Passage VII

A double-stranded DNA molecule is shown in Figure 1. Each single strand, or polynucleotide, is composed of repeating subunits called nucleotides. A nucleotide is composed of a sugar, a phosphate group, and a base. There are 4 types of bases in DNA: adenine, guanine, cytosine, and thymine.

Nucleotides are joined in a ladderlike structure. The sugars and phosphate groups form the sides of the ladder, or backbones, while pairs of bases form the rungs. Pairing between bases is specific:adenine alway bind with thymine, and guanine always binds with cytosine. Basepairs are connected together with hydrogen bonds, weak chemical bonds.

Scientists have proposed 3 models to explain how DNA replicates.

Semiconservative Model

DNA is separated into 2 polynucleotides by breaking the bonds between the base pairs. Each parental polynucleotide serves as a template for the synthesis of a new polynucleotide. Each parental polynucleotide then combines with the new polynucleotide synthesized from it. Thus, after replication, each DNA molecule is composed of a parental polynucleotide and a new polynucleotide（Figure 2).

Conservative Model

DNA is separated into 2 polynucleotides by breaking the bonds between the base pairs. Each polynucleotide serves as a template for the synthesis of a new polynucleotide. The parental polynucleotides then rejoin; the new polynucleotides also combine. Thus, after replication, 1 DNA molecule is composed of parental DNA; the other is composed of new DNA (Figure 3).

Dispersive Model

DNA is separated into 2 polynucleotides by breaking the bonds between the base pairs. Each polynucleotide is then fragmented along the length of the sugar-phosphate backbone into many short, single-stranded polynucleotide fragments. Each fragment serves as a template for the synthesis of a new complementary fragment. The fragments then combine so that, after replication, each DNA molecule contains many pieces of parental DNA and many pieces of new DNA (Figure 4).