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