Topic 1.6 Cell Division

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Topic 1.6 Cell Division

• Chapter 12
• Campbell Reece

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5 Ws

• Who does cell division? All cells (except most
  brain cells)
• What is cell division? Dividing of the DNA and
  cytoplasm/cell contents into two separate
  identical cells
• Where does it occur? In most places of all
  organisms
• When does it happen? During the mitotic stage of
  the cell cycle (depends on the type of cell) ie.
  Red blood cells every day, heart cells 50 times
  in our lives, embyro cells are the most rapid
• Why does it happen? For growth/development,
  repair damaged cells, replace old cells or
  infected ones, and asexual reproduction (bacteria)

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Cell division is more complex in eukaryotes than
prokaryotes
The process of cell division in eukaryotes may be
divided into two steps Mitosis The allocation of
two complete sets of chromosomes to each of the
daughter cells Cytokinesis The division of the
parent cell into two daughter cells with an
equivalent complement of cytoplasm and organelles
from the parent cell. http//highered.mcgraw-hill
.com/sites/9834092339/student_view0/chapter10/anim
ation_-_cytokinesis.html
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Cell Division

• As you will recall from Topic 1.5(U1) all cells
  can only be formed by division of pre-existing
  cells
• All eukaryotic cells go through a cycle over and
  over again known as the cell division cycle. It
  involves interphase, mitosis, and cytokinesis.

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1.6 (U1) Mitosis is division of the nucleus into
two genetically identical daughter cells.

• The cell cycle outlines and describes how cells
  behave during their growth and division stages.
• Once cells reach a certain size they will divide
  into two genetically identical daughter cells in
  a process known as mitosis.
• The cell cycle involves two major phases a
  growth phase and a division phase. The largest
  portion of the cell cycle is spent in interphase
  (growth phase).

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1.6 (U4) Interphase is a very active phase of the
cell cycle with many processes occurring in the
nucleus and cytoplasm.

• Interphase is the longest phase in the cell cycle
  and it includes three phases G1 , S, and G2 .

Phases of Interphase Major Events
G1 Growth of the cell and increase in number of organelles.
S Replication of chromosomes with copies remaining attached to one another
G2 Further growth occurs, organelles increase in number, DNA condenses to form visible chromosomes, microtubules begin to form.
Overall Cell is performing the tasks appropriate to its type along with many metabolic reactions such as protein synthesis . For example, a cell of the pancreas may be actively secreting insulin to lower high glucose levels in the body.
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Interphase under the microscope!
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1.6 (U2) Chromosomes condense by supercoiling
during mitosis.

• You will learn that DNA condenses into
  chromosomes during mitosis and this occurs during
  a process known as condensation of chromosomes.
• The DNA will wrap itself around proteins
  (histones) which will then group together and
  followed by more wrapping and grouping together.
  This process is called supercoiling.
• In the end a chromosome is produced which
  consists of two identical molecules of DNA known
  as sister chromatids held together in the middle
  by a centromere.

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1.6 (U2) Chromosomes condense by supercoiling
during mitosis.

• Chromosomes must be supercoiled or they would not
  fit inside the nucleus (5µm) of human cells
  (10µm.
• Human chromosomes are between 15 000µm and 85 000
  µm in length!

http//www.youtube.com/watch?vN5zFOScowqo
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Four Phases of Mitosis

• The process of mitosis in the cell cycle is when
  the nucleus of the cell divides and forms two new
  genetically identical nuclei.
• This takes place over the four phases of mitosis
  and towards the end there is a division of the
  cytoplasm and all its contents.
• Then the cell begins to form two separate new
  daughter cells, each containing one nucleus and
  they are an exact copy of each other.

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Prophase

• During prophase the chromatin (DNA) begins to
  condense by super-coiling.
• Spindle fibres begin to form from the pair of
  centrioles in animal cells that begin to move to
  opposite poles of the cell.
• The nuclear envelope surrounding the nucleus
  begins to dissolve.

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Metaphase

• During this stage of mitosis, the spindle fibres
  attach to the sister chromatids at the
  centromere.
• The chromosomes are guided by the spindle fibres
  to line up along the equatorial plate in the
  cell. One spindle fibre microtubule from each
  pole is attached to each of the sister
  chromatids.
• This is the longest phase in mitosis lasting
  about 20 minutes.

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Anaphase

• During this phase of mitosis the spindle fibres
  begin pulling the sister chromatids to opposite
  poles.
• The sister chromatids separate at the centromere
  and are pulled toward opposite ends of the cell
  as the spindle fibres shorten.
• This phase is very short lasting only a few
  minutes.

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Telophase

• During this phase in mitosis, the chromosomes
  have arrived at each end and a nuclear membrane
  begins to form around them.
• The chromosomes uncoil and become less dense
  which makes them no longer visible.
• The spindle fibres begin to break down and the
  process of cytokinesis begins.

http//highered.mheducation.com/sites/0072495855/s
tudent_view0/chapter2/animation__mitosis_and_cytok
inesis.html
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1.6 (U3) Cytokinesis occurs after mitosis and is
different in plant and animal cells

• This involves the division of the cytoplasm and
  all its contents.
• In animal cells the process of cytokinesis is
  known as cleavage.
• In plant cells cytokinesis is quite different due
  to the presence of the cell wall.

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Cytokinesis in Animal Cells

• The first sign of cytokinesis in animal cells is
  the appearance of a cleavage furrow near the
  location where the metaphase plate was located.
• There is a contractile ring of actin
  microfilaments and myosin molecules that interact
  together to cause the ring to contract which
  deepens the furrow and separates the cell
  contents. The process is like pulling on the
  drawstrings of your hoodie!
• The parent cell is pinched into two separate,
  identical daughter cells.

http//highered.mheducation.com/sites/9834092339/s
tudent_view0/chapter10/animation_-_cytokinesis.htm
l
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Cytokinesis in Plant Cells

• During telophase vessicles from the Golgi
  apparatus follow microtubules to the middle of
  the cell. Once they reach the middle they
  coalesce and form a cell plate which will develop
  into the cell membranes of the daughter cells.
• The vessicles carry the materials such as pectin
  and other substances needed for the new cell wall
  and they collect in the cell plate as it grows.
  This will develop into the middle lamella that
  will attach the new cell walls to one another.
• The cell plate grows until it joins with the
  membrane surrounding the cell. This separates
  the cell into two identical daughter cells and a
  new cell wall forms from the contents of the cell
  plate and cellulose that has been deposited by
  each daughter cell.

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1.6 (U5) Cyclins are involved in the control of
the cell cycle.

• Cyclin is a regulatory protein that assists in
  the control of the progression of cells through
  the cell cycle.
• Cyclins ensure that each task in the cell cycle
  is not only performed on time but in the proper
  order. The next task will not begin until the
  previous tack has been completed.

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1.6 (U5) Cyclins are involved in the control of
the cell cycle.

• The cell contains enzymes known as
  cyclin-dependent kinases. When the cyclin binds
  to these enzymes they are activated and will
  attach phosphate groups to other proteins
  contained in the cell.
• The attachment of the phosphates activates these
  proteins and they begin to carry out the specific
  tasks of the cell cycle.

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Four main Cyclins

• Cyclin D triggers cells to move fromG0 to G1 and
  G1 into S phase
• Cyclin E prepares the cell for DNA replication in
  S phase
• Cyclin A activates DNA replication inside the
  nucleus in S phase
• Cyclin B promotes the assembly of the mitotic
  spindle and other tasks in the cytoplasm to
  prepare for mitosis.

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1.6 (U6) Mutagens, oncogenes and metastasis are
involved in the development of primary and
secondary tumors.

• Benign tumors

• Malignant Tumors

• Tumors are abnormal groups of cells that develop
  in the tissues of the body. In benign tumors,
  the cells adhere to one another and do not invade
  other tissues or move to other body parts.
  Benign tumors do not usually cause much harm.

• When cells in a tumor become detached and move to
  other parts of the body and develop into
  secondary tumors these are classified as
  malignant. Malignant tumors, known as carcinomas
  are more often than not life-threatening. These
  tumors cause a disease known as cancer.

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Causes of Cancer

• Carcinogens are chemicals and agents that are
  known to cause cancer.
• Some known carcinogens include some viruses,
  chemical mutagens, high energy radiation such as
  x-rays and short-wave UV light.
• Mutagens are agents that cause gene mutations and
  these mutations can cause cancer.

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Mutagens Mutations

• When random changes occur that alter the base
  sequence of genes we call them mutations. Most
  of the time these mutations in genes do not cause
  cancer unless they occur in genes that are known
  to become cancer causing following mutation known
  as oncogenes.
• Oncogenes are involved in controlling the cell
  cycle and cell division. If there is a mutation
  in these genes it can cause uncontrolled cell
  division which can lead to the formation of
  tumors.

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Mutations

• The chance of a cell becoming a tumor cell is
  small due to the fact that several mutations
  would have to occur.
• However, due to the significant number and types
  of cells in the human body, the chance of tumor
  formation during our lifetime is significant.
• A primary tumor forms when a tumor cell begin
  dividing by mitosis into a large group of cells.
• If cells from the primary tumor move in a process
  known as metastasis and begin to develop in
  another part of the body it is known as a
  secondary tumor.

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1.6 (A1) The correlation between smoking and
incidence of cancers

• See handout given in class.