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1.2What Is a Living Organism?

What Is a Living Organism? Living organisms have the following characteristics (Fig. 1-1).

  • They are made from structures called "cells."
  • They reproduce by genetic material called "DNA."
  • They respond to stimuli from the environment.
  • They synthesize an energy substance called "adenosine triphosphate (ATP)" from the environment, and they live and grow using that energy.

Fig. 1-1. Four Characteristics of Living Organisms

Now let's look at these characteristics in more detail.

(1) All living organisms are made of cells, which are the units of life. A cell comprises a plasma membrane consisting of a phospholipid bilayer (see Chapter 2, Fig. 2-4). There are various kinds of cells, from liver cells, which are several dozen micrometers in diameter, to nerve cells, which are several meters long.
The smallest size visible through the naked eye is generally about 0.1 mm (100 µm) and through a light microscope is about 0.2 µm (200 nm); anything less than 1 µm cannot be seen clearly without an electron microscope. However, living organisms cannot be observed using an electron microscope.

(2) Another obvious major characteristic of living organisms is that they produce offspring that are the same as themselves. Unicellular organisms under normal nutrition conditions produce offspring by asexual reproduction, such as by dividing (protozoa, etc.) or budding (baker's yeast). In a sexual reproduction, the cells of the offspring have the same traits as the cells of their parents (the characteristics that surface) as long as their DNA does not mutate. Multicellular organisms, however, undergo sexual reproduction and produce offspring that inherits half of each parent's genes.
The genetic material (DNA) of both humans and bacteria is similar in that it consists of four bases—adenine, guanine, cytosine, and thymine—which are the characters in the DNA code, but differences between the DNA of these organisms are produced by the differences in the ratios of these bases. The fact that these organisms share the same genetic material is evidence for the assertion that all organisms on the earth evolved from a single organism.
However, if a mutation occurs in DNA during self-replication, it is sometimes expressed in the traits of the offspring. The bases of DNA change randomly at a constant rate, and the resulting evolution can also be considered a characteristic of living organisms (see Column at the bottom).

(3) The third characteristic of living organisms is their response to stimuli. The plasma membrane contains proteins called "receptors," which receive stimuli from the outside environment (see Chapter 6). When outside stimuli such as chemical substances or heat reach the receptors, a series of various chemical reactions occurs in the cytoplasm. Finally, DNA is transcribed and new proteins are synthesized. This system of chain reactions is called "signal transduction" (see Chapter 7). Genes for various receptors are present in the genes of all living organisms, from Escherichia coli bacteria to humans. These organisms have many receptors in common, such as potassium channels through which potassium ions travel. The abundance of common receptors also suggests that all living organisms on the earth evolved from a single primitive organism.

(4) The final characteristic of living organisms is that they carry out metabolism (synthesis and degradation of substances) in their cells (see Chapter 8). Metabolism is a process in which organisms synthesize ATP to store energy, and then release that energy by hydrolysis, obtaining heat in the process.

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Is a Virus a Living Organism?

Viruses do not fit the definition of living organisms. Viruses are molecular aggregates of proteins and nucleic acids (see Chapter 2) and lack cells or metabolic systems. Furthermore, although viruses self-replicate, they can only do so by using materials from their hosts, and they are only able to multiply inside host cells. Viruses also differ from normal living organisms in that their genetic material is not limited to DNA but can also be RNA. Viruses that use RNA as genetic material are called "RNA viruses." Some RNA viruses create DNA using a reverse transcriptase enzyme (an enzyme that makes DNA from RNA), and then produce proteins in the usual manner.

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Why Do Mutations Occur in the Bases of DNA?

DNA is made up of four bases: adenine (A), guanine (G), cytosine (C), and thymine (T), and complementary pairs are formed between A and T and between C and G (see Chapter 2, Fig. 2-5). Several Cs and Gs align at a site in a region upstream of each gene, and if a methyl group is added (methylated) at the 5th C position of this site, the gene is inactivated. If the amine group is removed, this 5-methylcytosine (Cm) becomes T, as shown in the figure. In other words, C changes into T. When this happens, the C-G pair becomes T-G via Cm-G, and no complementary pair is formed. In such a case, one of the bases is judged to be an error. If G is judged to be an error, a T-A complementary pair is formed, and if T is judged to be an error, the T is removed and a C-G pair is reformed. In the former case, a base substitution ultimately occurs (Column Fig. 1-1).

Column Fig. 1-1. Methylation of Cytosine and Changes in Bases

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