Biotechnology and Bioinformatics (1+1)

1.4.1. Introduction

Prior to cell division, the DNA material in the original cell must be duplicated so that after cell division, each new cell contains the full amount of DNA material. The process of DNA duplication is usually called replication. In this process each strand of the original double-stranded DNA molecule serves as template for the reproduction of the complementary strand. Through DNA replication, two identical DNA molecules have been produced from a single double-stranded DNA molecule. The replication is termed semi conservative since each new cell contains one strand of original DNA and one newly synthesized strand of DNA.

In a cell , DNA replication begins at specific locations in the genome , called " origins ". Unwinding of DNA at the origin, and synthesis of new strands, forms a replication fork .

At a specific point, the double helix of DNA is caused to unwind possibly in response to an initial synthesis of a short RNA strand using the enzyme helicase. Protein s are available to hold the unwound DNA strands in position. Each strand of DNA then serves as a template to guide the synthesis of its complementary strand of DNA.

• DNA polymerases are a family of enzymes that carry out all forms of DNA replication. DNA polymerase III is used to join the appropriate nucleotide units together. A DNA polymerase can only extend an existing DNA strand paired with a template strand; it cannot begin the synthesis of a new strand.
• To begin synthesis of a new strand, a short fragment of DNA or RNA , called a primer , must be created and paired with the template strand before DNA polymerase can synthesize new DNA.
• Once a primer pairs with DNA to be replicated, DNA polymerase synthesizes a new strand of DNA by extending the 3' end of an existing nucleotide chain, adding new nucleotides matched to the template strand one at a time via the creation of phosphodiester bonds .
• The energy for this process of DNA polymerization comes from two of the three total phosphates attached to each unincorporated base . (Free bases with their attached phosphate groups are called nucleoside triphosphates ))
• When a nucleotide is being added to a growing DNA strand, two of the phosphates are removed and the energy produced creates a phosphodiester bond that attaches the remaining phosphate to the growing chain.
• DNA polymerases are gene rally extremely accurate, making less than one error for every 10⁷nucleotides added.
• DNA polymerases also have proofreading ability; they can remove nucleotides from the end of a strand in order to correct mismatched bases.