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DNA and Replication


    DNA is a double stranded molecule which stands for deoxyribonucleic acid. DNA is made up of four different nitrogenous bases: guanine, cytosine, adenine, and thymine. These bases are complementary to each other in which adenine and thymine are paired by 2 hydrogen bonds and cytosine and guanine are paired by three hydrogen bonds. DNA also has a sugar group and phosphate back bone. DNA is anti parallel, therefore one side runs from the 5' to 3' direction and the other runs from the 3' to 5' direction. 


    The replication of DNA depends on whether you are looking at it from the leading or the lagging strand. DNA has a semi conservative replication. Each strand serves as a template for the daughter DNA. Replication always builds in the 5' to 3' direction. 

  • Replication begins at the origin of replication.
  •  Helicase unwinds the DNA making the replication fork (Y). Single-stranded binding proteins prevent the DNA from recombining and keep the 2 strands apart.
  •  Topoisomerase removes all twist and knots and keeps the DNA from not super coiling again. 

Leading Strand:

  1. Primase begins the replication process using RNA primers. 
  2. DNA polymerase attaches to the RNA primers and begins elongation, adding DNA nucleotides to the strand. 
  3. It continuously replicates toward the replication fork, as the DNA uncoils slowly. 

Lagging Strand

Due to the fact that this DNA strand goes from 3' to 5' (from the replication fork toward the outer edge) and the fact that DNA replication must build in the 5' to 3' end, The replication is created in Okazaki fragments. 
  1. DNA replication is made with short Okazaki fragments
  2. Between each Okazaki fragments are RNA primers. The RNA primers are replaced with nucleotides.
  3. The Okazaki fragments are binded with DNA ligase. 

Proteins used in DNA Replication

function for leading and lagging strands: unwinds parental double helix at replication forks
-Single-strand binding protein
function for leading and lagging strands: binds to and stabilizes single-stranded DNA until it can be used as a template
function for leading and lagging strands: corrects "overwinding" ahead of replication forks by breaking, swiveling, and rejoining DNA strands
function for leading strand: synthesizes a single RNA primer at the 5' end of the lagging strand
function for lagging strand: synthesizes an RNA primer at the end of the 5' end of each Okazaki fragment
-DNA polymerase II
function for leading strand: continuously synthesizes the leading strand, adding on to the primer
function for lagging strand: Elongates the Okazaki fragment, adding on to its primer
-DNA polymerase I
function for leading strand: Removes primer from the 5' end of leading strand and replaces it with DNA, adding to the adjacent 3' end
function for lagging strand: removes the primer from the 5' end of each fragment and replaces it with DNA, adding on to the 3' end of the adjacent fragment
-DNA Ligase
function for leading strand: Joins the 3' end of the DNA that replaces the primer to the rest of the leading strand
function for lagging strand: Joins the Okazaki fragments


There are now 2 copies of the original DNA, each having one template strand from the parent DNA.