dna replication

• DNA strands

  • The carbon atoms of the sugar molecule are numbered as 1′, 2′, 3′, 4′, and 5′ (1′ is read as “one prime”).
  • The phosphate residue attaches to the hydroxyl group of the 5′ carbon of one sugar and the hydroxyl group of the 3′ carbon of the sugar of the next nucleotide.
  • The bond is called a 5′–3′ phosphodiester linkage.
• Antiparallel

  • This is called an antiparallel orientation because the helix's two strands run in opposite directions.
  • The 5′ carbon end of one strand will face the 3′ carbon end of its matching strand.
• DNA polymerase

  • DNA polymerase catalyses the condensation reaction between the nucleotides in a DNA strand.
  • DNA polymerase forms phosphodiester bonds between adjacent nucleotides.
• Watson and Crick

  • In 1953, two scientists called Watson and Crick found that specific base pairing was involved in a way of copying sequences of genetic information.
  • They thought that each strand acts as a template from which the new complementary strand is copied.
  • But they didn't know how replication took place.
  • They proposed three models of replication including conservative replication.
• Meselson and Stahl

  • Meselson and Stahl were interested in understanding how DNA replicates.
  • They grew E. coli for several generations in one medium containing a “heavy” isotope of nitrogen (15N) and another medium containing a 'light' isotope of nitrogen (14N).
  • During replication, the nitrogen gets included into nitrogenous bases, and eventually into the DNA.
• Isolating DNA

  • After each of the first few generations, the cells were harvested and the DNA was isolated, then centrifuged.
  • The DNA formed separate bands according to its density. DNA grown in "heavy" 15N formed a band lower down the centrifuge tube than that grown in "light" 14N.
• The middle band

  • Meselson and Stahl moved E. coli from the 15N batch to a broth containing 14N only.
  • They noted that after one generation of growth in 14N, the single band observed was in the space between DNA of cells grown exclusively in 15N and 14N.
  • This suggested a semi-conservative mode of replication.
• Conclusive evidence

  • The DNA harvested from cells grown for two generations in 14N formed two bands:
  • One DNA band was between 15N and 14N.
  • The other DNA band corresponded to the band of 14N DNA.
  • These results could only be explained if DNA replicates in a semi-conservative manner.