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tiny-mole [99]
2 years ago
8

Uncontrolled Cell Growth (page 252)

Biology
1 answer:
KiRa [710]2 years ago
4 0

The larger a cell becomes, the more demands

the cell places on its DNA. As a cell increases

in size, it usually does not make copies of

DNA. If a cell were to grow without limit, an

“information crisis” would occur. In addition, as a cell increases in size, the more trouble it has moving enough nutrients (food)

and wastes across its cell membrane. The

rate at which materials move through the

cell membrane depends on the surface area

of the cell—the total area of its cell membrane. However, the rate at which food and

oxygen are used up and waste products are

produced depends on the volume of the cell.

If a cell were a cube, you could determine surface area by multiplying length !

width ! number of sides. You could determine volume by multiplying length !

width ! height. You then could determine

the cell’s ratio of surface area to volume by

dividing the surface area by the volume. As

a cell grows, its volume increases more

rapidly than its surface area. That is, as a

cell becomes larger, its ratio of surface area

to volume decreases.

Before a cell becomes too large, a growing cell divides, forming two “daughter”

cells. The process by which a cell divides into

two new daughter cells is called cell division.

10–2 Cell Division

Each cell has only one set of genetic information. For that reason, a cell must first

copy its genetic information before cell division begins. Each daughter cell then gets a

complete copy of that information. In most

prokaryotes, cell division is a simple matter

of separating the contents of the cell into

two parts. In eukaryotes, cell division

occurs in two main stages. The first stage is

division of the nucleus, called mitosis. The

second stage is division of the cytoplasm,

called cytokinesis.

In eukaryotes, genetic information is

passed on by chromosomes. Well before cell

division, each chromosome is replicated

(copied). When copying occurs, each chromosome consists of two identical “sister”

chromatids. Each pair of chromatids is

attached at an area called a centromere.

The cell cycle is a series of events that

cells go through as they grow and divide.

During the cell cycle, a cell grows, prepares

for division, and divides to form two daughter cells, each of which then begins the cycle

again. The cell cycle consists of four phases.

The M phase includes mitosis and cytokinesis. The other three phases are sometimes

grouped together and called interphase.

Interphase is divided into three phases: G1

, S,

and G2

. During the G1 phase, cells increase in

size and make new proteins and organelles.

During the next phase, the S phase, the replication (copying) of chromosomes takes

place. When the S phase is complete, the cell

enters the G2 phase. During the G2 phase,

many of the organelles and molecules

required for cell division are produced.

Mitosis consists of four phases: prophase,

metaphase, anaphase, and telophase. The

first and longest phase is prophase. During

prophase, the chromosomes condense and

become visible. The centrioles separate and

take up positions on opposite sides of the

nucleus. Centrioles are two tiny structures

located in the cytoplasm near the nuclear

envelope. The centrioles lie in a region

called the centrosome that helps to organize

the spindle, a fanlike microtubule structure

that helps separate the chromosomes.

Summary .

During the second phase, called

metaphase, chromosomes line up across the

center of the cell. During the third phase,

called anaphase, the centromeres that join the

sister chromatids split and the sister chromatids become individual chromosomes. The

two sets of chromosomes move apart. During

the fourth and final phase, called telophase,

the chromosomes gather at opposite ends of

the cell and lose their distinct shapes. Two

new nuclear envelopes form.

Cytokinesis usually occurs at the same

time as telophase. In most animal cells, the

cell membrane is drawn inward until the

cytoplasm is pinched into two nearly equal

parts. In plant cells, a structure known as a

cell plate forms midway between the divided nuclei. A cell wall then begins to

appear in the cell plate.

10–3 Regulating the Cell Cycle

In a multicellular organism, cell growth and

cell division are carefully controlled. For

instance, when an injury such as a cut in the

skin occurs, cells at the edge of the cut will

divide rapidly. When the healing process

nears completion, the rate of cell division

slows down and then returns to normal.

Cyclins—a group of proteins—regulate

the timing of the cell cycle in eukaryotic

cells. There are two types of these regulatory proteins: internal regulators and

external regulators.

Internal regulators are proteins that

respond to events inside the cell. They

allow the cell cycle to proceed only when

certain processes have happened inside the

cell. External regulators are proteins that

respond to even

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