The Chemistry of Biology
Nucleic acids, which are composed of nucleotides, are very large and complex organic molecules that contain the genetic code for that organism. Two closely related types are needed to transmit the genetic information from parent to offspring: DNA and RNA. Not surprisingly, they also share structural similarities.
Both DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are polymers of individual nucleotides. Each nucleotide has three components:
- Five-carbon-ring sugar (deoxyribose or ribose)
- Phosphate group
- Nitrogen base
Both DNA and RNA have four nitrogen bases available to construct nucleotides. Three of the nitrogen bases are the same. Review the following table for similarities and differences between DNA and RNA.
Both DNA and RNA are large molecules of successive nucleotides bonded to the main chain by the dehydration synthesis reaction. Additional nucleotides connect when the phosphate of one nucleotide bonds with the nitrogen base of the next nucleotide.
During asexual reproduction, an exact copy of the parent DNA is copied and supplied to all offspring so that both parent and offspring are identical.
Francis Watson and James Crick discovered the structure of DNA, which today we refer to as a double helix. The DNA double helix is actually two complementary strands of DNA that wrap around each other and are held in place by hydrogen bonding between the two strands. The resulting structure is a spiraling geometric molecule that looks like a twisted ladder, a.k.a. a double helix. It is important to remember that DNA molecules are so large they cannot escape the nuclear membrane that surrounds the nucleus of a cell.
Each DNA molecule consists of many genes. Each gene is composed of hundreds or usually thousands of nucleotides arranged in a specific order. The unique sequencing of the nitrogenous bases associated with the nucleotides dictates the primary structure of the protein to be created by the cell. Your chromosomes and those of all living creatures consist of long strands of DNA that store the heredity information, or genetic code, for that organism.
RNA (ribonucleic acid) is similar to DNA in that both are composed of long chains of nucleotides. However, RNA is a single strand and therefore much smaller than DNA. Although DNA contains the genetic information that determines all cell characteristics and functions, RNA stores and transfers the genetic code contained in the DNA. Its smaller size allows RNA to move freely through the nuclear membrane to transport the genetic instructions, usually to the ribosomes, for protein assembly.
Excerpted from The Complete Idiot's Guide to Biology © 2004 by Glen E. Moulton, Ed.D.. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.