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UNIT 2:  GENETIC CONTINUITY

Mitosis and Meiosis

B.  Meiosis

·         mitosis ensures that the offspring cells have the exact same number of chromosomes as the parent cell – this is to maintain a certain species characteristic number, called a diploid number

·         mitosis occurs in all cells of the body, called somatic cells

·         when special sex cells, called gametes, fuse to form an offspring cell, called a zygote, the chromosome number is crucial

·         this is because the offspring must have the diploid number of chromosomes, as found in either of the parents, in order to be viable and to survive

·         therefore, each sex cell must have exactly one-half the information – called haploid -- so that when it fuses with the other sex cell, the zygote will have the complete set of chromosomal information

·         to prevent a doubling of chromosomal information in the zygote, a process called meiosis occurs

1.  Chromosome Number and Structure

·         the diploid number found in human somatic cells is 46 – this number is designated 2n

·         the 46 chromosomes are arranged as 23 pairs of homologs – each homolog coming from each parent, thus 23 chromosomes constitute the maternal set, and 23 chromosomes constitute the paternal set

·         each homolog carries information for the same hereditary traits as its partner

·         the information for any specific trait is in the form of a gene – a heredity unit which is in fact, a length of DNA

·         each gene has a particular position or locus on a chromosome

·         each homolog of the pair carry genes for the same trait at the same locus

·         there are usually at least two forms of the same gene – each form is called an allele

·         each homologous pair of chromosomes carries genetic information for thousands of hereditary traits

·         homologous pairs of chromosomes are matched together by their lengths, centromere position, and the patterns of banding that result from staining the chromosomes at the metaphase stage of mitosis

·         Figure 5.15, p. 132, illustrates the characteristics of homologous chromosomes in a colchicine metaphase preparation

·         Figure 5.16, p. 132, relates the roles of meiosis and mitosis in our life cycle – the diploid phase is shaded in blue

·         when fertilization occurs two haploid gametes, the egg or ovum (female) and sperm (male), join to make a single diploid cell called the zygote, the first cell of the new organism

·         the next step is mitosis and differentiation

·         the zygote contains one haploid set of chromosomes from the father, and one from the mother

·         therefore, even though the zygote contains two forms of the same gene (i.e. two alleles) in its genetic makeup, the individual organism will possess the characteristics dictated by only one of the forms

·         both forms must be present to ensure viability in the zygote

2.  Meiosis Stages

·         meiosis ensures that the zygote possesses the right number of chromosomal information after fertilization

·         it occurs in the reproductive tissues of sexually reproducing organisms

·         for humans, sperm are produced by specialized cells found in the testicles called spermatogonia, and eggs are produced by specialized cells found in the ovaries called oogonia

·         for plants, pollen is the male sex cell, and ovules are the female sex cell

·         meiosis has two parts to it:  meiosis I, and meiosis II

·         for a meiosis tutorial click on http://www.biology.arizona.edu/cell_bio/tutorials/meiosis/page3.html, or http://www4.ncsu.edu/unity/users/b/bnchorle/www/index.htm

·         for a mitosis review and a meiosis tutorial click on http://www.biology.iupui.edu/biocourses/N100/2k2ch9meiosis.html

Meiosis I

·         the genetic material has already been replicated before meiosis I in the pre-meiotic S phase

·         after the chromosomes thicken and become more visible, the homologous pairs come together (side by side) in a process called synapsis (see Figure 5.18, p. 134 – just before crossing over takes place)

·         at the synapsis point, the homologous pair of chromosomes form what is called a tetrad – meaning “four strands”

·         soon after the tetrad is formed, certain unpredictable parts of each chromatid strand of each homolog, cross over with sections of the chromatid strands of the other homolog – it looks like the homologs are tangled up (see Figure 5.17, p. 133)

·         the points where the chromatid strands of each homolog cross over each other are called chiasmata

·         after the homologous chromosomes become “untangled” each has obtained a section or “piece” from the other

·         this breakage and reunion allows chromosomes to exchange genetic material

·         the process is called genetic recombination or crossing over because at the point of recombination, the genetic material on the chromosome crosses from one homolog to the other

·         this process allows for greater genetic variation in a population of a species

·         other than this process, the events that take place in prophase I of meiosis I are the same as those that occur in prophase f mitosis – nuclear membrane disappears, nucleolus disappears, centrioles migrate to opposite poles of the cell, etc.

·         at the end of prophase I, the tetrads move to the equator of the cell

·         at metaphase I, the tetrad orients itself such that each homolog of the pair is prepared to move to opposite sides of the cell

·         during anaphase I, each homolog is pulled along the spindles to opposite sides of the cell

·         at the end of telophase I of meiosis I, there are two daughter cells, each with half the amount of genetic information (n) – thus meiosis I is often called the reduction division stage of meiosis

Meiosis II

·         after a brief, and sometimes lacking, interphase, the second stage of meiosis begins – meiosis II

·         this process looks identical to mitosis, except that it begins with half the genetic material of mitotic cells

·         at the end of prophase II, each chromosome, made up of two chromatids, lines up at the middle of the cell

·         during anaphase II, the centromere of each chromosome splits, and each chromatid is pulled to opposite ends of the cell, along the spindle fibres

·         telophase II of meiosis II, re-establishes the conditions just before prophase I

·         by the end, each diploid (2n) cell produces four haploid (n) daughter cells, each containing genetic information that mayor may not be similar to each parent

·         Figure 5.18, pp. 134-135, illustrates the process of meiosis

·         for a meiosis I animation click on http://www.biology.arizona.edu/cell_bio/tutorials/meiosis/meiosis1_movie.html, and a meiosis II animation click on http://www.biology.arizona.edu/cell_bio/tutorials/meiosis/meiosis2_movie.html

3.  The Variation Contributors:  Random Assortment of Chromosomes and Crossing Over

·         during the later stages of prophase I, the tetrads make their way to the middle of the cell

·         there are many different arrangements/orientations in which the tetrad can position itself at the equator of the cell

·         it is not completely necessary for the paternal homologs to end up on one side of the cell, and the maternal homologs to end up on the other side

·         for humans, for example, each of the 23 tetrads stay together, and move to the middle of the cell together up until the separation occurs, but the assortment and orientation of each tetrad is independent of any of the other tetrads – there is random assortment of the chromosomes (see Figure 5.19, p. 136)

·         because maternal and paternal sets of chromosomes do not need to stay together, humans with a diploid number of 46, that is 23 pairs of chromosomes, can produce 2²³ or 8 388 608 different combinations of chromosomes

·         any one of these 8 388 608 different combinations in an egg could come together with any one of the 8 388 608 possible combinations in a sperm

·         a second way that meiosis produces variability is from crossing over or genetic recombination as a result of breakage and reunion of chromatids during prophase of meiosis I

·         crossing over may occur at different points along the chromosomes – therefore, each chiasma will affect different genes

·         the number of possible gene combinations that result from genetic recombination is extremely large, which is added to those that are possible as a result of independent assortment

·         this large potential in variability explains why some siblings look very much alike while others look very different

·         for a crossing over animation click on http://www.people.virginia.edu/~rjh9u/meiosisx.html, and for an animation of independent assortment click on http://www.people.virginia.edu/~rjh9u/meiosis1.html

4.  The Gametes

·         at the end of meiosis in males, four functional spermatids are produced, which in turn undergo differentiation to become sperm cells – a head containing 23 chromatid strands, a flagellum for locomotion, and a middle portion loaded with mitochondria to make ATP for the whiplash movement of the tail

·         sperm are very small compared to egg cells (see Figure 5.20, p. 137, for a comparison)

·         the events leading up to the first division of meiosis are the same in both male and female gametogenesis – what’s different are the events occurring in the cytoplasm

·         the first division of the cytoplasm in female meiosis is unequal, producing one large cell, called a secondary oocyte, and one small cell, called the first polar body, which may divide once more and then deteriorate

·         in meiosis II, the secondary oocyte divides unequally once again, producing one large functional egg cell, and another small cell, called the secondary polar body which also deteriorates

·         the only contribution that the sperm makes to the zygote are the 23 chromatid strands – the egg contributes the other 23 chromatids, all the cytoplasmic organelles, and the rest of the cytoplasm with its contents

·         this is why the egg is much larger than the sperm – to ensure that the zygote has enough cytoplasm and nutrients for its development following fertilization

·         Figure 5.22, p. 138, illustrates the difference between male and female meiosis

5. Meiosis Mistakes

·         there are two possible sources of error that can occur in meiosis:  a mistake can happen when the chromosomes separate during division, or a mistake can happen when an incorrect exchange of genetic information occurs during chiasma formation

·         the two results of such mistakes are:

1.  ABNORMAL CHROMOSOME NUMBER

·         if an egg or sperm cell contain a number of chromosomes other than the haploid number due to a nondisjunction, then the zygote will either have too few or too many chromosomes – a condition called aneuploidy

·         nondisjunction is when the homologous chromosomes fail to move apart properly during meiosis I, or the sister chromatids do not separate during meiosis II

·         if fertilization involves a sex cell that is missing a chromosome, the resulting child will have only one copy of a particular chromosome, a condition known as monosomy

·         if fertilization involves a sex cell that has an extra chromosome, the resulting child will have three copies of a particular chromosome, a condition known as polysomy

·         both cases may result in a viable offspring, but the child will show effects associated with the genetic information carried on the chromosome involved in the aneuploidy

·         Down syndrome is an example of polysomy that results from an extra chromosome 21

·         Figure 5.24, p. 140, illustrates the effects of nondisjunction

·         in most animal gametes for example, if all of the chromosomes did not “disjoin” during the first or second meiotic division, and this type of gamete combined with another normal haploid gamete, the zygote would be considered polyploidy or triploid (3n), and would most likely die before complete development

·         in plants, polyploidy is not uncommon

·         in a plant called the evening primrose, 100% nondisjunction occurs that results in diploid gametes

·         this results in a diploid gamete fertilizing another diploid gamete to make a viable, and perfectly normal, tetraploid (4n) offspring, which in turn develops normally and reaches sexual maturity

·         these perfectly normal tetraploids can only self-fertilize or mate with other tetraploids

·         if a tetraploid mates with a diploid, for example, the resulting offspring is triploid, which may be viable, but is completely infertile – it cannot produce normal gametes

·         this means that the original diploid organism, and the new triploid organism can co-exist but they cannot produce fertile offspring, making them a different species

·         the ultimate effect is that a new species is formed – this process is called sympatric speciation

2.  ABNORMAL CHROMOSOME STRUCTURE

·         when homologous chromosomes exchange information incorrectly during the crossing over process of meiosis, the resulting sex cells, and ultimately the zygote produced form fertilization involving these cells, will have the correct number of chromosomes, but the genetic information may be altered or the chromosomes may contain duplications, deletions, inversions, or translocations of genetic material

·         a deletion is when a chromosome fails to reattach to the homolog and is somehow lost

·         a duplication is if this “lost” segment reattaches to a complete homolog

·         if the reattachment occurs to the correct homolog, but is in the reverse order, the alteration is called an inversion

·         if the reattachment occurs to a nonhomologous chromosome, the alteration is called a translocation

·         if fertilization involves a gamete that has one of these four alterations, the child produced may exhibit a wide range of symptoms depending on the severity of the genetic alteration and the specific genetic information involved

·         Figure 5.25, p. 141, illustrates each of the four kinds of alterations that can occur to chromosomes

·         for an example of nondisjunction at the 21^st chromosome – Down’s syndrome, click on http://www.tokyo-med.ac.jp/genet/anm/domov.gif, for a deletion illustration click on http://www.accessexcellence.com/AB/GG/deletion.html, for a duplication illustration click on http://www.accessexcellence.com/AB/GG/duplication.html, for a translocation illustration click on http://www.accessexcellence.com/AB/GG/translocation.html, and for an insertion illustration click on http://www.accessexcellence.com/AB/GG/translocation.html, and for an overview of all types of abnormal chromosomal structure click on http://csep10.phys.utk.edu/klug_instructorCD/biology/ch7/animations/mod07_01.swf

6.  The Classic Comparison – Meiosis vs. Mitosis

Homework:         1-10, p. 142