THE BASICS OF CELL BIOLOGY
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The human body is made up of organ systems, which are composed of cells (the ‘building blocks
of life’). The millions of cells that make up the body are composed of water, salt, proteins, lipids
and carbohydrates. They also contain nucleic acids, which are responsible for transmitting and
expressing hereditary material.
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Eukaryotic cells: these cells contain a membrane-bound nucleus (where genetic information is
stored), and other organelles that carry out specific functions. Examples include: the cells that
make up single-celled organisms such as protozoa, as well as all of the multi-cellular life on earth:
fungi, plants and animals (which include humans).
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Prokaryotic cells: these cells do not contain a nucleus (the DNA molecules/ chromosomes are
not bound by a membrane), or other membrane-bound organelles. Examples include: some of
the single celled organisms, such as bacteria and blue-green algae.
Somatic vs. Sex cells
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Somatic cells: make up the body and all its parts (e.g. cells of the skin, bones, lungs, eyes,
etc.). Each one contains the complete set of chromosomes for the organism in its nucleus (46 in
humans, known as the “diploid number”).
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Sex cells (gametes): the eggs/ova produced by a female’s ovaries, and the sperm produced
by a male’s testes. Each cell contains half the total number of chromosomes for the organism (23
in humans, known as the “haploid number”).
The Two Types of Chromosomes
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Sex chromosomes: the chromosomes that determine an individual’s sex (X and Y). Humans
normally have two (XX for females, XY for males).
Autosomes: chromosomes other than sex chromosomes (#1-22). Humans normally have 44.
Homologous chromosomes: sex chromosomes and autosomes that share the same genes
and line up together during cell division.
Cell Division
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Humans, like other multi-cellular organisms, begin life as a single cell formed by an egg and a
sperm cell. The result is a zygote (fertilized egg), which is a somatic cell, not a sex cell. All
somatic cells undergo a process called mitosis: the single cell divides into two cells, each of
which divide into two more, etc. Through the process of mitosis the zygote becomes an embryo.
Mitosis also occurs in the cells of mature organisms (e.g. this is how hair grows, new skin cells
form, etc.).
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Gametes (sex cells) are produced in the testes and ovaries by meiosis: process is similar to
mitosis, but with a few differences:
1) whereas mitosis produces two somatic cells identical to the “parent” cell, meiosis produces
four sex cells
2) mitosis produces new somatic cells containing the total number of chromosomes (they are
diploid), while meiosis results in sex cells that contain half the number of chromosomes (they
are haploid