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DNADeoxyribo Nucleic Acid
DNA or Proteins?
Scientists debated which was the genetic material
A couple of experiments showed that altering DNA changed an organisms make up
Experimental Evidence
•Avery-MacLeod-McCarty experiment showed that DNA could transform bacteria
•Hershey-Chase experiments showed that viruses insert DNA into hosts, not proteins
Avery-MacLeod-McCarty
Hershey-Chase
DNA is Made of Nucleotides
Nitrogenous Base (determines whether nucleotide is A, T, G or C)
Deoxyribose sugar
Phosphate group
Solving the Structure of DNA
Solved by relatively unknown biologists James Watson and Francis Crick (and Rosalind Franklin)
They published their theory in a one-page paper
The Ribose-Phosphate Backbone
•Ribose sugar of one nucleotide connects to the phosphate group of the next
•Bonds are called phosphodiester bonds
The Two Types of Bases
Purines•2 ringed nitrogenous base
•Adenine and guanine
Pyrimidines•1 ringed nitrogenous base
•thYmine and cYtosine
Base Pairing 2 hydrogen bonds
form between adenine and thymine
3 hydrogen bonds form between cytosine and guanine
Notice a purine always bonds with a pyrimidine
The Double Helix
Discovered by Watson and Crick
The two strands are held together by hydrogen bonding between base pairs
Chromosomes
•A single molecule of DNA
•Eukaryotes usually have many linear chromosomes
•Prokaryotes usually have one circular chromosome
•Prokaryotes also have plasmids
DNA Replication
Watson and Crick noticed the huge benefit of double strands
Each strand can serve as a template for making the other
Semiconservative Model Each strand serves
as a template for the creation of a new strand of DNA
2 DNA molecules are created, each containing 1 strand of the original DNA
Semiconservative Replication
DNA Replication is Remarkably Fast and Accurate!
Humans have 46 chromosomes, and thus 46 DNA molecules
About 6 billion base pairs
DNA replication takes just a few hours, even in humans
Only 1 error per 1 billion nucleotides
The Basics of DNA Replication Requires more
than a dozen enzymes and proteins
Appears to operate pretty similarly in prokaryotes and eukaryotes (except DNA is in one circular molecule in prokaryotes)
Origins of Replication
Replication begins in hundreds to thousands of sites at once in eukaryotes
Replication occurs in both directions
The Beginning
•Topoisomerase unwinds the helix
•Helicase separates the strands
•Single stranded binding proteins (SSBs) keep the strands separated
•This forms a replication fork
Topoisomerase, Helicase and Single-Strand Binding Protein
DNA Polymerases
Each nucleotide is added one by one by DNA polymerases
Nucleoside Triphosphates are added
Nucleoside loses 2 phosphate groups, providing energy to synthesize new strand
Antiparallel DNA Strands The two strands are
arranged in opposite directions
3' end contains hydroxyl group
5' end contains phosphate group
Nucleotides are ONLY added to the 3' end of a strand
The Leading Strand
Synthesis always occurs in the 5' to 3' direction (the 5' end of the new strand is synthesized first) (attaches to the 3’
end of the template) One strand can
grow continuously as the fork opens in front of it
The Lagging Strand
•Built discontinuously in the opposite direction of replication
•Built in fragments, called Okazaki fragments
•DNA ligase connects fragments
Review Helicase separates strands, SSBs help keep
strands apart DNA polymerase adds nucleotides DNA ligase fuses Okazaki fragments of lagging
strands together
Priming DNA
DNA polymerases can only add nucleotides to an existing strand
A RNA primer, first binds to the template strand with the help of primase (aka RNA Polymerase)
Eventually replaced by DNA molecules
Primer continued...
The leading strand requires only one primer For the lagging strand, each fragment requires
a new primer The primer is replaced by DNA polymerase
DNA Polymerase – An Amazing Enzyme
DNA pol proofreads each nucleotide that it adds against the template
If an error is made, the enzyme deletes the nucleotide and continues synthesizing DNA
Other proteins do the same thing
DNA is also repaired after damage, such as exposure to X-rays
Over 100 DNA repair enzymes!
Extremely, extremely important
DNA Repair/ Excision Repair
Nucleases cut out (incise) the incorrect nucleotide
DNA polymerase adds the correct nucleotide
Ligase connects the new nucleotide to the strand
Topoisomerase
Helicase Single Stranded Binding Proteins
Primase
3’
5’
DNA Polymerase
Leading Strand
Lagging Strand
Okazaki Fragments
DNA Ligase