After a long recess, let's get on with this:
Why is DNA so important as a molecule?
Because it has two basic properties to play the role of heredity's molecule: it can hold
information (represented by the
nucleotide sequence on either of the double-helix strands) and it can be precisely
replicated and give rise to new copies of itself (one molecule can make 2 molecules). Information in this case means all the instructions the cell needs to know in order to build a full new organism. And to replicate is to reproduce (to pass the instructions to new generations ahead).
Remember, [genetic] information is not knowledge and 'they need to reproduce'...
The replication of DNA, the way one molecule can generate exact copies of itself happens in a process called
semi-conservative replication. It means that from one double-helix (two strands), two new complete molecules will be generated, each containing one old strand (which will act as a template) and one new strand, newly synthesized according to instructions on the template strand (the base pairing rules, discussed above).
The catch here is that nature only knows how to polymerize (build) a new DNA strand in one single direction (called 5'->3') [please, check the diagram on my
first 14 Jul 2009 entry]. New building blocks (each nucleotide) can only be added to a nucleotide chain (strand) exhibiting a free 3'-OH [hydroxyl] group on one end. This chemical group is necessary to make a new [strong] chemical bond to attach the entering nucleotide to the forming chain. In order to make this possible, 1) the twisted molecule needs to be unwinded and rest 'flat'; 2) both strands (running on opposite directions, the 69 configuration), being held by relatively weak hydrogen bonds formed by the opposite strands base pairs need to be separated (as in an opening zipper), so the new strands can be constructed over the two old strands...
Animation:
Notice that all this process are mediated by a host of
Enzymes (functional proteins that can foster chemical reactions to take place). Also notice that because of the strict direction limitation in the making of a new strand, as the replication fork advances (the direction of the separation of the two original strands), one new strand can be constructed in a direct continuous fashion, following the direction of the DNA's unzipping, while the other new nascent strand needs to be made discontinuously, as the unzipping takes place. The old strand acts as a template for the new strand being created, in a way that if in a particular position, an A is found on the old strand, a T will be incorporated in the new strand. If a G is found on the old strand, a C will be placed in the new strand and vice-versa [A always pairs with a T, and vice versa; and a G always pairs with a C, and vice-versa]
Illustration:
Legend:
Helicase=Enzime responsible to unzip the double strand and bring each strand apart (an enzyme called Girase, untwist the double helix before that)
SSB=Single strand Binding Proteins (stabilizes the single strands, preventing them to renaturate - to conform to their stable complimentary double strand configuration)
Primase= Adds a short primer (a temporary RNA which pairs with the template DNA) to supply a 3'-OH group necessary to the further addition of each new nucleotide used to build the new forming strand)
DNA Pol. III or DNA Polymerase III (the marvellous enzyme which adds the free nucleotides floating inside the cell to for the new chain of linked nucleotides) - It not only does that, as it can proofread it's work done to avoid errors in the sequence, also known as mutations)
DNA Pol I = Replaces the RNA primers with DNA filling
Ligase = Form the lacking [phosohodiester] bonds among adjacent nucleotides, left behind by primers, among DNA segments (called Okazaki Fragments) in the newly formed discontinuous lagging strand (lower chain in the animation)...
One image is worth more that a thousand words:
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