Discovering DNA If you can’t see it how can you tell what it is made of? (Also: Replication & RNA)

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 – 1st 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 – 1st 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 chains 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 http://www.youtube.com/watch?v=yqESR7E4b_8

Semi conservative Replication http://www.wiley.com/college/boyer/0470003790/animations/replication/replication.swf 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

Replication origins and replication Forks In eukaryotes replication takes place at multiple replication origins at the same time. The area where the 2 strands of parental DNA are being separated is a Replication fork

Parent vs Daughter Strands The original DNA strands (chains of nucleotides) are called Parent strands. The strands formed of new nucleotides are called daughter strands Daughter strands are complimentary to parent strands The two strands are antiparallel (5’ -> 3’ and 3’ -> 5’)

DNA Replication Enzymes to know Helicase Topoisomerase Primase DNA polymerase DNA ligase

Dna replication Animations http://sites.fas.harvard.edu/~biotext/ani mations/replication1.swf http://www.wiley.com/college/pratt/047 1393878/instructor/animations/dna_rep lication/index.html

1) Helicase unwinds double helix & breaks H-bonds DNA Replication Location = nucleus 1) Helicase unwinds double helix & breaks H-bonds 2) SSBP proteins keep parental strands apart 3) Topoisomerase cuts DNA to let it untwist then puts it back together 4) Primase matches RNA primer to parent strand 5) DNA Polymerase adds DNA nucleotides to 3’ end of RNA primer to grow the daughter strand from 5’ -> 3’ 6) Ligase fuses Okazaki fragments together

DNA proof reading & repair DNA Polymerases proofread as they build After replication…. Nuclease enzymes cut out errors and DNA Polymerase fills in correct code where the error was cut out

Every time a chromosome is replicated it gets shorter Aging chromosomes Every time a chromosome is replicated it gets shorter To keep from losing genes eukaryotes have telomeres … long stretches of non-coding DNA at the end of each chromosome Telomere length is restored in germ cells by telomerase Gene for telomerase is not active in somatic cells

Deoxyribonucleic acid Both use phosphate Bases are GCAT The other nucleic Acid DNA Deoxyribonucleic acid Both use phosphate Bases are GCAT Double stranded molecule RNA Ribonucleic acid Both use phosphate Bases are GCAU Single stranded molecule

Nitrogenous BASES in Nucleotides Pyrimidines include : Cytosine, Thymine, Uracil Pyrimidines have only one carbon ring Purine include; Guanine, and Adenosine Purines have 2 carbon rings

Nucleotides and evolution Nucleotide structure and base pairing conserved in all organisms and viruses Evidence of evolution from a common ancestor Semiconservative replication in all organisms & viruses Prokaryotes : circular double helix DNA Eukaryotes : multiple liner double helix DNA molecules (combined with histones form chromosomes) Evidence that all eukaryotes are more closely related to each other than they are to prokaryotes

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]