Cell cycle

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Cell cycle

• How Do Cells Divide?

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What you will learn

• 1. Why Do Cells Divide?
• 2. Chromosome structure
• 3. Cell Division in Prokaryotes
• 4. Cell Cycle
• 5. Mitosis
• 6. Cytokinesis
• 7. Control of Cell Division and Cancer
• 8. Meiosis
• 9.Why do cells need two types of cell division?
• 10. Gamete Formation

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1. Why Do Cells Divide?

• Virchow Cells can only come from preexisting
  cells
• In unicellular organisms, can reproduce an entire
  organism
• Allows multicellular organisms to reproduce
  asexually
• Basis of sexual reproduction? sperm and egg
• Allows fertilized egg, or zygote, to develop into
  an adult organism
• Replaces worn-out or damaged cells
• Enables multicellular organism to grow to adult
  size
• http//www.pbs.org/wgbh/nova/miracle/program.html

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2. Chromosome Structure

• DNA can be in the form of Chromatin
• Diffuse mass of long, thin fibers, not seen under
  the microscope, less tightly coiled
• Combination of DNA and protein
• DNA must be tightly packaged before cell
  division, so it can be evenly divided between the
  two new cells.
• DNA will now be in the form of Chromosome!

http//www.dnalc.org/resources/3d/07-how-dna-is-pa
ckaged-basic.html
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2. Chromosome Structure

• Chromosomes
• Rod-shaped structure
• Coiled up, compact forms of chromatin
• Contains one long DNA molecule bearing hundreds
  or thousands of genes.
• DNA is attached to protein molecules called
  histones
• DNA wraps with protein like wrapping paper on a
  present giving it the X-shape
• Only found in eukaryotic cells (prokaryotes have
  naked, circular shaped chromosomes)

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2. Chromosome Structure
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2. Chromosome Structure

• Sister chromatids
• Each duplicated chromosome contains two identical
  copies.
• Centromere
• The point by which two chromatids are joined.
• Chromatin
• Diffuse mass of long, thin fibers, not seen under
  the microscope, less tightly coiled
• Combination of DNA and protein

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2. Chromosome Structure
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3. Cell Division in Prokaryotes

• Binary fission
• Process by prokaryotes reproduce by cell
  division.
• Steps
• Duplication of chromosomes and separation of
  copies.
• Cell elongates
• Divides into two daughter cells

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3. Cell Division in Prokaryotes
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4. Cell Cycle

• In your own body, millions of cells must divide
  every second to maintain the total number of
  about 100 trillion cells.
• Some cells divide once a day, and some do not at
  all (mature muscle cells, brain cells)

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4. Cell Cycle

• Starts out with Interphase
• Occurs when the cell is between cell division
• Interphase stages
• G1 Cells grow to mature size
• S DNA is copied
• G2 Cell prepares for division
• Cells exit the cell cycle via
• G0 Cells do not copy DNA or prepare for
  mitosis, but are still alive (e.g. nervous
  system)

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5. Mitosis

• The last stage of the cell cycle when the nucleus
  of a cell divides to produce two new daughter
  cells (after cytokinesis) each with the same
  amount and type of chromosomes as the parent
  cells.
• Mitosis is divided into four phases
• A.Prophase
• B. Metaphase
• C. Anaphase
• D. Telophase

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5. Mitosis

• A.Prophase
• What does the cell look like?
• Centrioles and spindle fibers appear
• Nuclear envelope disappears, and chromosomes are
  visible
• What happens to the DNA and nucleus?
• Chromosomes form when chromatin tightens and
  coils
• Nuclear membrane breaks down and disappears
• What two things appear near where the nucleus
  was?
• Centrioles and spindle fibers

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5. Mitosis

• A. Prophase

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5. Mitosis

• B. Metaphase
• What does the cell look like?
• Chromosomes move to the middle
• Where are the chromosomes during metaphase?
• Middle of the cell

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5. Mitosis
B. Metaphase
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5. Mitosis

• C. Anaphase
• What does the cell look like?
• Chromosomes move to the end of cell
• What happens to the chromosomes?
• Chromosome splits at centromere into 2 chromatids
  and moves to end of cell

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5. Mitosis

• C. Anaphase

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5. Mitosis

• D. Telophase
• What does the cell look like?
• Cell starts to pinch in
• Nucleus starts to reform
• Chromosomes are at opposite ends
• What happens to the chromosomes and nucleus?
• Nucleus forms back around single chromatids

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5. Mitosis

• D. Telophase

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6. Cytokinesis

• What is cytokinesis?
• Cytoplasm and contents (other organelles) divide
• Whats special about cytokinesis in plants?
• Cell wall also divides with new cell plate in
  middle
• Whats special about cytokinesis in animals?
• Takes place when the cell membrane pinches in
  until the cytoplasm is pinched into two equal
  halfs

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7. Control of Cell Division and Cancer

• Cell division is a complex process that needs to
  be regulated.
• These regulators determine when and how the cell
  should divide.
• External Regulators
• Internal Regulators

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7. Control of Cell Division and Cancer

• External regulators
• Various proteins produced by other cells that
  speed up or slow down the cycle.
• If the cell touches other cells, than cell
  division slows down.
• If enough space between cells and nutrients are
  available, growth factors and other proteins make
  cells divide or speed up their cell cycle.

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7. Control of Cell Division and Cancer

• Internal regulators
• Cyclins
• proteins that regulate the timing of the cell
  cycle in eukaryotic cells.
• Other regulator proteins (checkpoints)
• they make sure that certain things happen in the
  cell before the cell moves to the next phase of
  the cell cycle
• 3 major checkpoints in the cell cycle.
• The age of the cell.

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7. Control of Cell Division and Cancer

• Cancer cells
• lack normal checkpoints and continue to grow
  without inhibition
• do not respond to normal signals within the
  cell
• are not inhibited by other cells
• will divide indefinitely

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7. Control of Cell Division and Cancer

• Mutations in the genes of these checkpoint
  proteins may lead to cancer
• The uncontrolled growth of cells.
• Tumor an abnormally growing mass of body cells
• Benign tumor
• If abnormal cells remain at original site
• Can be problematic if disrupt certain organs, but
  usually easily removed by surgery
• Malignant tumor
• If abnormal cells spread into other tissues and
  body parts, interrupting organ function

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7. Control of Cell Division and Cancer
Tumor Progression
2. Blood vessels feed tumor
3. Tumor cells enter blood and lymph vessels
4. Secondary tumors form in other parts of the
body
1. Tumor growth

• Movie clips on cancer, its nature and experiments
  to treat it (Parts 2 and 6)
• http//www.pbs.org/wgbh/nova/cancer/program.html
• http//www.youtube.com/watch?vHonoQ6mE6dYfeature
  related

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7. Control of Cell Division and Cancer

• Treatment of Cancer
• Surgical removal of tumor Most effective when
  tumor is in a defined area
• Chemotherapy Medicines that disrupt the process
  of mitosis in rapidly growing cells
• Radiation Therapy - High energy gamma radiation
  is aimed at the growing tumour. This damages the
  DNA in rapidly dividing cells and helps to
  destroy the tumor.

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8. Meiosis

• Many of the stages of meiosis closely resemble
  corresponding stages in mitosis.
• Type of cell division that produces haploid
  gametes in diploid organisms.

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8. Meiosis
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8. Meiosis

• Like mitosis, is preceded by the replication of
  chromosomes.
• However, this single replication is followed by
  two consecutive cell divisions, called Meiosis I
  and Meiosis II.
• These divisions result in four daughter cells,
  each with a single haploid set of chromosomes.
• Produces daughter cells with only half as many
  chromosomes as the parent cell.

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8. Meiosis

• We will be looking at chromosome pairing up
• called homologous chromosomes (or homologues)
  because they both carry genes controlling the
  same inherited characteristics.

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8. Meiosis

• Any cell with two homologous (the same) sets of
  chromosomes is called a diploid cell
• the total number of chromosomes is called the
  diploid number (abbreviated 2n)
• For humans, the diploid number is 46 that is
  2n46
• Almost all human cells are diploid

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8. Meiosis

• The exception are the egg and sperm cells,
  collectively known as gametes.
• A cell with a single chromosome set is called a
  haploid cell.
• For humans, the haploid number (abbreviated n) is
  23 that is n23

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8. Meiosis

• Prophase I
• Each chromosome pairs with its corresponding
  homologous chromosome to form a tetrad. The
  tetrads overlap and exchange some of their
  genetic material crossing-over.

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8. Meiosis

• Crossing over in Prophase I results in great
  diversity because new genetic variations can
  result from it.

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8. Meiosis

• Metaphase I
• Spindle fibers attach to the chromosomes.

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8. Meiosis

• Anaphase I
• The fibers pull the homologous chromosomes toward
  opposite ends of the cell.
• The cells are now containing half of the genetic
  information from the original parent cell and are
  thus considered HAPLOID!

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8. Meiosis

• Telophase I and cytokinesis
• Nuclear membranes reforms, the cell separates
  into two cells.

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8. Meiosis

• Prophase II
• Meiosis I results in two haploid (N) daughter
  cells, each with half the number of chromosomes
  as the original cell.

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8. Meiosis

• Metaphase II
• The chromosomes line up in a similar way to the
  metaphase stage of mitosis.

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• Anaphase II
• The sister chromatids separate and move toward
  opposite ends of the cell.

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8. Meiosis

• Telophase II and cytokinesis
• Meiosis II results in four haploid (N) daughter
  cells.
• http//www.sumanasinc.com/webcontent/animations/co
  ntent/meiosis.html
• http//www.pbs.org/wgbh/nova/baby/divi_flash.html

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9.Why do cells need two types of cell division?

• Mitosis
• Provides growth, tissue repair, and asexual
  reproduction
• Produces daughter cells genetically identical to
  the parent cell
• Involves one division of the nucleus, and is
  usually accompanied by cytokinesis, producing two
  diploid daughter cells.
• Meiosis
• Need for sexual reproduction? human egg and sperm
  cells
• Entails two nuclear and cytoplasmic divisions
• Yields four haploid daughter cells, with one
  member of each homologous chromosome pair.
• Form tetrads crossing over occurs.

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10. Gamete Formation

• In females

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10. Gamete Formation

• In males

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11. Karyotype

• The term karyotype refers to the chromosome
  complement of a cell or a whole organism.
• A karyotype is an ordered display of magnified
  images of an individuals chromosomes arranged in
  pairs, starting with the longest.
• In particular, it shows the number, size, and
  shape of the chromosomes as seen during metaphase
  of mitosis.
• Chromosome numbers vary considerably among
  organisms and may differ between closely related
  species.

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11. Karytype

• Karyotypes are prepared from the nuclei of
  cultured white blood cells that are frozen at
  the metaphase stage of mitosis.
• Shows the chromosomes condensed and doubled
• A photograph of the chromosomes is then cut up
  and the chromosomes are rearranged on a grid so
  that the homologous pairs are placed together.
• Homologous pairs are identified by their general
  shape, length, and the pattern of banding
  produced by a special staining technique.

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11. Karyotype

• Male karyotype
• Has 44 autosomes, a single X chromosome, and a Y
  chromosome (written as 44 XY)
• Female karyotype
• Shows two X chromosomes (written as 44 XX)

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11. Karyotype- Normal
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11. Karyotype- Abnormal
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12a. Mutations- Chromosome Number

• Nondisjunction
• Members of a chromosome fail to separate.
• Can lead to an abnormal chromosome number in any
  sexually reproducing diploid organism.
• For example, if there is nondisjunction affecting
  human chromosome 21 during meiosis I, half the
  resulting gametes will carry an extra chromosome
  21.
• Then, if one of these gametes unites with a
  normal gamete, trisomy 21 (Down Syndrome) will
  result.

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12a. Mutations- Chromosome Number
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12b. Mutations- Chromosome Structure

• Abnormalities in chromosome structure
• Breakage of a chromosome can lead to a variety of
  rearrangements affecting the genes of that
  chromosome
• 1. deletion if a fragment of a chromosome is
  lost.
• Usually cause serious physical and mental
  problems.
• Deletion of chromosome 5 causes cri du chat
  syndrome child is mentally retarded, has a small
  head with unusual facial features, and has a cry
  that sounds like the mewing of a distressed cats.
  Usually die in infancy or early childhood.

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12b. Mutations- Chromosome Structure

• 2.duplication if a fragment from one chromosome
  joins to a sister chromatid or homologous
  chromosome.
• 3.inversion if a fragment reattaches to the
  original chromosome but in the reverse direction.
• Less likely than deletions or duplications to
  produce harmful effects, because all genes are
  still present in normal number
• 4. translocation moves a segment from one
  chromosome to another nonhomologous chromosome
• Crossing over between nonhomologous chromosomes!

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12b. Mutations- Chromosome Structure