When Chargaff separated the parts of a sample DNA what did he find out about the matching bases?

Answer 1

Because of the asymmetry in pyrimidine and purine use in coding sequences, the strand with the greater coding content will tend to have the greater number of purine bases (Szybalski's rule). Because the number of purine bases will to a very good approximation equal the number of their complementary pyrimidines within the same strand and because the coding sequences occupy 80-90% of the strand, there appears to be a selective pressure on the third base to minimize the number of purine bases in the strand with the greater coding content and that this pressure is proportional to the mismatch in the length of the coding sequences between the two strands.

The origin of the deviation from Chargaff's rule in the organelles has been suggested to be a consequence of the mechanism of replication. During replication the DNA strands separate. In single stranded DNA, cytosine spontaneously slowly deaminates to adenosine (a C to A transversion). The longer the strands are separated the greater the quantity of deamination. For reasons that are not yet clear the strands tend to exist longer in single form in mitochondria than in chromsomal DNA. This process tends to yield one strand that is enriched in guanine (G) and thymine (T) with its complement enriched in cytosine (C) and adenosine (A), and this process may have given rise to the deviations found in the mitochondria.

Chargaff's second rule appears to be the consequence of a more complex parity rule: within a single strand of DNA any oligonucleotide is present in equal numbers to its reverse complementary nucleotide. Because of the computational requirements this has not been verified in all genomes for all oligonucleotides. It has been verified for triplet oligonucleotides for a large data set. Albrecht-Buehler has suggested that this rule is the consequence of genomes evolving by a process of inversion and transposition. This process does not appear to have acted on the mitochondrial genomes. Chargaff's second parity rule appears to be extended from the nucleotide-level to populations of codon triplets, in the case of whole single-stranded Human genome DNA

A kind of "codon-level second Chargaff's parity rule" is proposed as follows:

Codon populations where 1st base position is T are identical to codon populations where 3rd base position is A:

« % codons Twx ~ % codons yzA » (where Twx and yzA are mirror codons i.e TCG and CGA).

Codon populations where 1st base position is C are identical to codon populations where 3rd base position is G:

« % codons Cwx ~ % codons yzG » (where Cwx and yzG are mirror codons i.e CTA and TAG).

Codon populations where 2nd base position is T are identical to codon populations where 2nd base position is A:

« % codons wTx ~ % codons yAz » (where wTx and yAz are mirror codons i.e CTG and CAG).

Codon populations where 2nd base position is C are identical to codon populations where 2nd base position is G:

« % codons wCx ~ % codons yGz » (where wCx and yGz are mirror codons i.e TCT and AGA).

Codon populations where 3rd base position is T are identical to codon populations where 1st base position is A:

« % codons wxT ~ % codons Ayz » (where wxT and Ayz are mirror codons i.e CTT and AAG).

Codon populations where 3rd base position is C are identical to codon populations where 1st base position is G:

« % codons wxC ~ % codons Gyz » (where wxC and Gyz are mirror codons i.e GGC and GCC).

Answer 2

He found out that the percentages of adenine and thymine were the same, and the percentages of cytosine and guanine were the same.