•Regulating progression of cell cycles
•Mitotic spindle formation – the mitotic spindle is a macromolecular process that separates chromosomes into two daughter cells during mitosis. The spindle itself is constructed of microtubule polymers with intrinsic polarity (minus and plus).
Figure 2-8 Microtubules.
Microtubule formation is defined as any minute tubule found in eukaryotic cytoplasm. The microtubules are composed of tubulin protein and are vital components of the cytoskeleton, mitotic spindle, cilia, and flagella.
Figure 2-9 Cilia extending from cellular surface.
The structural form of centrosomes is based on an assembly of nine microtubules. Microtubules appear as hollow, cylinder-like structures constructed of rings of proto-filaments. They’re involved in a number of cellular functions and activities including motion. The motion aspect of microtubules is the result of proteins that utilize energy from ATP to literally propel movement along the microtubule.
The microtubule at the attached end is defined as the “minus” end while the other is the “plus” end. Microtubules are slender but can grow up to one thousand times longer than they are wide.
Microtubules are constructed of alpha (α) and beta (β) tubulin dimers. A dimer is defined as a molecule or complex molecular structure that consists of two identical molecules linked together. More simply defined, a dimer is an oligomer that is formed from two similarly structured monomers joined by a bond that can be intermolecular, covalent, weak, or strong.
Alpha (α) tubulin dimers and beta (β) tubulin dimers are assembled into microtubules, which are in turn involved in numerous functions and processes in the cytoskeleton including:
•DNA segregation
•Intracellular transport
•Structural support
Cilia and flagella are rope-like appendages that extend outward from the surface of numerous types of eukaryotic cells. Their main function is to move liquids over the surface of cells.
Microtubule “motors” facilitate movement. The two major motor groups are defined as:
•Dyneins – move toward the “minus” end of the microtubule
•Kinesins – move toward the “positive or plus” end of the microtubule
A prime example of such function is sperm. Cilia and flagella enable single-celled sperm to “swim”. In a multicellular structure, such as those lining bronchial tubes, the cilia and flagella move or encourage movement of mucus upward toward the throat.
The structure of cilia and flagella are identical and each contain nine filaments situated in a cylindrical array. These filaments contain a fully structured microtubule, a partial microtubule, and what are known as cross bridges of dynein, or one of the “motor” proteins that facilitate movement.
A membrane encloses the entire assembly of cilia and flagella like a sheath.
The functions of mitochondria and its components are vital, but so too are microvilli. These tiny, hair-like projections jut out from the surface of plasma membranes that serve to enhance the overall surface area of numerous types of cells.
The endomembrane system (including the organelles already mentioned) work as a team to:
•Produce
•Degrade
•Store
•Transfer (export) molecules
They also destroy potentially damaging substances.
■The Cell Nucleus
The nucleus is composed of five major parts, each with a specific role to play.
The largest organelle is the nucleus of a cell. The nucleus is responsible for regulating the activities or functions of a cell, whether it be a blood cell, a muscle cell, or a brain cell. The nucleus is responsible for two specific functions:
•Storing hereditary material (DNA)
•Coordinating cellular activities including but not limited to reproduction, protein synthesis, growth, and intermediary metabolism
Most cells only have one nucleus, although some types of algae and slime molds have more. Bacteria and cyanobacteria (prokaryotes) are known as one-celled organisms and don’t have a nucleus. Rather, in such organisms, the cytoplasm contains all the “instructions” for function as well as cellular information.
The five major parts of the nucleus include:
•Nuclear membrane or envelope
•Nuclear “sap”
•Chromatin fibers
•Nucleolus
•Endosomes
Nuclear membrane or envelope – The nucleus is typically spherical and takes up approximately 10% of the volume of a eukaryotic cell.6 The nucleus is separated from other parts of the cell by a nuclear envelope, or more specifically, a double-layered membrane. This space between the double layers is defined as the perinuclear space and connects with the endoplasmic reticulum. The inner surface area of the nuclear envelope is lined with protein. This lining is called the nuclear lamina. In turn, the lamina binds to chromatin and other components found inside the nucleus.
Very small holes occupy the surface of the envelope. These holes are called nuclear pores and allow for the passage of molecules between the cytoplasm and the nucleus. The nuclear pores are responsible for overseeing passage of molecules between the cytoplasm in the nucleus. Some of these molecules are allowed to pass through the membrane.
Eukaryotic and prokaryotic cells both contain common features such as DNA, cytoplasm, a plasma membrane, a nuclear weight region, a nucleus, and ribosomes.
DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are allowed to pass into the nucleus through these nuclear pores, and so too are molecules that will provide the energy for genetic material construction.
During cellular division (mitosis) the nuclear envelope disintegrates, but will later form again as the two cells formed from mitosis complete formation and as the chromatin unravels and also eventually evaporates.
Chromatin or chromatin fibers are defined as a complex arrangement of DNA and proteins found inside the cell nucleus. The nucleus contains molecules that when laid out would encompass nearly 2 meters (6 feet) of DNA.7 They’re literally packed inside every nucleus of every human cell. The chromatin fibers are an extraordinary feat of “packaging” wonder that is precisely structured and organized into very dense fibers or streams called chromatin.
This packaging is facilitated by special proteins that bind and fold DNA into an incredibly complex series of loops and coils.
Nuclear “sap” is considered similar to the cytoplasm of a cell, but it’s not exactly the same. Nuclear sap is a substance found inside a new nucleus, also known as nucleoplasm. Its main function is to act as a suspension substance for organelles that also helps the nucleus maintain its structure and shape. It’s also a major component of transportation for cellular metabolism. Enzymes and nucleotides are dissolved in the nucleoplasm.
This fluid is typically found in the nuclei of eukaryotic cells. This sap-like structure is likened to protoplasm and is constructed of various molecules, dissolved ions, and water. Nuclear sap is completely encompassed within the nuclear membrane, also known as the nuclear envelope. This fluid, also gel-like in nature as cytoplasm or cytosol, contains