Chapter 3
Cells: The Living Units
99
3
Check Your Understanding
26.
If one of the DNA strands being replicated “reads” CGAATG,
what will be the base sequence of the corresponding DNA
strand?
27.
During what phase of the cell cycle is DNA synthesized?
28.
What are three events occurring in prophase that are undone
in telophase?
For answers, see Appendix H.
Protein Synthesis
Define gene and genetic code and explain the function
of genes.
Name the two phases of protein synthesis and describe the
roles of DNA, mRNA, tRNA, and rRNA in each phase.
Contrast triplets, codons, and anticodons.
In addition to directing its own replication, DNA serves as the
master blueprint for protein synthesis. Although cells also make
lipids and carbohydrates, DNA does not dictate their structure.
Historically, DNA is said to specify
only
the structure of protein
molecules, including the enzymes that catalyze the synthesis of
all classes of biological molecules.
Much of the metabolic machinery of the cell is concerned in
some way with protein synthesis. Tis is not surprising, seeing
as structural proteins constitute most of the dry cell material,
and functional proteins direct almost all cellular activities. Es-
sentially, cells are miniature protein factories that synthesize the
huge variety of proteins that determine the chemical and physi-
cal nature of cells—and therefore of the whole body.
Recall from Chapter 2 that proteins are composed of
polypeptide chains, which in turn are made up of amino acids.
For purposes of this discussion, we define a
gene
as a segment
of a DNA molecule that carries instructions for creating one
polypeptide chain. Supposedly we humans have an estimated
25,000 protein coding genes. (Note, however, that some genes
specify the structure of certain varieties of RNA as their final
product.)
Te four nucleotide bases (A, G, ±, and C) are the “letters”
used in the genetic alphabet, and the information of DNA is
found in the sequence of these bases. Each sequence of three
bases, called a
triplet
, can be thought of as a “word” that speci-
fies a particular amino acid. For example, the triplet AAA calls
for the amino acid phenylalanine, and CC± calls for glycine.
Te sequence of triplets in each gene forms a “sentence” that
tells exactly how a particular polypeptide is to be made: It spec-
ifies the number, kinds, and order of amino acids needed to
build a particular polypeptide.
Variations in the arrangement of A, ±, C, and G allow our
cells to make all the different kinds of proteins needed. Even a
“small” gene has an estimated 210 base pairs in sequence. Te
ratio between DNA bases in the gene and amino acids in the
polypeptide is 3:1 (because each triplet stands for one amino
acid), so we would expect the polypeptide specified by such a
gene to contain 70 amino acids.
Control of Cell Division
Te signals that prod cells to divide
are incompletely understood, but we know that the ratio of cell
surface area to cell volume is important. Te amount of nutri-
ents a growing cell requires is directly related to its volume. Vol-
ume increases with the cube of cell radius, whereas surface area
increases more slowly with the square of the radius.
For example, a 64-fold (4
3
) increase in cell volume is ac-
companied by only a 16-fold (4
2
) increase in surface area. Con-
sequently, the surface area of the plasma membrane becomes
inadequate for nutrient and waste exchange when a cell reaches
a certain critical size. Cell division solves this problem because
the smaller daughter cells have a favorable ratio of surface area
to volume. Tese surface-volume relationships help explain why
most cells are microscopic in size.
±wo other factors that influence when cells divide are chemi-
cal signals (growth factors, hormones, and others) released by
other cells and the availability of space. Normal cells stop pro-
liferating when they begin touching, a phenomenon called
con-
tact inhibition
. Te system that controls the cell cycle has been
compared to the timer on a washing machine. Like that timer,
the control system for the cell cycle is driven by a built-in clock.
However, just as the washer’s cycle is subject to adjustments
(by regulating the flow from the faucet, say, or by an internal
water-level sensor), the cell cycle is regulated by both internal
and external factors.
±wo groups of proteins are crucial to the ability of a cell to
accomplish the S phase and enter mitosis:
Cyclins
, regulatory proteins whose levels rise and fall during
each cell cycle
Cdks (cyclin-dependent kinases)
, which are present in a
constant concentration in the cell and are activated by bind-
ing to particular cyclins
A new batch of cyclins accumulates during each interphase.
Subsequent joining of specific Cdk and cyclin proteins initi-
ates enzymatic cascades that phosphorylate histones and other
proteins needed for cell division. At the end of mitosis, enzymes
destroy the cyclins.
A number of “switches” and crucial checkpoints for cell di-
vision occur throughout interphase. Tese built-in stop sig-
nals halt the cell cycle until overridden by internal or external
go-ahead signals. In many cells, a G
1
checkpoint, called the
restriction point
, seems to be most important (see Figure 3.31).
If the cell is prevented from progressing past this checkpoint,
it enters the nondividing state (G
0
). Another important check-
point, and the first to be understood, occurs late in G
2
, when a
threshold amount of a protein complex called
MPF (M-phase
promoting factor)
is required to give the okay signal to pass
the G
2
checkpoint and enter M phase. Later in M phase, MPF
is inactivated.
Besides these “go” signals, there are a number of so-called
repressor genes that inhibit cell division. One example is the
p53
gene that initiates a series of enzymatic events that pro-
duce growth-inhibiting factors. Roughly half of all cancers have
abnormal
p53
genes. Contact with other cells does not inhibit
these cancerous cells and they divide wildly, making them dan-
gerous to their host.
(Text continues on p. 102.)
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