Prokaryotic Cells

• Prokaryotic cells lack a nucleus and other membranous organelles.

• Cell division can be utilized by unicellular organisms for reproduction and by multicellular organisms for growth and repair.

• Both prokaryotic and eukaryotic organisms contain DNA in chromosomes.

Prokaryotic Cells

• Prokaryotes have a chromosome which is referred to as a nucleoid.

Prokaryotic chromosome contains DNA and proteins but much less protein than eukaryotes.

This chromosome is a circular loop attached to the inside of the plasma membrane.

Prokaryotic Cells

• Reproduce by binary fission - produces two genetically identical daughter cells.

• First DNA is replicated, then two chromosomes separate, cell lengthens and pulls them apart

• Eukaryotes have a more complex manner of division.

Eukaryotic Cell

• In eukaryotes histone proteins organize the chromosomes.

•When a cell is not undergoing division the DNA in the nucleus is a tangled mass of threads called chromatin.•At cell division, chromatin becomes highly coiled and condensed and now visible as chromosomes.

Haploid vs. Diploid

• Each species has a characteristic number of chromosomes.

• Diploid number (2n) includes two sets of chromosomes of each type.

• Haploid number (n) contains one of each kind of chromosome.

• Cell division in eukaryotes involves nuclear division and cytokinesis.

Eukaryotic Cell• Cytokinesis is the division of the

cytoplasm.• Somatic cells undergo mitosis for

development, growth, and repair.• Nuclear division leaves the

chromosome number constant.• A chromosome begins cell division

with two sister chromatids.

Chromosomes• Sister chromatids are two

strands of genetically identical chromosomes.

• The centromere is a region of constriction on a chromosome where sister chromatids are attached.

• Centromeres are believed to be responsible for organizing the spindle fibers. Sister

Chromatids

Centromere

The Mitotic Spindle• Centrosome organizes spindle.• Each centrosome contains a pair of

barrel-shaped organelles called centrioles.

• The spindle contains many fibers, each composed of a bundle of microtubules.

• Microtubules assemble when tubulin subunits join, disassemble when tubulin subunits become free, and form interconnected filaments of cytoskeleton.

Mitosis Overview• Mitosis is divided into five phases:

• 1. Prophase

• 2. Prometaphase

• 3. Metaphase

• 4. Anaphase

• 5. Telophase

Prophase

• Chromatin condenses and chromosomes become visible.

• Nucleolus disappears and the nuclear envelope fragments.

• Already duplicated chromosomes are composed of two sister chromatids held together by a centromere.

Prophase• Kinetochores develop for future

chromosome orientation (attach to the centromere).

• Spindle begins to assemble as pairs of centrosomes migrate away from each other.

• Short microtubules radiate out from the pair of centrioles located in each centrosome to form starlike asters.

Prometaphase

• Important event during this phase is the attachment of chromosomes to the spindle and their movement as they align at the metaphase plate of the spindle fibers.

• The kinetochores of sister chromatids capture kinetochore spindle fibers.

Metaphase

• Chromosomes, attached to kinetochore fibers are aligned at the metaphase plate.

• Non-attached spindle fibers, called polar spindle fibers, can reach beyond the metaphase plate and overlap.

Anaphase

• Two sister chromatids of each duplicated chromosome separate at the centromere.

• Daughter chromosomes move to opposite poles.

Telophase• Spindle disappears.

• Chromosomes decondense and return to chromatin

• Nuclear envelope reforms and nucleoli reappear.

• Cytokinesis nearly complete.

Plant Mitosis

• We have outlined mitosis in animal cells. Mitosis in plant cells occurs in a similar way.

• Although plant cells have a centrosome and spindle, there are no centrioles and asters do not form.

Cytokinesis Overview• 1. Animal cells – Cleavage Furrow

• 2. Plant cells – Cell Plate

• 3. Other eukaryotes – Protists and Fungi

Cytokinesis in Plant Cells• The golgi apparatus produces vesicles that

move to the midpoint between the daughter nuclei.

• Vesicles fuse forming a cell plate which indicates the location of the new plasma membranes and cell walls.

Cytokinesis in Animal CellsCleavage furrow indents the plasma

membrane between the two daughter nuclei at a midpoint.

• Cleavage furrow deepens as a band of actin filaments constricts between the two daughter cells.

Cleavage Furrow

Cleavage Furrow

Cleavage Furrow

Cell Cycle

• Cytokinesis in other eukaryotic cells:

• Protists and fungi also undergo mitosis and cytokinesis.

• In some organisms the nuclear envelope does not fragment but divides and one nucleus goes to each daughter cell.

Cell Cycle Overview• 4-stage sequence of

events.• 1. M stage• 2. G1 stage• 3. S stage • 4. G2 stage • Interphase consists

of G1, S, G2 stages.

Cell Cycle

• 1. M stage (mitosis) is the entire cell division state

• 2. G1 stage is when a cell grows in size and organelles increase in number.

• 3. S stage is DNA synthesis

• 4. G2 stage occurs just prior to cell division; preparation for mitotic cell division.

Cell Cycle

• Some cells divide continuously throughout the life of an organism.

• There appear to be stimulatory substances causing a cell to proceed through two critical checkpoints:– 1. G1 stage S stage– 2. G2 stage M stage

Enzymes and the Cell Cycle

• Enzymes regulate passage of cells through these points. (cyclins and kinases)

• Kinases are enzymes that remove a phosphate group from ATP and add it to another protein.

• Cyclin proteins activate kinases, which in turn activate enzymes.

• Growth factors are molecules that attach to the plasma membrane receptors and bring about cell growth.

Cancer

• Carcinogenesis is the development of cancer.• Cancer is a genetic disease requiring a series of

mutations toward developing a tumor.• Carcinogens are agents that cause cancer.• A tumor indicates a failure in controlling cell

division.• The tumor-suppressor gene p53 normally stops

the cell cycle when DNA mutates.

Cancer

• A p53 protein mobilzes repair enzymes and stops the cell cycle; only when repaired does the cell cycle start again.

• If DNA repair is not possible the p53 protein promotes cell death. (apoptosis)

• Apoptosis is a sequence of cellular changes involving:– the shattering of the nucleus– chopping up of chromosomes– packaging cellular remains into vesicles.

Cancer

• Apoptosis is caused by cells harboring enzymes called caspases.

• Caspases can be released at two times:– during development

– in adulthood

Cancer

• Characteristics of Cancer cells:• 1. Cancer cells lack differentiation• 2. Cancer cells have abnormal nuclei• 3. Cancer cells form tumors• 4. Cancer cells undergo angiogenesis and

metastasis.

Cancer

• Angiogenesis is the formation of new blood vessels that bring nutrients and oxygen to a cancerous tumor.

• Cancer in situ is still in its place of origin and has not spread to other tissues.

• When metastasis spreads new tumors distant from the primary tumor malignancy has occured.

Cancer

• Prevention of cancer:• Avoiding carcinogenic agents• Early detection-self examinations• Diet