Concept Introduction

Within the nucleus of every cell are long strings of DNA, the code that holds all the information needed to make and control every cell within a living organism.

DNA, which stands for deoxyribonucleic acid, resembles a long, spiraling ladder. It consists of just a few kinds of atoms: carbon, hydrogen, oxygen, nitrogen, and phosphorus. Combinations of these atoms form the sugar-phosphate backbone of the DNA -- the sides of the ladder, in other words.

Other combinations of the atoms form the four bases: thymine (T), adenine (A), cytosine (C), and guanine (G). These bases are the rungs of the DNA ladder. (It takes two bases to form a rung -- one for each side of the ladder.)

A sugar molecule, a base, and a phosphate molecule group together to make up a nucleotide. Nucleotides are abundant in the cell's nucleus. Nucleotides are the units which, when linked sugar to phosphate, make up one side of a DNA ladder.

During DNA replication, special enzymes move up along the DNA ladder, unzipping the molecule as it moves along. New nucleotides move in to each side of the unzipped ladder. The bases on these nucleotides are very particular about what they connect to. Cytosine (C) will "pair" to guanine (G), and adenine (A) will "pair" to thymine (T). How the bases are arranged in the DNA is what determines the genetic code.

When the enzyme has passed the end of the DNA, two identical molecules of DNA are left behind. Each contains one side of the original DNA and one side made of "new" nucleotides.

Replication Fork - structure formed at the point where the two strands of the original molecule are being separated and the complementary strands are being synthesized.

Helicase - breaks hydrogen bonds linking the complimentary base pairs; opens the helix so replication can occur

RNA Primase - catalyzes the synthesis of RNA primers on the exposed single strand of DNA; this needs to be in place for DNA polymerase to begin to synthesize new complimentary DNA strands along the template strands.

DNA Polymerase - catalyzes synthesis of the new strand; the enzyme that joins all the nucleotide components to one another to form a long strand of nucleotides.

Leading strand - In DNA replication, the 5' to 3' strand of the DNA double helix, which is synthesized continuously.

Lagging strand - In DNA replication, the 3' to 5' strand of the DNA double helix, synthesized as a series of Okazaki fragments in the 5' to 3' direction; these segments are linked to one another in condensation reactions catalyzed by DNA ligase.

Okasaki fragments - the discontinuous segments in which the 3' to 5' strand (the lagging strand) of the DNA double helix is synthesized.

DNA Ligase - connects newly synthesized DNA segments to the growing DNA strand by catalyzing the condensation reaction that bonds adjacent phosphate and sugar groups.