Chapter 9

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Chapter 9 Cell reproduction
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Sizes of living things
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• Surface area represents ability to take in/get
  rid of materials.
• Volume represents needs of the cell

• Rather than grow bigger, cells divide to increase
  in number

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• Small cube 1 mm tall
• Surface area 6 mm2
• Volume 1 mm3
• Surface area to volume ratio 61
• Larger cube 2 mm tall
• Surface area 24 mm2
• Volume 8 mm3
• Surface area to volume ratio 31

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• Why are cells so small?
• Cells take in nutrients and expel wastes across
  the plasma membrane, which surrounds the cell.
• Staying small ensures that the cell can more
  efficiently take in nutrients and expel wastes
• Movement of substances within the cell are better
  managed with small cell than large
• Substances move by diffusion or through movement
  of the cytoskeleton occurs too slowly if cell
  is too big
• Cells communicate better when small
• Movement of signaling proteins within the cell
  can only work in smaller cells

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• Cell Increase and Decrease
• Cell division increases the number of somatic
  (body) cells
• Two parts of cell division
• Mitosis (division of nucleus)
• Cytokinesis (division of cytoplasm)
• Apoptosis (cell death) decreases the number of
  cells.

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• Cell division occurs when
• Body growth
• Maintenance and repair
• Fighting infection
• Replacing worn/dead cells
• Apoptosis occurs when
• Tail of tadpole disappears ? frog
• Skin between human fingers and toes dies during
  development
• Death of cells leading to leaves falling from
  trees in fall
• Both cell increase and apoptosis occur during
  normal development and growth example of
  homeostasis.

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• The Cell Cycle
• An orderly sequence of events that occurs from
  the time a cell is first formed until it divides
  into two new cells.

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http//www.cellsalive.com/cell_cycle.htm
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• Most of the cell cycle is spent in interphase
• G1 stage cell growth, cell doubles its
  organelles (cell structures), prepares for DNA
  replication
• S stage DNA replication occurs
• G2 stage cell makes proteins needed for cell
  division
• Amount of time spent in interphase varies
  average for adult mammals is 20 hours
• Nerve cells and muscle cells exit the cell cycle
  ? G0 phase

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• Following interphase is the M stage, including
  mitosis and the C stage, when cytokinesis occurs
  (definitions slide 5).
• During mitosis, two copies of DNA made during
  replication are separated, and become the nuclei
  of the two daughter cells takes about 4 hours.
• The cell cycle ends when cytokinesis, the
  splitting of the cytoplasm, is complete.

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• Chromosome Structure
• In a non-dividing cell genetic material is in
  the form of chromatin (DNA protein)
• In a dividing cell, chromatin undergoes coiling
  to form chromosomes
• Proteins called histones package the DNA so it
  can fit into the nucleus (2 meters of DNA fit
  into nucleus that is 5 micrometers)

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• After replication, there are 2 identical sister
  chromatids, held together by a centromere

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• Each species has a set number of chromosomes
• Humans 46
• Crayfish 200
• Corn 20
• Adders tongue fern 1262
• Chimpanzee - 48
• Sand dollar 52
• Dog 78
• Cat - 32

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• Body cells contain the diploid (2n) number of
  chromosomes 2 chromosomes of each kind (1 from
  each parent)
• Sex cells (eggs and sperm) contain only 1
  chromosome of each kind haploid (n) number of
  chromosomes

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• Mitosis occurs in body cells diploid cells
  divide to produce diploid cells daughter cells
  are genetically identical to parent cells

http//www.cellsalive.com/mitosis.htm
Animation of mitosis
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Mitosis overview
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1. Centriole
2. Chromatin
3. Nucleolus (in yellow)
4. Nuclear membrane
5. Spindle fibers
6. Chromosome (replicated)
7. Centromere
8. Sister Chromatids (each half of replicated
   chromosomes)
9. Daughter Chromosomes (once the replicated
   chromosome splits)
10. Cell membrane
11. Cleavage furrow
12. Asters
13. Centrosome (aster centriole)

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Late Interphase

• Centrosomes (which contain pair of centrioles and
  an aster which are short microtubules)
  duplicate
• Chromatin condenses into chromosomes

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Early Prophase (sometimes referred to as prophase)

• Chromosomes become visible
• Centrosomes move to opposite ends of the cell
• Nucleolus disappears

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Late Prophase (sometimes referred to as
prometaphase)

• Nuclear membrane disappears
• Spindle fibers form
• Chromosomes become attached to spindle fibers
  centromere attaches to spindle fibers

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Metaphase

• Chromosomes line up at metaphase plate
  equidistant from poles

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Anaphase

• Centromeres holding sister chromatids divide
• Sister chromatids separate, becoming daughter
  chromosomes, and move toward opposite ends of cell

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Telophase

• Spindle disappears
• Nuclear membrane reappears
• Chromosomes turn into chromatin
• Nucleolus reappears

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• Mitosis in Plant Cells
• Same phases as in animal cells
• Have centrosome and spindle, but no centrioles or
  asters

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• Cytokinesis, or division of cytoplasm,
  accompanies mitosis.
• Cleavage of the cytoplasm begins in anaphase, but
  is not completed until just before the next
  interphase.
• Newly-formed cells receive a share of organelles
  made during interphase.

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Cytokinesis in Animal Cells

• A cleavage furrow (indentation of membrane where
  cell will divide) begins at the end of anaphase.
• A band of actin and myosin filaments, called the
  contractile ring, slowly forms a constriction
  between the two daughter cells.
• A narrow bridge between the two cells is apparent
  during telophase, then the contractile ring
  completes the division.

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Cytokinesis in animal cells
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Cytokinesis in Plant Cells

• The rigid cell wall surrounding plant cells
  cannot form a cleavage furrow.
• Instead, a cell plate forms from vesicles
  released by the Golgi apparatus (a part of the
  cell that processes proteins)
• New plant cell walls form and are later
  strengthened by cellulose fibers.

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Cytokinesis in plant cells
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Cell Division in Prokaryotes

• The process of asexual reproduction in
  prokaryotes is called binary fission.
• The two daughter cells are identical to the
  original parent cell, each with a single
  chromosome.
• Following DNA replication, the two resulting
  chromosomes separate as the cell elongates.
• Cell divides without cell structures seen in
  plants animals

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Animation of binary fission
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• Meiosis
• Produces sex cells (gametes) eggs sperm
• Reduces the chromosome number so that egg or
  sperm cells each have only one of each kind of
  chromosome (2n ? 1n).
• The process ensures that the next generation will
  have
• the diploid number of chromosomes
• a combination of traits that differs from that of
  either parent.

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Overview of meiosis
http//www.cellsalive.com/meiosis.htm
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• Meiosis involves two cell divisions and produces
  four haploid cells.
• Humans have 23 pairs of homologous chromosomes
  (chromosomes with the same genes), or 46
  chromosomes total.
• Prior to meiosis I, DNA replication occurs.

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• In many organisms, haploid daughter cells mature
  into gametes (sex cells eggs and sperm)
• Fertilization (fusion of egg and sperm) -restores
  the diploid number of chromosomes

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Phases of Meiosis

• The same four phases seen in mitosis prophase,
  metaphase, anaphase, and telophase occur during
  both meiosis I and meiosis II.
• The period of time between meiosis I and meiosis
  II is called interkinesis.
• No replication of DNA occurs during interkinesis

Animation
Meiosis square dance
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Meiosis I

• Prophase I
• Nuclear memebrane nucleolus disappear
• Spindle forms
• Homologous chromosomes pair during synapsis pair
  of homologous chromosomes are referred to as a
  tetrad
• Metaphase I
• Homologous chromosomes line up at metaphase plate

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• Anaphase I
• Homologous chromosomes separate move to
  opposite poles
• Telophase I
• Nuclear membrane and nucleolus reappear
• Cytokinesis occurs
• Interkinesis
• Period of time between meiosis I and II

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• Meiosis II
• Prophase II
• Spindle reappears, nucleolus and nuclear membrane
  disappear
• Chromosomes attach to spindle
• Metaphase II
• Chromosomes line up at metaphase plate

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• Anaphase II
• Sister chromatids separate, becoming daughter
  chromosomes
• Telophase II
• Spindle disappears, nuclear membrane and
  nucleolus reappear
• Cytokinesis divides the cells

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Genetic Recombination

• Genetic variation occurs in several ways
• Crossing-over of nonsister chromatids occurs
  during prophase I

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2) Independent assortment of homologous
chromosomes separate in a random manner 223
or 8,388,608 possible combinations of the 23
pairs of chromosomes
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• Combining of chromosomes of genetically different
  gametes during fertilization
• Between crossing over, combinations of gametes
  produced, and random combining of sperm and egg,
  variation is endless in the human population

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Meiosis vs. Mitosis

• Mitosis
• DNA replication occurs only once during
  interphase.
• One cell division
• Two diploid daughter cells genetically
  identical to parent

• Meiosis
• DNA replication occurs only once during
  interphase.
• Two cell divisions.
• Four haploid daughter cells genetically
  different from parent

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• Mitosis
• Daughter cells are identical to each other
• Occurs in all somatic cells for growth and repair

• Meiosis
• Daughter cells are different from each other
• Occurs only in the reproductive organs for the
  production of gametes

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Meiosis compared to mitosis
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• The Human Life Cycle
• Requires both mitosis and meiosis.
• In males, meiosis occurs as spermatogenesis and
  produces 4 haploid sperm.
• In females, meiosis occurs as oogenesis and
  produces 1 egg cell.
• Mitosis is involved in the growth of a child and
  repair of tissues during life.

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Life cycle of humans
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• Spermatogenesis
• Diploid primary spermatocytes undergo meiosis I
  to produce haploid secondary spermatocytes.
• Secondary spermatocytes divide by meiosis II to
  produce 4 haploid spermatids.
• Spermatids mature into sperm with 23 chromosomes.

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Spermatogenesis
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• Oogenesis
• Diploid primary oocyte undergoes meiosis I to
  produce one haploid secondary oocyte and one
  haploid polar body.
• Secondary oocyte begins meiosis II, stops at
  metaphase II, and is released from the ovary.
• Meiosis II will be completed only if sperm are
  present.
• Following meiosis II, there is one haploid egg
  cell with 23 chromosomes and up to three polar
  bodies.
• Polar bodies serve as a dumping ground for extra
  chromosomes will disintegrate.

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Oogenesis
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• In humans, both sperm cells and the egg cell have
  23 chromosomes each (1n)
• Following fertilization of the egg cell by a
  single sperm, the zygote has 46 chromosomes, the
  diploid number (2n) found in human somatic cells.
• The 46 chromosomes represent 23 pairs of
  homologous chromosomes.
• Cell differentiation occurs during development
  resulting in a variety of cell types

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• Control of the cell cycle
• Regulated by proteins called cyclins which bind
  to enzymes called cyclin-dependent kinases
• Different combinations of these at different
  stages of the cell cycle control different
  activities

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• Three checkpoints
• During G1 prior to the S stage if DNA is
  damaged, apoptosis occurs
• During S and G2 stage will not proceed if DNA
  is damaged or not copied
• During the M stage prior to the end of mitosis
  if chromosomes are not properly aligned

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• Abnormal cell cycle
• Cancer the uncontrolled growth and division of
  cells
• Results in a tumor, an abnormal mass of cells.
• Cancer cells crowd out normal cells

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• Carcinogenesis, the development of cancer, is a
  gradual process could take decades.
• Apoptosis - programmed cell death
• Angiogenesis - the formation of new blood vessels
  to bring additional nutrients and oxygen to a
  tumor cancer cells stimulate angiogenesis
• Metastasis - the invasion of other tissues by
  establishment of tumors at new sites

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• A patients prognosis is dependent on the degree
  to which the cancer has progressed
• Whether tumor has invaded surrounding tissues
• Whether there is any lymph node involvement
• Whether there are metastatic tumors in distant
  parts of body
• Early diagnosis and treatment is critical to
  survival.

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Information on staging of cancer
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• Causes of cancer
• Mutations in segments of DNA that control the
  production of proteins, such as those that
  regulate the cell cycle
• Environmental factors such as carcinogens
  agents that can cause cancer
• Ex radiation, tobacco, chemicals