Figure 2.3: Like all living things, cats reproduce themselves and make a new generation of
cats. When animals and plants reproduce they make tiny undeveloped versions of themselves
calledembryos, which grow up and develop into adults. A kitten is a partly developed cat.
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individual. (Cats are one such species,Figure2.3.) Other organisms can reproduce without
sex (”asexually”). For example, bacteria can simply split in two, producing two identical
new cells. But it’s not just bacteria that can reproduce without sex. Some lizards can
produce clones of themselves. In such species, all individuals are female and simply lay their
eggs when they are ready to reproduce. During all reproduction, the parents pass genetic
information to their offspring, a process calledheredity. Heredity is the passing of genes
to the next generation. These genes influence all the traits of an organism, including overall
body shape, size, whether it has fur or feathers, teeth or a beak, eye color, and so on. This
genetic information is essential to an organism. In all organisms made of cells, this genetic
information comes in the form of deoxyribonucleic acid, or DNA, which we will discuss
in lesson 2. (In viruses, which are not made of cells, the genetic information is sometimes
in the form of RNA, a different nucleic acid.) DNA contains the ”instructions” for building
important molecules inside of cells.
Living Things are Composed of Cells
All living things are composed of cells (Figure2.4), the tiny units that are the building
blocks of life. Cells are the smallest possible unit of life that is still considered living. Most
cells are so small that they are usually visible only through a microscope. Some organisms,
like the tiny plankton that live in the ocean, are composed of just one cell (Figure2.5).