Question 1

Cells divide by two mechanisms called mitosis and meiosis. Meiosis, however, is restricted to one purpose, the production of eggs and sperm, and is therefore treated in chapter 27 on reproduction. Mitosis serves all the other functions of cell division:

Accepted Answer

development of an individual, composed of some 50 trillion cells, from a one-celled fertilized egg;
⚫ growth of all tissues and organs after birth;
⚫ replacement of cells that die; and
⚫ repair of damaged tissues.

Question 2

Four phases of mitosis are recognizable:

Accepted Answer

prophase, metaphase, anaphase, and telophase (fig. 4.15).

Question 3

Prophase. At the outset of mitosis, the chromosomes shorten and thicken, eventually coiling into compact rods that are easier to distribute to daughter cells than the long, delicate chromatin of interphase.

Accepted Answer

There are 46 chromosomes, each with two chromatids and one molecule of DNA per chromatid. The nuclear envelope disintegrates during prophase and releases the chromosomes into the cytosol.

Question 4

The centrioles begin to sprout elongated microtubules called spindle fibers, which push the centrioles apart as they grow. Eventually, a pair of centrioles comes to lie at each pole of the cell.

Accepted Answer

Some spindle fibers grow toward the chromosomes and become attached to the kinetochore on each side of the centromere (see fig. 4.5). The spindle fibers then tug the chromosomes back and forth until they line up along the midline of the cell.

Question 5

Metaphase.

Accepted Answer

The chromosomes are aligned on the cell equator, oscillating slightly and awaiting a signal that stimulates each of them to split in two at the centromere.

Question 6

Metaphase.

Accepted Answer

The spindle fibers now form a lemon-shaped array called the mitotic spindle.

Question 7

Long microtubules reach out from each centriole to the chromosomes,

Accepted Answer

and shorter microtubules form a starlike aster, which anchors the assembly to the inside of the plasma membrane at each end of the cell.

Question 8

Anaphase." This phase begins with activation of an enzyme that cleaves the two sister chromatids from each other at the centromere. Each chromatid is now regarded as a separate, single-stranded daughter chromosome.

Accepted Answer

One daughter chromosome migrates to each pole of the cell, with its centromere leading the way and the arms trailing behind.

Question 9

Anaphase.

Accepted Answer

Migration is achieved by means of motor proteins in the kinetochore crawling along the spindle fiber as the fiber itself is "chewed up" and disassembled at the chromosomal end.

Question 10

Telophase. The daughter chromosomes cluster on each side of the cell. The rough ER produces a new nuclear envelope around each cluster, and the chromosomes begin to uncoil and return to the thinly dispersed chromatin form.

Accepted Answer

The mitotic spindle breaks up and vanishes. Each new nucleus forms nucleoli, indicating it has already begun making RNA and is preparing for protein synthesis.

Question 11

Telophase is the end of nuclear division but overlaps with cytokinesis 11 (SY-toe-kih-NEE-sis), division of the cytoplasm into two cells. Early traces of cytokinesis are visible even at anaphase

Accepted Answer

. It is achieved by the motor protein myosin pulling on microfilaments of actin in the terminal web of the cytoskeleton. This creates a crease called the cleavage furrow around the equator of the cell, and the cell eventually pinches in two.

Question 12

Interphase has now begun for these new cells. Be aware, however, that mitosis (nuclear division)

Accepted Answer

can occur without cytokinesis (cellular division). This is why some cells acquire two or more nuclei or multiple identical sets of chromosomes.

Question 13

This is why some cells acquire two or more nuclei or multiple identical sets of chromosomes.

Accepted Answer

Interphase has now begun for these new cells. Be aware, however, that mitosis (nuclear division) can occur without cytokinesis (cellular division).

Question 14

One might think that since DNA is faithfully replicated by complementary base pairing in the S phase,

Accepted Answer

and then the two copies are distributed to the two daughter cells at each mitotic anaphase, the two daughter cells would be genetically identical.

Question 15

One might reasonably expect that all cells of the body, with the exception of the sex cells, are genetically identical.

Accepted Answer

But they're not.

Question 16

We saw earlier that ----- isn't error-free, and even after replication, new mutations can arise spontaneously in either strand independently of the other.

Accepted Answer

DNA replication

Question 17

For this and several other reasons, our somatic (non-sex) cells are genetically variable. One's body is a patchwork of genetic variation—a state known as genetic mosaicism.

Accepted Answer

This variation is especially great among our immune cells, sex cells, and neurons.

Question 18

Even so-called identical twins, which arise from a single fertilized egg, aren't really genetically identical.

Accepted Answer

They're born with as many as 100 to 200 gene differences, arising by new, spontaneous mutations acquired independently of each other while still in the womb.

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