Download Terms to brush up on before this unit: DNA – deoxyribonucleic acid

Chapter 12: The Reproduction of
Cells
Prokaryotic Cell Division: no nucleus
 Prokaryotes contain most vital genes in a
singular circular chromosome-They
reproduce through binary fission
 Eukaryotic cells have a nucleus and divide via
mitosis and cytokinesis
The Cell Cycle:
I - Interphase includes a period of intense
biochemical activity in preparation for cell
division
1) G1 phase – first growth phase (G = “gap”)
2) S phase – synthesis phase when DNA is
synthesized as chromosomes are duplicated
3) G2 phase – second growth phase
I I- M Phase is the shortest part of the cell cycle
and the phase during which the cell actually
divides, includes:
1) Mitosis – division of the nucleus
a)prophase b)prometaphase
c)metaphase d) anaphase
e)telophase
2) Cytokinesis – division of the cytoplasm
I. Interphase
most proteins and other materials are synthesized
throughout this phase
 G1 phase – growth and increase of enzymes for
replication of DNA
 S phase – DNA replication
 G2 phase – protein synthesis and preparation for
division -characterized by well defined nucleus and
distinct nucleoli bound by a nuclear envelope, but
duplicated chromosomes not distinguished individually
yet
M – Phase (Mitosis)
I.Prophase
 In the nucleus:
 Nucleoli disappear
 Chromatin fibers tightly coil and condense into
discrete chromosomes that can be viewed with a
light microscope
 Each chromosome is composed of two identical
sister chromatids joined at the centromere
 In the cytoplasm
 Mitotic spindle fibers form (microtubules between
two centrosomes)
 Centrosomes move apart, propelled along the
surface of the nucleus
II. Prometaphase
 Nuclear envelope dissolves, allowing microtubules
to interact with the highly condensed chromosomes
 Spindle fibers (bundles of microtubles) extend from
each pole toward the cell’s equator
 Each chromatid now has a specialized structure, the
kinetochore, located at the centromere region
 Kinetochore microtubules become attached to the
kinetochores and put the chromosomes into motion
III. Metaphase
 Centrosomes positioned at opposite poles of
the cell
 Chromosomes move to the metaphase plate, a
line equidistant between the two spindle poles
 Centromeres of all chromosomes are aligned
along the metaphase plate with help of
kinetochore microtubules
IV. Anaphase
 Anaphase is characterized by movement of
chromosomes toward opposite poles
 In order for this to happen, sister chromatids split
apart into separate chromosomes
 Because kinetochore fibers are attached to the
centromeres, the chromosomes move “centromere
first” in a V shape
 Simultaneously, the poles of the cell move further
apart, enlongating the cell
 At the end of anaphase, the two poles have
identical collections of chromosomes
V. Telophase
 During telephase…
- Daughter nuclei begin to form at the two poles
- Nuclear envelopes form around the
chromosomes
- Nucleoli reappear
- Chromatin fiber of each chromosome uncoils
and the chromosomes become less distinct
 At the end of telephase…
- Mitosis, the division of one nucleus into two
genetically identical nuclei, is complete
- Cytokinesis has begun and the appearance of
two separate daughter cells occurs shortly
after mitosis is completed
M-PHASE (cytokinesis)
Division of the cytosol
a) involves a band of actin(microfilaments) in
animal cells- forms a cleavage furrow
b) involves a cell plate in plant cells
A Closer Look at the Mitotic Spindles:
 The mitotic spindle is composed of microtubules and
associated proteins. Its function is to help pull
chromosomes toward the poles.
 By late prometaphase, each chromatid of a replicated
chromosome develops its own kinetochore (a protein
structure on the centromere).
1) some spindle microtubules attach to the kinetochores and are called kinetochore microtubules
2) some spindle microtubules are not directly attached
to the chromosomes, but overlap at the metaphase
plate, are called nonkinetochore microtubules
Control of Cell Division: (external
and internal cues
1) Contents of the culture medium
 essential nutrients and special
regulatory substances called growth factors
2) Cell density
 Crowding inhibits cell division
density-dependent inhibition.
cancer cells do not exhibit this.
3) G1 phase of the cell cycle
 restriction point which occurs late in G1
 can result in a nondividing state called
the G0 phase (most human body cells
Some cells such as nerve and muscle, stay in
G0 and never divide. Other cells, such as the
liver, can be induced by environmental cues
(such as injury) to continue through the cell
cycle and divide
4) Cell size
 The most important indicator -ratio of
cytoplasmic volume to genome size
prevents daughter cells from becoming
progressively smaller with each cell cycle
5) Cdks – cyclin dependent kinases
a) kinases (proteins) are always in cell
b) cyclins(proteins) fluctuate in concentration
c) a kinase must join a cyclin to be activated
Ex) MPF – M-phase promoting factor triggers
passage through G2 checkpoint by
phosphorylation (nuclear envelope)
Checkpoint Summary- not fully
understood
G1 checkpoint – cell size/ Cdks
G2 checkpoint – Cdks (MPF)
M checkpoint – Cdks and kinetochore
attachment –triggers a cycle to destroy cyclins
and the centromere proteins (APC-anaphase
promoting complex)
Cancer Cells
do not stop growing in response to cell density
do not have a restriction point in G1 phase
immortal (they continue to divide indefinitely);
normal mammalian cells in culture only divide
about 20 to 50 times
If abnormal cells evade destruction, they may
gather to form a tumor (an unregulated
growing mass of cells)
 If the cells remain at this original site, the
mass is benign can be removed by surgery
 If the cells can spread to other parts of the
body, the tumor is called malignant. Only a
malignant tumor is said to be cancer.