Rudiments of Eukaryotic Cell Structure
A typical eukaryotic cell is 10 µ in diameter, making its volume about 1,000 times that of a bacterial cell. Like bacteria, eukaryotic cells contain cell membranes, cytoplasmic proteins, DNA, and ribosomes, albeit of somewhat different structure from the corresponding prokaryotic elements (Fig. 1.5). Eukaryotic cells, however, possess many structural features that even more clearly distinguish them from prokaryotic cells. Within the eukaryotic cytoplasm are a number of structural proteins that form networks. Microtubules, actin, intermediate filaments, and thin filaments form four main categories of fibers found within eu-karyotic cells. Fibers within the cell provide a rigid structural skeleton, participate in vesicle and chromosome movement, and participate in changing the cell shape so that it can move. They also bind the majority of the ribosomes.
The DNA of eukaryotic cells does not freely mix with the cytoplasm, but is confined within a nuclear membrane. Normally only small pro-teins of molecular weight less than 20 to 40,000 can freely enter the nucleus through the nuclear membrane. Larger proteins and nuclear RNAs enter the nucleus through special nuclear pores. These are large structures that actively transport proteins or RNAs into or out of the nucleus. In each cell cycle, the nuclear membrane dissociates, and then later reaggregates. The DNA itself is tightly complexed with a class of proteins called histones, whose main function appears to be to help DNA retain a condensed state. When the cell divides, a special apparatus called the spindle, and consisting in part of microtubules, is necessary to pull the chromosomes into the daughter cells.
Eukaryotic cells also contain specialized organelles such as mito-chondria, which perform oxidative phosphorylation to generate the cell’s needed chemical energy. In many respects mitochondria resemble bacteria and, in fact, appear to have evolved from bacteria. They contain DNA, usually in the form of a circular chromosome like that of E. coli
and ribosomes that often more closely resemble those found in bacteria than the ribosomes located in the cytoplasm of the eukaryotic cell. Chloroplasts carry out photosynthesis in plant cells, and are another type of specialized organelle found within some eukaryotic cells. Like mitochondria, chloroplasts also contain DNA and ribosomes different from the analogous structures located elsewhere in the cell.
Most eukaryotic cells also contain internal membranes. The nucleus is surrounded by two membranes. The endoplasmic reticulum is an-other membrane found in eukaryotic cells. It is contiguous with the outer nuclear membrane but extends throughout the cytoplasm in many types of cells and is involved with the synthesis and transport of membrane proteins. The Golgi apparatus is another structure contain-ing membranes. It is involved with modifying proteins for their trans-port to other cellular organelles or for export out of the cell.
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