand: red Slide # 13 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 # 14 Steps in DNA Replication Daughter strand Parent strand Strands are antiparallel Slide # 15 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