DNA Replication “A Play”

The Cast Replication Bubbles Replication Forks Helicase Primase RNA Primer DNA Polymerase Leading Strand Lagging Strand Okazaki Fragments DNA Ligase Telomeres

Replication Bubble DNA is unwound at certain sites called replication origins The unwinding forms a replication bubble where replication will take place by using the exposed DNA strands as templates Replication Bubbles form at many sites along the DNA molecule in order to make replication as fast a possible!

Replication Forks Replication Bubbles have two replication forks The Replication Fork is where the new strands of DNA will develop

Helicase An enzyme that unwinds and separates the DNA Works just ahead of the replication fork to keep replication moving quickly along the entire DNA molecule

Primase An enzyme that joins an RNA primer to the template DNA strand to start replication

RNA Primer DNA cannot start a new DNA strand using the template strand of DNA unless it has something to attach to. RNA primer is a made up of a few nucleotides that are complementary to the DNA template and can serve as a starting point for replication Remember RNA has Uracil instead of Thymine…so A would bond with U

DNA Polymerase An enzyme with many functions 1. Adds new nucleotides to 3’ end of elongating strand 2. Dismantles RNA primer 3. Proofreads base pairing

Leading Strand DNA can only be synthesized in a 5’ to 3’ direction Since the template strands run antiparallel this means that only one strand can be replicated continuously as the replication fork is separating This strand is called the leading strand

Lagging Strand Because DNA is replicated in only the 5’ to 3’ one strand must be replicated discontinuously This is known as the lagging strand This is done by building Okazaki fragments and then joining them together

Okazaki Fragments Short Fragments of DNA that make up the lagging strand because it is replicated discontinously Each Okazaki Fragment requires its own RNA primer The Okazaki Fragments must be joined together for replication of the lagging strand to be complete

DNA Ligase The enzyme that joins DNA fragments (mainly Okazaki Fragments) together

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One Final Step After ligase joins the gaps between Okazaki fragments and the gaps where replication bubbles meet one more thing must happen DNA polymerase proofreads the new DNA strand to make sure that complementary base pairing is correct DNA polymerase cuts out any incorrect base pairing on the new strand and replaces it with the proper base

Wait! There’s a little problem! The RNA primer at the 5’ end of the daughter strand must be removed Once this is done there is nothing for any new DNA nucleotides to attach to to fill in the gap. This means the new DNA strand is slightly shorter (about 100 base pairs)

So, what does that mean? The loss of genetic material could be disastrous and even result in cell death Luckily there are special regions at the end of DNA that helps this problem

Telomeres Telomeres are stretches of repetitive nucleotides that act as a buffer zone Telomeres do not direct cell development The erosion of telomeres helps protect against the loss of other genetic material The complete erosion of telomeres can cause the loss of important genetic material or cell death Telomeres can be extended using the enzyme telomerase