DNA Denaturation, Annealing and Replication
Now we'll learn why it is that the double-strandedness of DNA is so important.
You'll recall that, in the center of a double-stranded DNA molecule, the 'A' nucleotides are weakly attracted to 'T' nucleotides, and 'G' is attracted to 'C'. This has some critically important consequences. When two strands can pair like that, they MUST have exactly opposite and complementary chemical structures. That means:
If we heat up a tube of DNA dissolved in water, the energy of the heat can pull the two
strands of DNA apart (there's a critical temperature called the T|
The two strands still have the same nucleotide sequences, however, so they are still complementry. If we cool the tube again, then in the course of the normal, random molecular motion they'll eventually bump into each other ... and stick tightly, reforming double-stranded DNA. This process is called 'annealing' or 'hybridization', and it is very specific; only complementary strands will come together if it is done right. This process is used in many crime labs to identify specific strands of DNA in a mixture.
Now, when we've denatured the two strands, there's something else we can do - replicate
the DNA. The key here is that any single-stranded piece of DNA can only hybridize
with another if their sequences are complementary. If we have just one strand, we
can actually build another strand to match it.
Here's how it's done, either in a test tube or in a live cell:
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