Quiz Me!	Cell Division
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Last revised: Thursday, October 7, 1999
Reading: Ch. 12, 13 in text

Note: These notes are provided as a guide to topics the instructor hopes to cover during lecture. Actual coverage will always differ somewhat from what is printed here. These notes are not a substitute for the actual lecture!

Copyright 1999. Thomas M. Terry

Introduction

• It is important that cells only divide when appropriate. Usually cells are not dividing
• Cancers =cell anarchy. Individual cell keeps dividing, ignores communication
• Capacity for division is important: mechanism = MITOSIS Normally, cells divide to replace dead or missing cells
  • intestinal cells divide every 3 days, are broken down by digestion
  • blood cells last 3 months, are replaced by new cell division
  • nerve cells usually don't divide, last for life
  • Embryo: almost constant cell division. Every 30 minutes.
• Sex cells are unique: in humans, males produce 500,000,000 /day. Females 1/month. Special type of cell division = MEIOSIS

CELL CYCLE

PROKARYOTIC ORGANISMS (unicellular)

• Cells eat, grow, divide by binary fission
• View movie of dividing bacteria (550K)
• DNA organized into circular bacterial chromosome.
• Not complexed with histone proteins, does not condense into compact structure during division
• before division, single chromosome is replicated into two, separated to two sides of cell (by growth of membrane between attachment points?), pinch off, two new cells.

EUKARYOTIC ORGANISMS (unicellular or multicellular)

• Cells normally divide by mitosis
  • View movie of Mitosis
• Cells contain multiple chromosomes. In humans, 23 pairs of chromosomes = 46.
  • View karyotype of human chromosomes.
  • Practice creating a karyotype
• Chromosomes made of chromatin; DNA (long, string-like molecule) complexed with histone proteins (~50% DNA, 50% histones)
• chromatin organized into nucleosomes, "beads on a string".
  • View EM of DNA
  • View animation movie showing formation of a nucleosome (1037K)
• chromatin has the ability to condense into tightly packed structure (= heterochromatin).
• View nucleosomes folding into heterochromatin
• chromosome = most compactly folded state of chromatin, only visible by light microscope when fully condensed (during mitosis or meiosis); at other times, dispersed in nucleus
  
  
• Diagram above shows a metaphase chromosome, made up of two sister chromatids. Chromatids will separate in anaphase, one going to each cell.
• View movie showing chromosome organization and folding (Note: includes information on transcribing genes to be discussed later) (1312K)
• Chromatids held together at centromere; location of centromere is always characteristic for any chromosome.
• kinetochore = proteinaceous region that forms at both sides of centromere, binds to microtubules (spindle fibers) to pull sister chromatids apart.

Stages of cell cycle: G1, S, G2, M

• Once a cell has complete dividing, it enters interphase, the stage between divisions
• Interphase is "normal" cell life -- it is busy making proteins and other polymers, housecleaning, and doing whatever its special adaptations require of it.
• The stage following division (M), but preceding DNA synthesis (S) is called G1, or gap 1 phase. Actually, many cells sit in this phase for long periods of time, and some never leave it. Except for rapidly dividing cells, it makes more sense to think of this phase as Go, a point of exit from the cell cycle altogether.
• At some point, the cell may be "triggered" to begin preparing for another round of cell division. This critical step commits the cell to DNA synthesis, during which every chromosome is replicated. At this point the cell leaves G1 (or Go). This fairly long process is called "S" phase, for synthesis.
• Once S is completed, the cell continues to prepare for division, called the "G2", or gap 2 phase (gap between S and division).
• Once mitosis begins, the cell is in the "M" phase. Mitosis consumes most cell energy for its duration, so much normal cell activity ceases.
• The M phase terminates with cytokinesis, the physical separation of the two daughter cells.

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Stages of MITOSIS & CYTOKINESIS

• Work through Mitosis: An interactive study guide. from The Biology Place.
• View animation of mitosis from The Biology Place
• View slides of whitefish mitosis
• View slides of onion root tip mitosis

Interphase

• cell is not dividing
• cell is metabolically active
• nucleolus visible, ribosomes being made
• DNA duplicates (defines the S phase of interphase; before this is G1, after this is G2)
• nucleus intact

Prophase

• nucleolar material disperses
• centrosomes move to opposite ends of cells (in animal cells, centrioles involved)
• mitotic spindle forms -- set of microtubules running from each pole to approximate middle of cell, overlap with polar microtubules from other end
• chromatin condenses, chromosomes appear as pairs of identical sister chromatids, held together at centromere
• kinetochores appear at centromere region (one for each chromatid)
• chromosomes attach to kinetochore microtubules
• nuclear membrane disappears
• Review diagram of prophase

Metaphase

• microtubules from both poles manage to attach to kinetochores, pull chromosomes into a line at middle of cell = equatorial plate
• anatomy of metaphase chromosome: two identical chromatids, essentially two separate chromosomes, but still attached at centromere.
• Note: certain drugs (e.g. colchicine) block separation of chromosomes. All mitotic cells reach metaphase, stop = "metaphase arrest"
• Review diagram of metaphase

Anaphase

• chromosomes separate. Dynein protein involved (same protein used in ciliary contraction)
• very little energy needed; as little as 20 ATP's per chromosome can pull it from equator to pole of cell
• Review diagram of anaphase

Telophase

• chromosomes arrive at poles
• chromosomes decondense, return to tangled mass of chromatin
• spindle disappears
• nuclear membrane reappears
• nucleolus reappears, ribosome synthesis begins anew
• Review diagram of telophase

Cytokinesis

• daughter cells are pinched apart
• In plants: new cell membrane appears as vesicles, wall grows
• In animals: cells are "pinched apart" by contraction of microfilaments
• View movie showing cells undergoing cytokinesis (2138K)
• Time line for stages of mitosis/cytokinesis

CONTROL OF MITOSIS

1. Critical problem: how to tell cell to divide?
   • If too often --> cancer
   • If not often enough --> death
   • Cell cycle control points and cancer (from U. of Virginia)
2. Growth factors
   • one of "hot" areas of research today; made by genetic engineering
   • Special proteins produced in extremely tiny amounts stimulate growth
   • Some of these genes have been cloned ---> allows production of significant quantities, can use in research and clinical applications
   • Real life example: Ammonia splashed in eye of refrigeration worker. Coating of eye failed to heal, only see blurs. Treat with drops of growth factor, cause new cell division, sight restored quickly.
   • Several growth factors have been isolated, tissue specific. Nerve growth factor, fibroblast growth factor, etc.
3. Growth suppressors
   • also discovered a number of suppressors that block cell division.
   • Dozens of different proteins affect division, either positively or negatively.
   • How to understand?
   • Cells contain mechanisms to die as well as grow and divide. Cell death = apoptosis.
4. p53
   • In human cancers, p53 is most common gene mutation observed.
   • exact role of p53 in the cell cycle is unknown
   • p53 induces apoptosis and exerts cell cycle inhibition in G1 phase.
   • Mutations in p53 often --> altered proteins that bind and inactivate normal p53, leading to loss of regulation of cell cycle control.
   • The antitumor effect of p53 gene transfer is also correlated with a reduction in blood vessel density in tumors suggesting mediation in anti-angiogenesis mechanisms.
   • Numerous experiments have been performed to transfer the p53 gene to cells and tissues via lipid carriers and viral mediated methods. Some success in reducing
5. Other genes involved in cancer
6. Positive and negative regulators of growth and division
   • recent theory: process of cell proliferation tightly linked with mechanism of cell death = `Two Signal: Death/Survival Model.'
   • cell that departs from interphase will either divide or die with roughly equal probability.
   • signals such as growth factors have the ability to influence this `decision' and thus promote the growth of tissues.
   • Prediction: at least two signals must cooperate in cell transformation into cancerous cell -- one signal drives cells out of rest toward either cell proliferation or cell death, and one signal blocks cell death.
   • recent discovery: c-myc, a gene known to be involved in cell proliferation, can participate in apoptosis (active cell death)
   • another protein, bcl-2, known to cooperate with c-myc in cell transformation into cancer, blocked c-myc-induced apoptosis.
   • Genes involved in cancer (from U. of Virginia)

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SEXUAL REPRODUCTION

Gametes = sex cells produced in special organs
Male: sperm in testicles (anthers in plants) ---> sperm or pollen
Female: eggs in ovaries

Meiosis

• unique type of cell division
• basically similar to mitosis, except:
  • only produces gametes
  • # of chromosomes reduced by half:Ex. Humans 46 -> 23
  • requires two successive divisions without intervening DNA replication (no S phase)
  • involves gene recombination ---> each gamete is genetically unique
  • Work through Tutorial on meiosis from The Biology Place

Stages of Meiosis

• see text Fig. 13.5 for further explanation
• involves 2 successive cell divisions Meiosis I & II -> 4 daughter cells, not 2
• superficially similar to mitosis: each division involves formation of spindle, progression through prophase, metaphase, anaphase, and telophase.
• But some differences, noted below:

prophase I

• homologous chromosomes pair up (synapsis)
• crossing over of segments -> new chromosome combinations diff. from parents. Morphologically, this looks like an X-shaped form, called chiasma (pl. chiasmata).
• 1st division pulls homologues apart (doesn't separate at centromeres)
• View animation of prophase I from The Biology Place

metaphase I

• separates the maternal and paternal chromosomes
• note: the centromeres do not split; so what gets pulled to each pole of the cell is a chromosome that looks like a mitotic metaphase chromosome (with paired sister chromatids)

telophase I

• Note: in some organisms, no real telophase; 2nd division follows immediately.
• No DNA synthesis (S phase) preceding meiosis II
• View animation of Meiosis I from The Biology Place

metaphase II

• this division looks just like metaphase of mitosis, except there are only half as many chromosomes. In mitosis, there would be both a maternal and a paternal chromosome. In meiosis II, only one of these per cell.
• View animation of Meiosis II from The Biology Place

Results of Meiosis

• Each gamete is different from others
• One reason for diversity: because maternal and paternal chromosomes sorted randomly; e.g. one human egg cell might have chromosome #1(M), 2(M), 3(P), 4(M), 5(P), 6(P), .... 23(M); a second egg might have #1(M), 2(P), 3(P), 4(P), 5(M), 6(M), .... 23(P). Note that each chromosome has 50% chance of being from father (P) or mother (M).
• What is chance that any two sex cells will be identical? 1 in 2 raised to the 23rd power = 1 in 8,388,608. So if someone tells you "you're 1 a million", you should answer "No, I'm 1 in 8 million" (more or less) ;-)
• A second reason for this diversity = "Shuffling" of chromosomes. Each synaptic pair of homologous chromosomes undergoes recombination with random breakage and joining -- so the actual chances of finding two gametes with identical chromosomes is not 1 in 8+ million, more like 1 in a trillion or less.

Meiotic errors

• Process of meiosis is incredibly complex; involves forming 23 homologous pairs, pulling them apart after chromosomes have been cut and pasted back together
• Mistakes can occur, and do occur.
• Nondisjunction: failure of one synaptic pair of chromosomes to separate during anaphase I. Results in one cell with 1 extra chromosome, 1 cell with one missing chromosome.
• Visit the Cytogenetics Gallery and explore some common results of nondisjunction.
• Note: nondisjunction is relatively common. Estimated that 1 in 5 normal human pregnancies spontaneously abort in first two months, due to fertilized egg having too many or too few chromosomes.
• Common error is failure of a pair of chromosomes to separate during anaphase I, resulting in gamete with 24 or 22 chromosomes.
• When fertilized by normal gamete of opposite type, get 47 or 45 chromosome. For one of these chromosomes, will have 2N + 1N ---> 3N in embryo (trisomy).
  • Trisomy example: Down syndrome (extra copy of chromosome 21)
  • View karyotype (chromosome spread)
  • Visit Down Syndrome WWW page
• If gamete lacks a chromosome (0N) , is fertilized ---> 1N in embryo (monosomy).
• Very rare for successful pregnancy to result after nondisjunction -- only with smallest chromosomes or sex chromosomes. Even then, almost always leads to developmental abnormalities, some degree of mental retardation. E.g. Down syndrome, Turner's syndrome, etc.
• View statistics on frequency of chromosomal abnormalities
• Translocation: piece of one chromosome becomes attached to another chromosome. Probably results from error during crossing over. Can also produce Down syndrome, and many other errors.

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