McDougal Littell Science Cells and Heredity 

Table of Contents
Page 82 (continued)

Cell division produces two genetically identical cells.

Recall that many cells in your body are continually dividing into new cells. The new cells help your body grow, develop, repair itself, and replace worn-out parts. Though your body cells divide at different rates, the same process—mitosis—divides their genetic material.

Cell division produces daughter cells that are genetically identical to each other, as well as to their parent cell, which no longer exists. Being genetically identical to their parent cell helps the new cells function properly. A skin cell, for example, divides and produces skin cells genetically identical to it.

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How are daughter cells like the parent cell?

Steps of Mitosis

The process of mitosis is essential in evenly dividing the genetic material between the daughter cells. Although mitosis is a continuous process, scientists divide the events of mitosis into four phases.

  1. 1 Chromosomes form. During prophase, the DNA in the nucleus of a cell condenses and becomes visible under a light microscope. Each chromosome consists of two identical chromatids held together by a centromere. The membrane around the nucleus disappears.
  2. 2 Chromosomes line up. The chromosomes line up in the middle of the cell. This stage is called metaphase.
  3. 3 Chromosomes separate. During the stage called anaphase, the chromatids split, resulting in two separate identical chromosomes. These chromosomes are pulled to opposite sides of the cell.
  4. 4 Nuclei form. A new nuclear membrane forms around each group of chromosomes during telophase. The chromosomes return to their threadlike form.

Mitosis is finished, and the cell's genetic material has been divided. Following telophase the parent cell's cytoplasm is divided to complete the parent cell's division into two entirely separate daughter cells.

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Cell Division

Before mitosis, the cell's DNA is copied during interphase.

Interphase

Cell

The cell has grown and is ready to divide.

Cell

Mitosis produces two new cells with identical copies of DNA.

  1. 1. Chromosomes condense. Prophase
    Cell
    Cell
  2. 2. Chromosomes line up. Metaphase
    Cell
    Cell
  3. 3. Chromosomes separate. Anaphase
    Cell
    Cell
  4. 4. Nuclei form. Telophase, Cytokinesis
    Cell
    Cell
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Division of the Cytoplasm

Cytokinesis, or the division of the parent cell's cytoplasm, immediately follows mitosis in eukaryotic cells. Cytokinesis differs slightly in animal cells and plant cells.

During cytokinesis in an animal cell, a fiber ring forms in the center of the dividing cell. The fiber ring contracts, pulling the cell membrane inward. Eventually, the cell is pinched into two daughter cells.

In a plant cell, the cell wall prevents the cell membrane from being pulled inward. A structure called a cell plate grows between the two new nuclei. The cell plate develops into a membrane and eventually becomes part of the cell wall of each of the new cells.

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How does cytokinesis differ in plant cells and animal cells?

INVESTIGATE Cell Division

How can you model mitosis?

PROCEDURE

  1. 1 Divide the poster board into six spaces, and draw arrows from one space to the next to indicate a cycle. Label the spaces, in order, “Interphase,” “Prophase,” “Metaphase,” “Anaphase,” “Telophase,” and “Cytokinesis.”
  2. 2 In each space, make a model of a cell and its DNA in the indicated phase. Make sure you represent the cell membrane, the nuclear membrane—when it is present—and the DNA.

WHAT DO YOU THINK?

  • In which phases is the nuclear membrane present?
  • In which phases are the chromosomes condensed?
  • What do the arrows in your model show?

CHALLENGE How do you think cell division would differ in prokaryotic cells? Do you think cell division in prokaryotic cells would be more or less complex than in eukaryotic cells? Make drawings to show how you think a prokaryotic cell might divide.

Material
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Cytokinesis

Cytokinesis happens in both plant and animal cells.

Animal Cell
 
Plant Cell

READING ViSUALS

COMPARE AND CONTRAST How does the process of cytokinesis in the animal cell on the left differ from that of the plant cell on the right?

The two daughter cells are now completely separated. Each is surrounded by a cell membrane. Each daughter cell has some of its parent cell's cytoplasm. Though daughter cells are genetically identical to their parent cell, they are smaller. After division, cells may enter a period of growth, during which they take in the resources they need to increase the amount of their cytoplasm and to grow to full size. When cells are fully grown, they are about the same size as the parent cell was before division.

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What happens to cells after cytokinesis?