Transcript The Cell Cycle

The Cell Cycle
Chapter 12
Biology – Campbell • Reece
The Role of Cell Division
Reproduction – unicellular organisms
(Amoeba) can use cell division to create
new offspring
Growth – a fertilized egg divides
continuously to produce a multicellular
organism
Repair – replace cells that die from normal
wear and tear or accidents
Cell Division Terminology
Genome – the genetic information of a cell
Chromosome – one very long, linear DNA
molecule
Chromatin – DNA and the proteins that
maintain the structure of the chromosome
Sister chromatids – duplicate copies of
the chromosome held together at the
centromere
Sister Chromatids
More Terminology
Somatic cells – all body cells except the
reproductive cells
– 46 chromosomes in human somatic cells
Gametes – reproductive cells (sperm and
eggs)
– ½ the number of chromosomes (23 in humans)
Mitosis – division of the nucleus
Cytokinesis – division of the cytoplasm
The Mitotic Cell Cycle
Interphase (~90% of the cycle) – the cell
grows and copies the chromosomes
– G1 phase – “first gap”, the cell grows
– S phase – duplication of DNA
– G2 phase “second gap”, the cell grows &
prepares for mitosis
Mitotic (M) phase (~10%)
– Mitosis – Prophase, prometaphase,
metaphase, anaphase, telophase
– Cytokinesis
The Mitotic Cell Cycle
The Mitotic Spindle
Begins to form in the cytoplasm during prophase
Consists of microtubules and associated
proteins
Centrosome – a region containing material that
functions to organize the microtubules
– The spindle microtubules grow out from them
Aster – a radial array of short microtubules
extending from each centrosome
The spindle includes the centrosomes, the
spindle microtubules, and the asters
The Mitotic Spindle
Interphase
Nucleus is well
defined
One or more nucleoli
Two centrosomes
present (each has a
pair of centrioles in
animal cells)
Microtubules extend
from asters
Chromosomes not
visible
Prophase
Chromatin becomes
tightly coiled
Chromosomes are
visible (sister
chromatids)
Nucleoli disappear
Mitotic spindle
appears
Centrosomes move
away from each other
Prometaphase
Nuclear envelope
disappears
Microtubules extend
from each pole
Microtubules attach to
the kinetochore (a
structure at the
centromere of each
chromatid)
Metaphase
Centrosomes are now
at opposite poles
Chromosomes line up
at the middle of the
cell (on the
metaphase plate)
The spindle is fully
formed
Longest stage of
mitosis
Anaphase
The paired
centromeres of each
chromosome
separate
Each chromatid
moves to opposite
poles of the cell
By the end, each pole
has an equal set of
chromosomes
Telophase and Cytokinesis
Nuclear envelope
reforms around the
chromosomes
Chromatin becomes
less condensed
(chromosomes less
visible)
Cytokinesis begins
A cleavage furrow
forms & pinches the
cell in two
Cytokinesis: Animals vs. Plants
Animals:
– Occurs by a process
called cleavage
– A cleavage furrow (a
shallow groove on the
surface of the cell near
the metaphase plate)
forms
– The cleavage furrow
deepens until the
parent cell is pinched
in two
Plants:
– No cleavage furrow
– A cell plate forms
near the middle of the
cell
– The cell wall continues
to grow until it divides
the cell in two
Cytokinesis: Animals vs. Plants
Binary Fission
“division in half”
A type of cell
division used by
prokaryotes to
reproduce
It is believed that
binary fission gave
rise to mitosis
Regulation of the Cell Cycle
Some cells divide often (skin cells), some
only when needed (liver cells), and some
not at all in a mature human (muscle &
nerve cells)
Cell cycle control system – a cyclically
operating set of molecules that triggers &
coordinates events in the cell cycle
Cell Cycle Checkpoints
Checkpoint – a critical control point where
stop and go-ahead signals can regulate the
cycle (found in the G1, G2 & M phases)
Cell Cycle Clock
Cell cycle clock – the
fluctuation in the
abundance and
activity of cell cycle
control molecules set
the pace for the cycle
– Protein kinases,
cyclins, and MPF
(“maturation-promoting
factor”)
Internal & External Cues
Internal signals (from kinetochores) –
anaphase does not begin until all of the
chromosomes are attached to the spindle
External signals
– Growth factors – proteins that stimulate cell
division
– Density-dependent inhibition – cells continue
dividing until they run out of room
– Anchorage dependence – cells must be
attached to something to divide
Cancer
Cancer cells do not respond normally to the
body’s control mechanisms
Cell divide excessively & invade other tissues
Tumors
– Benign – remain at the original site (usually safe)
– Malignant – become invasive enough to impair
the functions of one or more organs (cancer)
Metastasis – spread of cancer cells away
from the original site