AH Biology: Unit 1
The Cell Cycle
Cell theory
• Cells are the basic unit of all organisms
(Schleiden and Schwann, 1839).
• All cells are produced by the division of preexisting cells (Virchow, 1858).
The cell cycle
• All organisms are the result of a repeated
sequence of cell growth and division.
• The sequence of events a cell goes through in
order to duplicate its contents and divide is
known as the cell cycle.
• The products of the cell cycle are two
genetically identical daughter cells.
• What are the purposes of cell
division?
• Why is it important that the daughter
cells produced are genetically
identical?
• Are there any circumstances in which
the daughter cells produced should
not be genetically identical?
Cell division in a unicellular organism
• Cell division is the organism’s mode of
reproduction.
• Each round of cell division produces new
individual organisms, eg yeast budding, binary
fission of bacterial cells.
The cell cycle of a prokaryote
Cells can cycle
every 20
minutes in
optimal
conditions
Replication of
single circular
DNA molecule
Cell growth
Ingrowth of
plasma
membrane
and cell wall
Cell division
(fission)
Cell division in a multicellular organism
• A functioning fully grown organism is the
result of a series of tightly controlled cell
divisions.
• Cell division occurs in the adult body in
order to replace cells that have died.
Lifespan of human cells
• Mucus-secreting cells and absorptive cells of the small
intestine: 5–6 days.
• Skin cells ~1 month.
• Olfactory neurons ~1 month.
• Red blood cells: 120 days.
• Auditory hair cells and photoreceptors: a lifetime (and not
replaced).
Stopping your cells’ cycles of duplication and division would
be lethal!
Why would you expect the cell
cycle of a eukaryotic cell to be
more complex than that of a
prokaryotic cell?
• DNA is packaged into
multiple chromosomes
as chromatin and
stored within the
nuclear envelope.
• Cell division must be
preceded by the
segregation of
replicated
chromosomes.
The cell cycle of a eukaryotic cell is
more complex
• The progress of a cell through the cell cycle is
regulated by a control system, which can
respond to intracellular and extracellular
signals.
Why do multicellular eukaryotes require the
ability to control the rate of the cell cycle?
Cells that proliferate rapidly and uncontrollably
can form tumours.
Rate of cell production > rate of cell death
= net gain of cells
Rate of cell production < rate of cell death
= net loss of cells
Neurons are generally terminally differentiated and no longer
participate in the cell cycle.
Healthy
brain
Effects of
Alzheimer’s
disease
The death of large populations of neurons is associated with
neurodegenerative disorders.
Two major phases in the cell cycle of a
eukaryotic cell
1. Interphase: period of cell growth and
DNA replication.
2. M phase: segregation of replicated
chromosomes and division of cytoplasm
to form two genetically identical daughter
cells.
The major phases of the eukaryotic cell
cycle
Interphase
M phase
1. Interphase
The interphase is divided into three parts:
1. G1: The initial growth phase. Protein synthesis
occurs and new organelles are formed.
2. Synthesis (S): Replication of nuclear DNA.
3. G2: Second phase of growth prior to mitosis. By
the end of this phase the centrosome has been
duplicated.
1. Interphase
• At the end of the interphase the parent cell has
usually doubled in size.
• The interphase comprises approximately 95%
of the length of the cell cycle.
How long would the interphase last in cultured
human cells that spend 1 hour in the M phase?
The cell cycle
G1
Interphase
M
M
S
G2
2. M phase
Two processes occur during the M phase:
1. Mitosis: The accurate separation and
distribution (segregation) of replicated
chromosomes and the formation of daughter
nuclei.
2. Cytokinesis: The division of the cytoplasm
into two separate daughter cells.
Mitosis
Mitosis can be divided into four stages:
1.
2.
3.
4.
Prophase
Metaphase
Anaphase
Telophase
Each stage can be recognised by the appearance
and location of the cell’s chromosomes and the
nucleus.
Interphase chromosomes
Prophase in an animal cell
• Replicated chromosomes condense (become shorter and
fatter) and appear as pairs of identical sister chromatids.
They are held together at their centromeres and along
their length by protein ‘glue’.
• The centrosomes move apart and microtubules radiate out
from them to form spindle fibres.
• The nuclear envelope is broken down.
A prophase chromosome
Prophase in an animal cell
Metaphase in an animal cell
• Multiple spindle fibres become attached to each sister
chromatid.
• The sister chromatids in each pair are ‘captured’ by
spindle fibres radiating from opposite poles of the spindle.
• The replicated chromosomes are aligned at the equator of
the spindle fibres that are attached to them. This position
is called the metaphase plate.
A metaphase chromosome
Metaphase in an animal cell
Anaphase in an animal cell
• The proteins holding the sister chromatids together are
degraded.
• The sister chromatids separate and are now known as
daughter chromosomes.
• The spindle fibres attached to each of the daughter
chromosomes shorten to pull them to opposite poles of the
cell.
Anaphase in an animal cell
Telophase in an animal cell
• The daughter chromosomes arrive at the cell poles and
decondense (unravel).
• A new nuclear envelope is assembled around each group
of daughter chromosomes to form two daughter nuclei.
Telophase in an animal cell
The underlying principle of mitosis
Conservation of the cell’s chromosome
complement is achieved by:
- DNA replication prior to the M phase
- the arrangement of replicated chromosomes
on the spindle fibres at the metaphase plate.
Reviewing mitosis
• Mitosis animation 1
• Mitosis animation 2
• Mitosis in a cultured lung cell
Cytokinesis
• The cell cycle ends with the division of the
cytoplasm into two daughter cells. The onset of
cytokinesis overlaps with the final two stages of
mitosis.
• Cytokinesis is accompanied by the insertion of new
sections of plasma membrane to compensate for
the increased surface area of the daughter cells.
• Cytokinesis is regulated to ensure that it occurs in
the correct location at the correct stage of the M
phase.
Cytokinesis in animal cells
Cleavage furrow
Contractile
ring
Cytokinesis in animal cells
• Cytokinesis is achieved by the action of a
contractile ring of structural and regulatory
proteins.
• The location of the ring within the dividing cell is
specified by the spindle fibres.
• Cell division is usually symmetrical.
Why would you expect cytokinesis in
plant cells to be different to
cytokinesis in animal cells?
Cytokinesis in plant cells
Telophase
Cytokinesis
G1
Cell
plate
Animation of cytokinesis in animal and plant cells.
Cytokinesis in plant cells
• The cytoplasm is sectioned off by the formation
of a new cell wall (the cell plate).
• The formation of the cell plate is guided by the
remains of the spindle fibres.
• Division may be asymmetrical or symmetrical.
The cell cycle: summary
G1
Cytokinesis
Telophase
Anaphase
Metaphase
Prophase
Mitosis
Interphase
M
S
G2
Remember: I Picked My Apple To Cook
Animated overviews of the cell
cycle
Cells Alive animation
McGraw-Hill Higher Education animation
Virtual Cell animation
Cell cycle tutorial activity
• Onion root tip cell analysis website
What’s a cell’s favourite
sport?
Cycling!