Section F - DNA structure and replication

Bases In DNA there are four bases adenine (abbreviated A), guanine (G), thymine (T) and cytosine (C). Adenine and guanine are purines thymine and cytosine are pyrimidines. 

Nucleosides A nucleoside is a pyrimidine or purine base covalently bonded to a sugar. In DNA, the sugar is deoxyribose and so this is a deoxynucleoside. There are four types of deoxynucleoside in DNA deoxyadenosine, deoxyguanosine, deoxythymidine and deoxycytidine. 

Nucleotides A nucleotide is base + sugar + phosphate covalently bonded together. In DNA, where the sugar is deoxyribose, this unit is a deoxynucleotide. 

3 5 phosphodiester In DNA the nucleotides are covalently joined together by 3 5 phosphodiester 

DNA sequence The DNA sequence is the sequence of A, C, G and T along the DNA molecule which carries the genetic information. 

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Exceptions to this repeat-unit-size rule (see Section 2.3) were found, however, even when the parental mitochondrial genomes carried the same ori sequence (compare the supppressivities of petites bl7/b20, bll/b7, a 1/7/8. f in/20 of Fig. 2.16 and the predominance in crosses of b7 over bll, of b over hp5, of b 13/1 over a-23/3 see also Rayko et al., 1988). This indicates that noncoding, intergenic sequences flanking ori sequences also play a role in the modulation of replication efficiency. Since in different petites such sequences differ in primary structure, size, and position relative to ori sequences, this modulation is likely to take place through an indirect effect on DNA and nucleoid structure (Rayko et al., 1988 see Fig. 2.21). 

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