Question 1

One of the most important questions in biology is what signals cells when to divide and when to stop.

Accepted Answer

The activation and inhibition of cell division are subjects of intense research for obvious reasons such as management of cancer and tissue repair.

Question 2

Cells divide when (1) they grow large enough to have enough cytoplasm to distribute to their two daughter cells; (2) they have replicated their DNA, so they can give each daughter cell a duplicate set of genes;

Accepted Answer

(3) they receive an adequate supply of nutrients;

Question 3

Cells divide when

Accepted Answer

(4) they are stimulated by growth factors, chemical signals secreted by blood platelets, kidney cells, and other sources; or (5) neighboring cells die, opening up space in a tissue to be occupied by new cells.

Question 4

Growth factors and their receptors are a central issue in understanding the uncontrolled growth of cancer (see Deeper Insight 4.3).

Accepted Answer

Cells stop dividing when they snugly contact neighboring cells or when nutrients or growth factors are withdrawn. The cessation of cell division in response to contact with other cells is called contact inhibition.

Question 5

An absence of contact inhibition

Accepted Answer

, leading to uncontrolled cell division, is one of the characteristics of cancer.

Question 6

The cell cycle is regulated by a molecular timer and certain checkpoints at which the cell checks its own status before moving on to the next phase.

Accepted Answer

Two key elements of the timer are families of proteins called cyclins and cyclin-dependent kinases (Cdks).

Question 7

A kinase is an enzyme that adds phosphate to other proteins, thereby activating or suppressing their function.

Accepted Answer

Cdks exist at fairly stable levels in cells, but without cyclins, they lie dormant and perform no function.

Question 8

Cyclin levels rise and fall through the cell cycle. In early interphase, cyclin genes are transcribed and cyclin levels rise. As they bind to Cdks, those enzymes phosphorylate proteins with various effects described shortly.

Accepted Answer

At the end of mitosis, proteasomes degrade the cyclins, their levels fall, and the Cdks become dormant again until the next cycle.

Question 9

At specific checkpoints during the cell cycle, a cyclin binds to a Cdk and activates a cascade of biochemical reactions that prepare a cell to move on to the next phase of the cycle.

Accepted Answer

One of these, called the Start or G1 checkpoint, either allows the cell to proceed toward the S phase or, if it doesn't, the cell goes into the noncycling Go phase.

Question 10

A G2/M checkpoint late in the G2 phase determines whether the cell is able to proceed to mitosis.

Accepted Answer

A third checkpoint at the transition from metaphase to anaphase determines whether the cell can proceed to anaphase, leading to separation of its sister chromatids.

Question 11

Malfunctions of this process can be disastrous to the cell and even to the life of the whole person. If it fails to allow a cell to progress through the cell cycle, the result can be a failure of tissue maintenance and repair.

Accepted Answer

On the other hand, if the cell cycle progresses too readily, out of control, the result can be uncontrolled tissue growth (neoplasia) and cancer.

Question 12

Among other functions of cyclin-Cdk complexes, they control (1) the replication of DNA and centrioles in the S phase; (2) the condensation of chromosomes, breakdown of the nuclear envelope,e.

Accepted Answer

formation of the mitotic spindle, and attachment of chromosomes to the spindle in prophase; and (3) splitting of the centromere and separation of the sister chromatids at anaphas

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4.3e