Chromosome

DNA is the genetic material of all cells and contains information necessary for the synthesis of proteins. DNA is composed of two strands of nucleic acids arranged in a double helix. The nucleic acid strands are composed of a sequence of nucleotides. The nucleotides A scanning electron micrograph (SEM) of a human X-chromosome. © Biophoto Associates/Photo Researchers, Inc. Reproduced by permission. in DNA have four kinds of nitrogen-containing bases: adenine, guanine, cytosine, and thymine. Within DNA, each strand of nucleic acid is partnered with the other strand by bonds that form between these nucleotides. Complementary base pairing dictates that adenine pairs only with thymine, and guanine pairs only with cytosine (and vice versa). Thus, by knowing the sequence of bases in one strand of the DNA helix, you can determine the sequence on the other strand. For instance, if the sequence in one strand of DNA were ATTCG, the other strand's sequence would be TAAGC.

DNA functions in the cell by providing a template by which another nucleic acid, called ribonucleic acid (RNA), is formed. Like DNA, RNA is also composed of nucleotides. Unlike DNA, RNA is single stranded and does not form a helix. In addition, the RNA bases are the same as in DNA, except that uracil replaces thymine. RNA is transcribed from DNA in the nucleus of the cell. Gene are expressed when the chromosome uncoils with the help of enzymes called helicases and specific DNA binding proteins. DNA is transcribed into RNA.

Newly transcribed RNA is called messenger RNA (mRNA). Messenger RNA leaves the nucleus through the nuclear pore and enters into the cytoplasm. There, the mRNA molecule binds to a ribosome (also composed of RNA) and initiates protein synthesis. Each block of three nucleotides called codons in the mRNA sequence encodes for a specific amino acid, the building blocks of a protein.