If you can’t see it how can you tell what it is made of? DISCOVERING DNA

DNA GROUND ZERO: WHAT WE KNEW TO START WITH 1) Parents pass genetic traits on to their offspring (inheritance) 2) Chromosomes in a cell’s nucleus carry the traits 3) Chromosomes are made of DNA and Proteins (histones) So we can conclude that….. Genetic info is carried by either DNA or protein

MANY SCIENTISTS CONTRIBUTED TO DISCOVERING THE STRUCTURE OF DNA 1) Griffith – bacteria give genetic traits to other bacteria. the trait they passed on was ability to secrete a capsule Didn’t know if DNA or protein was being traded

2) Avery – 1 st to show DNA is the genetic material. Showed that only DNA could transfer a trait from one bacteria to another…. give rough bacteria the ability to make a capsule

3) HERSHEY AND CHASE – CONCLUSIVE EVIDENCE THAT DNA IS GENETIC MATERIAL Background: 1) Viruses are only made of DNA and Protein 2) Viruses transfer their genes to other cells 3) DNA has phosphate, but Protein does not 4) Bacteriophage Viruses inject genetic material into bacteria

Hershey and Chase Experiment: 1) create viruses with radioactive DNA 2) Let viruses put their genetic material into a bacteria 3) If the bacteria is then radioactive, the genetic material is DNA 4) If the bacteria is NOT radioactive, the genetic material is Protein

4) CHARGAFF - Showed that there was always the same amount of A nucleotides as T nucleotides and There were always the same amount of G as C Chargaff’s Rule: A=T G=C Clue this gave Watson and Crick….. A is linked to T and G is linked to C

5) FRANKLIN- Background: DNA molecules are too small to see with a microscope Visible light’s wavelenth is too long & goes around it X – rays have shorter wavelenth and bend around DNA Bending light is called diffraction The pattern of bending can be used to identify the shape of the object that bends it

Franklin’s Experiment – Took X-ray diffraction photos of DNA Measured the diffraction patterns and concluded DNA was a helix and calculated the diameter of the DNA molecule

6) Watson & Crick Based on the fact that DNA contains the 5-carbon sugar Deoxyribose. Based on Chargaff’s Rule : DNA contains the nitrogenous bases A, T, G and C Based on Franklin’s photo showing a helical shape

Watson & Crick Constructed the first working Model of DNA’s structure

THE PLAYERS: 1) Griffith – bacteria give genetic traits to other bacteria. 2) Avery – 1 st to show that DNA is the genetic material. 3) Hershey & Chase – Showed conclusively that DNA is the genetic material 4) Chargaff – A = T and G = C 5) Franklin – x-ray diffraction 6) Watson & Crick – model of DNA

DNA structure DNA is made of subunits called Nucleotides Nucleotides are made of 3 parts 1) one 5 carbon sugar 2) phosphate 3) nitrogen containing base

Nucleotides are bonded together in 2 chains covalent bonds hold one nucleotide to the next The strong bonds between phosphates and sugars forms the backbone of the double helix

2 chains of nucleotides are held together by weak Hydrogen bonds between their bases

The two joined strands are twisted into a double helix. The cell stores DNA wrapped around proteins called histones to form a bundle called a nucleosome Further wrapping of DNA forms the X-shape Seen in mitosis

SEMI CONSERVATIVE REPLICATION One double helix is replicated so there are 2 double helixes. 1 molecule of DNA becomes 2 molecules of DNA Each molecule retains one of the original chains of nucleotides

PARENT VS DAUGHTER STRANDS Original chains of nucleotides = Parent strand. New chain of nucleotides = daughter strands Daughter strands are complimentary to parent strands The two strands are antiparallel (5’ -> 3’ and 3’ -> 5’)

REPLICATION Helicase – breaks H-bonds between bases and unwinds double helix Single strand binding proteins – keep strands apart RNA primase – adds RNA primer to get daughter strand started DNA polymerase – assembles new nucleotides on daughter chain of nucleotides covalently bonds them in place Ligase – bonds okazaki fragments together

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PROKARYOTIC REPLICATION Starts at origin One replication fork moves in each direction Two identical circular chromosomes formed

In order to help overcome these types of topological problems caused by the double helix, topoisomerases bind to either single-stranded or double-stranded DNA and cut the phosphate backbone of the DNA. This intermediate break allows the DNA to be untangled or unwound, and, at the end of these processes, the DNA backbone is resealed again. Since the overall chemical composition and connectivity of the DNA does not change, the tangled and untangled DNAs are chemical isomers, differing only in their global topology, thus their name. Topoisomerases are isomerase enzymes that act on the topology of DNA. [1]isomeraseenzymestopology DNA [1]