BIO 101 Ch.9: Mitosis

Growth process of cellular reproduction

The cell duplicates its contents (DNA and organelles)

Cell division process of mitosis

The contents of the parent cell divides into 2 daughter cells

After the DNA is replicated, what happens to it?

It is packaged into chromosomes.

• Thickened complex of DNA and proteins

• Allows easier distribution to daughter cells

Chromatin

The thin, thread-like material of the DNA + associated proteins that condenses into chromosomes

Structure of chromatin (or the contents in it)

DNA periodically wound around histones to form nucleosomes.

Humans have __ chromosomes

46

The Cell Cycle

• Interphase, mitosis, and cytokinesis

• Orderly sequence of stages that go from when the cell has arisen to when it is reproducing

Stages of interphase

G1, S, G2

G1 stage

Stage before DNA replication

• Cell doubles organelles

• Accumulates materials for DNA synthesis

• Makes “decision” whether to divide or not

  • If not, cell goes into G0 (arrested) and is not dividing nor preparing to divide

S stage

DNA synthesis

• Results in each chromosome being composed of two sister chromatids

G2

• Stage following DNA synthesis

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M (Mitotic) Phase

• Cell division occurs

• Division of nucleus (mitosis)

  • Creates two identical daughter nuclei

  • Prophase, metaphase, anaphase, telophase

• Division of cytoplasm (cytokinesis)

• Distributes duplicated nuclear contents of parent cell equally to daughter cells

Sister chromatid

The added chromosome that results from DNA replication

• Makes an “x” shape with the other chromatid

• The pair is called this

Daughter chromosomes

Separated sister chromatids

Daughter nuclei

• produced by mitosis

• in mitosis, genetically identical to each other and the parent nucleus

Why does every animal have an even number of chromosomes?

Each parent can contribute half

Spindle

• Most eukaryotic cells rely on this structure to pull chromatids apart.

• Part of the cytoskeleton

• The fibers are made of microtubules

• The fibers may overlap at the spindle equator or attach to duplicated chromosomes.

Centrosome

Primary microtubule organizing center

Prophase

• Chromosomes are visible under microscope in condensed pairs

• Each chromosome consists of two sister chromatids held together at a centromere

• Outside of nucleus, spindle begins to assemble between separating chromosomes

• Nucleolus (ribosome-building organelle) disappears

• Nuclear envelope breaks down

• Spindle fibers from each pole attach to chromosomes on either side of the centromere

  • Chromosome moves toward one pole, and then the other

Metaphase

• Chromosomes line up along equatorial plate (the middle)

• Spindle fibers extend to opposite poles of the spindle

  • Unattached spindle fibers reach beyond the equator and overlap

Anaphase

• Chromosomes are pulled to opposite poles of the cell

  • In this way, sister chromatids separate -→ become daughter chromosomes

• Spindle fibers attached to the chromosomes disassemble; each pole receives set of daughter chromosomes

• Spindle poles move apart; unattached fibers slide past one another

  • Contributes to chromosome separation

Telophase and Cytokinesis

• Spindle disappears

• New nuclear envelopes form around daughter chromosomes; nucleoli reappear

• Chromosomes become indistinct chromatin

• Each nucleus contains the same kind and amount of chromosomes as the parent cell

• Remnants of spindle fibers still visible between the two nuclei

• Division of cytoplasm begins + continues after daughter nuclei have formed

• Two distinct cells visible under microscope

Characteristics of interphase

• Eukaryotic cell duplicates the contents of its cytoplasm; DNA replicates in the nucleus

• Duplicated chromosomes not yet visible

• A pair of centrosomes (microtubule-organizing organelles) are present outside the nucleus

Centrioles

• Small organelles found in the cytoplasm of most animal cells

• Located near nucleolus, inside centrosome

• Help organize mitotic spindle

Plant cells

• Have centrosomes, lack centrioles

• Cell membrane forms from center outward along a cell plate

Animal cells

• Each centrosome has two centrioles and an aster (array of microtubules)

• Cell membrane forms through a cleavage furrow (outer membrane toward the center)

Nucleolus

• Located inside eukaryotic nuclei

• Makes ribosomes

Prophase in plant cells

• Nucleolus disappears

• Duplicated chromosomes visible

• Centrosomes begin moving apart; spindle in process of forming

• Kinetochore of each chromatid attached to a kinetochore spindle fiber

• Polar spindle fibers stretch from each spindle pole and overlap

Metaphase in plant cells

• Centromeres of duplicated chromosomes aligned at the metaphase plate (center of fully formed spindle)

• Kinetochore spindle fibers attached to sister chromatids come from opposite spindle poles

Anaphase in plant cells

Same as anaphase in animal cells

Telophase in plant cells

Same as telophase in animal cells

Cytokinesis

• Accompanies mitosis in most but not all cells

• Mitosis w/o this stage results in multinucleated cell

  • Muscle cells in vertebrate animals

  • Embryo sac in flowering plants

Cytokinesis (animal cells)

• Cleavage furrow forms as anaphase ends

• Contractile ring (a band of actin filaments) forms a constriction

• Like pulling a drawstring

Cytokinesis (plant cells)

• Rigid cell wall prevents furrowing

• Involves building of new plasma membrane and cell walls between daughter cells

• Golgi apparatus produces vesicles

• Cell plate - newly formed plasma membrane

• New membrane releases molecules that build cell wall

Cell cycle checkpoints

• G1

• G2

• Mitotic stage

G1 checkpoint

• Occurs at the end of G1 phase

• Cell committed to divide after this point

  • Enters G0 if not

• Proper growth signals must be present to pass

• DNA integrity checked

  • if repair is not possible, apoptosis occurs

G2 checkpoint

• occurs at the end of G2 phase

• Verifies that DNA is replicated

• DNA damage repaired

Mitotic stage checkpoint

• Occurs between metaphase and anaphase

• All chromosomes must be attached to spindle to pass

Signal

a molecule that stimulates or inhibits an event

• can be external or internal

Kinases

Internal signals that remove a phosphate from ATP and add it to other molecules

Cyclins

Internal signals present during only certain stages of the cell cycle

• destruction at the appropriate time is necessary for normal cell cycle progression

External signals

Growth factors (ex. epidermal GF stimulates skin near injury to finish cell cycle and repair injury) and hormones (ex. estrogen stimulates lining of uterus to divide and prepare for egg implantation)

Contact inhibition

• Cells stop dividing when they touch

• Important in tissue repair

Cells divide about 70 times in a culture and then die. Why?

Due to shortening of telomeres (repeating DNA sequence at end of chromosome)

Apoptosis

• Programmed cell death

• Remaining cell fragments engulfed by white blood cells

• Unleashed by internal or external signals

• Helps keep number of cells at appropriate level

• Normal part of growth and development (ex. tails of tadpoles, webbing between human digits)

Steps of apoptosis

1. Cell rounds up, nucleus collapses

2. Chromatin condenses, DNA fragments

3. Plasma membrane blisters, blebs form

4. Cell fragments and engulfed

Cancer

Disease of the cell cycle in which cellular reproduction occurs repeatedly without end

Steps of cancer development

1. Cell acquires a mutation for repeated cell division

2. New mutations arise, and another cell has ability to start a tumor

3. Cancer in situ. The tumor is at its place of origin; another cell mutates further

4. Cells gain the ability to invade underlying tissues by producing a proteinase enzyme

5. Cancer cells have ability to invade lymphatic and blood vessels

6. New metastatic tumors are found some distance from the original tumor

Proto-oncogenes

• Code for proteins that promote the cell cycle and inhibit apoptosis

• Accelerates the cell cycle

• Activated by growth factors

• When mutate, become cancer-causing genes “oncogenes”

Tumor suppressor genes

• Code for proteins that inhibit the cell cycle and promote apoptosis

• When they mutate, their products no longer inhibit the cell cycle or promote apoptosis

Genetic changes other than cancer

• Absence of telomere shortening

• Chromosomal rearrangements (chromosomes of cancer cells become unstable → portions of DNA helix lost, duplicated, or scrambled; when reattached to another chromosome, a translocation occurs

Cell cycle genes associated with cancer other than proto-oncogene and tumor supressor

• BRCA1 + BRCA2

• RB gene

• RET gene

Characteristics of cancer cells

• Carcinogenesis - development

• Lack differentiation; do not contribute to body function

• May be immortal; divide repeatedly

• Abnormal nuclei w/ abnormal number of chromosomes

• Do not undergo apoptosis

• Form tumors; do not respond to inhibitory signals

• Metastasis (cells travel to start new tumors)

• Angiogenesis (form new blood vessels to to nourish themselves)

  • Benign - contained within capsule

  • Malignant - invasive and may spread

Cancer treatment

• Removes tumor or interferes with ability to reproduce

• Radiation therapy and chemotherapy

  • Damages DNA or some aspect of mitosis

  • Leads to side effects

• Hormone therapy designed to prevent cells from receiving signals to continue growth and division