1. What instructions are necessary for inheritance?
2. How are those instructions duplicated for distribution into daughter cells?
3. By what mechanisms are those instructions divided into daughter cells?
B. In cell division, parent cells must provide their daughter cells with hereditary instructions (encoded in DNA) and enough cytoplasmic machinery to start up their own operation.
I. Dividing Cells: The Bridge Between Generations
A. Overview of Division Mechanisms
1. Before a cell of an organism reproduces, it undergoes mitosis or meiosis.
2. Both are nuclear division mechanisms that sort out the parent's DNA into new nuclei, followed by a mechanism that divides the cytoplasm into daughter cells.
a. Multicelled organisms grow by way of mitosis of the body, or somatic, cells.
b. Meiosis occurs in germ cells, which generate gametes necessary for sexual reproduction.
B. Key Points About Chromosomes
1. Chromosomes are molecules of DNA complexed with proteins.
2. Between divisions, each threadlike chromosome is duplicated to form sister chromatids joined by a centromere—the region where the chromosome will attach to microtubules during nuclear division.
3. The DNA of humans and other eukaryotes is highly organized to prevent tangling.
a. Some histones (a type of protein) act as spools to wind the DNA into units called nucleosomes.
b. Another histone stabilizes the arrangement and allows the beaded chain to form looped regions.
C. Mitosis and the Chromosome Number
1. All somatic cells of a particular species have the same number of chromosomes; example: humans have forty-six.
a. Chromosomes come in pairs—one member from each parent.
b. Chromosome pairs carry genes for the same traits.
2. Chromosome number (n) tells how many of each type of chromosome is present in a cell; 2n is diploid.
II. The Cell Cycle
A. The cell cycle is a recurring sequence of events that extends from the time of a cell’s formation until its division is completed.
B. Wonder of Interphase
1. The control of cell division resides in the subphases of interphase.
a. During G1, cells assemble most of the carbohydrates, lipids, and proteins that are needed by the cell and for export.
b. During the S (SYNTHESIS) phase the DNA molecules and histones are copied.
c. During G2, further protein synthesis drives the cell toward mitosis.
2. Most of a cell’s existence (about 90%) is spent in interphase; mitosis occupies only a small portion.
a. During interphase the cell’s mass increases, the cytoplasmic components approximately double in number, and the DNA is doubled.
b. Some cells are arrested in interphase and usually never divide again (example: brain cells).
C. Mitosis Proceeds Through Four Stages
1. Major changes in mitosis proceed through four stages: prophase, metaphase, anaphase, and telophase.
2. Chromosomes are moved by a spindle apparatus composed of two sets of microtubules that extend from each pole (centriole) of the cell and overlap at the equator.
A. Prophase: Mitosis Begins
1. Chromosomes (already duplicated during interphase) become visible as rodlike units, each consisting of two sister chromatids joined at the centromere.
2. Nuclear membrane breaks up; spindle forms.
3. Microtubules move one pair of centrioles to opposite pole of the cell.
B. Transition to Metaphase
1. Sister chromatids become oriented toward opposite poles.
2. When all the chromosomes are aligned at the cell’s equator halfway between the poles, the cell is said to be in metaphase.
C. From Anaphase Through Telophase
1. Sister chromatids separate and move toward opposite poles.
2. Now each chromatid is an independent (daughter chromosome) chromosome.
3. Telophase begins when chromosomes arrive at the poles.
4. The nuclear envelope forms from the fusion of small vesicles; mitosis is complete.
D. At the conclusion of mitosis, each new cell has the same chromosome number as the parent nucleus.
IV. Division of the Cytoplasm (CYTOKINESIS)
A. Cell Plate Formation in Plants
1. Plant cells form a cell plate (cellulose) that separates the two new cells.
2. Vesicles containing building materials fuse with one another to form the disklike cell plate between the two new cells.
B. Cytoplasmic Division of Animal Cells (Cleavage Furrow)
1. In animal cells, cleavage furrow on the outer surface indicates that two new cells are forming.
2. Contractile microfilaments pull the plasma membrane inward.
V. Focus on Science: Henrietta's Immortal (Cancer) Cells (HeLa Cells)