Chapter 12

Question 1

genome

Answer 1

A cell’s DNA, its genetic information, is called its genome.

Question 2

When a cell is not dividing, and even as it replicates its DNA
in preparation for cell division, each chromosome is in the
form of

Answer 2

a long, thin chromatin fiber.

Question 3

. This attachment is mediated by

proteins that

Answer 3

recognize and bind to the
centromeric DNA; other bound proteins
condense the DNA, giving the duplicated chromosome a narrow “waist.”

Question 4

mitosis and cytokinesis

Answer 4

Mitosis, the division of the genetic material in the nucleus,
is usually followed immediately by cytokinesis, the division
of the cytoplasm. One cell has become two, each the genetic
equivalent of the parent cell.

Question 5

explain what happens in each cell cycle phase in like a word or two

Answer 5

Thus, a cell
grows (G1), continues to grow as it copies its chromosomes
(S), grows more as it completes preparations for cell division
(G2), and divides (M). The daughter cells may then repeat
the cycle.

Question 6

explain what happens in each cell cycle phase in some numbers

Answer 6

A particular human cell might undergo one division in
24 hours. Of this time, the M phase would occupy less than
1 hour, while the S phase might occupy 10–12 hours, or
about half the cycle. The rest of the time would be apportioned between the G1 and G2 phases. The G2 phase usually
takes 4–6 hours; in our example, G1 would occupy about
5–6 hours. G1 is the most variable in length in different
types of cells. Some cells in a multicellular organism divide
very infrequently or not at all. These cells spend their time
in G1 (or a related phase called G0, to be discussed later in
the chapter) doing their job in the organism—a cell of the
pancreas secretes digestive enzymes, for example.

Question 7

e. Overlapping with the latter stages of mitosis,

cytokinesis

Answer 7

completes the mitotic phase.

Question 8

MPF- what do they do and what stand for, what are they

Answer 8

shows the fluctuating activity of MPF,
the cyclin-Cdk complex that was discovered first (in frog
eggs). Note that the peaks of MPF activity correspond to the
peaks of cyclin concentration. The cyclin level rises during
the S and G2 phases and then falls abruptly during M phase.
The initials MPF stand for “maturation-promoting factor,”
but we can think of MPF as “M-phase-promoting factor”
because it triggers the cell’s passage into the M phase, past
the G2 checkpoint. When cyclins that accumulate during
G2 associate with Cdk molecules, the resulting MPF complex
is active—it phosphorylates a variety of proteins, initiating MPF acts both directly as a kinase and
indirectly by activating other kinases. For example, MPF causes
phosphorylation of various proteins of the nuclear lamina (see
Figure 6.9), which promotes fragmentation of the nuclear envelope during prometaphase of mitosis. There is also evidence that
MPF contributes to molecular events required for chromosome
condensation and spindle formation during prophase.

Question 9

Molecular control of the cell cycle at the

G2 checkpoint.

Answer 9

The steps of the cell cycle are timed by rhythmic
fluctuations in the activity of cyclin-dependent kinases (Cdks). Here we
focus on a cyclin-Cdk complex in animal cells called MPF, which acts at
the G2 checkpoint as a go-ahead signal, triggering the events of mitosis.

Question 10

PDGF- where are they? what are they? how are they related to fibroblasts

Answer 10

Consider, for example, platelet-derived growth factor (PDGF),
which is made by blood cell fragments called platelets. The
experiment illustrated in Figure 12.18 demonstrates that
PDGF is required for the division of cultured fibroblasts,
a type of connective tissue cell. Fibroblasts have PDGF receptors on their plasma membranes. The binding of PDGF
molecules to these receptors (which are receptor tyrosine
kinases; (more on pg 295 if u want)

Question 11

density dep inhibition

Answer 11

The effect of an external physical factor on cell division is
clearly seen in density-dependent inhibition, a phenomenon in which crowded cells stop dividing (Figure 12.19a). As
first observed many years ago, cultured cells normally divide
until they form a single layer of cells on the inner surface of
a culture flask, at which point the cells stop dividing

Question 12

anchorage dependence

Answer 12

Most animal cells also exhibit anchorage dependence
(see Figure 12.19a). To divide, they must be attached to
a substratum, such as the inside of a culture flask or the

Question 13

Density-dependent inhibition and anchorage dependence
appear to function not only in cell culture but also in the
body’s tissues,

Answer 13

checking the growth of cells at some optimal
density and location during embryonic development and
throughout an organism’s life. Cancer cells, which we discuss next, exhibit neither density-dependent inhibition nor
anchorage dependence

Question 14

transformation

Answer 14

Cells
in culture that acquire the ability to divide indefinitely are
said to have undergone transformation, the process that
causes them to behave like cancer cells. By contrast, nearly all
normal, nontransformed mammalian cells growing in culture
divide only about 20 to 50 times before they stop dividing,
age, and die

Question 15

benign tumor

Answer 15

The
abnormal cells may remain at the original site if they have
too few genetic and cellular changes to survive at another
site. In that case, the tumor is called a benign tumor. Most
benign tumors do not cause serious problems (depending on
their location) and can be removed by surgery.

Question 16

malignant tumor

Answer 16

y. In contrast,
a malignant tumor includes cells whose genetic and cellular changes enable them to spread to new tissues and impair
the functions of one or more organs; these cells are also sometimes called transformed cells (although usage of this term is
generally restricted to cells in culture). An individual with a
malignant tumor is said to have cancer

Question 17

metastasis

Answer 17

Cancer cells may also secrete signaling molecules that cause blood vessels to grow toward the
tumor. A few tumor cells may separate from the original tumor,
enter blood vessels and lymph vessels, and travel to other parts
of the body. There, they may proliferate and form a new tumor.
This spread of cancer cells to locations distant from their original site is called metastasis

Question 18

what happens in cytokinesis in plants

Answer 18

The cell plate enlarges until its surrounding
membrane fuses with the plasma membrane along the perimeter of the cell. Two daughter cells result, each with its own
plasma membrane. Meanwhile, a new cell wall arising from
the contents of the cell plate forms between the daughter cells.

Question 19

what is a cell plate

Answer 19

Cytokinesis in plant cells, which have cell walls, is markedly different. There is no cleavage furrow. Instead, during telophase, vesicles derived from the Golgi apparatus
move along microtubules to the middle of the cell, where
they coalesce, producing a cell plate. Cell wall materials carried in the vesicles collect inside the cell plate as it grows

Question 20

binary fission

Answer 20

The term binary fission,
meaning “division in half,” refers to this process and to the
asexual reproduction of single-celled eukaryotes, such as the
amoeba in Figure 12.2a. However, the process in eukaryotes
involves mitosis, while that in prokaryotes does not

Question 21

origin of replication

Answer 21

In some bacteria, the process of cell division is initiated
when the DNA of the bacterial chromosome begins to replicate at a specific place on the chromosome called the origin
of replication, producing two origins. A

Question 22

bacteria

Answer 22

During binary fission in bacteria, the origins of the
daughter chromosomes move to opposite ends of the cell. The
mechanism involves polymerization of actin-like molecules, and
possibly proteins that may anchor the daughter chromosomes to
specific sites on the plasma membrane.
(a

Question 23

dinoflagellate

Answer 23

. In unicellular protists called dinoflagellates, the
chromosomes attach to the nuclear envelope, which remains
intact during cell division. Microtubules pass through the nucleus
inside cytoplasmic tunnels, reinforcing the spatial orientation of
the nucleus, which then divides in a process reminiscent of
bacterial binary fission.

Question 24

diatoms nd some yeast

Answer 24

. In these two other groups of
unicellular eukaryotes, the nuclear envelope also remains intact
during cell division. In these organisms, the microtubules form a
spindle within the nucleus. Microtubules separate the
chromosomes, and the nucleus splits into two daughter nuclei

Question 25

most eukaryotes

Answer 25

. In most other eukaryotes, including plants and
animals, the spindle forms outside the nucleus, and the nuclear
envelope breaks down during mitosis. Microtubules separate the
chromosomes, and two nuclear envelopes then form

Question 26

what controls the cell cycle 1970s hyp

Answer 26

? In the early 1970s, a variety of
experiments led to the hypothesis that the cell cycle is driven by
specific signaling molecules present in the cytoplasm

Question 27

Some of

the first strong evidence for this hypothesis came from (the hypothesis of who controls the cell cycle)

Answer 27

experiments with mammalian cells grown in culture. In these experiments, two cells in different phases of the cell cycle were fused
to form a single cell with two nuclei (Figure 12.14). If one of the
original cells was in the S phase and the other was in G1, the G1
nucleus immediately entered the S phase, as though stimulated
by signaling molecules present in the cytoplasm of the first cell.
Similarly, if a cell undergoing mitosis (M phase) was fused with
another cell in any stage of its cell cycle, even G1, the second
nucleus immediately entered mitosis, with condensation of the
chromatin and formation of a mitotic spindle.

Question 28

cell cycle control system- d nd how was it found

Answer 28

The experiment shown in Figure 12.14 and other experiments on animal cells and yeasts demonstrated that the
sequential events of the cell cycle are directed by a distinct molecules in the cell that both triggers and coordinates key
events in the cell cycle