DNA replication

Before each cell divides, its DNA is replicated to ensure that each daughter cell receives an identical copy. 

This accurate transmission of genetic information is crucial, as errors in replication could result in the production of faulty proteins.

The process of DNA replication occurs as follows:

1. The enzyme DNA helicase unwinds the double helix and breaks the hydrogen bonds between the complementary base pairs on the polynucleotide strands.

2. Free-floating nucleotides (adenine, cytosine, guanine, and thymine) in the nucleus align with their complementary bases on the exposed strands, forming new hydrogen bonds. Each original strand serves as a template for constructing a new complementary strand.

3. The enzyme DNA polymerase then joins the newly aligned nucleotides together through condensation reactions, forming phosphodiester bonds. This creates the sugar-phosphate backbone of the new DNA strands.

This process is known as semi-conservative replication because each new DNA molecule consists of one original strand and one newly synthesised strand. 

Semi-conservative replication ensures genetic continuity between generations of cells.

Semi-conservative replication of DNA

Alternative theories and evidence

Alternative theories, such as conservative replication, suggested that the original DNA molecule remained intact while a separate copy was made. 

However, an experiment by Meselson and Stahl in 1958, using E. coli bacteria and different nitrogen isotopes, provided evidence supporting the semi-conservative model. 

This experiment also corroborated Watson and Crick's DNA double helix model by demonstrating the mechanism of complementary base pairing.

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