Deoxyribonucleic Acid The Molecular Basis of Inheritance Who are these guys?

What is DNA? Primary source of genetic information RNA can be used in some cases Eukaryotic cells – multiple, linear chromosomes, found in nucleus ____________________________________________________________________ _____________ = separate extra-piece of circular DNA

Watson and Crick – 1953 Discovered the structure of DNA Nobel prize in 1962 (with Wilkins) Deduced that DNA was a ____________ Through observations of the X-ray crystallographic images of DNA from Rosalind Franklin

Erwin Chargaff – 1950’s ____________________________________________________________________ In any DNA: ___________________________________________________________

DNA Structure Monomers = nucleotides Nucleotide structure: _________________ Sugar (deoxyribose) Nitrogen base _______________________________

Anti-parallel strands Nucleotides in DNA on one side run ____________and the opposing side runs from ____________ This gives the DNA molecule “direction” ________________________run in opposite directions 5 3 3 5

Bonding in DNA 5 3 3 5 hydrogen bonds covalent bonds ….strong or weak bonds? How do the bonds fit the mechanism for copying DNA?

Nitrogen Bases and Pairing in DNA – Purines _________ guanine (G) Pyrimidines thymine (T) ___________ Pairing A : T 2 Hydrogen bonds C : G _______________

Semi-Conservative Replication Replication of DNA base pairing allows each strand to serve as a template for a new strand _________________________________________________________

Prokaryotic DNA Replication Replication moves in two directions. ______________________________

Eukaryotic DNA Replication ______________________

single-stranded binding proteins Replication: 1st step __________________ DNA is unwound by _______________________ Makes the replication fork Helicase breaks the hydrogen bonds between the two strands separating them Free nucleotides are present in the nucleus single-stranded binding proteins replication fork

Leading and Lagging Strands 5 3 3 5 DNA polymerase III leading strand 5 3 5 3 5 5 3 lagging strand 5 3 5 3 5 3 5 lagging strand leading strand growing replication fork growing replication fork 5 leading strand lagging strand 3 5 5 5

Replication: Leading Strand ____________________________________________________________________________ _______________________lays down the nucleotides 5’ to 3’ direction Can only add nucleotides to 3 end of a growing DNA strand

Replication: Lagging Strand Runs in the opposite direction of leading strand. __________________________________________________________________________________ DNA polymerase lays down nucleotides from 5’ to 3’ direction forming fragments: Okazaki fragments RNA primer is removed from the fragments and replaced with DNA nucleotides _____________________

direction of replication Replication fork Animation DNA polymerase lagging strand 3’ RNA primase Okazaki fragments 5’ 5’ DNA ligase SSB 3’ 5’ 3’ helicase DNA polymerase 5’ leading strand 3’ direction of replication SSB = single-stranded binding proteins

Houston, we have a problem! Chromosome erosion __________________________________________________________________ DNA polymerase I 5 3 3 5 growing replication fork 3 DNA polymerase III 5 RNA 5 Loss of bases at 5 ends in every replication chromosomes get shorter with each replication limit to number of cell divisions? 3

Telomeres _____________________________________________________________________________ limit to ~50 cell divisions 5 3 3 5 growing replication fork 3 telomerase 5 5 Telomerase _________________________ can add DNA bases at 5 end different level of activity in different cells high in stem cells & cancers -- Why? TTAAGGG TTAAGGG TTAAGGG 3

Editing & proofreading DNA Many different types of polymerases and nucleases Cuts and removes abnormal bases ________________________________________ repairs mismatched bases Reduces error rate to 1 in 10 billion

DNA replication on the lagging strand RNA primer is added _____________________________ serves as starter sequence for DNA polymerase III HOWEVER short segments called Okazaki fragments are made because it can only go in a 5 3 direction 5 5 3 5 3 5 3 3 growing replication fork 5 3 primase 5 DNA polymerase III RNA 3

Replacing RNA primers with DNA ________________________ removes sections of RNA primer and replaces with DNA nucleotides DNA polymerase I 5 3 ligase 3 5 growing replication fork 3 5 RNA 5 3 __________________________________________________