DNA Replication

Slide 7

DNA replication occurs in the nucleus.  The cell copies & transmits genetic information via a process known as “semi-conservative replication.”  Each strand of the helix, or each side of the ladder, serves as a template for a new complementary and anti-parallel molecule as seen in slide 7.

DNA replication requires several different components: 

• Helicase– enzyme that catalyzes the “unzipping” of the DNA strand or breaking of the H bonds
• DNA polymerase - enzyme that catalyzes the attachment of one complementary nucleotide at a time to make the complementary DNA (cDNA)
• Ligase – enzyme that anneals or joins two strands of DNA
• Primer - a short nucleotide strand or oligonucleotide; primer attaches to a site on DNA so synthesis can begin; is actually a short RNA strand which eventually gets removed
• Nucleotides - deoxynucleotidetriphospate’s with adenine, cytosine, thymine & guanine (dATP, dCTP, dTTP, dGTP)

The enzymes, primer & nucleotides are also used in PCR (polymerase chain reaction) & other amplification technologies, so this is an important process to know to better understand the principle of these technologies.

Slide 8

In slide 8 a double stranded DNA molecule has been unzipped or H bonds have been broken by helicase.  The primer needed to begin replication always attaches to the 3’ end of the template strand.  Polymerase will assist in the lengthening of the new nucleic acid strand by adding one nucleotide at a time so that the resulting “copy” will be complementary & anti-parallel, as pictured in slide 8 with the leading strand on the right.  The new strand is always synthesized in the 5’ to 3’ direction. 

On the left, the short strands are called lagging strands.  Since replication of the new strand always takes place from the 5’ to 3’ end, the primer must bind to the 3’ end of the template which results in many short strands, called Okazaki fragments.  These eventually are linked together by DNA ligase to form a single strand that is complementary & anti-parallel to the template.  Eventually the RNA primers are replaced by complementary DNA nucleotides.  To view an animation of the process, visit the web site http://www.ncc.gmu.edu/dna/repanim.htm.

The result of DNA replication is two double helices, as illustrated in slide 9.  Replication begins with 1 DNA molecule and ends with 2 identical copies – each with a strand from the original DNA and a newly synthesized strand that is complementary & anti-parallel to the original.