Chapter 3: The Cell

The Cell: Anatomy and Division

cell

structural and functional unit of all living things;

nucleus

is near the center of the cell, is surrounded by cytoplasm, which is enclosed by the plasma membrane; contains the genetic material, DNA; control center of the cell; necessary for cell reproduction; contains nucleoli; bound nuclear envelope

chromatin

when the cell is not dividing, the genetic material is loosely dispersed throughout the nucleus in a threadlike form called ____; when the cell is in the process of dividing to form daughter cells, ___ coils and condenses to form chromosomes

chromosomes

dense, rodlike bodies formed when the cell is in the process of dividing to form daughter cells, which causes chromatin to coil and condense, forming ____

nucleoli

small spherical bodies, composed primarily of proteins and ribonucleic acid RNA; are assembly sites for ribosomes that are particularly abundant in the cytoplasm

nuclear envelope

double-layered porous membrane that encloses the nucleus; similar in composition to other cellular membranes, but is distinguished by large nuclear pores

nuclear pores

spanned by protein complexes that regulate what passes through, and they permit easy passage of protein and RNA molecules

plasma membrane

separates cell contents from the surrounding environment, providing a protective barrier; phospholipids and globular protein molecules are its main structural building blocks; is selective about what passes through it; allows nutrients to enter cell but keeps out undesirable substances; valuable cell proteins and other substances are kept within the cells, and excreta or wastes pass to the exterior, in other words, is selectively permeable; maintains a resting potential

microvilli

greatly increase the surface are of the cell available for absorption or passage of materials and for the binding of signaling molecules

cytoplasm

consists of the cell contents outside the nucleus; cytosol + organelles

cytosol

fluid cytoplasmic material; suspended in this are many small structures called organelles

organelles

metabolic machinery of the cell; are highly organized to carry out specific functions for the cell as a whole

cytoplasmic organelles

ribosomes, SER and RER, Golgi apparatus, lysosomes, peroxisomes, mitochondria, cytoskeletal elements, and centrioles

ribosomes

tiny spherical bodies composed of RNA and protein; floating free or attached to the RER in the cytoplasm; actual sites of protein synthesis

endoplasmic reticulum

membranous system of tubules that extends throughout the cytoplasm

rough ER

studded with ribosomes; tubules of the ___ provide an area for storage and transport of the proteins made on the ribosomes to other cell areas

smooth ER

has no function in protein synthesis; rather, it is a site of steroid and lipid synthesis, lipid metabolism, and drug detoxification

golgi apparatus

stack of flattened sacs with bulbous ends and associated small vesicles; found close to the nucleus; role in packaging proteins and other substances for export from the cell or incorporation into the plasma membrane and in packaging lysosomal enzymes

lysosomes

various-sized membranous sacs containing digestive enzymes (acid hydrolases); function to digest worn-out cell organelles and foreign substances that enter the cell; have the capacity to destroy the cell entirely if they are ruptured and are for this reason referred to as “suicide sacs”

peroxisomes

small lysosome-like membranous sacs containing oxidase enzymes that detoxify alcohol, free radicals, and other harmful chemicals; are particularly abundant in liver and kidney cells

mitochondria

generally rod-shaped bodies with a double-membrane wall; inner membrane shaped into folds or cristae; contain enzymes that oxidize foodstuffs to produce cellular energy ATP; referred to as powerhouses of the cell

centrioles

paired, cylindrical bodies that lie at right angles to each other close to the nucleus; each is composed of nine triplets of microtubules; as part of the centrosome, they direct the formation of the mitotic spindle during cell division; form the bases of cilia and flagella

cytoskeletal elements

microfilaments, intermediate filaments, microtubules; form an internal scaffolding called the cytoskeleton

microfilaments

formed largely of actin, a contractile protein, and thus are important in cell mobility (particularly in muscle cells)

intermediate filaments

stable elements composed of a variety of proteins and resist mechanical forces acting on cells

microtubules

form the internal structure of the centrioles and help determine cell shape

cell cycle

series of changes a cell goes through from the time it is formed until it reproduces

interphase

longer period of the cell cycle, during which the cell grows and carries out its usual activities

mitotic (M) phase

cell division; period of cell cycle when the cell reproduces itself by dividing; essential for growth and repair; has two events, mitosis and cytokinesis

mitosis

the division of the copied DNA of the mother cell into two daughter nuclei; product is two daughter nuclei, generally identical to the mother nucleus; phases include prophase, metaphase, anaphase, and telophase

cytokinesis

the division of the cytoplasm; begins when mitosis is nearly complete

meiosis

specialized type of nuclear division that occurs only in the reproductive organs (testes or ovaries); yields four daughter nuclei that differ genetically in composition from the mother nucleus; used only for producing gametes (eggs and sperm) for sexual reproduction

interphase

period of cell’s life where it carries out its normal metabolic activities and grows; not part of mitosis; during this phase. the DNA-containing material is in the form of chromatin. The nuclear envelope and one or more nucleoli are visible; has three distinct periods: G1, S, and G2

G1

part of interphase when the centrioles begin replicating

S

part of interphase when DNA is replicated

G2

part of interphase when final preparations for mitosis are completed, and centrioles finish replicating

prophase

first phase of mitosis; has early and late stages

early prophase

chromatin condenses, forming barlike chromosomes; each duplicated chromosome consists of two identical threads called sister chromatids, held together at the centromere; as the chromosomes appear, the nucleoli disappear, and the two centrosomes separate from one another; the centrosomes act as focal points for growth of a microtubule assembly called the mitotic spindle. As the microtubules lengthen, they propel the centrosomes toward opposite poles of the cell; microtubule arrays called asters extend from the centrosome matrix

late prophase

the nuclear envelope breaks up, allowing the spindle to interact with the chromosomes; some of the growing spindle microtubules attach to kinetochores, special protein structures at each chromosome’s centromere. Such microtubules are called kinetochore microtubules. ; The remaining spindle microtubules (not attached to chromosomes) are called nonkinetochore microtubules. The microtubules slide past each other, forcing the poles apart.; the kinetochore microtubules pull on each chromosome from both poles in a tug-of-war that ultimately draws the chromosome to the center, or equator, of the cell

metaphase

second phase of mitosis; the two centromeres are at opposite poles of the cell; the chromosomes cluster at the midline of the cell, with their centromeres precisely aligned at the equator of the spindle. This imaginary plane midway between the poles is called the metaphase plate; enzymes act to separate the chromatids from each other

anaphase

third and shortest phase of mitosis; begins abruptly as the centromeres of the chromosomes split simultaneously. Each chromatid now becomes a chromosome in its own right. The kinetochore microtubules moved along by motor proteins in the kinetochores, gradually pull each chromosome toward the pole it faces. At the same time, the nonkinetochore microtubules slide past each other, and push the two poles of the cell apart. The moving chromosomes look V-shaped. The centrimeres lead the way and the chromosomal arms dangle behind them. The short compact shape of the chromosomes makes it easier for them to move and separate. Diffuse threads of chromatin would trail, tangle, and break, resulting in imprecise “parceling out” to the daughter cells.

telophase

final phase of mitosis; begins as soon as chromosomal movement stops; the final phase is like prophase in reverse; the identical sets of chromosomes at the opposite poles of the cell uncoil and resume their threadlike chromatin form; a new nuclear envelope forms around each chromatin mass, nucleoli reappear within the nuclei, and the spindle breaks down and disappears; mitosis is now ended. The cell, for just a brief period, is binucleate and each new nucleus is identical to the original parent nucleus.

cytokinesis

division of cytoplasm; begins during late anaphase and continues through and beyond telophase; a contractile ring of actin microfilaments forms the cleavage furrow and pinches the cell apart