DNA Review

The Size of DNA... If the DNA of one cell is stretched out, it makes a 7 ft. long string There are about 5 trillion cells in the human body If you connected all the DNA together, it would be long enough that it would take light over 10 hours to travel its length (ruptured bacteria) This is a whole lot of information

DNA Structure DNA is a Double Helix - like a twisted rubber ladder made from three main components Sides of the ladder are composed of alternating sugar and phosphate pieces Each “rung” of the ladder is made up of two complementary bases A bound to T C bound to G DNA is put together in chunks called NUCLEOTIDES Each nucleotide has a sugar, PO43- and base

Nucleotides A nucleotide is held together with strong molecular bonds 5’ A nucleotide is held together with strong molecular bonds Oxygens provide the “hooks” necessary to bind to other NTP’s We identify where these Oxygens are by labeling the carbons of the deoxyribose sugar 1’ 4’ 3’ 2’ (see, no oxygen at 2’…)

DNA Strand Each of the components of the nucleotide binds to another nucleotide to form a single DNA strand Since Oxygen easily forms two strong bonds, the phosphate and sugars both bind most strongly The Hydrogens on the base bind less strongly with an Oxygen or Nitrogen from another base to bind one strand to another

ds DNA Helix Finally we have the characteristics of double stranded DNA STRONG bonds between the deoxyribose sugars and phosphates WEAK bonds between the Hydrogen and Oxygen or Nitrogen (weaker between A and T than C and G) Twisting of the two anti-parallel strands to form a double helix Heat it up, which bonds break first?...

The Replicator DNA is a replicator because of the base pairing reality Since only one of the bases can pair with only one other of the other bases, each strand is a negative of the other The process of pulling the strands apart from each other and replacing each single strand with its complement is called REPLICATION We start with some ds DNA

Replication #1 DNA polymerase comes on the scene UNWINDS the DNA upstream UNZIPS the DNA This site is known as the replication bubble

Replication #2 Complementary Bases begin adding into both sides of the ds DNA A binds with T, C binds with G (no other possibility because of the shape of the bases!) The DNA Polymerase precedes the paired bases and clips out any that are already added ahead of it (on the 3’ end) with an exonuclease activity

Replication #3 Finally you have 2 identical copies of ds DNA The final job of the Polymerase is to Proofread the NTP’s after they are added (back on the 5’ end) and to clip out any that are incorrectly paired

Review: DNA Polymerase Unwinds DNA at the replication fork Unzips DNA (breaks apart Hydrogen Bonds) Cuts out nucleotides attaching upstream on the 3’ end Proofreads back on the 5’ end and cuts out improperly paired bases

DNA Structure and Replication