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Basic Biology - Molecules of Life - DNA
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 Complementary chains
Two complementary polynucleotide chains form a stable structure, which resembles a helix and is known as a the DNA double helix. About 10 bp in this structure takes a full turn, which is about 3.4 nm long.
This structure was first figured out in 1953 in Cambridge by Watson and Crick (with the help of others), and the birthplace of this structure is often thought to be the Eagle pub on Bene't street. Later they got the Nobel Prize for this discovery, for more see the book by Watson – The Double Helix.
It is remarkable that two complementary DNA polypeptides form a stable double helix almost regardless of the sequence of the nucleotides. This makes the DNA molecule a perfect medium for information storage. Note that as the strands are complementary, each one of them fully determining the other, therefore for the information purposes it is enough to give only one strand of the genome molecules. Thus, for many information related purposes, the molecule used on the example above, can be represented as CGATTCAACGATGC. The maximal amount of information that can be encoded in such a molecule is therefore 2 bits times the length of the sequence. Noting that the distance between nucleotide pairs in a DNA is about 0.34 nm, we can calculate that the linear information storage density in DNA is about 6x10 8 bits/cm, which is approximately 75 GB or 12.5 CD-Roms per cm.
Complementarity of two strands in the DNA is exploited for copying (multiplying) DNA molecules in a process known as the DNA replication , in which one double stranded DNA is replicated into two identical ones. (The DNA double helix unwinds and forks during the process, and a new complimentary strand is synthesised by specific molecular machinery on each branch of the fork. After the process is finished there are two DNA molecules identical to the original one.) In a cell this happens during the cell division and a copy identical to the original goes to each of the new cells.
Note that mismatched components between polynucleotide strands are possible, if the total sum of weak forces between the complementary nucleotides are strong enough. So the molecules like
C-G-A-T-T-G-C-C-A-C-G-A-T-G-C
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G-C-T-T-A-C-G-T-T-G-C-A-A-C-G
are chemically possible, though they may be rare in a living cell. More bonds, i.e., more complementary pairs, makes the molecule more stable. If there are not enough bonds, the two stranded molecular structure may become weak and the strands may come apart. The number of links needed to keep the double-helix together depends on the temperature (so-called melting temperature) and other environmental factors. DNA which is no longer in the helical form is said to be denatured.
Next we will look at RNA molecules >>>
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