The Cell Cycle
Prokaryotes
- Cell division takes place in two stages:
DNA is copied, then the cell splits in a process called binary
fission. Fig
12.10
Each daughter cell contains one identical copy of the parent
cell's DNA and is a functioning cell.
Eukaryotes
- Somatic,
or body cells undergo mitosis, while germ or stem cells
in reproductive organs undergo meiosis. The life cycle
of a cell is called the cell cycle.
Mitosis results in two identical daughter cells. The purposes
of mitosis include asexual reprodution (Fig.
12.1a), growth (Fig.
12.1b), and renewal and repair of tissues
(Fig.
12.1c).
Chromosomes (Fig.
12.2) are made of DNA
Chromosome Number
- Most eukaryotic somatic cells
have two copies (diploid or 2n) of each
chromosome, known as homologous chromosomes, one from
each parent. In some plants, this number may be much greater.
Gametes (sex cells or sperm and egg) are called haploid
(1n) because they have half the normal chromosome number
(one copy of each chromosome).
Humans have 46, or 23 pairs, of chromosomes.
- Before cell division, each homologue
replicates into two identical copies called sister chromatids.
joined at the centromere. Fig.
12.3
Stages of the cell cycle
Fig
12.4
- Interphase:
the time between divisions. It is not part of mitosis or meiosis.
- Subphases of interphase:
- G1 phase (first gap)
-
- S phase
-
- G2 phase (second gap)
-
- Mitosis
(M) follows, during which the nucleus and chromosomes of the
cell are divided. Fig
12.5.Mitosis,
the movie! Animal
mitosis video.
Prophase
(Fig.
12.5b) and movie
- -
in nucleus - duplicated chromatin becomes tightly coiled
into sister chromatids; nucleolus disappears.
- -
in cytoplasem - mitotic spindle begins to form.
-
- Prometaphase
(Fig.
12.5c) and movie
- -
nuclear envelope fragments
- -
microtubules of spindle can now approach the chromosomes
- -
each of the 2 sister chromatid pairs now have a kinetochore
at the centromere.
- -
some of the microtubules attach to the kinetochore
-
- Metaphase
(Fig
12.5d) and movie
- -
centrosomes at opposite ends (poles) of the cell
- -
chromosomes line up at the metaphasal plate with microtubules
extending to centrosomes to form the spindle (Fig.
12.6)
-
- Anaphase (Fig
12.5e) and movie
- - sister
chromatids pull apart and are now considered chromosomes
- *
hypothesis - the motor proteins in the kinetochore
move the chromosome along the microtubule toward the poles. (Fig
12.7)
- - nonkinetochore
microtubules lengthen, pushing the centrosomes further apart.
- - at the
end of anaphase, each group of chromosomes is clustered at opposite
poles.
-
- Telophase (Fig
12.5f) and movie
- - nonkinetochore
microtubules elongate the cell more
- - daughter
nuclei form - nuclear envelopes form from fragments of the parent
nucleus and ER.
- - chromosomes
uncoil and nuclear division ends
-
- Last is cytokinesis, in which
the cytoplasm is cleaved, resulting in two daughter cells.
- -
In animal cells, cytokinesis begins when a cleavage
furrow (Fig
12.8a) forms, splitting the cytoplasm
between two daughter cells.
- Plant cells
(Fig
12.9) instead have a cell plate
(Fig
12.8b) forming to separate the cytoplasm
of daughter cells.
-
- One hypothesis for the evolution of
mitosis (Fig
12.11)
