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CELL DIVISION
Since organisms start from one cell, yet adults may contain billions of
cells, a process for producing new identical cells is required. Likewise, most
cells have a relatively short life so they must constantly be replaced.
I. How prokaryotic cells divide
A. Prokaryotic cells lack a nucleus and organelles
B. Prokaryotes have a single circular loop of DNA attached to the inside of
the plasma membrane; about 1,000 times the length of the cell
C. Prokaryotes reproduce asexually via binary fission
D. Before cell division takes place, DNA is replicated so two
"chromosomes" are attached inside the plasma membrane
E. Following DNA replication, the two chromosomes separate when cell
lengthens and pulls them apart
F. When cell is approximately twice its original length, the plasma
membrane grows inward, a new cell wall forms, dividing the cell into two
approximately equal daughter cells
II. Eukaryotic chromosomes
A. DNA in the chromosomes of eukaryotic cells is associated with histone
proteins which organize the chromosomes
B. When a eukaryotic cell is not undergoing division, genetic material
within a nucleus is a tangled mass of threads called chromatin
C. At cell division, chromatin becomes tightly coiled and condensed, and
visible as chromosomes
D. Each species has a characteristic number of chromosomes
1. With the exception of bacteria virtually all organisms have two sets
of each chromosome in their non-sex cells, one came from each parent
2. Diploid (2n) number = 2 sets of chromosomes in somatic
(body) cells, e.g. humans have 2n = 46
3. Haploid (n) number = one set of chromosomes,
usually restricted to sex cells, e.g. human sperm and eggs have n= 23
4. Diploid organisms have pairs of homologous
chromosomes - corresponding chromosomes,
one from each parent, which contain the same genes
E. Cell division in eukaryotes involves nuclear division and cytokinesis
(division of the cytoplasm)
1. Somatic (body) cells undergo mitosis in the development,
growth, and repair of multicellular organisms
a. This nuclear division leaves the chromosome number constant
b. A 2n nucleus divides to provide daughter nuclei that are also
2n
2. A chromosome begins cell division with two sister chromatids
1) Sister chromatids are two parts of a chromosome, at the
beginning of cell division, that are attached at a centromere; each
consists of a DNA molecule identical to the DNA molecule of the other
chromatid
2) Centromere = a region of constriction on a chromosome, where
sister chromatids are attached
III. The cell cycle has three continuous overlapping phases:
A. Interphase - the time between cell divisions
B. Mitosis - division of the nuclear material
C. Cytokinesis - division of the cytoplasm
A. Interphase - the phase between successive mitotic divisions. It is
the period of normal metabolic activity
1. Three phases:
a. G1 - period of normal metabolic cellular activities:
transcription, translation (protein synthesis) and growth of the cytoplasmic
materials including the organelles
b. S - the genetic material (DNA) is duplicated, chromosomes
now consist of two chromatids joined at the centromere
c. G2 - metabolic activities in preparation for mitosis
B. Mitosis - the process by which a nucleus gives rise to two
identical daughter nuclei, 4 overlapping phases:
1. Prophase
a. Chromatin condenses and chromosomes become visible
b. Nucleolus and nuclear envelope disappear
c. Spindle begins to assemble
2. Metaphase
a. spindle attaches to chromosomes in region of centromere
b. chromosomes align on the equatorial plane (metaphase plate)
3. Anaphase
a. centromeres divide
b. daughter chromosomes move towards opposite poles
5. Telophase
a. Spindle disappears
b. Chromosomes decondense and return to chromatin
c. Nuclear envelope reforms and nucleoli reappear
C. Cytokinesis (cytoplasmic cleavage)
1. In plant cells golgi apparatus produces vesicles that fuse, forming a cell
plate
2. In animals a cleavage furrow indents the plasma membrane between
the two daughter nuclei at a midpoint; then constriction separates the
cytoplasm
IV. Consequences of the cell division
1. Nuclear material was duplicated prior to division
2. Therefore two new genetically identical cells are formed
V. When Control Is Lost
A. Cell Cycle
1. Cell cycle has built-in checkpoints where proteins can advance, delay,
or block forward progress of the cycle
2. e.g., kinases can signal the end of DNA replication; growth factors
signal the start of mitosis
B. Checkpoint Failure and Tumors
1. When checkpoint mechanisms fail, a cell loses control over its
replication cycle
a. In some cases, mitosis repeats over and over
b. In others, the cells do not die as they are supposed to
c. Other times damaged DNA is replicated
2. Proto-oncogenes code for proteins that stimulate mitosis; tumor
suppressors inhibit mitosis.
a. Neoplasms are abnormal masses of cells that have lost controls over
their growth and cell division
b. Benign growths pose no threat to the body
C. Characteristics of Cancer
1. Cancers are abnormally growing and dividing cells of a malignant
neoplasm
2. All cancer cells display four characteristics:
a. They grow and divide abnormally
b. The cell membrane is leaky, and the cytoskeleton is disorganized
c. Cells have a weakened capacity for adhesion and may break away to
move to other sites in the body (metastasis)
VI. Duration of cell cycle - varies with the tissues and organism
involved. Average time for events in plant root tip:
- Interphase: 12 - 30 hrs.
- Prophase: 1 - 2 hrs.
- Metaphase: 5 - 15 min.
- Anaphase: 2 - 10 min.
- Telophase: 10 - 30 min.
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