1. Chapter 18 – Part I
Mitosis

2. The Cell Cycle
G1: first gap or growth phase during which most of the carbohydrates, lipids, and proteins for the cell’s own use are assembled. The cell is busy doing its job.
S: cell stops doing its job and enters the synthesis phase where DNA and centriole replication occurs.
G2: second gap or growth phase during which proteins that will drive mitosis are produced. Organelle duplication occurs.
M: mitosis involving replication of the nuclear material and separation of the chromosomes followed by cytoplasmic division.
The length of the cell cycle varies depending on cell type.

3. G1
Interval of cell growth, before DNA replication (chromosomes unduplicated) S
Interval of cell growth, when DNA replication is completed (chromosomes duplicated)
Each daughter cell starts interphase
Cytoplasmic division
Telophase G2
Anaphase
Figure 19.5 Animated! A general diagram of the cell cycle. The duration of each phase differs among different cell types.
Interphase ends for parent cell
Metaphase
Interval following DNA replication;
cell prepares to divide
Prophase

4. Cell reproduction occurs during the M phase of the cell cycle
The nuclei of eukaryotic cells divide by one of two mechanisms—mitosis or meiosis.
Mitosis occurs
in somatic cells.
Meiosis occurs
only in germ cells
that divide to
form gametes.

5. mitosis, division of cytoplasm
Figure 19.2 A simple way to think about how mitosis maintains the diploid chromosome number from one generation to the next.
a. Two chromosomes in a parent cell at G1 of the cell cycle.
b. The same two chromosomes after the S phase of the cell cycle. Each chromosome has two chromatids.
c. Each “daughter” cell has the same number of chromosomes has the parent cell after mitosis. Both cells get back to work (G1) to start life in interphase.

6. A Closer Look at Chromosomes
In a chromosome, DNA is wrapped with proteins.
DNA is wound up on proteins called histones to form units called nucleosomes.
Prior to mitosis, each chromosome is duplicated to form two sister chromatids held together by a centromere.

7. a One unduplicated chromosome
one chromatid
two sister
chromatids
one chromatid
b One chromosome (duplicated)

8. Centrioles form a mitotic organizing center
Once the nuclear membrane dissolves, spindle fibers attach to chromosomes and move them around in the cytoplasm.
A cell’s chromosomes move into new positions with the help of the spindle apparatus.
The spindle apparatus is formed after the centrioles migrate and anchor themselves to opposite poles.
Spindle fibers connect to the centromere holding the sister chromatids together.

9. pole
Figure 19.4 Example of a spindle that moves chromosomes in a dividing cell. The barrel-shaped structures are centrioles.
one spindle
microtubule
pole

10. Mitosis begins with prophase.
Chromatin becomes chromosomes as they thicken and condense.
Nuclear and nucleolar membranes disintegrate and their separate compartmental fluids mix with the cytoplasm.
Centrioles (having already replicated during the S phase of the cell cycle) begin their migration to opposite poles of the cell.

11. Next comes metaphase.
Microtubules of the spindle apparatus interact with the chromosomes to move them to the equatorial plane of the cell and orient them so that sister chromatids are aligned toward opposite poles.

12. Anaphase is the third stage of Mitosis.
Anaphase is marked by the separation of the sister chromatids and their movement toward opposite poles; at the end of anaphase each chromatid is an independent chromosome.
Microtubules attached to centromeres shorten, pulling the chromosomes to the opposite poles.
Great care is taken to separate the chromatids equally. This is called disjunction.

13. Telophase is the final stage of mitosis.
The chromosomes, no longer attached to the microtubules, return to their threadlike form (they “decondense” to form chromatin).
The nuclear and nucleolar envelopes reform.
At the end of mitosis, each new daughter cell is identical to its original parent cell.
In humans, the parent cell has 46 chromosomes and each of the daughter cells will also have 46 chromosomes.

14. Cell at
Interphase
nucleus
cytoplasm
Telophase
Prophase
Figure 19.6 Light micrographs showing the progress of mitosis in a cell from the African blood lily ( Haemanthus). The chromosomes are stained blue, and microtubules that are moving them about are stained red.
Anaphase
Metaphase

15. How the Cytoplasm Divides
Cytokinesis, the division of the cytoplasm, begins at the end of anaphase.
Midway between poles, the plasma membrane sinks inward to form the cleavage furrow.
Contractile microfilaments pull the plasma membrane inward to divide the cell in two.
Each new cell has a
nucleus, cytoplasm,
and a plasma
membrane.

16. Cytokinesis