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Wednesday, October 05, 2016

PROKARYOTIC CELL DIVISION

Prokaryotes, such as bacteria, propagate by binary fission. For unicellular organisms, cell division is the only method to produce new individuals. In both prokaryotic and eukaryotic cells, the outcome of cell reproduction is a pair of daughter cells that are genetically identical to the parent cell. In unicellular organisms, daughter cells are individuals.
To achieve the outcome of cloned offspring, certain steps are essential. The genomic DNA must be replicated and then allocated into the daughter cells; the cytoplasmic contents must also be divided to give both new cells the machinery to sustain life. In bacterial cells, the genome consists of a single, circular DNA chromosome; therefore, the process of cell division is simplified. Karyokinesis is unnecessary because there is no nucleus and thus no need to direct one copy of the multiple chromosomes into each daughter cell. This type of cell division is called binary (prokaryotic) fission.

Binary Fission
Due to the relative simplicity of the prokaryotes, the cell division process, called binary fission, is a less complicated and much more rapid process than cell division in eukaryotes. The single, circular DNA chromosome of bacteria is not enclosed in a nucleus, but instead occupies a specific location, the nucleoid, within the cell Although the DNA of the nucleoid is associated with proteins that aid in packaging the molecule into a compact size, there are no histone proteins and thus no nucleosomes in prokaryotes. The packing proteins of bacteria are, however, related to the cohesin and condensin proteins involved in the chromosome compaction of eukaryotes.
The bacterial chromosome is attached to the plasma membrane at about the midpoint of the cell. The starting point of replication, the origin, is close to the binding site of the chromosome to the plasma membrane (Figure 1). Replication of the DNA is bidirectional, moving away from the origin on both strands of the loop simultaneously. As the new double strands are formed, each origin point moves away from the cell wall attachment toward the opposite ends of the cell. As the cell elongates, the growing membrane aids in the transport of the chromosomes. After the chromosomes have cleared the midpoint of the elongated cell, cytoplasmic separation begins. The formation of a ring composed of repeating units of a protein called FtsZ directs the partition between the nucleoids. Formation of the FtsZ ring triggers the accumulation of other proteins that work together to recruit new membrane and cell wall materials to the site. A septum is formed between the nucleoids, extending gradually from the periphery toward the center of the cell. When the new cell walls are in place, the daughter cells separate.



Figure  1  These images show the steps of  binary  fission  in  prokaryotes.  (credit:   modification   of  work  by “Mcstrother”/Wikimedia Commons)


Mitotic Spindle Apparatus
The precise timing and  formation  of the mitotic spindle  is critical to the success of eukaryotic cell division. Prokaryotic cells, on the other hand, do not undergo karyokinesis and  therefore have  no need for a mitotic spindle. However, the FtsZ protein  that  plays  such a  vital role in prokaryotic  cytokinesis is  structurally and functionally  very  similar  to  tubulin,  the building  block  of the  microtubules that make up  the  mitotic spindle  fibers  that  are necessary for eukaryotes. FtsZ  proteins can form filaments,  rings,  and  other  three- dimensional  structures that resemble the way tubulin forms microtubules, centrioles, and various cytoskeletal components. In addition, both FtsZ and tubulin employ the same energy source, GTP (guanosine triphosphate), to rapidly assemble and disassemble complex  structures.
FtsZ and tubulin are homologous structures derived from common evolutionary origins. In this  example, FtsZ is the ancestor protein to tubulin (a modern protein). While both proteins are found in extant  organisms, tubulin function  has  evolved  and diversified tremendously since  evolving  from its FtsZ  prokaryotic origin. A survey of mitotic assembly components found in present-day unicellular eukaryotes reveals crucial intermediary steps to the complex membrane-enclosed genomes of multicellular eukaryotes (Table 1).

Cell Division Apparatus among Various  Organisms


Structure of
genetic material
Division of nuclear material
Separation of daughter cells
Prokaryotes
There is no nucleus. The single, circular chromosome exists in a region of cytoplasm called the nucleoid.
Occurs through binary fission. As the chromosome is replicated, the two copies move to opposite ends of the cell by an unknown mechanism
FtsZ proteins assemble into a ring that pinches the cell in two.
Some protists
Linear chromosomes exist in the nucleus.
Chromosomes attach to the nuclear envelope, which remains intact. The mitotic spindle  passes through  the envelope and elongates the cell. No centrioles exist.
Microfilaments form a cleavage furrow that pinches the cell in two.
Other protists
Linear chromosomes exist in the nucleus.
A mitotic spindle  forms from the centrioles and passes through  the nuclear membrane, which remains intact. Chromosomes attach to the mitotic spindle,  which separates the chromosomes and elongates the cell.
Microfilaments form a cleavage furrow that pinches the cell in two.
Animal cells
Linear chromosomes exist in the nucleus.
A mitotic spindle  forms from the centrosomes. The nuclear envelope dissolves. Chromosomes attach to the mitotic spindle,  which separates the chromosomes and elongates the cell.

Microfilaments form a cleavage furrow that pinches the cell in two.

Tabel 1.

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