Download The Cell cycle

THE CELL CYCLE
Chapter 12
YOU MUST KNOW
• The structure of a duplicated chromosome.
• The events that occur in the cell cycle (G1, S, G2).
• The role of cyclins and cyclin-dependent kinases in the
regulation of the cell cycle.
• Ways in which normal cell cycle is disrupted to cause cancer,
or halted in certain specialized cells,
• The features of mitosis that result in the production of
genetically identical daughter cells including replication,
alignment of chromosomes (metaphase), and separation of
chromosomes (anaphase).
Cell Cycle – life of a cell from when it is first formed from
a dividing parent cell until its own division into two cells
• Somatic cells – all “body” cells except for gametes
(eggs and sperm)
• Called diploid – contain two full sets of chromosomes
(one from each parent)
• Gametes are haploid – contain only one set of
chromosomes so that when they combine, the zygote
formed will contain 2 complete sets again
• The Chromosome
• Long sequence of DNA
• During interphase, they are found in
chromatin form (thin, stringy)
• When replicated, consist of two sister chromatids
attached by a centromere which
is composed of protein kinetochores
on each chromatid
• Each of the chromatids have
identical DNA sequences
Interphase
• 90% of the cell cycle
• G1 phase – cell grows and carries out normal cell
functions (protein synthesis, cellular respiration, etc.)
• S phase – DNA replication – every chromosome is
copied to form sister chromatids (total chromosome #
does not change!!!)
• G2 phase – more growth, cell functions, preparation for
mitosis
Mitosis
• Division of genetic material (chromosomes) in
preparation for cell division
Note: AP does not require you to know the names of the different
phases, only the sequence of events
1. Prophase
• Chromatin becomes tightly coiled into distinct
chromosomes
• This also causes the nucleolus to disappear
Prophase (continued)
• Mitotic spindle begins to form
in cytoplasm
• Consists of microtubules
extending from two centrioles
in the centrosomes
2. Prometaphase
• The nuclear membrane
breaks down, allowing
microtubules to attach to
chromosomes at
centromeres
3. Metaphase
• Centrioles have
migrated to opposite
poles
• Microtubules move
chromosomes to
metaphase plate
(equator of the cell) –
aligned “single file”
4. Anaphase
• Sister chromatids are pulled
apart by kinetochore
microtubules
• Cell elongates as nonkinetochore microtubules push
away from each other
• Opposite poles end up with
complete, equal sets of
chromosomes
5. Telophase
• Nuclear envelopes reform
around sets of chromosomes
at each pole
• Chromosomes start to decondense to chromotin
• Cytokinesis (division of the
cell) begins
Cytokinesis
• Physical division of the cell into two cells
• Animals – cleavage furrow forms and cell pinches in
two
• Plants – cell plate forms to divide cytoplasm and then
cell wall & membrane are built upon it
Binary Fission
• Prokaryotic version of cell division
Cell Cycle Regulation
• Involves a series of checkpoints in which molecular
signals tell the cell whether to pause or continue on in
the cell cycle
G1 phase checkpoint
• Most important – if cell passes this checkpoint, will
likely complete cell cycle
• Cells that do not pass this checkpoint go to G0 phase –
non-dividing
• Most adult cells remain in G0 (muscle, nerve cells) but
some can respond to signals to move back to G1 (liver
cells)
Checkpoint mechanisms
• Cyclin-dependent kinases (Cdks)
• Cyclins are proteins – production controlled by genes
being on/off
• Cdks are activated when they bond to a cyclins
• Activation of Cdks allows them to produce MPFs –
Mitosis Promoting Factors
• Accumulation of enough MPFs signal cell to continue on
in the cell cycle
• During anaphase, MPFs start to “turn off” by initiating the
break down of the cyclins – causes mitosis to end
Normal Cell Division Control
• Density-dependent inhibition – crowded cells don’t
divide
• Anchorage dependency – cells must be attached to
something, such as ECM of a tissue, to divide
Cancer – uncontrolled cell division
• Transformation – normal cells become cancerous
• Do no exhibit density-dependent or anchorage inhibition
• Tumor – mass of abnormal cells that divide uncontrollably
• Benign – stays in one place
• Malignant – tumor impairs organ function
• Metastasis – cells of tumor break off and travel to other parts
of body