All Cells Come
From Cells
Why Would A Cell Divide?
• Growth
• Repair
The Cell Cycle, Mitosis, and Meiosis
New cells can only be made when existing cells divide. All cells have the
ability to divide – but some cells lose this ability.
• Intestinal epithelial lining - replaced every five days by cell division
• Liver cells - divide only to repair damage, and then stop dividing
• Meristem cells (tips of roots and shoots) – divide to produce new growth
In eukaryotic cells, there are two types of cell division – mitosis
and meiosis. Mitosis is used to produce new cells for
growth and repair. Meiosis is used in the formation of
gametes (sex cells) only.
In prokaryotes (bacteria), cell division does not involve mitosis
or meiosis – bacteria reproduce asexually, by a type of cell
division termed by binary fission.
The cell cycle is the process that all body cells from
multicellular organisms use to grow and divide. The cell cycle
starts when a cell has been produced by cell division and ends
with the cell dividing to produce two identical cells.
Reproduction
• Asexual reproduction - Mitosis
– Produces offspring that inherit all their
genetic material from just one parent
– Ex – Paramecium and Sponges
• Sexual reproduction - Meiosis
– Genetic material from each of two
parents combines
– Ex: Most animals
(Including you)
Reproduction in bacteria
Binary Fision (Bacteria)
INTERPHASE
G1
Period of cell growth; cell prepares cell for
cell division (mitosis); genetic material
(DNA) is copied and checked for errors –
prevents mutations being passed on
No apparent activity
S phase
CELL
CYCLE
G2
New organelles and proteins are made
Divided into three phases (G1, S, and
G2 phase)
MITOSIS (M)
Mitosis (M)
Process by which a nucleus divides into
two – each with an identical set of
chromosomes – the nuclei are genetically
identical
Four phases – prophase, metaphase,
anaphase, and telophase
Two daughter cells – genetically identical
Followed by cytokinesis – division of
the cell into two genetically identical
daughter cells
The Cell Cycle
Mitosis : Cell division in which
two cells, with the same
number of chromosomes
identical to the parent, are
produced from one cell
Most cells in the
Human body have
46 chromosomes
with 23 pairs
The Cell Cycle
G2 - Second growth
phase - short
Short gap before mitosis
(cell division)
Cytoskeleton of cell
breaks down and the
protein microtubule
components begin to
reassemble into spindle
fibres – required for cell
division
DNA content = 46
S - Replication phase
DNA replication – this
must occur if mitosis is
to take place
The cell enters this
phase only if cell division
is to follow
DNA content = 46
Cytokinesis – cell
divides into two
DNA content = 20
G1 - First growth phase –
longest phase
Protein synthesis – cell “grows”
Most organelles produced
Volume of cytoplasm increases
Cell differentiation (switching
on or off of genes)
Length depends on internal
and external factors
If cell is not going to divide
again it remains in this phase
DNA content = 23 (arbitary)
G1 + S + G2 = INTERPHASE
No apparent observable activity
I Pro Mat
Interphase
• Up to 90% of a cells time is here
• Normal cell functions:
–Producing energy
–Producing enzymes
• G1 phase – cell grows
• S phase – genetic material replicates
• G2 phase – prepares for mitosis
Centrioles
show up
Cell is in the
first stage of
mitosis.
Prophase (1st phase)
• Centrioles divide and go to
opposite sides of the cell
• Spindle fibers – appear from
the centrioles and stretch to
both poles
• Nuclear envelope breaks down
• Nucleolus disappears (this
results in what organelle no
longer being produced?)
Metaphase (2nd Phase)
• Chromosomes attach to spindle fibers
• Chromosomes migrate to the center of
the spindle fibers
Anaphase (3rd phase)
• Centromeres separate
–Each sister chromatid = 2 daughter
chromosomes (move to the poles)
Telophase
th
(4
Phase)
• Spindle fibers disappear
• Nuclear envelopes reform
• Daughter chromosomes begin to
uncoil
• Nucleolus reforms – protein
production returns
• Cytokinesis occurs (cytoplasm divides)
– Two complete identical cells are formed
INTERPHASE
Before a cell divides, its chromosomes are copied exactly in INTERPHASE
to form two chromatids (“sister” chromosomes). This process is called
replication; ATP is synthesised – provides energy for cell division; organelles
are replicated and proteins are made, chromosomes condense – becoming
shorter and fatter
PROPHASE
Nuclear envelope breaks down; chromosomes lie freely in cytoplasm;
centrioles move to opposite ends of the cell, forming protein (tubulin) fibres
across it called a spindle – fibres extend to the equator of the cell
METAPHASE
Chromosomes line up at the equator; the spindle fibres from each pole
become attached to the centromere of the chromosomes
ANAPHASE
The spindle fibres contract; the centromeres are split and the pairs of sister
chromatids are separated and dragged to opposite poles assuming a “V”
shape – the centromeres lead; a complete set of chromosomes is therefore
found at each pole; energy (ATP) is required
TELOPHASE
Chromatids reach their respective poles and uncoil – become thin and long
again – now called chromosomes again – no longer visible under LM; spindle
fibres break down; nuclear envelope forms around each group of
chromosomes – forming two nuclei; cytokinesis follows – cytoplasm divides
and a plasma membrane forms two form two individual cells; cell enters
interphase once again
Animation
• http://www.cellsalive.com/mitosis.htm