DNA Part IV: Cellular

Reproduction- Mitosis and Cytokinesis

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• Cell division includes mitosis and cytokinesis.

• Mitosis it the division of the nucleus.

• Cytokinesis is the division of the cytoplasm.

• Daughter cells are genetically identical.

Eukaryotic Cell Reproduction

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1. Unicellular organisms undergo cell division to reproduce themselves.

2. Multicellular organisms undergo cell division for growth or repair, or to make a new organism from a fertilized egg,.

Unicellular vs. Multicellular Mitosis

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Purpose of Chromosomes

There are six feet of DNA in a human somatic cell.

• DNA is wrapped around histone proteins and coiled forming chromosomes.

• Packaging DNA in chromosomes prevents DNA breakage and helps ensure that each cell gets one copy of each chromosome.

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During interphase, the DNA forms chromatin. There are areas of DNA that are tightly wound around histones, and there are areas of DNA that are just loosely wound around histones. This depends on the DNA is being transcribed or replicated.

Coiling of DNA to Form Chromosomes

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DNA Coiling to Form Chromosomes

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• Double stranded chromosomes are held together by centromere.

• One half of a double stranded chromosome is called a chromatid.

• The lengths of the chromatids may also be held together by proteins called cohesins.

Chromatids Versus Chromosome

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Chromatids Versus Chromosome

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Centromere vs. Kinetochore

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•Chromatin fibers begin to condense into chromosomes and are visible under the microscope.•Cohesins hold chromatid arms together (vertebrates only at the centromere).•Nucleoli disappears.•Mitotic spindle forms asters radiating out from the centrosome.•After replicating, the centrosomes are moving to opposite poles.

Prophase

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Prometaphase•Nuclear envelopes fragments and nucleolus is no longer visible.•Centrosomes are at opposite ends of the nuclear area.•The microtubules extend through the nuclear area•Two opposing kinetochores form on the centromere on each chromatid. •Kinetochore microtubules attach to the kinetochores. Moving the chromosomes back and forth until they reach the middle of the cell.

Prometaphase

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Nonkinetochore microtubules overlap from opposite poles .

Nonkinetochore Microtubules

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Metaphase

• Longest phase of mitosis.• Double stranded chromosomes line

up on the metaphase plate.

Metaphase

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Anaphase-•Cohesin proteins are cleaved and the sister chromatids separate. •The chromosomes are pulled to opposite poles. •The kinetochore microtubules are disassembled at the chromosome end.•Spindle poles move apart by interacting with nonkinetochore microtubules.

Anaphase

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Which End of the Microtubule is Shortened?

Experiment• During anaphase, mark the

microtubules to form a stripe.

• Observe which side of the microtubules shorten.

• They shortened on the side of the chromatids, so therefore the kinetochores are disassembling the kinetochores microtubules and not the centrosome.

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• Two daughter nuclei form in the cell.

• Nuclear envelope forms from the fragments of the disassembled nuclei and the endomembrane system.

• Chromosomes unwind forming chromatin.

• Beginning of cytokinesis

Telophase

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•Mitosis without cytokinesis results in multinucleated cells. This happens in certain algae, plants, fungi, and even a few animals.

•Animals cells do cytokinesis by the pinching in of the cell membrane.

•Rings of actin form under the cell membrane associated with myosin contracts like a “pull-string” purse forming a cleavage furrow.

Cytokinesis in Animals

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In algal cells, cytokinesis occurs by an inward growth of new cell wall and membrane.

Cytokinesis in Algae

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In higher plants, cytokinesis begins in the middle and proceeds toward the periphery as membranous vesicles fuse to form the cell plate..

Cytokinesis in Higher Plants

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Comparing Cytokinesis in Plants and Animals

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Mitosis in a plant occurs in the root tips, shoot tips and other specialized areas.

Mitosis in a Plant Root Tip

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This is a much more primitive, thus simpler mechanism of cell division!

Prokaryotes Reproduce by Binary Fission

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Yeast cells and diatoms do not breakdown the nuclear membrane. Instead it pinches inward like the binary fission of prokaryotes. The microtubules are contained within in the nucleus.

Dinoflagellates also do not breakdown the nuclear membrane but the spindle fibers penetrate the nuclear membrane and attach to the plasma membrane.

Evolution of Mitosis