Science 2—9 How do cells produce new cells?
Cell Division You have grown a great deal since you were born. This growth did not happen because
your cells got larger. It happened because the total number of cells in your body increased. The cells
of most organisms, including humans, are able to reproduce and make new cells. This process is called
cell division.
Chromosomes The nucleus controls cell division. Inside the nucleus are chromosomes.
Chromosomes are made up of a threadlike material chromatin. Chromosomes control cell processes
and determine the traits of the entire organism. During cell division, each chromosome makes an
exact copy of itself.
Mitosis After chromosomes make copies of themselves, the nucleus divides. This is called mitosis.
There are four phases in mitosis: prophase, metaphase, anaphase, and telophase.
During prophase, the chromosomes group tightly together, and the nuclear membrane disappears.
In metaphase, all the copied chromosomes line up across the center of the cell. During anaphase, the
chromosome copies separate and move to opposite ends of the cell. Special organelles called
centrioles and spindle fibers help with this process. During telophase, the cell membrane pinches
together in between the two nuclei. Finally the cell splits, forming two identical cells.
The two identical cells formed by cell division are called daughter cells. The nuclei of the two
daughter cells are exactly alike. Each daughter cell is about half the size of the original cell. In time,
each of the daughter cells will grow and divide to form two new daughter cells.
Cell Division in Plants
Plant cells also reproduce by cell division. Like
animal cells, plant cells make copies of their
chromosomes and carry out mitosis. However, in
plant cells, the cell membrane does not pinch
together to form two daughter cells. In plant cells,
a new cell wall and new cell membrane form down
the middle of the cell. They form a wall between
the two new nuclei. Two daughter cells are formed,
one on each side of the new cell wall.
CHECKING CONCEPTS
1. What is mitosis?
2. What controls cell division?
3. What are chromosomes?
4. What are daughter cells?