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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.

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mitosis meiosis and the prokaryotes
Mitosis, Meiosis, and the Prokaryotes
  • Eukaryotic cells
    • Mitosis copies DNA and divides a nucleus, producing two identical nuclei
    • Meiosisnuclear division that produces haploid gametes for sexual reproduction
key points about chromosome structure
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

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

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

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).


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 histones

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


Stepped Art

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




Interval of cell growth before

DNA replication (chromosomes unduplicated)

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


Interval after DNA replication; the cell prepares to divide

cytoplasmic division; each descendant cell enters interphase






Interphase ends for parent cell

Stepped Art

Fig. 9-4, p. 144

  • Interphasecell 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 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)
a closer look at mitosis
A Closer Look at Mitosis
  • When a nucleus divides each new nucleus has same chromosome number as parent cell
  • Four main stages of mitosis: prophase, metaphase, anaphase, and telophase

A Early Prophase

Mitosis begins. In the nucleus, the chromatin begins to

appear grainy as it organizes and condenses. The

centrosome is duplicated.

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


B Prophase

The 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 (2b), p. 147


C Transition to Metaphase

The 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 (2c), p. 147


D Metaphase

All 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 (2d), p. 147


E Anaphase

Motor 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 (2e), p. 147


F Telophase

The 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.

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

9 4 cytoplasmic division mechanisms
9.4 Cytoplasmic Division Mechanisms
  • Cytokinesis
    • The process of cytoplasmic division

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

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


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

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

when control is lost
When Control is Lost
  • Sometimes, controls over cell division are lost
    • Cancer may be the outcome
cell cycle controls
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
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
    • 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
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)

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.

Fig. 9-11, p. 151