Overview

This chapter introduces several basic genetic concepts, without going into detail about any of them. These genetic concepts are as follows:

• life form

• nucleic acid

• gene

• chromosome

• organism

• virus

• semiautonomous organelle

The origin of life and the evolution of the three domains of life are described briefly.

Life Forms Are Genetic Systems

Two essential components of every life form are proteins and nucleic acids. Nucleic acids (DNA and RNA) are thread-like coding molecules, the building material of genes and chromosomes. Genetics is about genes and chromosomes – their structure and function, their behavior and misbehavior, their evolution, and methods of studying them. Because genes are the coding molecules of life, they are complicated and varied. It is difficult to pin down the term “gene” in a simple definition, but, to a first approximation, a gene is a segment of nucleic acid whose immediate function is to encode a piece of RNA (Figure 1.1). The key concepts here are replication (copying) of genes and coding. The replication of genes and their coding properties are described in detail in later chapters.

From a genetic point of view, a life form is an assemblage of large molecules capable of reproducing itself and including at least one chromosome. A chromosome is a long, thin thread made of DNA or, in some cases, RNA and may also contain proteins.