− Atsuko Iwamoto-Kihara, Akihiko Kawaguchi −
Living cells have a set of genes containg information to make and maintain thier bodies. Genes are inheritated information that is carried with long chain(s) of genomic DNA (deoxyribonucleic acid), actually encode proteins. Genetic information in the DNA is described as the sequence of four letters, G (guanine), A (adenine), T (thymine) and C (cytosine). Comparison of sequences between various organisms is used for evolutional study such as where ancient Japanease people come from.
Almost all of DNA chains are stored in "nucleous" of the cell, which carry 6 billion letters of genetic information inheritated from father and mother. Moreover, mitochondria which organella contribute to cellular energy production also have small circular mitochondrial DNA carring 17,000 letters derived only from mother (Fig. 1). Accumuration of mutations (replacements of letters by accident) in mitochondrial DNA sequence lets us know where did our anciant people come from, what kind of relationship is between Jomon people and contemporary Japanese. To analyse these, it is nessesary to extract enough amount of mitochondrial DNA from Jomon bones. But almost of DNA molecules in very old cells usually degraded. Recently, a technique of chain-reaction of DNA replication (PCR; polymerase chain reaction) let us make 1 million copies of small amount of DNA in a few hours. We can now compare the DNA sequence of Jomon people to ours. It reveal that Jomon people and early modern Ainu people have relationship each other, and they share the same feature with southeast asian people. By PCR, we are able to amplify a part of fossil plant DNA and other parts of mammoth's DNA. PCR method is used widly in genetics and biotechnology.
The principle of PCR was found by Dr. K. B. Mullis when he drove his car in one night of April weekend. Long double helix DNA molecule denatured by heating at 95OC in vitro becomes two single-strand DNA chains. Primers bind to each chain when it is low tempreture. Primers are short single-strand DNAs, which are nessesary to start polymerization of new DNA chains. The part of double strand by binding of a primer strarts to polymerize new DNA strand catalyzed with an enzyme, DNA polymerase (Fig. 2). New double-stranded chain has completely the same sequence with the original chain (Fig. 2-1 and Fig. 2-4). Thus, the reaction consist of 3 steps (step 1, denature double-stranded DNA at 95OC; step 2, binding of a primer; step 3, polymerization of the new DNA strand) increases the number of double-stranded DNA twice a cycle. Because it takes 2-3 hours to finish 20-30 cycles, 1 copy of DNA chain increases to 1 million copies in 1 day.
PCR is able to make copies of DNA chain that has length between a few hundreds and 10 thousands letters, however whole genome sequence is much longer than 10 thausand. For examle, chromosome 22 which is the shortest in human chromosomes has 34 million letters. Even though Escherichia coli has 4 million letters in its genome. It is impossible to amplify the whole genome of one living organism in one time, whole genome of fossil or Jomon people.
