CELL GROWTH AND DIVISION
CELL DIVISION
CELL DIVISION
• Every cell must copy its genetic information
before cell division takes place.
• Each daughter cell then gets a complete copy
of that information.
• In prokaryotes, the process after that is to
separate the contents of the cell into two
parts.
• In eukaryotes, the process is more complex
and occurs in two main stages.
CELL DIVISION
• The first stage is the division of the nucleus
called mitosis.
• The second stage is the division of the
cytoplasm and is called cytokinesis.
• Many organisms, especially unicellular ones,
reproduce just by mitosis and cytokinesis.
• Reproduction by mitosis is classified as
asexual, since the cells produced by mitosis
are genetically identical to the parent cell.
CELL DIVISION
• Mitosis is also the source of new cells in
multicellular organisms.
• In humans, mitosis begins just after the egg is
fertilized, producing the vast numbers of cells
needed for the embryo to take form.
• Chromosomes
– In eukaryotes, genetic information is passed from
one generation to the next on chromosomes.
CELL DIVISION
– DNA makes up chromosomes.
– DNA carries the genetic information.
– Each organism has its own specific number of
chromosomes.
• Fruit flies have eight
• Humans have 46
• Carrots have 18
– Chromosomes are not visible except during division.
• The DNA and protein molecules are spread throughout the
nucleus.
• They appear at the beginning of cell division and condense into
compact structures that can be seen with a light microscope.
CELL DIVISION
– Before division each chromosome is replicated or
copied.
• There are two identical chromatids.
• When division occurs one chromotid goes to each of
the two new cells.
• Each pair of chromotids are attached at an area called
the centromere.
• The centromeres are located near the middle of the
chromotids.
• Humans have 46 chromosomes, each with two
chromotids.
CHROMOSOMES
CHROMOSOMES
CHROMOSOMES
CHROMOSOMES
CHROMOSOMES
MITOSIS
MITOSIS
• Biologists divide the events of mitosis into
four phases: prophase, metaphase,
anaphase, and telophase.
• Prophase
– This is the first and longest phase and can take 5060% of the total time required to complete
mitosis.
– Chromosomes become visible
MITOSIS
CENTRIOLES
CENTRIOLES
CENTRIOLES
MITOSIS
– The centrioles, two tiny structures located near the
nuclear envelope in the cytoplasm, separate and take
up positions on opposite sides of the nucleus.
– The centrioles are in a region called the centrosome
that helps to organize the spindle.
• Helps to separate the chromosomes.
– The chromososmes become attached to fibers in the
spindle near the centromere of each chromatid.
• Plant cells don’t have centrioles but organize the spindles in
similar regions.
MITOSIS
• Near the end of prophase the nucleolus disappears, the
nuclear envelope breaks down, and the chromosome coil.
• Metaphase
– This is the second phase, lasting only a few
minutes.
– The chromosomes line up across the center of the
cell.
– Microtubules connect the centromere of each
chromosome to the two poles of the spindle.
MITOSIS
METAPHASE
METAPHASE
METAPHASE
MITOSIS
MITOSIS
• Anaphase
– This is the third phase of mitosis.
– The centromeres that join the sister chromatids
split, allowing them to separate and become
individual chromosomes.
– The chromosomes continue splitting and move
toward the poles of the spindle.
– This phase ends when the chromosomes stop
moving.
ANAPHASE
ANAPHASE
ANAPHASE
ANAPHASE
MITOSIS
MITOSIS
• Telophase
– This is the fourth and final phase.
– The chromosome become less distinct and
disperse into dense material.
– The nuclear envelope reforms around each cluster
of chromosomes.
– The spindle breaks apart.
– The nucleolus becomes visible.
– Mitosis is complete but cell division is not.
TELOPHASE
TELOPHASE
TELOPHASE
TELOPHASE
MITOSIS
• Cytokinesis
– This is the division of the cytoplasm itself.
– This usually occurs at the same time as telophase.
– The process in animals is:
• Cell membrane is drawn inward and pinched in nearly two
equal parts.
• Each part then contains its own nucleus and cytoplasmic
organelles.
– The process in plants is:
• A cell plate forms midway between the divided nuclei.
• This gradually forms a membrane and cell wall.
ANIMAL CYTOKINESIS
ANIMAL CYTOKINESIS
PLANT CYTOKINESIS
PLANT AND ANIMAL CYTOKINESIS
ANIMAL CYTOKINESIS
PLANT CYTOKINESIS