A septum in
cell biology
Cell biology (also cellular biology or cytology) is a branch of biology that studies the structure, function, and behavior of cells. All living organisms are made of cells. A cell is the basic unit of life that is responsible for the living an ...
is the new
cell wall
A cell wall is a structural layer that surrounds some Cell type, cell types, found immediately outside the cell membrane. It can be tough, flexible, and sometimes rigid. Primarily, it provides the cell with structural support, shape, protection, ...
that forms between two daughter cells as a result of
cell division
Cell division is the process by which a parent cell (biology), cell divides into two daughter cells. Cell division usually occurs as part of a larger cell cycle in which the cell grows and replicates its chromosome(s) before dividing. In eukar ...
. Cell division is an extremely complex process that contains four different subprocesses.
These processes included the growth of a cell,
DNA replication
In molecular biology, DNA replication is the biological process of producing two identical replicas of DNA from one original DNA molecule. DNA replication occurs in all life, living organisms, acting as the most essential part of heredity, biolog ...
, the process of allocating replicated chromosomes to daughter cells, and septum formation.
Ultimately, the septum is the crucial ending to
mitosis
Mitosis () is a part of the cell cycle in eukaryote, eukaryotic cells in which replicated chromosomes are separated into two new Cell nucleus, nuclei. Cell division by mitosis is an equational division which gives rise to genetically identic ...
,
meiosis
Meiosis () is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, the sperm or egg cells. It involves two rounds of division that ultimately result in four cells, each with only one c ...
, and the division of bacterial cells. The formation of the septum (a new cell wall) allows the two daughter cells to be separate from one another and perform their respective functions independently.
Composition
In ''
Schizosaccharomyces pombe
''Schizosaccharomyces pombe'', also called "fission yeast", is a species of yeast used in traditional brewing and as a model organism in molecular and cell biology. It is a unicellular eukaryote, whose cells are rod-shaped. Cells typically meas ...
'', the primary septum is composed of linear β(1,3)-D-glucan, β(1,6) branches, and α(1,3)-D-glucan.
The secondary septum in
Schizosaccharomyces pombe
''Schizosaccharomyces pombe'', also called "fission yeast", is a species of yeast used in traditional brewing and as a model organism in molecular and cell biology. It is a unicellular eukaryote, whose cells are rod-shaped. Cells typically meas ...
is composed of β(1,6)-D-glucan, β(1,6) branches, and α(1,3)-D-glucan.
The synthesis of linear β(1,3)-D-glucan for the primary septum is done by the enzyme β(1,3)-D-glucan synthase and regulated by a Rho GTPase.
Ags1/Mok1 enzyme is responsible for the synthesis of α(1,3)-D-glucan in the primary septum and secondary septum.
Septum formation during binary fission
The process of bacterial cell division is defined as
binary fission
Binary may refer to:
Science and technology Mathematics
* Binary number, a representation of numbers using only two values (0 and 1) for each digit
* Binary function, a function that takes two arguments
* Binary operation, a mathematical o ...
, where a bacterium splits to produce two daughter cells.
This division occurs during cytokinesis, which in bacteria is made possible due to the divisome (a specific large protein complex) and
FtsZ (the ancestor to tubulin for bacteria that drives cytokinesis itself).
This protein machinery works to form the barrier known as the septum between the two daughter cells. At the core of this protein complex is the
Z-ring- protofilaments that assemble around the cell at the specific site of cell division.
The Z-ring formation is made possible due to certain positioning proteins that depend on the species of the cell.
The FtsZ portion of the divisome are protofilaments that are tightly attached to the inside of the cytoplasmic membrane by other proteins, for example in ''E. coli'', FtsA and ZipA assist in securing the protofilaments to the membrane.
This FtsZ complex and the membrane attachments are termed as the proto-ring.
Once the proto-ring is assembled, FtsA (the ancestor to actin for bacteria) connects the Z ring to the other proteins within the divisome and the constriction of the Z-ring and cytoplasmic membrane begins inward.
Such inward constriction causes the cells to form a septum and separate from one another, forming two distinct bacteria cells.
Septum formation for eukaryotic cells
Animal cells
During cytokinesis in animal cells, a contractile ring made up of
actin filaments
Microfilaments, also called actin filaments, are protein filaments in the cytoplasm of eukaryotic cells that form part of the cytoskeleton. They are primarily composed of polymers of actin, but are modified by and interact with numerous other p ...
forms, and this ring pinches to divide the cell into two daughter cells.
The cells are able to separate due to the formation of a
cleavage furrow, which pinches in a centripetal fashion (from the outside of the cell towards the center of the cell).
This cleavage furrow is able to pinch together due to the actin filaments that form the contractile ring.
Thus, in animal cells it can be observed that the septum is not a true wall, rather the pinching of a cleavage furrow.
Plant cells
The manner in which plant cells form a septum is drastically different than that of animal cells. This is because in a plant cell there is no cleavage furrow or pinching of the plasma membrane, rather a
cell plate forms in the middle of the cell that then allows the division into two daughter cells.
The cell plate formation occurs due to vesicles budding from the golgi apparatus
and adding to the plant cell in a centrifugal manner thanks to the directed movement of microtubules (from the center to the outside of the cell).
Septum formation in fungi
In
yeast
Yeasts are eukaryotic, single-celled microorganisms classified as members of the fungus kingdom (biology), kingdom. The first yeast originated hundreds of millions of years ago, and at least 1,500 species are currently recognized. They are est ...
,
septin
Septins are a group of guanosine triphosphate, GTP-molecular binding, binding proteins gene expression, expressed in all eukaryote, eukaryotic cells except plants. Different septins form protein complexes with each other. These complexes can fur ...
s form a ring structure, to which other proteins are recruited.
In particular,
chitin synthase 2 is required, an
enzyme
An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different mol ...
that synthesises
chitin
Chitin (carbon, C8hydrogen, H13oxygen, O5nitrogen, N)n ( ) is a long-chain polymer of N-Acetylglucosamine, ''N''-acetylglucosamine, an amide derivative of glucose. Chitin is the second most abundant polysaccharide in nature (behind only cell ...
thereby building up the primary septum. A secondary septum of
β-glucan
Beta-glucans, β-glucans comprise a group of β-D-glucose polysaccharides (glucans) naturally occurring in the cell walls of cereals, bacteria, and Fungus, fungi, with significantly differing Physical chemistry, physicochemical properties depen ...
s and
mannoproteins is then assembled using the enzyme
1,3-Beta-glucan synthase, and the primary septum degraded during cell separation. After degradation of the primary septum, a chitinous bud scar remains on both the mother and daughter cell.
Septum formation–related diseases and treatments
In regard to septum formation–related diseases,
cancer
Cancer is a group of diseases involving Cell growth#Disorders, abnormal cell growth with the potential to Invasion (cancer), invade or Metastasis, spread to other parts of the body. These contrast with benign tumors, which do not spread. Po ...
in eukaryotic cells can occur due to mutations that cause different errors in cytokinesis and in the septum formation itself.
This is because defects in cytokinesis can affect the number of sets of chromosomes in the cell, and if a defect occurs that leads to a tetraploid cell, there is a high possibility that
aneuploid cells could generate from it.
This is an issue because the majority of tumors in humans are made up of aneuploid cells.
Additionally, if one of the steps of cytokinesis is negatively affected, the formation of the septum could be made impossible which could lead to the formation of more aneuploid cells.
For instance, if the cleavage furrow in an animal cell fails to cleave inwards due to the absence of one of its activators such as polo-like kinase 1, the cell remains with double the amount of chromosomes and could lead to cancerous tumors.
Along with this, outside factors could influence cytokinesis and septum formation, such as asbestos fibres.
These fibres block the process of cytokinesis of occurring due to their carcinogenic nature.
Furthermore, certain breast cancers have been linked to the loss of the adhesion between the cell and the matrix.
This causes a process known as
entosis to occur, which is the uptake of the cell by cells that neighbor it.
Such uptake results in multi-nucleation which can cause human breast cancers.
Since the failure of cytokinesis and septum formation can lead to diseases, researchers were able to discover that blocking the formation of the septum by blocking cytokinesis could treat certain bacterial diseases such as ''
Streptococcus''.
Researchers found that FtsZ could be used as a target, as it has the ability to stop the division of the cell, which causes diseases such as ''Streptococcus'' to halt in cell division, and then
lyse, causing the diseased cell to no longer be functional.
While some inhibitors of FtsZ have been discovered such as sanguinarine, further work is still required for the majority of these inhibitors to be utilized in a clinical setting.
References
Organelles
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