This module presents some basic concepts of cell biology as they pertain to the study of genetics. Specifically, we will concentrate on the cell cycle, look at chromosomes, and review cellular organelles involved in gene expression and inheritance.
Prokaryotes vs. Eukaryotes
Prokaryotes are the simpler type of cellular life form. These cells have no nucleus to contain their genetic material (the prefix pro means 'before', and the word root karyo means 'nucleus', so these organisms existed before the nucleus). The prokaryotes include the bacteria, single-celled organisms that are enclosed by a cell wall and one or two plasma membranes. The cytoplasm of a bacterial cell contains DNA (the bacterial chromosome and any episomes), RNA, and a variety of proteins. Viruses are also usually included with the prokaryotes, because they have relatively small genomes and simple construction. Most viruses consist of nucleic acid surrounded by a protein coat (no cytoplasm), although some are also enclosed by a lipid envelope.
Eukaryotes are considerably more complex, with delegation of certain cellular functions to discrete subcellular compartments called organelles. The eukaryotes include all animals and plants, as well as the fungi. Eukaryotic cells contain a true nucleus (the prefix eu means 'good', therefore they have a 'good nucleus'). The nucleus is a membrane-bound compartment that contains the cell's genome (the nucleus is shown in purple in the diagram of a eukaryotic cell at right). Because the nucleus contains the genetic material, it is the most important organelle that we will consider.
Extending outward from the nucleus is a series of tubular membranes called the endoplasmic reticulum (ER, for short). The ER (the mustard-colored structure in the diagram) is responsible for coordinating the transport of certain proteins to specific locations in the cell. ER is subdivided into smooth ER and rough ER, according to their appearance. Rough ER has a rough appearance because it has ribosomes embedded in the membrane. Ribosomes are small structures (appearing as tiny dots next to the ER in the diagram) made of RNA and protein that play a very important role in protein synthesis (this will be considered in detail in the module on translation).
Eukaryotic cells are surrounded by a plasma membrane. In addition to this, plant cells are surrounded by a cell wall, which provides structural support. Animal cells have no such cell wall. Surrounding the nucleus, and contained within the plasma membrane is the cytoplasm. The cytoplasm is sometimes thought to be nothing more than the liquid contents of the cell, but it is very highly organized. The cytoplasm contains a cytoskeleton, a protein scaffold made of microtubules, microfilaments, and intermediate filaments that provides shape and support to the cell and organizes the contents of the cell. Animal cells contain a pair of centrioles, which organize spindle microtubules during cell division. Plant cells do not contain centrioles; it is not known what organizes the spindle during cell division in plants.
This is by no means a comprehensive description of cellular organelles, simply an introduction to organelles that will be discussed in subsequent modules.