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C8. Cells arise from pre-existing cells, and cell division leads to an increase in cell number.


Student Outcome: C8.1

Explain why the amount of DNA in a cell doubles before division.


What happens during "S" phase of the Cell Cycle

The most complex structure that must be doubled in size in preparation for cell division is the cell's chromatin--it's chromosomal DNA together with associated proteins. This process of doubling the genome must occur with extraordinary precision, as we have discussed earlier. In the absence of precise, exact genomic duplication, one of the daughter cells will receive a flawed, mutant genome that will threaten its ability to survive, or even worse, cause it to start to grow uncontrollably like a cancer cell.


The period during which DNA replication occurs is not spread evenly throughout the cell cycle between successive mitoses. Instead, DNA replication is accomplished during a discrete window of time, termed S (synthetic) phase. S phase in mammalian cells usually takes 6-8 hours, during which time the entire complement of chromosomal DNA is replicated. The period of mitosis, termed M phase, usually takes less than an hour, and encompasses the aforementioned processes of chromosomal condensation, breakdown of the nuclear membrane, alignment of the condensed chromosomes in the mitotic apparatus, segregation of two sets of condensed chromosomes to opposite poles of the cell, reformation of two nuclear membranes around the two sets of recently segregated chromosomes, decondensation of the chromosomes, and the pinching off and separation of the two daughter cells.


Importantly, S phase does not follow hard on the heels of M phase. Instead, there is a period, often as long as 10-12 hours, after M phase during which time the recently divided cell prepares itself for S phase. This long preparation period allows the cell to synthesize a number of macromolecular constituents and build up mass. Cells that rush too quickly into S phase following mitosis end up being abnormally small.


Source: web.mit.edu



Student Outcome: C8.2

Describe how prokaryotic cells divide by binary fission.


Binary fission is the form of asexual reproduction used by most prokaryotes and protists to reproduce. This process results in the reproduction of a living cell by division into two equal or near-equal parts.


Binary fission begins when the DNA replication occurs. Each circular DNA strand then attaches to the cell membrane. The cell elongates, causing the two chromosomes to separate. The cell membrane then invaginates (grows inwards) and splits the cell into two daughter cells through a process called cytokinesis.


Source of picture: http://www.anselm.edu/homepage/jpitocch/genbio/mitosisnot.html


Organisms that reproduce through binary fission generally grow exponentially.


This type of asexual reproduction normally results in two identical cells. However, bacterial DNA has a relatively high mutation rate. This rapid rate of genetic change is what makes bacteria capable of developing resistance to antibiotics and helps them exploit invasion into a wide range of environments.


Many organisms reproduce by binary fissions, such as:


  • Bacteria (for example, Rickettsia species, that cause diseases such as Rocky Mountain spotted fever)
  • Most protists (for example, Amoeba proteus)
  • Entamoeba histolytica (a protozoan that is a human intestinal parasite)
  • Pyrodictium abyssi (an anaerobic hyperthermophilic archaea of deep-sea hydrothermal vents)
  • Schizosaccharomyces pombe (a fungal organism that is a species of yeast)


Source: http://www.answers.com/topic/binary-fission


This video (with a robot commentary) is an animated version of binary fission

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This is an almost complete opposite - done by a rock and roll freak and with a real organism (Paramecium)

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Student Outcome: C8.3

Illustrate the process of mitosis in eukaryotic cells.


The process of replicating and dividing genetic material is much more complex in eukaryotes than it is in prokaryotes. A typical eukaryotic cell contains about a thousand times more DNA than a prokaryotic cell. And instead of being neatly contained in a single ring, eukaryotic DNA forms many distinct molecules, called chromosomes. Human cells have 46 chromosomes. To ensure that each daughter cell receives one and only one of each chromosome, eukaryotic cells undergo a complex, multistep process called mitosis. Before mitosis can begin, eukaryotes must first replicate their DNA. This process is similar to but takes much longer than DNA replication in prokaryotes. Then, during mitosis, the replicated chromosomes line up along the center line of the cell. Tiny fibers, called spindle fibers, attach to the individual chromosomes and pull them apart, toward opposite poles of the cell, where they remain until the cell divides.


Source: http://www.teachersdomain.org/6-8/sci/life/repro/dnadivide/index.html


Here is a video that might be useful. Go to the link if it is not working.


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Here is another! However, there is no commentary and you need to be able to identify the organelles.


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Here is a third video showing mitosis in real cells

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Many different views of mitosis in real cells


Student Outcome: C8.4

Know that the products of mitotic division or binary fission have the same number and type of chromosomes as the parent.


Mitosis is the process that most cells use to divide. It is the process whereby a single cell divides to form two identical daughter cells. These daughter cells carry the same diploid number of chromosomes and exactly the same genetic information as the parent cell.


The DNA within the nucleus has already duplicated itself when the cell starts to divide. The chromosomes shorten and thicken, becoming visible under a microscope. Each chromosome can now be seen to consist of two strands attached at only one point, the centromere. Each chromosome has made a copy of itself and now consists of two identical strands or chromatids.


The process is precise. The ‘double-stranded’ chromosomes line up down the middle of the cell and split into their two chromatids. The chromatids move to opposite ends of the cell and when one of each chromatid reaches its destination, the cell divides into two identical cells, each with the same number of chromosomes as its parent.


Source: http://www.hi.com.au/resource/rfacts.asp?kla=1&subtopicid=1330


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