Biomedical Engineering Reference
In-Depth Information
MICROTUBULES
MICROVILLI
(a)
(b)
(c)
FIGURE 3.11 Centrioles (a) contain microtubules and are located at right angles to each other in the cell's
centrosome. These organelles play an important part in cell division by anchoring the microtubules that are used
to divide the cell's genetic material. Microvilli (b), which are extensions of the plasma membrane, line the villi, tiny
fingerlike protrusions in the mucosa of the small intestine, and help increase the area available for the absorption of
nutrients. Cilia (c) line the respiratory tract. The beating of these organelles helps move bacteria and particles
trapped in mucus out of the lungs.
as humans, come from the mother's egg cell, since the father's sperm contributes little more
than the DNA in a haploid (half) set of chromosomes to the developing offspring.
Microtubules, intermediate filaments, and microfilaments provide structural support and
assist with movement. Microtubules are long, hollow, cylindrical structures that radiate
from microtubule organizing centers and, during cell division, from centrosomes, a special-
ized region of the cytoplasm that is located near the nucleus and contains two centrioles
(Figures 3.4 and 3.11a) oriented at right angles to each other. Microtubules consist of spiraling
subunits of a protein called tubulin, whereas centrioles consist of nine triplet microtubules that
radiate from their centers like the spokes of a wheel. Intermediate filaments are hollow and
provide structure to the plasma membrane and nuclear envelope. They also aid in cell-to-cell
junctions and in maintaining the spatial organization of organelles. Myofilaments are found
in most cells and are composed of strings of protein molecules. Cell movement can occur when
actin and myosin, protein subunits of myofilaments, interact. Microvilli (Figure 3.11b)are
extensions of the plasma membrane that containmicrofilaments. They increase the surface area
of a cell to facilitate absorption of extracellular materials.
Cilia (Figure 3.11c) and flagella are parts of the cytoskeleton that have shafts composed
of nine pairs of outer microtubules and two single microtubules in the center. Both types
of shafts are anchored by a basal body that has the same structure as a centriole. Flagella
function as whiplike tails that propel cells such as sperm. Cilia are generally shorter and
more profuse than flagella and can be found on specialized cells such as those that line
the respiratory tract. The beating of the cilia helps move mucus-trapped bacteria and parti-
cles out of the lungs.
3.2.3 DNA and Gene Expression
DNA (see Figure 3.3) is found in the nucleus and mitochondria of eukaryotic cells. In
organisms that reproduce sexually, the DNA in the nucleus contains information from both
parents, while that in the mitochondria comes from the organism's mother. In the nucleus,
the DNA is wrapped around protein spools, called nucleosomes, and is organized into
pairs of chromosomes. Humans have 22 pairs of autosomal chromosomes and two sex
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