THE CELLULAR BASIS OF REPRODUCTION AND
DEVELOPMENT
I.
Like begets like, more or less
a. Strictly speaking, “like begets like” only applies to asexual
reproduction
b. CHROMOSOMES  the structures that contain most of the
organism’s DNA
c. Sexual reproduction leads to offspring that resemble their
parents more so than non-related organisms of the same species
II.
Cells arise only from pre-existing cells
a. The perpetuation of life, including all aspects of reproduction
and inheritance, its based on cellular reproduction
b. CELL DIVISION  cellular reproduction
c. Cell division plays two main roles
i. Cell division makes it possible for a fertilized egg to
develop through various embryonic stages and for an
embryo to develop into an adult organism
ii. Cell division ensures the continuity of life from
generation to generation; it is the basis of both asexual
reproduction and the formation of sperm and eggs in
sexual reproduction
III.
Prokaryotes reproduce by binary fission
a. BINARY FISSION  “dividing in half”; a type of cell division
b. Prokaryotes, most genes are carried on a circular DNA
molecule that, with associated proteins, constitutes the
organism’s single chromosome
c. While prokaryotes are simpler than eukaryotes, copying DNA
is still a formidable task
IV.
The large, complex chromosomes of eukaryotes duplicate with
each cell division
a. Eukaryotes are generally more complex and larger
b. Humans have about 100,000 genes versus 3,000 in bacterium
c. Chromosomes are only clearly visible when the cell is about to
divide
d. Rest of the time, the chromosomes exist as long, thing fibers
called CHROMATIN
e. Before the cell divides, the chromosomes will get copied
f. SISTER CHROMATIDS  single copy of the chromosomes
(contain identical genes)
g. CENTROMERE  hold chromatids together
h. When the cell divides, the sister chromatids will separate from
each other
V.
The cell cycle multiplies cells
a. Cells divide
i. To allow an organism to grow to adult size
ii. To replace dead or worn out cells
b. CELL CYCLE  an orderly sequence of events that extends
from the time a cell divides to form 2 daughter cells to the time
these daughter cells divide again
c. INTERPHASE  most of cell cycle spent here
i. G1 phase  “gap” 1; cell growth, protein and organelle
production
ii. S phase  DNA synthesis, chromosomes copied
iii. G2 phase  “gap” 2; metabolic activity, protein
production for cell division
d. Cell division involves two processes (M “Mitotic” Phase)
i. MITOSIS  nucleus and contents divide
ii. CYTOKINESIS  cytoplasm divides in two
VI.
Cell division is a continuum of dynamic changes
a. Mitosis is distinguished into 4 main stages
i.
ii.
iii.
iv.
PROPHASE
METAPHASE
ANAPHASE
TELOPHASE
b. MITOTIC SPINDLE  a football shaped structure of
microtubules that guides the separation of the two sets of
daughter chromosomes
c. CENTROSOMES  clouds of cytoplasmic material that in
animal cells contain centrioles
i. CENTRIOLES  mysterious function, since lacking
them has no ill-effect
VII. Cytokinesis differs for plant and animal cells
a. CYTOKINESIS  division of cell in two
b. Animal cells go through a process known as CLEAVAGE
c. CLEAVAGE FURROW  a shallow groove in the cell surface
i. There is a ring of microfilaments made of actin which
contract, pulling the membrane inward until it cleaves in
two
d. CELL PLATE  a membrane-enclosed disk formed at the
center of the cell
i. The cell plate grows outward as more vesicles fuse to it
until it fuses with the plasma membrane and eventually
the parental cell wall
VIII. Anchorage, cell density, and chemical growth factors affect cell
division
a. ANCHORAGE DEPENDENCE  cells must be attached, or
anchored, to a surface in order to divide
b. DENSITY-DEPENDENT INHIBITION  cell division slows
down as the number of cells increases
i. Example: skin cells replace lost skin, but stopped when
repair is complete
c. GROWTH FACTOR  a protein secreted by certain body cells
that stimulates cells in the vicinity to divide
IX.
Growth factors signal the cell cycle control system
a. CELL CYCLE CONTROL SYSTEM  a cyclically operating
set of proteins in the cell that both triggers and coordinates
major events in the cell cycle
b. Checkpoints exist within the cell cycle that halt (“brake”) the
cycle until conditions are correct for the cycle to continue
c. This area of biology is being closely studied to better
understand CANCER
X.
Growing out of control, cancer cells produce malignant tumors
a. CANCER CELLS  do not have a properly functioning cell
cycle control system; dividing excessively and invading other
tissues
b. TUMOR  an abnormal mass of cells
c. BENIGN TUMOR  an abnormal mass of essentially normal
cells
d. MALIGNANT TUMOR  a mass of cancer cells capable of
spreading into other tissues
e. METASTASIS  the spread of cancer cells beyond their
original site
f. Four categories of cancer
i. CARCINOMA  external or internal coverings of the
body (skin or intestinal lining)
ii. SARCOMA  arise in tissues that support the body
(bone and muscle)
iii. LEUKEMIA  blood-forming tissue
iv. LYMPHOMA  blood-forming tissue
g. Two types of cancer treatment
i. CHEMOTHERAPY  attempts to “halt” cell division
ii. RADIATION THERAPY  attempts to “halt” cell
division
XI.
Review of the functions of mitosis: Growth, cell replacement, and
asexual reproduction
a. Mitosis makes it possible for organisms to
i. Grow
ii. Regenerate and repair tissues
iii. Reproduce asexually
b. Mitosis leads to same number and type of chromosomes
XII. Chromosomes are matched in homologous pairs
a. SOMATIC CELLS  a typical body cell
b. Chromosomes have twins, nearly identical in length and
centromere position
c. HOMOLOGOUS CHROMOSOMES  both carry genes
controlling the same inherited characteristics
d. LOCUS  a particular place on a chromosome
e. Two general types of chromosomes
i. AUTOSOMES  found in both males and females
ii. SEX CHROMOSOMES  determines a person’s gender
1. X versus Y; not as homologous as autosomes
2.
XIII. Gametes have a single set of chromosomes
a. DIPLOID CELLS  cells whose nuclei contain two
homologous sets of chromosomes (2n)
b. GAMETES  the egg and sperm cells; the sex cells
c. HAPLOID CELLS  cells with a single set of chromosomes
(n)
d. FERTILIZATION  the fusing of the egg and sperm to restore
the diploid chromosome number
e. ZYGOTE  a fertilized egg
f. MEIOSIS  occurs only in reproductive organs; a special type
of cell division to give rise to sex cells with the haploid number
of chromosomes
XIV. Meiosis reduces the chromosome number from diploid to haploid
a. Meiosis resembles mitosis, but there are key differences
b.
c.
d.
e.
i. Cell undergoes two consecutive divisions, MEIOSIS I
and MEIOSIS II
ii. Four daughter cells result from these divisions
The two divisions are preceded by only one duplication of the
chromosomes
Each of the four daughter cells resulting from meiosis has only
half as many chromosomes as the starting cell; a single haploid
set of chromosomes
The actual halving of chromosome number occurs during
MEIOSIS I
STAGES OF MEIOSIS
i. MEIOSIS I : homologous chromosomes separate
1.
2.
3.
4.
PROPHASE I
METAPHASE I
ANAPHASE I
TELOPHASE I and CYTOKINESIS
ii. MEIOSIS II : sister chromatids separate
1.
2.
3.
4.
PROPHASE II
METAPHASE II
ANAPHASE II
TELOPHASE II and CYTOKINESIS
XV. Review : A comparison of mitosis and meiosis
a. Mitosis produces IDENTICAL daughter cells for growth, tissue
repair, and asexual reproduction
b. Meiosis produces HAPLOID daughter cells for sexual
reproduction
c. All events unique to meiosis occur in MEIOSIS I
d. In prophase I, duplicated homologous chromosomes pair to
form tetrads allowing for crossing-over
e. In metaphase I, tetrads are aligned at the metaphase plate
f. At the end of meiosis I, there are two haploid cells but each
chromosome still has two sister chromatids
XVI. Independent orientation of chromosomes in meiosis and random
fertilization lead to varied offspring
a. The orientation of the homologous pairs of chromosomes
(tetrads) at metaphase I is a matter of chance
b. For any species, the total number of combinations of
chromosomes that meiosis can package into gametes is 2 n,
where n is the haploid number
XVII. Homologous chromosomes carry different versions of genes
a. Homologous chromosomes can bear two, or more, different
kinds of genetic information for the same characteristics
b. Genes for the same characteristic are found at LOCI on the
chromosome
XVIII. Crossing over further increases genetic variability
a. CROSSING OVER  the exchange of corresponding segments
between two homologous chromosomes
b. CHIASMA  a place where two homologous (non sister)
chromatids are attached to each other
c. Crossing over takes place during SYNAPSIS and adds to
genetic variability
d. GENETIC RECOMBINATION  the production of gene
combinations different from those carried by the original
chromosomes
e. Steps in crossing over
i. Synapsis of tetrad
ii. Breakage of homologous chromatids
iii. Joining of homologous chromatids
iv. Separation of homologous chromosomes at Anaphase I
v. Separation of chromatids at Anaphase II and completion
of meiosis
XIX. A karyotype is a photographic inventory of an individual’s
chromosomes
a. Errors in meiosis can lead to gametes with abnormal
chromosome number and structure
b. KARYOTYPE  an orderly display of magnified images of
the individual’s chromosomes
XX. An extra copy of chromosome 21 causes Down Syndrome
a. TRISOMY 21  Three number 21 chromosomes
b. Most abnormal chromosome numbers will abort (miscarriage)
c. DOWN SYNDROME  characterized by John Langdon Down
in 1866
d. Most common chromosome abnormality (1 out of 700 children
born)
e. Characteristics include
i. Unique facial features; notably round face
ii. Flattened nose bridge
iii. Small irregular teeth
iv. Short stature
v. Heat defects
vi. Susceptibility to respiratory infection
vii. Leukemia
viii. Alzheimer’s disease
ix. Mental retardation
XXI. Accidents during meiosis can alter chromosome number
a. NONDISJUNCTION  the members of a chromosome pair
fail to separate
b. May occur in either MEIOSIS I or MEIOSIS II
XXII. Abnormal numbers of sex chromosomes do not usually affect
survival
Sex
Syndrome
Origin of
Frequency in
chromosomes
Nondisjunction
Population
XXY
Klinefelter
Meiosis in egg or
1 / 2,000
syndrome (male) sperm formation
XYY
None (normal
Meiosis in sperm
1 / 2,000
male)
formation
XXX
Metafemale
Meiosis in egg or
1 / 1,000
sperm formation
XO
Turner syndrome Meiosis in egg or
1 / 5,000
(female)
sperm formation
XXIII. Alterations of chromosome structure can cause birth defects and
cancer
a. Even when number of chromosomes is correct, changes can
still occur
b. Four major types of changes can occur
i. DELETION  a portion of a chromosome is lost
ii. DUPLICATION  a fragment from one chromosome
joins a homologous chromosome
iii. INVERSION  a fragment reattaches to the original
chromosome but in the reverse direction
iv. TRANSLOCATION  the attachment of a fragment of a
chromosome to a NON-HOMOLOUS chromosome
1. Translocations often lead to cancer by activating
portions of the non-homologous chromosomes