Toxic Plastic ■ 113● (^) The cell cycle is the set
sequence of events over the
life span of a eukaryotic cell
that will divide. Interphase
and cell division are the two
main stages of the cell cycle.
Interphase is longer, and
consists of the G 1 , S, and G 2
phases. DNA is replicated in
the S phase. Cells that will
not divide exit the cell cycle
and enter a G 0 phase.
● (^) Cell division is necessary
for growth and repair in
multicellular organisms,
and for asexual and sexual
reproduction in all types of
organisms. Many prokaryotes
divide through binary
fission, a form of asexual
reproduction.
● (^) Each chromosome in a
cell contains a single DNA
molecule compacted by
packaging proteins. The
somatic cells of eukaryotes
have two of each type of
chromosome matched
together in homologous pairs.
One chromosome in each pair
is inherited from the mother;
the other, from the father.
Chromosomal replication
produces two identical
sister chromatids that are
held together firmly at the
centromere.
● (^) Eukaryotes perform cell
division through mitosis
followed by cytokinesis,
producing daughter cells that
are genetically identical to
each other and to the parent
cell. The four main phases
of mitosis are prophase,
metaphase, anaphase, and
telophase. Through these
phases, the chromosomes of
a parent cell are condensed
and positioned appropriately,
and the sister chromatids are
separated to opposite ends
of the cell. During cytokinesis,
the cytoplasm of the parent
cell is physically divided to
create two daughter cells.
● (^) The cell cycle is carefully
regulated. Checkpoints
ensure that the cycle does
not proceed if conditions are
not right. Unregulated cell
growth and cancer occur
when checkpoints fail.
● (^) Meiosis is critical for
sexual reproduction. In
animals, the products
of meiosis are sex cells,
called gametes, that fuse
during fertilization to give
rise to a zygote. Meiosis—
consisting of two rounds
of nuclear and cytoplasmic
divisions—produces haploid
gametes containing only
REVIEWING THE SCIENCE
one chromosome from each
homologous pair. When
two gametes fuse during
fertilization, a diploid zygote
is formed, having one of each
homologous chromosome
from both parents.● (^) During meiosis I, the
maternal and paternal
members of each
homologous pair are sorted
into two daughter cells.
Meiosis II is similar to mitosis
in that sister chromatids are
segregated into separate
daughter cells at the end of
cytokinesis.
● (^) Meiosis produces genetically
diverse gametes through
crossing-over of homologous
chromosomes, leading to
genetic recombination
and then the independent
assortment of homologous
chromosomes. Meiosis
and fertilization together
introduce genetic variation
into populations.
THE QUESTIONS
The Basics
(^1) Homologous chromosomes are
(a) the same thing as sister chromatids.
(b) a pair of chromosomes of the same kind.
(c) identical copies of the same chromosome.
(d) always haploid.
(^2) Which of the following is not a contributor to genetic variation?
(a) binary fission
(b) crossing-over of homologous chromosomes
(c) random assortment of homologous chromosomes
(d) fertilization
(^3) Link each cell phase to the event that occurs within it.
CYTOKINESIS
S PHASE
G 1 PHASE
G 0 PHASE
- Each of the chromosomes in a human cell
contains two sister chromatids by the end of
this phase. - Most cell growth occurs during this phase.
- Cells that will never replicate leave the cell
cycle and enter this phase. - Two separate daughter cells are produced at
the end of this phase.