The Cell Cycle
Chapter 11
Objectives
• Understand the need for cell division
• Understand the process of Mitosis and
cytokinesis
• Know the parts of each phase of mitosis as
discussed in class
• Understand what regulates the cell cycle
and at what phases regulation may occur
Cell Cycle
The Cell’s Time Clock
• Smallest unit of life
– all living things must
reproduce
• Cells replicate for Cytokinesis
growth, replacement,
and repair
• Cell division requires
Mitosis &
Cytokinesis
Parts of the Cell Involved
• Chromosome
– Chromatid: Strand of DNA
– Kinetochore: Region of chromatid where spindle fibers attach
• Nucleus
– Nucleolus: Region of RNA
– Nuclear membrane: Borders the nucleus
• Centrosomes
– Centrioles in some organisms: produce microtubules (spindle
fibers and asters)
– Spindle fibers: microtubules that grow toward the chromatids
Interphase
• Cell performs normal
function
• Three subphases:
– G1: cell duplicates most
organelles
– S: quantity of DNA in the
cell is doubled as
chromosomes are
replicated. Each
chromosome has a pair of
sister chromatids
connected by a kinetochore
– G2: chemical components
stockpiled
• Nucleolus present
Mitosis
• Nuclear division • Mitotic events can be
without a reduction in categorized into
chromosome number discrete stages based
• Each new cell on what is happening
(daughter cell) will to structure of the cell
have the same quantity • Stage include:
of DNA as the – Prophase
parental cell • Prometaphase
• Why is this important? – Metaphase
– Anaphase
– Telophase
Prophase
(Including Prometaphase)
• Pro
• Three things visibly occur
– Chromosomes condense (shorten)
– Centrosomes migrate to the poles while producing
spindle fibers
– Nuclear membrane fragments
Metaphase Plate
Metaphase
• Meta
• Chromosomes are
moved by growing
spindle fibers to the
equator of the cell
(metaphase plate)
• Centrosomes are at the
poles, nuclear
membrane is gone
Anaphase
• Ana
• Centromere splits into
two
• Spindle fibers shorten
from kinetochore end
separating sister
chromatids and pulling
them toward the poles
Telophase
• Telo
• Nuclear membrane
reforms around each
region of
chromosomes
• Nucleolus reforms
• Cytokinesis (division
of the cytoplasm) may
occur
Cytokinesis May Vary Between
Major Taxonomic Groups
Cell Cycle Regulation
• Cell cycle events are triggered by the cell-
cycle control system; a set of molecules
found in the cytoplasm affected by internal
and external controls
• Checkpoints in G1, G2, and M phases of the
cycle
• G1 checkpoint is most critical. May throw
cells out of cyclic phase into G0, never to
divide again
Cyclins & Cylin-Dependent
Proteins
• Cyclin is a
cytoplasmic protein
that fluctuates in
concentration
• Cyclin-Dependent
Proteins: activate other
proteins in the cell by
phosphorylation
(protein kinases)
What does MPF do?
Other Internal and External Factors
• Internal
– M checkpoint does not proceed until signal is received
that all kinetochores are attached to spindle
microtubules
• External
– Growth factors: cycle will not proceed if requirements
are not met
– Social signals
• Density-dependent inhibition: under crowded conditions
chemical requirements are insufficient to allow cell growth
• Anchorage dependence: some cells must be attached to a
substrate in order to replicate
– DNA damage inhibits growth (gene p53)