12.1
In both prokaryotes and eukaryotes, most cell division involved the distribution of
identical genetic material, DNA, to two daughter cells. The exception is meiosis,
the special type of eukaryotic cell division that can produce sperm and eggs.
Cellular organization of the genetic material
A cell’s endowment of DNA, its genetic information, is called a Genome. A
prokaryotic genome is often a single DNA molecule. Eukaryotic genomes usually
consist of a number of DNA molecules. Before the cell can divide to from
genetically identical daughter cells, the DNA must be copied, and then the two
copies must be separated so that each daughter cell ends up with a complete
genome.
The replication and distribution of the amount of DNA is manageable because the
DNA molecules are packaged into structures called chromosomes. Each
eukaryotic chromosome consists of one very long, linear DNA molecule
associated with many proteins. The DNA molecule carries several hundred to a
few thousand genes, the units of information that specify an organism’s inherited
traits. The proteins maintain the structure of the chromosome and help control
the activity of the genes. The entire complex of DNA and proteins that is the
building material of chromosomes is referred to as chromatin. The chromatin of a
chromosome varies in its degree of condensation during the process of cell
division.
Every eukaryotic species has a characteristic number of chromosomes in each
cell nucleus.
Somatic cells: all body cells except the reproductive cells.
Gametes: reproductive cells, they have half as many chromosomes as somatic
cells.
The number of chromosomes in somatic cells vary widely among species.
Distribution of chromosomes during eukaryotic cell division.
When a cell is not dividing, and even as it replicates its DNA in preparation for
cell division, each chromosome is in the form of a long chromatin fiber. After DNA
replication, the chromosomes condense as a part of cell division: each chromatin
fiber becomes densely coiled and folded.
,Each duplicated chromosome has
two sister chromatids, which are
joined copies of the original
chromosome (figure 12.4). The two
chromatids, each containing an
identical DNA molecule, are initially
attached all along their lengths. Each
sister chromatid has a centromere, a
region containing specific DNA
sequences where the chromatid is attached most closely to its sister chromatid.
The part of a chromatid on either side of the centromere is referred to as an arm
of the chromatid.
Later in the cell division process, the two sister chromatids of each duplicated
chromosome separate and move into two new nuclei, one forming at each end of
the cell. The now separated sister chromatids are referred to as individual
chromosomes. Thus, each new nucleus receives
a collection of chromosomes identical to that of
the parent cell (figure 12.5).
,Mitosis, the division of genetic material in the nuleus, is usually followed
immediately by cytokinsis, the division of the cytoplasm. One cell has become
two, each genetic equilavent of the parant cell.
12.2
Phases of the cell cycle
, Mitosis is just one part of the cell cycle (figure 12.6). The mitotic (M) phase, which
includes both mitosis and cytokinesis, is the shortest part of the cell cycle. Mitotic
cell division alternates with a much longer stage, the interphase. During the
interphase, a cell that is about to divide grows and copies its chromosomes in
preparation of cell division. Interphase can be divided into sub-phases:
G1 phase
S phase
G2 phase
During all three sub-phases, a
cell that will eventually divide
grows by producing proteins
and cytoplasmic organelles.
Chromosomes are duplicated
during the S phase. Thus, a
cell grows (G1), continues to
grow as it copies its
chromosomes (S), grows
more as it completes
preparations for cell division
(G2), and divides (M).
Mitosis is conventionally broken down into five stages:
Prophase
Prometaphase
Metaphase
Anaphase
Telophase
Figure 12.7 describes these stages in an animal cell.
In both prokaryotes and eukaryotes, most cell division involved the distribution of
identical genetic material, DNA, to two daughter cells. The exception is meiosis,
the special type of eukaryotic cell division that can produce sperm and eggs.
Cellular organization of the genetic material
A cell’s endowment of DNA, its genetic information, is called a Genome. A
prokaryotic genome is often a single DNA molecule. Eukaryotic genomes usually
consist of a number of DNA molecules. Before the cell can divide to from
genetically identical daughter cells, the DNA must be copied, and then the two
copies must be separated so that each daughter cell ends up with a complete
genome.
The replication and distribution of the amount of DNA is manageable because the
DNA molecules are packaged into structures called chromosomes. Each
eukaryotic chromosome consists of one very long, linear DNA molecule
associated with many proteins. The DNA molecule carries several hundred to a
few thousand genes, the units of information that specify an organism’s inherited
traits. The proteins maintain the structure of the chromosome and help control
the activity of the genes. The entire complex of DNA and proteins that is the
building material of chromosomes is referred to as chromatin. The chromatin of a
chromosome varies in its degree of condensation during the process of cell
division.
Every eukaryotic species has a characteristic number of chromosomes in each
cell nucleus.
Somatic cells: all body cells except the reproductive cells.
Gametes: reproductive cells, they have half as many chromosomes as somatic
cells.
The number of chromosomes in somatic cells vary widely among species.
Distribution of chromosomes during eukaryotic cell division.
When a cell is not dividing, and even as it replicates its DNA in preparation for
cell division, each chromosome is in the form of a long chromatin fiber. After DNA
replication, the chromosomes condense as a part of cell division: each chromatin
fiber becomes densely coiled and folded.
,Each duplicated chromosome has
two sister chromatids, which are
joined copies of the original
chromosome (figure 12.4). The two
chromatids, each containing an
identical DNA molecule, are initially
attached all along their lengths. Each
sister chromatid has a centromere, a
region containing specific DNA
sequences where the chromatid is attached most closely to its sister chromatid.
The part of a chromatid on either side of the centromere is referred to as an arm
of the chromatid.
Later in the cell division process, the two sister chromatids of each duplicated
chromosome separate and move into two new nuclei, one forming at each end of
the cell. The now separated sister chromatids are referred to as individual
chromosomes. Thus, each new nucleus receives
a collection of chromosomes identical to that of
the parent cell (figure 12.5).
,Mitosis, the division of genetic material in the nuleus, is usually followed
immediately by cytokinsis, the division of the cytoplasm. One cell has become
two, each genetic equilavent of the parant cell.
12.2
Phases of the cell cycle
, Mitosis is just one part of the cell cycle (figure 12.6). The mitotic (M) phase, which
includes both mitosis and cytokinesis, is the shortest part of the cell cycle. Mitotic
cell division alternates with a much longer stage, the interphase. During the
interphase, a cell that is about to divide grows and copies its chromosomes in
preparation of cell division. Interphase can be divided into sub-phases:
G1 phase
S phase
G2 phase
During all three sub-phases, a
cell that will eventually divide
grows by producing proteins
and cytoplasmic organelles.
Chromosomes are duplicated
during the S phase. Thus, a
cell grows (G1), continues to
grow as it copies its
chromosomes (S), grows
more as it completes
preparations for cell division
(G2), and divides (M).
Mitosis is conventionally broken down into five stages:
Prophase
Prometaphase
Metaphase
Anaphase
Telophase
Figure 12.7 describes these stages in an animal cell.
