Download Bis2A 7.1 Binary Fission

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Bis2A 7.1 Binary Fission
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The BIS2A Team
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Section Summary
Prokaryotes, such as bacteria, propagate by binary ssion. For unicellular organisms, cell division is the only
method to produce new individuals. Before cell division, the single, circular chromosome is replicated and
then each copy is allocated into a daughter cell. In addition, the cytoplasmic contents are divided evenly
and distributed to the new cells. Bacterial cytokinesis is directed by a ring composed of a protein called
FtsZ. Ingrowth of membrane and cell wall material from the periphery of the cells results in the formation
of a septum that eventually constructs the separate cell walls of the daughter cells.
1 Binary Fission and Haploid Single Cell Organisms
Bacteria and Archaea
When we talk about haploid single cell organisms we are really discussing bacteria and archaea. Like all
other life forms, these organisms have one key evolutionary driver: to make more of themselves. Cells grow,
duplicate all major cellular constituents, like DNA, ribosomes agella, etc. then divide into two identical
daughter cells. This process is called binary ssion and is depicted in Figure 1 below.
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Figure 1:
Binary Fission in bacteria starts with DNA replication at the replication origin attached to
the cell wall, near the midpoint of the cell. New replication forks can form before the rst cell division
ends; this phenomenon allows an extremely rapid rate of reproduction.
Source: http://biology.kenyon.edu/courses/biol114/Chap01/week01.html
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2 Binary Fission
Figure 2:
Prokaryotes, including bacteria and archaea, have a single, circular chromosome located in a
central region called the nucleoid.
Due to the relative simplicity of the bacteria and archaea, the cell division process, called binary ssion,
is a less complicated and much more rapid process than cell division in eukaryotes. The single, circular
chromosome of bacteria is not enclosed in a nucleus, but instead occupies a specic location, the nucleoid,
within the cell. The DNA of the nucleoid is associated with proteins that aid in compacting the DNA into
a smaller, organized structure.
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 (review DNA replication in module 8.1). 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
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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.
note: How does attaching the replicating chromosome to the cell membrane aid in dividing the
two chromosomes after replication is complete?
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Figure 3:
These images show the steps of binary ssion in prokaryotes. (credit: modication of work
by Mcstrother/Wikimedia Commons)
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