Mitosis, Meiosis, and the Prokaryotes

Eukaryotic cells• Mitosis copies DNA and divides a nucleus,

producing two identical nuclei• Meiosis nuclear division that produces haploid

gametes for sexual reproduction

Comparison of Cell Division Mechanisms

Key Points About Chromosome Structure

Humans have 23 (or is it 46?) chromosomes that differ in length and shape• Each consists of one double strand of DNA• After duplication, each consists of two double

strands (sister chromatids) that remain attached to each other at a centromere

A Chromosome and Sister Chromatids

Fig. 9-3 (b-e), p. 143

C When the coiled coils unwind, a molecule of chromosomal DNA and its associated proteins are organized as a cylindrical fiber.fiber

D A loosened fiber shows a “beads-on-a-string” organization. The “string” is the DNA molecule; each “bead” is one nucleosome.

beads on a string

DNA double helix core of

histonesE A nucleosome consists of part of a DNA molecule looped twice around a core of histone proteins.

nucleosomeStepped Art

B When a chromosome is at its most condensed, the DNA is packed into tightly coiled coils.

multiple levels of coiling of DNA and proteins

centromere

A Duplicated human chromosome in its most condensed form. If this chromosome were actually the size shown in the micrograph, its two DNA strands would stretch out about 800 meters (0.5 miles).

Fig. 9-4, p. 144

G1Interval of cell growth before DNA replication (chromosomes unduplicated)

SInterval of cell growth when the DNA is replicated (all chromosomes duplicated)

G2Interval after DNA replication; the cell prepares to divide

Interphase ends for parent cellcytoplasmic division;

each descendant cell enters interphase

Stepped Art

Telophase

Anaphase Metaphase Prophase

Interphase

Interphase cell increases in size and duplicates its DNA• G1: growth and activity• S: DNA replication (synthesis)• G2: cell prepares for division

Mitosis and the Chromosome Number

Mitosis produce 2 diploid nuclei with same number and kind of chromosomes as parent

• Human somatic cells have 46 chromosomes paired in 23 sets (diploid number)

• Pairs have same shape and information about same traits (except sex chromosomes XY)

Bipolar Spindle Separates Sister Chromatids

Mitosis Maintains Chromosome Number

Karyotype

A Closer Look at Mitosis

** DO NOT WRITE DOWN & MEMORIZE EACH STEP!

When a nucleus divides each new nucleus has same chromosome number as parent cell

Four main stages of mitosis: prophase, metaphase, anaphase, and telophase

Fig. 9-6 (2), p. 147

Fig. 9-6 (2a), p. 147

A Early ProphaseMitosis begins. In the nucleus, the chromatin begins to appear grainy as it organizes and condenses. The centrosome is duplicated.

Fig. 9-6 (2b), p. 147

B ProphaseThe chromosomes become visible as discrete structures as they condense further. Microtubules assemble and move one of the two centrosomes to the opposite side of the nucleus, and the nuclear envelope breaks up.

Fig. 9-6 (2c), p. 147

C Transition to MetaphaseThe nuclear envelope is gone, and the chromosomes are at their most condensed. Microtubules of the bipolar spindle assemble and attach sister chromatids to opposite spindle poles.

Fig. 9-6 (2d), p. 147

D MetaphaseAll of the chromosomes are aligned midway between the spindle poles. Microtubules attach each chromatid to one of the spindle poles, and its sister to the opposite pole.

Fig. 9-6 (2e), p. 147

E AnaphaseMotor proteins moving along spindle microtubules drag the chromatids toward the spindle poles, and the sister chromatids separate. Each sister chromatid is now a separate chromosome.

Fig. 9-6 (2f), p. 147

F TelophaseThe chromosomes reach the spindle poles and decondense. A nuclear envelope begins to form around each cluster; new plasma membrane may assemble between them. Mitosis is over.

9.4 Cytoplasmic Division Mechanisms

Cytokinesis• The process of cytoplasmic division

Fig. 9-7a, p. 148

Fig. 9-7a (1), p. 148

1 Mitosis is completed, and the bipolar spindle is starting to disassemble.

Fig. 9-7a (2), p. 148

Fig. 9-7a (3), p. 148

Fig. 9-7a (3), p. 148

3 This contractile ring pulls the cell surface inward as it continues to contract.

Fig. 9-7a (4), p. 148

Fig. 9-7a (4), p. 148

4 The contractile ring contracts until the cytoplasm is partitioned and the cell pinches in two.

The Importance of Timing and Completion of Cell Cycle Events

When Control is Lost

Sometimes, controls over cell division are lost• Cancer may be the outcome

Cell Cycle Controls

Checkpoints in cell cycle allow problems to be corrected before cycle advances

Proteins produced by checkpoint genes interact to advance, delay, or stop the cell cycle• Kinases can activate other molecules to stop the

cell cycle or cause cells to die• Growth factors can activate kinases to start

mitosis

Checkpoint Failure and Tumors

When all checkpoint mechanisms fail, a cell loses control over its cell cycle and may form a tumor (abnormal mass) in surrounding tissue

Usually one or more checkpoint gene products are missing in tumor cells• Tumor suppressor gene products inhibit mitosis• Protooncogene products stimulate mitosis

Neoplasms

Neoplasms• Abnormal masses of cells that lack control over

how they grow and divide• Benign neoplasms (such as ordinary skin moles)

stay in one place and are not cancerous• Malignant neoplasms are cancerous

Characteristics of Cancer Cells

Cancers (malignant neoplasms) • Cells grow and divide abnormally; capillary blood

supply to the cells may increase abnormally• Cells may have altered plasma membrane and

cytoplasm; metabolism may shift toward fermentation

• Cells have altered recognition proteins and weakened adhesion; may break away and invade distant tissues (metastasis)

Fig. 9-11, p. 151

benign tumor malignant tumor

A Cancer cells break away from their home tissue.B The metastasizing cells become attached to the wall of a blood vessel or lymph vessel. They release digestive enzymes onto it. Then they cross the wall at the resulting breach.C Cancer cells creep or tumble along inside blood vessels, then leave the bloodstream the same way they got in. They start new tumors in new tissues.

Skin Cancers