III. The Eukaryotic Cell Cycle and Mitosis.
A. The large, complex chromosomes of eukaryotes duplicate with each cell division .
1. Whereas a typical bacterium might have 3000 genes, human cells, for example, have 50,000-100,000.
2. The majority of these genes are organized into several separate, linear chromosomes that are found inside the nucleus.
3. The DNA in eukaryotic chromosomes is complexed with protein in a much more complicated manner. This organizes and allows expression of much greater numbers of genes.
4. During the process of cell division, chromatin condenses and the chromosomes become visible under the light microscope.
5 . In multicellular plants and animals, the body cells (somatic cells) contain twice the number of chromosomes as the sex cells. Humans have 46 chromosomes in their somatic cells and 23 chromosomes in their sex cells. Different species may have different numbers of chromosomes.
6 . The DNA molecule in each chromosome is copied prior to the chromosomes' becoming visible.
7 . As the chromosomes become visible, each is seen to be composed of two identical sister chromatids, attached at the centromere.
B. The cell cycle multiplies cells .The result of this process (more or less) is two daughter cells that are genetically identical to each other and to their parental cell.
1 . Most cells in growing, and fully grown, organisms divide on a regular basis (once an hour, once a day), although some have stopped dividing. This process allows organisms to replace worn-out or damaged cells.
2 . Such dividing cells undergo a cycle, a sequence of steps that is repeated from the time of one division to the time of the next.
3 . Interphase represents 90% or more of the total cycle time and is divided into Gl, S, and G2 subphases.
4. During Gl, the cell increases its supply of proteins and organelles and grows in size.
5. During S, DNA synthesis (replication) occurs.
6. During G2, the cell continues to prepare for the actual division, increasing the supply of other proteins, particularly those used in the process.
7. Cell division itself is called the mitotic phase (it excludes interphase) and involves two subprocesses, mitosis (nuclear division, the M phase) and cytokinesis (cytoplasmic division).
8. The overall result is two daughter cells, each with identical sets of chromosomes.
9. Mitosis is very accurate. In yeasts, one error occurs every 100,000 divisions.
C. Cell division is a continuum of dynamic changes.
1 . Interphase: duplication of the genetic material; ends when chromosomes begin to become visible.
2. Prophase (the first stage of mitosis): mitotic spindle is forming, emerging from centrosomes (also known as microtubule-organizing centers. Prophase ends when the chromatin has completely coiled into chromosomes; nucleoli and nuclear membrane disperse.
3 . Metaphase: spindle fully formed; chromosomes are aligned single file with centromeres on the metaphase plate (the plane that cuts the spindle's equator).
4 . Anaphase: chromosome separation, from centromere dividing to arrival at poles.
5 . Telophase: the reverse of prophase.
6 . Cytokinesis: the division of the cytoplasm. Usually, but not always,
accompanies telophase.
D. Cytokinesis differs for plant and animal.
1 . In animals, a ring of microfilaments contracts around the periphery of the cell, forming a cleavage furrow that eventually cleaves the cytoplasm.
2. In plants, vesicles containing cell wall material collect among the spindle microtubules, in the center of the cell, then gradually fuse, from the inside out, forming a cell plate which gradually develops into a new wall between the two new cells. The membranes surrounding the vesicles fuse to form the new parts of the plasma membrane.
E. Anchorage, cell density, and chemical growth factors affect cell division.
1 . To grow and develop, or replenish and repair tissues, multicellular plants and animals must control when and where cell divisions take place.
2 . Most animal and plant cells will not divide unless they are in contact with a solid surface; this is known as anchorage dependence.
3 . Laboratory studies show that cells usually stop dividing when a single layer is formed and the cells touch each other. This density-dependent inhibition of cell growth is controlled by the depletion of growth factor proteins in masses of crowded cells.
F. Growth factors signal the cell-cycle control system
Cancer is a general term for many diseases in multicellular animals and plants involving uncontrolled cell division with the resultant tumor metastasizing
1. Cancer cells grown in culture are not affected by the growth factors that regulate density-dependent inhibition of cell division.
2. A malignant tumor consists of cancerous cells. These tumors metastasize. This is in contrast to benign tumors, which do not metastasize.
3. Cancers are named according to the tissue or organ of origin.
4. Usually, cancer cells do not exhibit density-dependent inhibition.
5. Some cancer cells divide even in the absence of growth factors.
6. Radiation and chemotherapy are two treatments for cancer. Radiation disrupts the process of cell division, and since cancer cells divide more often than most normal cells, they are more likely to be affected by radiation. Chemotherapy involves drugs that, like radiation, disrupt cell division. Some of these drugs-for example, taxol-target the mitotic spindle.
IV. Meiosis and Crossing Over.
A. Chromosomes are matched in homologous pairs
1 . In diploid organisms, somatic cells (non-sex cells), have pairs of homologous chromosomes. Homologous chromosomes share shape and genetic loci, and carry genes controlling the same inherited characteristics.
2. Each of the homologues is inherited from a separate parent.
3. In humans, 22 pairs, found in males and females, are autosomes. Two other chromosomes are sex chromosomes.
4. In mammalian females, there are two X chromosomes; in male mammals, an X and a Y.
B. Gametes have a single set of chromosomes
1. Adult animals have somatic cells with two sets of homologues (diploid, 2n).
2. Sex cells (gametes = eggs and sperm) have one set of homologues (haploid, n). These cells are produced by meiosis.
3. Sexual life cycles involve the alternation between a diploid phase and a haploid phase.
4. The fusion of haploid gametes in the process of fertilization results in the formation of a diploid zygote.
C. Meiosis reduces the chromosome number from diploid to haploid
1. An understanding of the cell cycle is needed for an understanding of meiosis.
2. Meiosis occurs only in diploid cells.
3. Like mitosis, meiosis is preceded by a single duplication of the chromosomes.
4. The overall result is four daughter cells, each with half the number of chromosomes.
5. Again, the process is dynamic but may stop at certain phases for long periods of time.
6. The process includes two consecutive divisions (meiosis I and meiosis 11).
7. The halving of the chromosome number occurs in meiosis I. The end result is two haploid cells, with each chromosome consisting of two chromatids.
8. Sister chromatids separate in meiosis II.
9. The end result is four haploid cells.
D. : A comparison of mitosis and meiosis (see your text for a comparison of mitosis and meiosis).
The following is a figure of meiosis I. Remember, there are 2 meiosis events. See your text.
E. Independent orientation of chromosomes in meiosis and random fertilization lead to varied offspring
1 . During prophase I of meiosis, each homologue pairs up with its "other." During this process, X and Y chromosomes behave as a homologous pair. This pairing of homologues is called synapsis.
2. When they separate at anaphase I, maternally (mother) and paternally (father) inherited homologues move to one pole or the other independently of other pairs.
3 . Given n chromosomes, there are 2 ways that different combinations of the half-pairs can move to one pole.
4. In humans, there are 223 ways of combining an individual's maternally inherited and paternally inherited homologues.
5 . Combining gametes into zygotes suggests there are 223 x 223 combinations in the zygote
.
F. Homologous chromosomes carry different versions of genes
1 - Simplified examples: coat color and eye color in mice.
2 . C (agouti = brown) and c (white) for different versions of the coat-color gene and E (black) and e (pink) for different eye-color genes.
G. Crossing over further increases genetic variability
1 . Crossing over is the exchange of corresponding segments between two homologues (sister chromatid exchange). The site of crossing over is called a chiasma.
2 . This happens between chromatids within tetrads as homologues pair up during synapsis (prophase 1).
3 . Crossing over produces new combinations of genes (genetic recombination).
4. Because crossing over can occur several times in variable locations among thousands of genes in each tetrad, the possibilities are much greater than calculated above. Essentially, two individual parents could never produce identical offspring from two separate fertilizations.
V. Alterations of Chromosome Number and Structure.
A. A karyotype is a photographic inventory of an individual's chromosomes
1. Alterations in chromosome number result in some of the most common genetic defects such as Down’s syndrome -- Trisomy 21--- the most common chromosome-number abnormality, occurring in about one out of 700 births.
The incidence of Down syndrome increases with the age of the mother .
Summary
Sexual life cycles produce genetic variation among offspring
Meiosis and fertilization are the primary sources of genetic variation in sexually reproducing organisms. Sexual reproduction provides genetic variation by:
∙ independent assortment;
∙ crossing over during prophase I of meiosis;
∙ random fusion of gametes during fertilization
A. Independent Assortment of Chromosomes
Independent assortment = The random distribution of maternal and paternal homologues to the gametes. (In a more specific sense, assortment refers to the random distribution of genes located or' different chromosomes.)
∙Since each homologous pair assorts independently from all the others, the process produces 2 n possible combinations of maternal and paternal chromosomes in gametes, where n is the haploid number.
∙In humans, the possible combinations would be 223, or about eight million.
∙Thus, each human gamete contains one of eight million possible assortments of chromosomes inherited from that person's mother arid father.
B. Crossing Over
Another mechanism that increases genetic variation is the process of crossing over, during which homologous chromosomes exchange genes.
Crossing over = The exchange of genetic material between homologues; occurs during prophase of meiosis I. This process:
∙Occurs when homologous portions of two nonsister chromatids trade places. During prophase I, X-shaped chiasmata become visible at places where this homologous strand exchange occurs.
∙Produces chromosomes that contain genes from both parents.
∙In humans, there is an average of two or three crossovers per chromosome pair.
C. Random Fertilization
Random fertilization is another source of genetic variation in offspring.
∙In humans, an egg cell that is one of eight million different possibilities will be fertilized by a sperm cell that is also one of eight million possibilities. Thus, the resulting zygote can have one of 64 trillion possible diploid combinations.
