Chapter 8
The Cellular Basis of
Reproduction and Inheritance
I. Cell
Division
1. Cell division is the reproduction
of a cell. The two daughter cells are genetically identical to each other and to
the parent cell.
2. Chromosomes are duplicated before
the parent cell splits into two cells.
3. Three functions of cell division:
replacement, growth and reproduction.
4. Sexual reproduction is the
creation of offspring by the fusion of two haploid gametes (sperm and egg),
forming a diploid zygote. Asexual reproduction is the creation of offspring by a
single parent without the participation of gametes.
5. There are 2 kinds of cell division
in sexually reproducing organisms: mitosis for replacement and growth, and
meiosis for reproduction.
II.
The Cell Cycle and Mitosis
1.
Eukaryotic chromosomes
(1)
Each
chromosome consists of one very long DNA molecule, containing thousands of
genes.
(2)
Chromosomes are made up by chromatin, containing DNA and histone proteins (like
beads on a string).
(3)
Nucleosome
is a unit of chromatin with about 200 base-pair DNA wound around a protein core
made up of 8 histones.
(4)
A sister
chromatid represents one of the two identical parts joined by the centromere of
a duplicated chromosome.
2.
The cell cycle
(1)
A sequence of events [interphase and mitotic
(M) phase] of a eukaryotic cell is called the cell cycle.
(2)
Interphase lasts for at least 90% of the
cell cycle and can be divided into G1 (gap), S (DNA synthesis)
and G2 phases. G1
à
S
à G2
à
M
3.
Mitosis and cytokinesis
(1)
Mitosis is divided into 5 main stages:
prophase, prometaphse, metaphase, anaphase, and telophase.
(2)
The movement of chromosomes depends on the
spindle microtubules generated from two centrosomes.
(3)
Cytokinesis usually occurs with telophase,
and is the division of the cytoplasm to form two separate daughter cells.
(4)
A cleavage furrow appears at the cytokinesis
process in an animal cell, while the cell plate occurs at the cytokinesis
process in a plant cell.
4. Cell culture
(1)
Cell culture
is a complex process by which cells are grown under controlled conditions,
generally outside of their natural environment, e.g., suspension (anchorage
independent) and anchorage (anchorage dependent) culture.
(2)
Density-dependent (contact) inhibition is a
phenomenon that crowded cells stop division in anchorage
culture.
(3)
The cell cycle control system is a
cyclically operating set of proteins that triggers and coordinates events in the
eukaryotic cell cycle. A checkpoint in the cell cycle is a critical control
point where stop-or-go signals can regulate the cell cycle.
(4)
There are 3 major checkpoints in the cell
cycle: G1, G2, and M phases.
(5) The G1 checkpoint is the most important. A cell will switch from G1 to the G0 phase (a permanently nondividing state), if a signal (e.g., a growth factor) required for the entry into the S phase never arrives.
5. Cancer
(1)
Cancer is a disease of the cell cycle.
(2)
The development of a cancer is a gradual
process. A single cell may change that convert a normal cell to a cancer cell.
Cancer cells divide excessively, and invade other normal tissues of the body.
(3)
If unchecked, cancer cells continue to grow
and produce malignant tumors that kill the organism.
(4)
A malignant tumor is a mass of abnormally
reproducing cells (growing out of control) that can spread and invade other
parts of the body
(metastasis).
(5)
A benign tumor is an abnormal mass of cells
that remains at its original site in the body. A benign tumor can be completely
removed by surgery or even left alone.
(6)
Many cancer cells have defective cell cycle
control systems that proceed past checkpoints even in the absence of growth
factors. Other cancer cells synthesize growth factors by themselves, causing the
cells to divide continuously.
(7)
Cancer cells can be cultured immortally in
the laboratory. For example, HeLa cells are named for the original donor,
Henrietta Lacks, who died of cervical cancer in 1951.
(8)
Metastasis is the spread of cancer cells
(malignant tumors) beyond their original site of the body.
(9)
Chemotherapy is used to treat widespread or
metastatic tumors. For instance, Taxol freezes the mitotic spindle after it
forms, which stops actively dividing cells after metaphase. Vinblastine obtained
from the periwinkle plant prevents the forming of the mitotic spindle.
(10)
The side effects of
chemotherapy are nausea, hair loss, and susceptibility, resulting from drug
effects on intestinal cells, hair follicle cells, and immune cell production,
respectively.
III.
Meiosis
1. Homologous chromosomes
(1)
Homologous chromosomes are the two
chromosomes that make up a matched pair in a diploid cell. Therefore, a somatic
cell has 46 chromosomes in humans; i.e., a somatic cell has 23 pairs of
homologous chromosomes in humans.
(2)
Chromosomes 1 to 22 are called autosomes,
while chromosomes 23 are called sex chromosomes to determine a person's sex.
2. Gametes and the life cycle
(1)
The life cycle of an organism represents the
entire sequence of stages in the life.
(2)
Diploid (2n)
organisms containing pairs of homologous chromosomes produce haploid (n is the number of chromosomes in a gamete) gametes (sperm and egg)
containing a single set of chromosomes by meiosis.
(3)
Fertilization is the fusion of two haploid
gametes to form a diploid zygote.
3. The process of meiosis
(1)
Meiosis reduces the chromosome number from
diploid to haploid.
(2)
Meiosis is the division of a single diploid
cell into 4 haploid daughter cells by meiosis I (separation of homologous
chromosomes) and meiosis II (separation of sister chromatids).
(3)
Meiosis is divided into 8 main stages:
prophase I, metaphase I, anaphase I and telophase I followed by prophase II,
metaphase II, anaphase II and telophase II.
4. Review: comparing mitosis and
meiosis
|
Items |
Mitosis |
Meiosis |
|
a.
Number of chromosomal
duplications |
1 |
1 |
|
b.
Number of cell
division |
1 |
2 |
|
c.
Number of daughter
cells produced |
2 |
4 |
|
d.
Number (n) of chromosomes in the daughter cells |
2n
(diploid) |
1n
(haploid) |
|
e.
How the chromosomes
line up during metaphase |
individual
chromosome (singly) |
homologous
chromosomes, then individual chromosome (in tetrads, then singly) |
|
f.
Genetic relationship
of the daughter cells to the parent cell |
identical |
Half (unique) |
|
g.
Functions performed in
the human body |
replacement and
growth |
reproduction |
5. Random fertilization
(1) Random fertilization leads to varied offspring.
(2) For any species, the total number of combinations of chromosomes that meiosis can produce in gametes is 2n. For a human (n = 23), there are 223 (about 8 million) possible chromosome combinations.
6. Crossing over
(1) Crossing over (crossover; chiasma (pl. chiasmata) Χ; fragment exchange; homologous recombination) is an exchange of corresponding segments between duplicated homologous chromosomes.
(2) Crossing over often occurs during prophase I of meiosis, when homologous chromosomes are closely paired.
7. Chromosomal abnormalities
(1)
Nondisjunction is an accident
of mitosis or meiosis that sister chromatids or a pair of homologous chromosomes
fail to separate at anaphase.
(2)
Karyotype is a display of
micrographs of the metaphase chromosomes of a cell, arranged by size and
centromere position.
(3)
Down syndrome is caused by an
extra copy of chromosome 21. This trisomy 21 with three number 21 chromosomes
makes 47 chromosomes in total.
(4)
Down syndrome named after Dr.
John Langdon Down in 1866 affects about one of every 850 children in US.
(5)
The incidence of Down syndrome
in the offspring of normal parents increases markedly with the age of the
mother. Down syndrome affects less than 0.05% (< 1 in 2,000) of children born to
women under age 30. However, the risk climbs to 1% (10 in 1,000) for mothers at
age 40, and is even higher for older mothers.
(6)
Nondisjunction in sex
chromosomes seems to upset the genetic balance less than unusual numbers of
autosomes. Y chromosome is very small and carries few genes. Mammalian cells
usually operate with only one functional chromosome in a pair of homologous
chromosome, while another copy of chromosome becomes inactived in each cell.
(7)
An extra X chromosome in a
male (XXY) causes Klinefelter syndrome. Females who lack of an X chromosome (XO;
monosomy X)
have Turner syndrome.