Cell Cycle

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Cell Cycle
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Why do cells divide?

• Cells divide for growth, cell repair and
  reproduction.
• There is a limit to cell growth.
• DNA cannot control activities in a large cell.
• Wastes and nutrients would take too long
  diffusing in and out of cell.
• Surface area / volume ratio is larger for small
  items than big ones

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Cell division

• Division of cell into two daughter cells.
• Prior to cell division, the DNA makes a copy of
  itself
• Each daughter cell gets an entire set of DNA and
  part of the cytoplasm with organelles

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What are chromosomes?

• Chromosomes Coiled structures that are made up
  of DNA and proteins
• DNA wraps around proteins that help to condense
  it at the start of mitosis
• During interphase the threads are loosely
  organized so that the DNA can be copied
• Chromosomes carry coded genetic information about
  the organisms make up.
• Number of chromosomes differ for each organism.
  Humans 46, one species of roundworm has 2 and
  some ferns gt 1200!
• An pictorial array of chromosomes is a karyotype.

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Karyotype
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Chromatin chromosomes - chromatid

• Chromatin is the collective term for the fibers
  of DNA and proteins that make up the chromosome.
• Histones are proteins that DNA wraps around at
  regular intervals
• Chromatid are identical chromosomes, replicated
  before cell division, joined together by a
  segment of DNA called a centromere. Together
  called sister chromatids.
• When they separate during mitosis they will be
  individual chromosomes in their own rite.

A condensed (metaphase) chromosome. (1) Chromatid
- one of the two identical parts of the
chromosome after S phase. (2) Centromere - the
point where the two chromatids touch, and where
the microtubules attach. (3) Short arm. (4) Long
arm
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• Telomeres- a repeating sequence of nucleotides
  that do not code for any proteins.
• They prevent the ends of chromosomes from
  attaching to each other. A little bit of the
  telomere is lost each time the chromosome is
  copied

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Cell cycle - eukaryotes

• Cell cycle Sequence of growth, DNA duplication,
  and cell division during an eukaryotic cells life
• An alternating cycle of cell growth and cell
  divisions, can take one to 24 hrs, depending on
  cell type. Some cells dont divide at all.
• Four phases of cell cycle
• G1 - gap 1
• S - synthesis
• G2 - gap 2
• Mitotic phase consisting of Mitosis and
  Cytokinesis

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• A Gap 1
• B Synthesis
• C Gap 2
• D mitosis
• E prophase
• F metaphase
• G anaphase
• H Telophase
• I Cytokinesis

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Interphase

• Interphase 90 of time of the cell cycle.
  Consists of three parts
• First growth G1 phase
• cell grows, organelles increase in number
• carries out routine functions, major portion of
  cells life.
• Muscle cells move joints, adrenal cells secrete
  hormones
• Critical checkpoint occurs before the cell
  proceeds to S phase
• Synthesis S phase
• Cells DNA is copied
• each chromosome consists of two chromatids
  attached at the centromere
• Second growth G2 phase
• preparations for cell division
• Microtubles are assembled, they move chromosomes
  during mitosis
• Critical checkpoint before the cell enters the
  mitotic phase

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Rate of cell division

• Rate of cell division is related to need for the
  cell
• S, G2, M together about 12 hrs.
• Embryos high rate of division
• Young children have more cell division than
  adults
• Stomach lining cells have wear and tear and are
  replaced often, while muscle cells are not. Some
  are replaced upon injury or death
• Cells that rarely divide enter a G0 stage,
  unlikely to divide, but carrying out normal
  functioning
• Neurons, lymphocytes (white blood cells) stay in
  G0 until an invader enters and then the
  lymphocytes actively divide to fight the
  infection.

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Mitotic phase Mitosis and Cytokinesis

• Mitosis nucleus divides into two nuclei. The
  purpose of mitosis is that each daughter cell
  ends up with identical nuclei with the same
  number and kinds of chromosomes as the original
  cell.
• Cytokinesis cytoplasm divides. Cell plate forms
  in plants, cleavage furrow forms in animal cells.
  Begins during telophase and ends after telophase

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Check points

• Cell growth G1 checkpoint decides whether cell
  will divide, if large and healthy, proteins will
  stimulate the cell to begin synthesis S phase. At
  this point some muscle and nerve cells pass into
  a resting period, G0, and do not divide.
• DNA synthesis G2 checkpoint DNA repair enzymes
  check DNA replication and cell prepares for
  mitosis
• Mitosis checkpoint triggers exit of mitosis and
  cell begins G1 phase

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How do the chromatids separate?

• Spindles are cell structures composed of both
  centrioles and microtubules individual
  microtubule fibers move the chromosomes during
  cell division
• Animal cells have one pair of centrioles at right
  angles
• Plants do not have centrioles but form spindles
  as well.
• In the G2 phase, centrioles are replicated so
  that two pairs of centrioles enter the mitotic
  phase.
• Centrioles start to move to opposite poles of cell

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Mitosis up close

• Mitosis is a continuous process but 4 parts are
  recognized
• Prophase
• Metaphase
• Anaphase
• Telophase
• Mitosis

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Late Interphase

• The cell gets ready to enter Mitosis. By the end
  of interphase the cell is large enough to divide
  and has two full sets of chromosomes

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Prophase

• Chromatin coils, compacts, chromosomes are
  identical, paired chromatids.
• Nuclear envelope and nucleolus break down.
• The centrosomes and centrioles begin to migrate
  to the opposite sides of the cell
• Spindle fibers called microtubles grow from the
  centrioles and attach to a structure called a
  kinetocore at the centromere.
• Chromosomes become visible
• animation

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Metaphase

• chromosomes line up at the equator of the cell
  called the metaphase plate
• best time to see the chromosomes they are coiled
  and condensed.
• animation

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Anaphase

• Centromeres have separated, individual
  chromosomes move to opposite ends of the cell.
• Microtubules shorten, moving the chromosomes to
  the poles of the cell
• animation

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Telophase

• The complete set of chromosomes are at the
  opposite poles of the cell. The spindle breaks
  down, chromosomes begin to uncoil, nuclear
  envelope and nucleolus reform. Two nuclei are now
  identical.

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Cytokinesis

• Cytokinesis starts toward the end of telophase
  dividing the cell into two new cells
• Plants cannot pinch inward like animal cells. A
  cell plate forms from vesicles of the Golgi body
• In animal cells a cleavage furrow forms

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Cell plate forms in plants
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Cytokinesis - animal cell
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The two new cells

• The new cells are about equal in size with about
  half of the original cells cytoplasm and
  organelles

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Regulation of cell division

• External factors include physical and chemical
  signals
• Cells growing in a petri dish will stop growing
  when they touch each other
• Growth factors are proteins that stimulate cell
  division. The bind to receptors that activate
  genes that trigger cell growth.
• Platelets help repair wounds by forming clots
  that help stop bleeding
• Erythropoietin stimulates red blood cell growth

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• Internal factors- when external factors bind to
  receptors they can trigger internal factors that
  affect the cell cycle.
• Kinases an enzyme that that transfers a
  phosphate from one molecule to another and
  changes the proteins shape
• Cyclins are proteins that work with kinases to
  help move the cell to different stages of the
  cell cycle.

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• Apoptosis programmed cell death. Signals that
  activate genes to produce self destructive
  enzymes. The nucleus of apoptotic cell shrinks
  and the cell is consumed and its molecular parts
  are used again by the cell
• Examples are the embryonic webbing of fingers and
  the metamorphosis of a tadpole to frog

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What is cancer?

• Certain genes make proteins that regulate growth
  and cell division. When one of these genes
  mutates, a protein may not function and cancer
  may result
• Cancer is the uncontrolled growth of cells. It is
  a disorder of cell cycle
• Cancer cells come from normal cells that have
  suffered damage to the genes that control cell
  cycle regulation

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• Cancer cells form tumors or clumps of
  disorganized cells
• Benign tumors have cancer cells that remain in
  one cluster
• Malignant tumors have cancer cells that travel
  away, or metastasize, from the original cluster.
• Malignant tumors are more difficult to treat
• Treatment involves radiation and or chemotherapy
• Radiation is targeted to cancer cells
• Chemotherapy is drugs that affect cancer cells
• Both treatments affect cells other than cancer
  cells and are responsible for the side affects
  associated with these treatments

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• Cancer cells carry mutations
• Oncogenes accelerate the cell cycle
• Mutations in genes is one of the causes, they can
  occur spontaneously or as a result of
  environmental factors such as temperature,
  chemicals or radiation
• Skin cancers are caused by exposure to the UV
  rays from the sun
• Carcinogens are cancer causing substances.
  Tobacco smoke, air pollutants
• Some mutations are inherited
• Some oncogenes are carried by viruses such as the
  one that causes cervical cancer.

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Asexual Reproduction

• Reproduction that does not involve the production
  of gametes or sex cells.
• The offspring are from a single parent and are
  genetically identical to each other.
• Prokaryotes reproduce through binary fission.
• A form of asexual reproduction by single celled
  organisms in which a cell divides into two
• Eukaryotes may undergo budding, fragmentation or
  vegetative propagation. These occur by mitotic
  divisions

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Prokaryotic cell division

• Prokaryotes have a circular chromosome as a loop
  of DNA, no nucleus, no spindle fibers
• Binary fission

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Advantages and disadvantages of Asexual
reproduction

• Advantages
• Efficient,
• no mates required
• all organisms reproduce
• If the organisms are well suited to their
  surroundings they will continue to live and grow.
• Disadvantages
• Does not provide genetic diversity which is
  necessary when environments change