¨ Every cell must first copy its genetic information before cell division
begins.
¨ Each daughter cell then gets a complete copy of that information.
¨ Cell division in eukaryotes is complex and occurs in two stages.
1. The first stage is mitosis, which
is the division of the cell nucleus.
2. The second stage is cytokinesis,
which is the division of the cytoplasm.
¨ Reproduction by mitosis is classified as asexual, since the cells
produced are genetically identical to the parent cell.
¨ Mitosis is also the source of new cells when a multicellular organism
grows and develops.
Chromosomes
¨ Chromosomes carry the genetic information that is passed on from
one generation of cells to the next.
¨ Chromosomes are made up of DNA and proteins.
¨ The cells of every organism have a specific number of chromosomes.
Ø Fruit flies – 8
Ø Human cells – 46
Ø Carrot cells – 18
¨ Chromosomes are not visible in most cells except during cell division.
¨ Well before cell division, each chromosome is copied.
¨ Each chromosome then consists of two identical “sister”
chromatids.
¨ When the cell divides, the “sister” chromatids separate
from each other with each one going to the two new cells.
¨ Each pair of chromatids is attached at an area called the centromere, which is located near the middle of the chromatids.
The Cell Cycle
¨ Interphase is the “in-between” period of growth that occurs when cells are not dividing.
¨ The cell cycle is
the series of events that cells go through as they grow and divide.
¨ During the cell cycle, a cell grows, prepares for division and divides
to form two daughter cells, each of which then begins the cycles again.
¨ The cell cycle consists of four phases:
1. M phase – mitosis and cytokinesis
2. S phase – chromosome replication, or synthesis (duplicate set of DNA)
3. G1 – cell growth
4. G2 – preparation for mitosis
(G stands
for “gap”)
Events of the Cell Cycle
¨ Interphase can be quite long, whereas cell division takes place
quickly.
¨ Interphase is divided into three phases: G1, S, and G2.
¨ During G1, cells increase in size and synthesize new
proteins and organelles.
¨ During the S phase, chromosomes are replicated and the synthesis
of DNA molecules takes place.
¨ During G2 (the shortest of the three phases), organelles
and proteins required for cell division are produced.
Mitosis
¨ Mitosis is divided into four phases: prophase, metaphase, anaphase
and telophase.
¨ Depending on the cell, mitosis may last a few minutes or several
days.
Prophase
¨ Prophase is the first and longest phase of mitosis and can take up as much as 50 to 60 percent
of the total time.
¨ During prophase, the chromosomes become visible.
¨ The centrioles separate and take up positions on opposite sides
of the nucleus.
¨ The centrioles lie in a region called the centrosome that helps
to organize the spindle, a fanlike microtubule that helps to separate the chromosomes.
¨ The condensed chromosomes become attached to the spindle fibers
at the centromere of each chromatid.
¨ Near the end, the chromosomes coil more tightly, the nucleolus disappears,
and the nuclear envelope breaks down.
Metaphase
¨ The second phase, metaphase only lasts a few minutes.
¨ The chromosomes line up across the center of the cell.
¨ Microtubules connect the centromere to the poles of the spindle.
Anaphase
¨ During the third phase, anaphase, the centromeres that join the sister chromatids split, allowing the sister chromatids to separate and
become individual chromosomes.
¨ The chromosomes move until they have separated into two groups near
the poles of the spindle.
¨ Anaphase ends when the chromosomes stop moving.
Telophase
¨ During fourth and final phase, telophase, the chromosomes begin to disperse into a tangle of dense material.
¨ A nuclear envelope re-forms around each cluster of chromosomes.
¨ The spindle breaks apart, and a nucleolus becomes visible in each
daughter nucleus making almost mitosis complete.
Cytokinesis
¨ As a result of mitosis, two nuclei are formed, usually within the
cytoplasm of a single cell.
¨ The final step is the division of the cytoplasm itself.
¨ Cytokinesis usually occurs at the same time as telophase.
¨ In animal cells, the cell membrane is drawn inward until the cytoplasm
is pinched into two nearly equal parts.
