Biology Now, 2e

Answers ■ A11

Figure 6.3

Q1: Name one similarity between cell division in prokaryotes and

cell division in eukaryotes.

A1: Both prokaryotes and eukaryotes replicate DNA, separate the

DNA to opposite poles of the cell, and then physically split the cell

into two parts.

Q2: Why is binary fission referred to as “asexual reproduction”?

A2: Because it does not involve sharing DNA between individuals,

as in sexual reproduction.

Q3: Name one difference between cell division in prokaryotes and

cell division in eukaryotes.

A3: Prokaryotic cells are smaller and simpler than eukaryotic

cells, so cell division happens more rapidly and is less complex; for

example, prokaryotes do not have organelles, so they do not have to

be divided up among daughter cells.

Figure 6.4

Q1: Do all cells in an organism enter each stage of mitosis at the

same time? (Hint: See image of onion root tip at far left in the

fig u re.)

A1: No.

Q2: What happens between the end of interphase and early

prophase that changes the appearance of the chromosomes?

A2: The DNA within the chromosomes is condensed to prepare for

division.

Q3: Explain in your own words the role of the mitotic spindle in

mitosis.

A3: The mitotic spindle is responsible for accurately separating

chromatids into daughter cells.

Figure 6.5

Q1: Why is it important for a chromosome to be copied before

mitosis?

A1: There need to be two copies of each chromosome, so that each

daughter cell has identical genetic material.

Q2: Are sister chromatids attached at the centromere considered

to be one or two chromosomes?

A2: Sister chromatids are, when attached, one chromosome. When

split apart, they are two separate but identical chromosomes.

Q3: Why is the chromosome’s DNA tightly packed for mitosis and

cy tokinesis? (Hint: Think about what would happen if it were

unpackaged, as during interphase.)

A3: Tightly packed chromosomes are more easily divided into

daughter cells.

Figure 6.6

Q1: What could happen if the cell’s checkpoints are disabled?

A1: The cell cycle could occur more quickly, because it will not

stop to ensure that the checkpoint conditions are met.

Q2: What is the advantage of stopping the cell cycle if the cell’s

DNA is damaged?

A2: If the nutrient supply is inadequate, the daughter cells cannot

grow. By stopping the cell cycle, the parent cell can wait until the

nutrient supply increases.

Q3: Which part of the cell cycle may have been influenced in Soto

and Sonnenschein’s breast tumor cell experiments?

A3: The G 0 or G 1 checkpoint.

Figure 6.7

Q1: Is a zygote haploid or diploid?

A1: Diploid.

Q2: What cellular process creates a baby from a zygote?

A2: Mitosis.

Q3: If a mother or father was exposed to BPA prior to conceiving

a child, how might that explain potential birth defects in the

fetus?

A3: If the parent’s BPA exposure caused mutation in sex cells (eggs

or sperm), that could lead to a birth defect in the fetus.

Figure 6.8

Q1: Is a daughter cell haploid or diploid after the first meiotic

division? How about after the second meiotic division?

A1: After meiosis I, a daughter cell is haploid (has one of each

homologous chromosome). After meiosis II, each daughter cell is

still haploid (has one of each homologous chromosome), and sister

chromatids have split into separate daughter cells.

Q2: What is the difference between homologous chromosomes and

sister chromatids?

A2: Sister chromatids are identical DNA molecules, replicated

from a single DNA molecule, that remain bound to each other.

They exist in a cell only from the S phase until anaphase of

mitosis, or anaphase II of meiosis. A homologous chromosome

pair consists of the two copies—one maternal, the other

paternal—of the same type of chromosome. The pair is present at

all times in diploid cells, but a haploid cell has just the paternal or

the maternal copy.

Q3: If the skin cells of house cats contain 38 homologous pairs of

chromosomes, how many chromosomes are present in the egg cells

they produce?

A3: 38.

Figure 6.9

Q1: Why is the term “crossing-over” appropriate for the exchange

of DNA segments between homologous chromosomes?

A1: Segments of DNA physically “cross over” between homologous

chromosomes.

Q2: At what stage of meiosis (I or II) does crossing-over occur?

A2: Meiosis I.

Q3: What would be the effect of crossing-over between two sister

chromatids?

A3: There would be no effect, because sister chromatids are

genetically identical.