CELLS

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CELLS
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Cell Theory

• All organisms are composed of cells.
• A cell is the smallest unit of living matter.
• Cells come only from preexisting cells.

Stem Cell
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Diagram of a typical animal cell. Organelles
are labeled as follows
8. Smooth endoplasmic reticulum 9.
Mitochondrion 10. Vacuole 11. Cytoplasm 12.
Lysosome 13. Centriole
1. Nucleolus 2. Nucleus 3. Ribosome 4. Vesicle
5. Rough endoplasmic reticulum 6. Golgi
apparatus 7. Cytoskeleton
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Typical Animal Cell Typical Plant Cell

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Eukaryote Nucleus

• Their DNA is organized into a true nucleus
  surrounded by a nuclear envelope which consists
  of two bilayer membranes.
• The nucleus of eukaryotic cells contains the
  genetic material which chemically directs all of
  the cells activities.
• Usually this is in the form of long strands of
  chromatin made of DNA and affiliated proteins.
• When a cell is getting ready to divide, the
  chromatin coils and condenses into individual,
  distinguishable chromosomes.
• Because the nuclear envelope consists of two
  bilayer membranes, there is a space between these
  two membranes called a lumen.

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Cell Division

• Cell division involves a single cell (called a
  mother cell) dividing into two daughter cells.
  This leads to growth in multicellular organisms
  (the growth of tissue) and to procreation
  (vegetative reproduction) in unicellular
  organisms. Prokaryotic cells divide by binary
  fission. Eukaryotic cells usually undergo a
  process of nuclear division, called mitosis,
  followed by division of the cell, called
  cytokinesis. A diploid cell may also undergo
  meiosis to produce haploid cells, usually four.
  Haploid cells serve as gametes in multicellular
  organisms, fusing to form new diploid cells. DNA
  replication, or the process of duplicating a
  cell's genome, is required every time a cell
  divides. Replication, like all cellular
  activities, requires specialized proteins for
  carrying out the job.

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Endoplasmic Reticulum

• Branching off from and continuous with the outer
  membrane of the nuclear envelope is a double
  walled space which zigzags through the cytoplasm.
  This is the endoplasmic reticulum (ER for short)
  and its central space or lumen is a continuation
  of the lumen between the membranes of the nuclear
  envelope. There are two kinds of ER smooth ER
  and rough ER. Typically ER closer to the nucleus
  is rough and that farther away is smooth. Smooth
  ER is a transition area where chemicals like
  proteins the cell has manufactured are stored in
  the lumen for transportation elsewhere in the
  cell. Pieces of the smooth ER called vesicles
  pinch off from the smooth ER and travel other
  places in the cell to transfer their contents.
  Rough ER gets its name because it has other
  organelles called ribosomes attached, which give
  it a rough appearance when viewed by an electron
  microscope. Rough ER and its associated ribosomes
  are involved in protein synthesis, with the new
  polypeptide being threaded into the lumen of the
  ER as it is formed.

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Ribosomes

• Ribosomes are special organelles that are
  directly involved in protein synthesis. They are
  made of RNA (ribonucleic acid) and protein and
  are manufactured in the nucleus (from a DNA
  template), then go out into the cytoplasm to
  function. Ribosomes of prokaryotes and eukaryotes
  are chemically different enough that some of our
  antibiotics such as tetracycline and
  streptomycin, can interfere with bacterial
  ribosomes ability to do protein synthesis
  without also interfering with our ribosomes.

The ribosome is plays a key role is the synthesis
of protein. When the polypeptide chain is growing
it must be kept aligned with the mRNA molecule so
that each codon still hooks up with the tRNA
molecule. After the addition of one amino acid
the chain is moved down three codons. This is
done using a large complex composed of protein
and RNA, called the ribosome. Ribosomes consist
of one large unit and one small unit. Half of the
eukaryotic ribosomal weight comes from RNA. The
ribosome contains a groove that guides the
polypeptide chain and another groove that holds
the mRNA molecule.
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Protein Synthesis

• An overview of protein synthesis.Within the
  nucleus of the cell (light blue), genes (DNA,
  dark blue) are transcribed into RNA. This RNA is
  then subject to post-transcriptional modification
  and control, resulting in a mature mRNA (red)
  that is then transported out of the nucleus and
  into the cytoplasm (peach), where it undergoes
  translation into a protein. mRNA is translated by
  ribosomes (purple) that match the three-base
  codons of the mRNA to the three-base anti-codons
  of the appropriate tRNA. Newly synthesized
  proteins (black) are often further modified, such
  as by binding to an effector molecule (orange),
  to become fully active.

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Protein Synthesis

• Protein synthesis is the process in which the
  cell builds proteins. DNA transcription refers to
  the synthesis of a messenger RNA (mRNA) molecule
  from a DNA template. This process is very similar
  to DNA replication. Once the mRNA has been
  generated, a new protein molecule is synthesized
  via the process of translation.
• The cellular machinery responsible for
  synthesizing proteins is the ribosome. The
  ribosome consists of structural RNA and about 80
  different proteins. When the ribosome encounters
  an mRNA, the process of translating an mRNA to a
  protein begins. The ribosome accepts a new
  transfer RNA, or tRNAthe adaptor molecule that
  acts as a translator between mRNA and
  proteinbearing an amino acid, the building block
  of the protein. Another site binds the tRNA that
  becomes attached to the growing chain of amino
  acids, forming the a polypeptide chain that will
  eventually be processed to become a protein.

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Stem Cells

• What are stem cells? A stem cell is an
  undifferentiated cell that has the potential to
  divide and differentiate into any of the 220
  types of cell in the human body. While most human
  cells such as heart cells or nerve cells do not
  replicate and replenish themselves, stem cells
  are capable of dividing without limit. The
  ability of stem cells to become any type of cell
  means that we could grow cells to replace those
  lost through disease or injury.
• Where do stem cells come from? The stem cells
  used for this type of research are harvested from
  4 or 5 day old embryos left over after in vitro
  fertilization. At this stage of development the
  embryo is no more that a ball of cells and it is
  the 30 or so cells known as the inner cell mass
  that are required by researchers. These cells are
  extracted in a process that kills the embryo and
  cultured in the lab. In this way, the 30 cells
  can divide into millions of stem cells each with
  the capability of becoming any type of cell.
• Why use embryonic stem cells? More recently it
  has become known that adults also possess stem
  cells. For around 30 years we have been
  transplanting bone marrow derived stem cells into
  cancer suffers to replenish their own cells
  destroyed by the disease. It was widely believed
  that these stem cells had lost the ability to
  differentiate into cells other than blood cells
  although evidence is appearing that this is not
  the case. There are a number of questions that
  need to be answered before we will be able to
  determine whether adult stem cells can be used to
  treat the debilitating diseases listed above. For
  example, which tissues contain stem cells, are
  there adult stem cells that can differentiate
  into any type of cells, can we enhance the
  proliferation of these cells, and what controls
  their differentiation?
• What are the cells used for? Researchers are
  currently working on ways to stimulate the stem
  cells to differentiate into the chosen type of
  cell. For example, by growing the stem cell on a
  certain type of media containing various
  components and growth hormones, scientists have
  been able to produce dopamine and serotonin
  secreting neurons, the type of cell lost in
  Parkinson's disease. Work is still required to
  understand completely what controls the
  differentiation of the cells and before they can
  be injected into humans, scientists need to
  combat the problem of potential rejection of the
  cells by the patient. Encouraging results have
  been seen during tests on animals and some
  treatments involving stem cells are beginning or
  are soon to start human trials.
• So what's the problem?If stem cell research
  could have such far reaching benefits, why is
  this area of science so controversial? The
  problem lies with the fact that stem cell
  research relies on cells taken from very young
  human embryos and many oppose this, arguing that
  this is killing a human being. Whether or not you
  take this opinion, or instead believe that a
  microscopic ball of cells is not 'alive' is your
  own choice, but with its potential to cure so
  many, stem cell research is here to stay.