⇒ Cellular Organization
Most cells reproduce asexually, without exchanging or acquiring new
hereditary information. Bacteria reproduce almost exclusively in this
fashion in a process called binary fission, during which the bacterium
grows, duplicates its hereditary information, segregates the duplicated
chromosome, and divides the cytoplasm. Most cells that form the bodies
of multicellular eukaryotes are also produced asexually in a process
termed mitosis. During mitotic division, the cells grow, duplicate their
genomes, separate their duplicated chromosome sets into nuclei at the opposite
poles of the cell, and divide the cytoplasm to form progeny cells.
The eukaryotic cell cycle contains four major phases (Figure
1-4). The S phase is when DNA synthesis occurs to replicate the chromosomes
by creating identical sister chromatids. The period between S
phase and the beginning of mitosis (M phase) is a gap, or growth period,
phase. Another gap or growth period called the G1
occurs between the M and S phases to complete the cycle.
Mitosis consists of four consecutive phases: prophase, metaphase,
anaphase, and telophase (Figure 1-5). During prophase, each chromosome
shortens and thickens by supercoiling on itself again and again.
|Figure 1-4 Eukaryotic cell cycle.
|Figure 1-5 Mitosis in animal cells.
The nuclear membrane dissolves, and a spindle of microtubules forms
from one pole of the cell to the other. During metaphase, the chromosomes
line up in the center of the spindle. At anaphase, the two chromatids
of each replicated chromosome are pulled to opposite poles by depolymerization
of the microtubules in the spindle apparatus that are
attached to the centromeres. These former sister chromatids are now considered
to be new chromosomes. Division of the cytoplasm (cytokinesis)
begins in telophase, as the chromosomes unwind and new nuclear membranes
form to enclose the sets of chromosomes at each pole of the cell.
When mitosis is completed, two progeny cells contain identical sets of
The somatic cells of most plants and animals are diploid, meaning
they have two sets of homologous chromosomes. One set is derived from
each parent through the gametes that produced the zygote from which the
organism developed. The process of meiosis reduces the chromosome
number from diploid to haploid in gametes, or sex cells; thus, each parent
contributes an equal number of chromosomes to their offspring.
The predominant form of reproduction in most multicellular eukaryotes
is sexual. At sexual maturity, some diploid germ line cells become
specialized to undergo meiosis and form haploid gametes. Meiosis
can be visualized as two highly modified cell cycles, back to back (see
Figure 1-6). A complete meiotic cycle involves one initial DNA replication
and two cytoplasmic divisions, yielding four haploid products, none
of which are genetically identical. The two cycles are labeled meiosis I
and II, each of which has its own prophase, metaphase, anaphase, and
The major events of these phases mirrors the events during mitosis.
However, during prophase I of meiosis, homologous chromosomes pair up in a process called synapsis. A synapsed pair of chromosomes contains
four chromatids. Each chromosome usually has one or more regions
in which two of the four chromatids break at corresponding sites and reunite
with one another, a process called crossing over, which increases
genetic variability. During anaphase I, the homologous chromosomes are
separated, yielding two haploid cells at the completion of the first stage
of meiosis. During anaphase II, sister chromatids are separated, as they
are during mitotic anaphase. The end result is four genetically different
Meiosis I is reductional division, since the number
of chromosomes is reduced; meiosis II is equational