The Cell Cycle
Types of Cell Division

Mitosis
– provides cells for growth, cell replacement, and
asexual reproduction
– maintains chromosome number

Meiosis
– produces cells for sexual reproduction
– reduces chromosome number by half
Chromosome Replication
All the DNA is duplicated.
 Duplicated chromosomes separate as cell
divides.

Asexual Reproduction in Bacteria
Bacteria reproduce asexually by binary
fission.
 A bacterium possesses a single, circular
chromosome which is replicated; and then
the replicates (chromosomes) are distributed
to two new cells formed by division of the
original cell.

The Eukaryotic Cell Cycle

Interphase
– G1 growth phase; synthesis of organelles
– S synthesis of DNA (replication)
– G2 growth; synthesis of proteins essential to
cell division

Mitosis
–
–
–
–
prophase
metaphase
anaphase
telophase
Important Terms
chromatin - the form taken by chromosomes
when a eukaryotic cell is not dividing
 monad chromosome - unreplicated
chromosome (single chromosome)
 dyad chromosome - replicated chromosome
(double chromosome)
 chromatid - half of a dyad chromosome
 centromere - structure by which chromatids
of a dyad chromosome are held together

Mitosis*

1. Prophase: dyad (replicated)
chromosomes resemble long threads
(chromatin); replicated centrioles move
apart toward opposite poles; (animal cell);
nucleolus disappears; dyad chromosomes
condense (coil tightly); nuclear membrane
disintegrates; spindle fibers appear between
centrioles; dyad chromosomes migrate
toward the equator of the cell
Mitosis*

2. Metaphase: dyad chromosomes line up
along the equator of the cell
Mitosis*

3. Anaphase: dyad chromosomes separate
forming monad chromosomes;
monad chromosomes move toward opposite
poles
Mitosis*

4. Telophase: monad chromosomes cluster
at poles; cytokinesis occurs gradually;
nuclear membrane forms; nucleolus
reappears; chromosomes uncoil; cytokinesis
is completed; two daughters cells are
formed
Daughter Cells of Mitosis
have the same number of chromosomes as
each other and as the original cell from
which they were formed
 e.g. Mitosis in a cell which possesses 4
chromosomes will result in the formation of
two daughter cells each having 4
chromosomes.

Can you identify the stages
(phases) of mitosis in both plant
cells and animal cells?
Can you cite differences between
plant cell mitosis and animal cell
mitosis?
Types of Cell Division

Mitosis
– provides cells for growth, cell replacement, and
asexual reproduction
– maintains chromosome number

Meiosis
– produces cells for sexual reproduction
– reduces chromosome number by half
Meiosis
preceded by interphase which includes
chromosome replication
 two meiotic divisions
 four daughter cells produced
 daughter cells contain half the number of
chromosomes as the original cell

Important terms in the study of
meiosis
monad chromosome - unreplicated
chromosome
 dyad chromosome - chromosome composed
of two chromatids (replicates)
 homologous - similar in size and shape
 synapsis - pairing of homologous
chromosomes
 crossover - exchange of segments between
chromatids of homologous chromosomes

Chromosomes are replicated
during S-phase of interphase. As
a result, monad chromosomes
become dyad chromosomes.
Meiosis I
(Figure 6.6, p. 106)




Prophase I: homologous chromosomes of diploid cell
synapse; homologous pairs migrate toward equator of cell;
crossover may occur during synapsis
Metaphase I: dyad pairs of homologous chromosomes
align at the equator of the cell
Anaphase I: homologous chromosomes move apart
toward opposite poles
Telophase 1: chromosomes cluster at poles of cell;
cytokinesis occurs forming two haploid daughter cells each
containing one member of each homologous pair thus
reducing chromosome by half
Meiosis II
(resembles mitosis)
(Figure 6.6, p. 106)




Prophase II: dyad chromosomes move toward the equator
of each cell (2)
Metaphase II: dyad chromosomes align at equator of cell
Anaphase II: dyad chromosomes separate forming monad
chromosomes (chromatids separate); monads migrate
toward opposite poles of each cell
Telophase II: cytokinesis occurs at right angles to the first
plane of division in each cell (2); four daughter cells are
formed each having half the number of chromosomes as
the original cell
Gamete formation - meiosis
meiosis in males - spermatogenesis
 meiosis in females - oogenesis

Spermatogenesis

4 mature sperm cells produced from meiosis
in a single spermatocyte
Oogenesis
one mature ovum (egg cell) and 3 polar
bodies formed from meiosis in a single
oocyte
 human female embryo - oocytes formed by
mitosis and then complete prophase I of
meiosis by time of birth
 puberty - one oocyte in alternating ovaries
resumes meiosis under the influence of
hormones; at ovulation the oocyte is at
metaphase II

Comparison of Mitosis and Meiosis
(Table 6.2; Figure 6.8)

Mitosis
–
–
–
–

synapsis does not occur
one division occurs
two daughter cells are formed
chromosome number is maintained
Meiosis
– synapsis occurs during Prophase I
– two divisions occur
– four daughter cells are formed
Nondisjunction
(Essay 6.1, p. 109)
failure of chromosomes to separate during
anaphase
 results: daughter cells contain abnormal
number of chromosomes

Chromosome anomalies
Down syndrome
 Turner syndrome
 Klinefelter syndrome

Down syndrome
Cause: trisomy 21
 Effects: mental retardation, weakened
cardiovascular system, shortened limbs,
widely-spaced eyes, receding forehead
and chin, protruding tongue

Turner syndrome
Cause: one X chromosome in female (XO)
 Effects: underdeveloped ovaries; failure of
secondary sex characteristics to appear;
sterility; webbing of neck; weakened
pancreas, liver, and heart in 40%; short
stature

Klinefelter syndrome
Cause: extra X chromosome in males
(XXY)
 Effects: underdeveloped testes, sterility,
failure of secondary sex characteristics to
appear, taller than average

Detection of chromosome
anomalies in a fetus:
amniocentesis
 chorionic villi sampling

Questions for next class meeting:

During which weeks of a pregnancy, may
the following procedures be conducted:
–
amniocentesis
–
chorionic villi sampling