DNA Basics Chp 14 Review of what you learned in biology DeoxyriboNucleic Acid

DNA Basics Chp 14 Review of what you learned in biology Plus More DeoxyriboNucleic Acid Summary of DNA structure and experiments 14 mins

DNA is made of units called nucleotides

Nucleotides are in 3 parts Nitrogen Base Phosphate Sugar (deoxyribose)

There are 4 Nitrogen Bases in DNA A – Adenine T – Thymine G – Guanine C – Cytosine

There are 2 types of Nitrogen Bases Purines – Adenine and Guanine double ring Pyrimidines – Thymine and Cytosine single ring

Base Pairing  The nitrogen bases bond together with weak hydrogen bonds  A only bonds to T  C only bonds to G

Example sequence A - T - C - G -

Example sequence A - T T - C - G -

Example sequence A - T T - A C - G -

Example sequence A - T T - A C - G G -

Example sequence A - T T - A C - G G - C

Bonding of Base pairs A-T has 2 hydrogen bonds C-G has 3 hydrogen bonds

Genetic code The chains of nucleotides form a sequence which is the genetic code which determines all protien structure

DNA Structure DNA is a two sided molecule that forms a twisted ladder structure called a double helix

DNA “Ladder” Model  Rungs (steps) of the ladder are the base pairs connected with weak hydrogen bonds  Sides of the ladder are the phosphates and the sugars (deoxyribose) connected with strong covalent bonds like a backbone

Discovery of DNA True or false Watson and Crick discovered DNA. FALSE. They were the first to publish the double helix structure They built on other ideas, including Franklin’s x-rays, and Pauling’s triple helix

Chargaff 1950 Discovered the four base pairs AT and CG and found the ratio of T to A was always the same as was C and G

Rosalind Franklin 1952 Used X-ray diffraction which suggested DNA had helix structure before Watson &Crick

Maurice Wilkins 1952 Also worked with Franklin. Helped her in X-ray diffraction

Hershey and Chase 1953 “Blender Experiment” Proved Nucleic acids carried genetic code, not protein

Hershey and Chase 1953 Used radioactive P and S in virus’ P is only in Nucleic acids. S is only in protein Infected e-coli with viruses with either P or S Radioactive P transferred showing code from DNA Radioactive S, did not transfer showing code not in protein

Watson and Crick 1954 Got Noble prize for discovering the Double Helix of DNA structure

Francis Crick 1955 Came up with Central Dogma DNA makes RNA and RNA makes protein

Meselson-Stahl 1957 Showed DNA replicates by splitting in half and each side is duplicated A-T C-G

DNA Replication DNA duplicates itself before mitosis This occurs in the S phase of cell cycle

3 possible models of replication a) Conservative – whole DNA replicated b) Dispersive – separate sections c) Semi-conservative – splits then duplicates Which One?

Semi conservative model The two DNA strands split before replication

The Meselson-Stahl experiment –Showed that DNA is replicated semi-conservatively DNA containing "heavy nitrogen" (15N) can be distinguished from DNA containing "light nitrogen" (14N)

3 rd generation half medium, half light

DNA Replication Video s/TromboneFINALd.swfhttp:// s/TromboneFINALd.swf Show beginning

Bonding direction 3’ carbon bonds to the 5’ of next ribose with phosphate in between

Replication direction rules 1)Moves - Replication always moves 5’ to 3’ 2)Read - Strand is always READ 3’to 5’

Two sides of DNA Leading strand replicates forward 5’ to 3’ Lagging strand replicates backwards one segment at a time 5’ to 3’

Prokaryotic replication Bacteria have circular DNA Replication is in both directions Uses enzyme gyrase to unwind

Origins of replication Where replication begins Replication fork

Replisome All the enzymes needed for replication in a single package

Proof reader enzymes  DNA must be replicated perfectly so the new cells that form are identical  There are proofreader polymerase enzymes to ensure copies are made correctly

Key ENZYMES There are many more Helicase – Breaks apart the DNA strands Primase – Builds a short RNA primer Polymerase – Elongates DNA strand by Attaching new nucleotides can only read in 3’ to 5’ direction Elongates in the 5’ to 3’ direction

Direction of DNA replication always 5’ to 3’ Leading strand - side made continuously in one direction Lagging strand – Side made away from replication fork

Okazaki Fragments Short segments of DNA that make lagging strand

Ligase Enzyme that connects Okazaki fragments

End of replication problem Removal of RNA primer leaves gap Telomeres – non coding repetitions at end Telomerase enzymes that lengthen telomeres

Telomeres PROTECT ends of Chromosome If telomeres did not wear away, some scientists think it would prolong life

DNA Replication Video s/TromboneFINALd.swfhttp:// s/TromboneFINALd.swf