DNA Replication
In reference to DNA and the genetic code, DNA replication is the means by which cells know what to do, and the reason why Junior has your blue eyes and crooked teeth. During DNA replication, your genetic sequences are copied so that the new strands can be passed on to developing cells and potential offspring. Without replication, neither mitosis nor meosis could occur.
What's truly amazing is how utterly precise the process of genetic replication has to be- one missing nucleotide here or one extra strand there and you are dealing with one seriously deformed organism. Special enzymes act like a sort of spell-check and make sure that everything is okay in the completed strand. In fact, it's the increasing incompetence of these enzymes that play a major factor in the aging process- as the spell-check gets rustier and rustier with time, so does the final product, i.e. your body.
Before you start reading about the process, it would probably be a good idea to check out the DNA nodes or read about the structure of DNA, as understanding the inherent structure is essential to grasping the ideas about DNA replication.
The Process
DNA replication of one helix of DNA results in two identical helixes. From now on I will refer to the original helix as the "parent", and the resulting helix as the "daughter." Remember first that DNA replication is semi-conserfative; that is, one parent DNA replication of one helix of DNA results in two identical helices. Each of these two daughter helices is a nearly exact copy of the parental helix. DNA creates "daughters" by using the parental strands of DNA as a template or guide. Each newly synthesized strand of DNA (daughter strand) is made by the addition of a nucleotide that is complementary to the parent strand of DNA. The process of adding the complementary nucleotides is referred to as complementary base pairing.
The very first step in DNA replication is the "unzipping" of the two DNA strands of the helix. An enzyme called a helicase unzips the two strands apart at places called the replication origins. The DNA strands are never fully separated (until they are replicated completely, of course) and form a Y shape known a the replication fork. The replication fork usually starts around the middle of the strand and works its way down. Usually there are two repliation forks going on at opposite ends of the DNA strand.
As nearly every chemistry student knows, every chemical and biological reaction that takes place needs some sort of catalyst, usually in the form of an enzyme. For replication purposes DNA polymerase fits the bill. DNA polymerase encourages the attachment of new nucleotides to the parental strand's substrates before and during replication.
Meanwhile, other proteins are also at work . Take single-stranded binding proteins, ( abbreviated SSB), that work with the helicase to keep the parental DNA helix unwound. It works by coating the unwound strands with rigid subunits of SSB that keep the strands from snapping back together in a helix. The SSB subunits coat the single-strands of DNA in a way as not to cover the bases, allowing the DNA to remain available for base-pairing with the newly synthesized daughter strands.
When the two strands of DNA are unzipped to begin replication, each strand is aligned in a different direction. The first direction is referred to as 5' to 3' while the other is oriented in the 3' to 5' direction. Because of the different orientations, the daugher strands synthesize differently, one in the direction of the replication fork, and the other can only add nucleotides in chunks. The strand in the direction of the replication fork is referred to as the leading strand ( the 5 to 3 strand) , and the chunk-adding strand is referred to as the lagging strand ( the 3 to 5 strand).
Because the method of replication for the lagging strand is a little less foolproof, the lagging strand must only replicate in tiny little segments. The segments, referred to as Okazaki fragments, are stretches of 100 to 200 nucleotides in humans (1000 to 2000 in bacteria) that are synthesized in the 5' to 3' direction away from the replication fork. Yet while each individual segment is replicated away from the replication fork, each subsequent Okazaki fragment is replicated more closely to the receding replication fork than the fragment before. These fragments are then stitched together by DNA ligase, creating a continuous strand. The lagging strand is referred to as such because it usually lags behind the leading strand for these reasons. This type of replication is called discontinuous replication and is a very vital development in preventing harmful mutations in our DNA sequence.
At the end of these pairings, the enzymes involved say arrivederci, and we are left with two new DNA helixes, ready to be synthesized in new young and nubile cells. The sum total of all of these strands make up the genome- literally hundreds of thousands of nucleotides that code the exact instructions for the manufacture of proteins. Yet another attribute of the beautiful structure and function of life on Earth.
Brought to you by the Node Your Homework project. P.S. Read about the Human Genome Project !