The Cell Cycle

Eukaryotic Chromosomes

LE 12-4 0.5 µm Chromosome duplication (including DNA synthesis) Centromere Sister chromatids Separation of sister chromatids Centromeres Sister chromatids

INTERPHASE S (DNA synthesis) LE 12-5 INTERPHASE S (DNA synthesis) G1 Cytokinesis Mitosis G2 MITOTIC (M) PHASE

G1 Phase: Cell Growth In the G1 phase, cells increase in size and synthesize new proteins and organelles.

S Phase: DNA Replication In the S (or synthesis) phase, new DNA is synthesized when the chromosomes are replicated.

G2 Phase: Preparing for Cell Division In the G2 phase, many of the organelles and molecules required for cell division are produced.

M Phase: Cell Division In eukaryotes, cell division occurs in two stages: mitosis and cytokinesis. Mitosis is the division of the cell nucleus. Cytokinesis is the division of the cytoplasm.

Prophase During prophase, the first phase of mitosis, the duplicated chromosome condenses and becomes visible.

Prophase The centrioles move to opposite sides of nucleus and help organize the spindle.

Prophase The spindle forms and DNA strands attach at a point called their centromere.

Prophase The nucleolus disappears and nuclear envelope breaks down.

Metaphase During metaphase, the second phase of mitosis, the centromeres of the duplicated chromosomes line up across the center of the cell.

Metaphase The spindle fibers connect the centromere of each chromosome to the two poles of the spindle.

Anaphase During anaphase, the third phase of mitosis, the centromeres are pulled apart and the chromatids separate to become individual chromosomes.

Anaphase The chromosomes separate into two groups near the poles of the spindle.

Telophase During telophase, the fourth and final phase of mitosis, the chromosomes spread out into a tangle of chromatin.

Telophase A nuclear envelope re-forms around each cluster of chromosomes.

Telophase The spindle breaks apart, and a nucleolus becomes visible in each daughter nucleus.

Cytokinesis Cytokinesis is the division of the cytoplasm. The process of cytokinesis is different in animal and plant cells.

Cytokinesis in Animal Cells The cell membrane is drawn in until the cytoplasm is pinched into two equal parts. Each part contains its own nucleus and organelles.

LE 12-9a 100 µm Cleavage furrow Contractile ring of microfilaments Daughter cells Cleavage of an animal cell (SEM)

Cytokinesis in Animal Cells In plants, the cell membrane is not flexible enough to draw inward because of the rigid cell wall. Instead, a cell plate forms between the divided nuclei that develops into cell membranes. A cell wall then forms in between the two new membranes.

LE 12-9b Vesicles forming cell plate Wall of parent cell 1 µm Cell plate New cell wall Daughter cells Cell plate formation in a plant cell (TEM)

LE 12-10 Chromatin condensing Nucleus Chromosomes Cell plate 10 µm Nucleolus Prophase. The chromatin is condensing. The nucleolus is beginning to disappear. Although not yet visible in the micrograph, the mitotic spindle is starting to form. Prometaphase. We now see discrete chromosomes; each consists of two identical sister chromatids. Later in prometaphase, the nuclear envelope will fragment. Metaphase. The spindle is complete, and the chromosomes, attached to microtubules at their kinetochores, are all at the metaphase plate. Anaphase. The chromatids of each chromosome have separated, and the daughter chromosomes are moving to the ends of the cell as their kinetochore micro- tubules shorten. Telophase. Daughter nuclei are forming. Meanwhile, cytokinesis has started: The cell plate, which will divide the cytoplasm in two, is growing toward the perimeter of the parent cell.

LE 12-6ca INTERPHASE PROPHASE PROMETAPHASE

LE 12-6da 10 µm METAPHASE ANAPHASE TELOPHASE AND CYTOKINESIS

Cells anchor to dish surface and divide (anchorage dependence). LE 12-18a Cells anchor to dish surface and divide (anchorage dependence). When cells have formed a complete single layer, they stop dividing (density-dependent inhibition). If some cells are scraped away, the remaining cells divide to fill the gap and then stop (density-dependent inhibition). 25 µm Normal mammalian cells

Cancer cells do not exhibit anchorage dependence LE 12-18b Cancer cells do not exhibit anchorage dependence or density-dependent inhibition. 25 µm Cancer cells

Lymph vessel Tumor Blood vessel Glandular tissue Metastatic tumor Cancer cell A tumor grows from a single cancer cell. Cancer cells invade neighboring tissue. Cancer cells spread through lymph and blood vessels to other parts of the body. A small percentage of cancer cells may survive and establish a new tumor in another part of the body.