9 — Cell division

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9 — Cell division Powered By Docstoc

9 — Cell division

I. Why is cell division necessary?

      A. How can an organism grow?
           1. could make each cell bigger — but can’t — why not?
           2. so need more cells
      B. How are damaged cells replaced?
           need more cells
      C. How are new organisms made?
           need more cells

      D. Remember, all cells must come from pre-existing cells (cell theory)
            So cells must divide to make more cells:
            1. for organisms to grow
            2. to replace damaged cells
            3. reproduction

      E. Generalized cell cycle
            1. cell grows by adding more cytoplasm
            2. cell replicates (makes a copy of) its DNA
            3. cell divides, one copy of DNA to each new cell

II. Types of cell division
      A. Prokaryotes (bacteria):
            1. no “growth” because they are single-celled
            2. reproduce by asexual reproduction (no change of DNA)
            3. Fission or budding (one parent only!)
            4. each cell is a clone (identical DNA)

      B. Eukaryotes
            1. Growth, replacement, and asexual reproduction
                    a. achieved by “somatic” (somatic means body) cell division
                    b. process called mitosis
                    c. result is a somatic cell (e.g. skin cell, blood, cell, liver cell, etc.)
                    d. each cell is a clone (identical DNA)

             2. Sexual reproduction
                    a. achieved by “germ (line)” cell division
                    b. process called meiosis
                    c. result is gametes (e.g. egg and sperm)
                    d. cells are genetically different from each other (not clones)


III. The cell cycle

      A. Each cell goes through a cycle which culminates in cell division

      B. Cell cycle length
             1. same for a cell type (e.g. all blood cells about 90–120 days)
             2. varies between cell types (e.g. intestinal lining about 3 days only)
             3. can be shorter than 12 hours, longer than 100 days

      C. Interphase (between mitosis)
             1. G1 (= gap)
                    a. growth and differentiation
                    b. synthesize carbohydrates, lipids, and proteins
                    c. G0: nondividing phase (“holding pattern”)
                    d. point of no-return is late in G1
                    e. variation in length of cell cycle due to length of G1 only!
             2. S (= synthesis): chromosomal DNA is replicated (2 copies now)
             3. G2
                    a. more growth and differentiation
                    b. prepare for cell division

      D. M phase
            1. includes mitosis: separation of chromosomes
                   a. replicated chromosomes separate (so each new cell gets a copy)
                   a. chromosomes move to opposite sides of cell
            1. and cytokinesis: separation of cytoplasm
                   a. cytoplasm divides to make two “daughter” cell
                   a. divided chromosomes go to each daughter cell

      E. Importance of cell cycle control
            1. normally there is a mechanism to keep cells from dividing too often
            2. if mechanism gets “damaged”, then cells divide out of control:
                   get cancer!!!

IV. Chromosomes

      A. DNA
           1. 2 polymers of nucleotides (bases: A, G, C, T) which connect by base pairing
                  a. A pairs with T (2 hydrogen bonds)
                  b. C pairs with G (3 hydrogen bonds)
           2. two strands twist into “double helix”
           3. DNA codes for proteins
           4. but not all DNA codes, some is “non-coding” (“junk”)
           5. Coding region is called a “gene” (so a gene codes for a protein)


       6. Variations in a gene may code for variations in a trait
              a. e.g. eye color gene may cause blue eyes vs. brown eyes
              b. variant genes are called alleles
              c. Alleles always appear at the same location on the same chromosome

B. Chromosomes are made of DNA with associated proteins called histones
     1. DNA winds around a cluster of histones like thread on a spool
     2. histones are + charged, DNA is –, so they stick together
     3. in “condensed form” of chromosome, histones coil to form a cylinder
     4. cylinders coil and loop to form chromosomes

C. In duplicated form (after DNA is replicated in S phase)
       1. 2 chromatids (sister chromatids) are identical
       2. chromatids are joined near center at centromere

D. Telomeres
      1. repetitive sequences at ends of chromosomes
      2. like the little plastic piece at end of shoelace? Prevents fraying?
      3. lose some each time cell divides
      4. cell “aging” (cell division stops) may be due to shortened telomeres
      5. in cancer, telomeres get replaced, so cells stay “young” and divide forever

E. Copy number of chromosomes per cell

       1. Diploid state (2n)
              a. Two (a pair) of each chromosome
              b. each pair has corresponding set of genes
                      (but could have different alleles)
              c. corresponding pairs called homologous chromosomes (homologues)
              d. one from father, one from mother
              e. Somatic cells are diploid, so are germ line cells

       2. Haploid state (n)
             a. only one of each chromosome
             b. could be from father or mother (random)
             c. gametes are haploid (e.g. egg, sperm)

       3. Total number of chromosomes varies between species
              a. from one to ?!
              b. Humans have 23 pairs, for a total of 46 in diploid state

F. Can visualize and identify chromosomes with a karyotype
      1. isolate condensed chromosomes during mitosis
      2. photograph stained chromosomes
      3. enlarge image, cut out
      4. pair up homologous chromosomes, arrange by size

       5. sizes and shapes of chromosomes differ!!
       6. centromere not always at middle


             7. in humans X chromosome is much bigger than the Y
                    a. males are XY
                    b. females are XX

V. Mitosis

     A. Review of cell structure
          1. nucleus contains DNA in discrete chromosomes
          2. nucleus is surrounded by nuclear envelope
          3. cytoplasm contains cytoskeleton
                  with microtubules (MT), microfilaments (MF), intermediate filaments (IF)

     B. Think of mitosis as a continuous process!

     C. Prophase

             1. (duplicated) DNA condenses into thick chromosomes
             2. microtubules, actin filaments in cytoplasm disappear
             3. nuclear envelope starts to disintegrate
             4. 2 pairs of centrioles (a.k.a. centrosomes) migrate to opposite “poles”
             5. MT form at centrioles and radiate to form spindle
                    a. remember, MTs are made of tubulin protein subunits
                    b. polymerize at a (+) end, depolymerize at a (–) end
                    b. (–) end can be protected from depolymerization by a MT organizing center (in
                         this case, the centrioles)
                    b. no centrioles in plants; not clear why
             6. randomly, MTs run into a chromosome, become attached to kinetochore
             7. each sister chromatid will have MT attached to a different pole

     D. Metaphase

             1. nuclear envelope is completely disintegrated
             2. kinetochores play tug-of-war, so chromosomes line up on equator
             3. Each chromosome lines up independently of its homologue

     D. Anaphase

             1. Sister chromatids separate
             2. one sister to one pole, other sister to other pole
                     a. kinetochores move along MT (like a train track) using motor proteins
                             (similar to cilia and flagella)
                     b. MT depolymerize behind moving chromosomes
             3. MT not attached to chromosomes slide past each other
                     a. motor proteins used here too
                     b. these MT do not depolymerize
                     c. causes cell to elongate


     D. Telophase

              1. chromosomes arrive at poles
              2. chromosomes decondense, MTs detach
              3. nuclear envelope re-forms around chromosomes

     G. Cytoplasmic division (a.k.a. cytokinesis)

              1. Animals
                    a. micro filaments parallel to plasma membrane
                    b. form ring around cell’s equator
                    c. MF slide past each other, cause cytoplasm to draw inward
                            (cleavage furrow)
                    d. cuts cell in two (daughter cells)
                    e. compare to drawstring of sweatpants

              2. Plants
                     a.   need to make a cell wall!
                     b.   vesicles which contain cell wall materials converge at equator
                     c.   fuse to make a “cell plate”
                     d.   separation of daughter cells
                     e.   build a new wall

     H. Interphase again

VI. Meiosis

     A. Cell division which enables sexual reproduction

     B. Strategy
            1. make gametes (haploid) from germ line cells (diploid) by meiosis
            2. combine gametes (fertilization forms zygote) to make diploid again
            1. zygote divides by mitosis to make multi-celled organism,
                   including germ line cells
            4. otherwise you’d always double the number of chromosomes at fertilization!

     A. Sexual vs. Asexual reproduction

              1. Asexual Reproduction
                    a. Mitosis: products are genetically identical
                    b. Advantages
                           1) only a single parent required
                           2) offspring will be just as successful as parents
                                   (assuming there is no change in the environment)
                           3) more offspring can be produced, and faster
                           4) requires less energy to reproduce
                           5) always works

       2. Sexual reproduction
              a. Meiosis: products are genetically different
              b. Advantage
                     1) Allows for adaptation to changes in the environment
                            by producing new combinations of traits
                     2) can get 2 favorable traits in one individual (1 from each parent)

D. Where gametes form from germ line cells

       1. plants
              a. eggs in ovary
              b. sperm in anthers
              c. may be on same or different flowers, same or different plants

       2. animals
              a. sperm in testes in male
              b. egg in ovary of female

E. Two stages of meiosis

       1. Meiosis I

              a. homologous chromosomes line up on opposite sides of equator
                    1) recall homologous chromosomes
                           — one from father, one from mother
                    2) somehow homologous chromosomes find each other
                    3) compare to all chromosomes lining up on equator in mitosis!
                    3) Lined up pairs are called tetrads (4 chromatids)
                    5) crossing over occurs here
                           a) exchange of DNA between maternal and paternal
                           b) leads to recombination: new combinations of alleles

              b. MT attached leads to only one pole (not both directions)
              c. so one of homologous pair goes to one pole,
                      other goes to other pole
              d. sister chromatids do not separate!
              e. Whole, duplicated chromosome goes to pole

              f. cytoplasm divides

              g. Resulting two cells are not identical
                    1) because lining up at equator was random
                    2) so some paternal chromosomes go to one cell, rest to other
                    3) how any one pair lines up is independent of other pairs:
                             =independent assortment


                           4) So then how many different combinations of chromosomes
                                  can be made?
                                  a) for each chromosome, 2 possible ways to line up
                                  b) so for n chromosomes, 2n combinations
                                  c) so for 2 chromosomes, 4 combinations,
                                          for 3, 8 combinations, etc.)
                                  d) for 23 chromosomes, 223 = over 8 million combinations!
                                  e) even more if you consider crossing over!!!

            2. Meiosis II
                  a. similar to mitosis, but only half the chromosomes now!
                  b. like mitosis, all chromosomes line up on equator
                  c. MTs attach to opposite pole, sister chromatids separate
                  d. sister chromatids go to opposite poles to make two new cells

     F. Results of Meiosis
           1. get 4 haploid cells from one diploid cell
           1. end up with 2 non-identical pairs of cells

VII. Production of gametes from meiosis
     A. Haploid cells formed by meiosis from diploid germ line cells are called gametes

     B. Male gamete is the sperm
           1. one germ line cell makes four sperm
           2. sperm are small, little cytoplasm
           3. donate only DNA at fertilization

     C. Female gamete is the egg (ovum)
           1. contributes cytoplasm as well as DNA
                   a. so mitochondria come from Mom!
                   b. remember, mitochondria have their own DNA
           2. to have enough cytoplasm, only 1 of 4 cells produced by meiosis becomes
                           an egg
                   a. that one egg takes all the cytoplasm
                   b. 3 smaller “polar bodies” unused

     D. At fertilization, 64 trillion (8 million x 8 million) different possible combinations
                     of the two human parents’ chromosomes!!!!!!

            1. not even counting crossing over!
            2. so no two kids will look exactly alike!!!
            3. (unless fertilized egg splits to make twins)


VIII. Life cycle in sexual reproduction

      A. animals
            1. In animals, two gametes, from mother and father (egg and sperm),
                   combine by fertilization to make a diploid zygote
            2. grows (by mitosis) to form a diploid multicelled organism, which then forms
                   new gametes (by meiosis) and the cycle continues

      B. Not all organisms use this kind of life cycle
            1. fungi and algae: multicelled organism is haploid
            2. plants:
                     a. alternation of haploid and diploid multicelled organisms
                     b. called alternation of generations life cycle
                     c. what we see is usually diploid (or even polyploid)

IX. Sources of genetic variablility
      A. Mutation produces different alleles
            1. mutations mean different DNA sequences in genes
            2. which produce different amino acid sequences in proteins
            3. so get different versions of traits

      B. Independent assortment during meiosis I produces novel combinations of alleles
             1. because lining up of homologous pairs at equator is random
             2. 2n combinations
             3. humans: 8 million combinations of mom and dad’s chromosomes

      C. Crossing over produces novel combinations of alleles
            1. exchanges pieces of mom’s and dad’s chromosomes during meiosis I
            2. makes combinations of alleles infinite

      D. Fertilization produces more novel combinations of alleles
             1. mom’s gamete and dad’s gamete join
             2. 8 million (actually infinite) x 8 million (also infinite)
                     = 64 trillion (still infinite) combinations

      E. Remember, genetic variability is desirable because it allows adaptation to changes
            in the environment!!