Chromosomes, Mitosis, and Meiosis. INTRODUCTION With few exceptions, nearly all the cells in your body contain 23 pairs of chromosomes. One of the pair you received from your mother; the other from your father. Although these chromosomes may carry slightly different information (say, brown eyes from your mother and blue from your father), the genes on these HOMOLOGOUS chromosomes affect the same traits (eye color in this example). Cells that carry a full set of paired chromosomes are said to be "DIPLOID". This term can also be used to describe the number of chromosomes in the cells of a particular organism. Our diploid (or 2N) number is 46 while that of the fruit fly is eight. MITOSIS permits a cell to make an exact genetic copy of itself during cellular division. This process insures that no cell in your body that requires both maternal and paternal genes is lacking either. Nearly all the cells of your body that are capable of division do so by the process of mitosis. While cell division is used by multicellular organisms to increase in size and repair wounds, it is used by one-celled organisms in their reproduction. Sexual reproduction with its need for eggs and sperm complicates the matter somewhat since it is not possible to produce GAMETES through mitosis alone. If, for example, both your mother and father made their sex cells by mitosis, their eggs and sperm would each contain 46 chromosomes. If such a egg were fertilized by a diploid sperm, the result would be a tetraploid ZYGOTE (and the offsprings cells would have a total of 92 chromosomes). Ten generations down the line, each of your great- great-great-great-great-great-great-great-great grandchildren would have cells with 94208 chromosomes and by 20 generations each cell would have over 96 million chromosomes. Obviously, this isn't going to work and so there must be some mechanism to insure that gametes receive only half of the diploid number (e.g., HAPLOID gametes are produced). MEIOSIS fulfills this need by starting with diploid cells and then distributing the chromosomes so that half of each homologous pair is deposited in the gametes. As you shall see, meiosis is essentially mitosis twice over. It also differs from mitosis in that four cells can potentially be produced from an original stem cell (while mitosis produces only two).
Welcome message from author
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Chromosomes, Mitosis, and Meiosis.
INTRODUCTION
With few exceptions, nearly all the cells in your body contain 23 pairs of chromosomes. One of the pair you received from your mother; the other from your father. Although these chromosomes may carry slightly different information (say, brown eyes from your mother and blue from your father), the genes on these HOMOLOGOUS chromosomes affect the same traits (eye color in this example). Cells that carry a full set of paired chromosomes are said to be "DIPLOID". This term can also be used to describe the number of chromosomes in the cells of a particular organism. Our diploid (or 2N) number is 46 while that of the fruit fly is eight. MITOSIS permits a cell to make an exact genetic copy of itself during cellular division. This process insures that no cell in your body that requires both maternal and paternal genes is lacking either. Nearly all the cells of your body that are capable of division do so by the process of mitosis. While cell division is used by multicellular organisms to increase in size and repair wounds, it is used by one-celled organisms in their reproduction. Sexual reproduction with its need for eggs and sperm complicates the matter somewhat since it is not possible to produce GAMETES through mitosis alone. If, for example, both your mother and father made their sex cells by mitosis, their eggs and sperm would each contain 46 chromosomes. If such a egg were fertilized by a diploid sperm, the result would be a tetraploid ZYGOTE (and the offsprings cells would have a total of 92 chromosomes). Ten generations down the line, each of your great-great-great-great-great-great-great-great-great grandchildren would have cells with 94208 chromosomes and by 20 generations each cell would have over 96 million chromosomes. Obviously, this isn't going to work and so there must be some mechanism to insure that gametes receive only half of the diploid number (e.g., HAPLOID gametes are produced). MEIOSIS fulfills this need by starting with diploid cells and then distributing the chromosomes so that half of each homologous pair is deposited in the gametes. As you shall see, meiosis is essentially mitosis twice over. It also differs from mitosis in that four cells can potentially be produced from an original stem cell (while mitosis produces only two).
CENTRIOLE NUCLEUS
CHROMOSOMES
METAPHASE
PLATE
SPINDEL
FIBERS
ASTER CYTOKINESISCONDENSED
Figure 1. The five stages of mitosis and some representative structures are indicated.
EXERCISE An Introduction to Mitosis.
PURPOSE: To introduce the process of mitosis to the student.MATERIALS NEEDED:
Models and wall charts of mitosis. "Flip book" page Shears or paper cutter.
As mentioned in the introduction, mitosis allows cells to make an exact genetic replicate of themselves during cell division. This process has is divided into a cycle of five stages (Fig 1). During INTERPHASE the chromosomes are metabolically active and the DNA is not visible as discrete chromosomes. The DNA is also replicated during this phase in preparation for cell division. To differentiate between the non-replicated and replicated chromosomes during interphase, this stage has been sub-divided into G1 ("growth 1"), SYNTHESIS, and G2 ("growth 2") phases (Fig 2). During the G1 portion the cell is metabolically active and busy with processes such as protein synthesis. In preparation for mitosis, DNA is replicated during the synthesis portion of interphase. Synthesis is then followed by a G2 phase.
Interphase is followed by PROPHASE. Chromosomes condense and become visible, the nucleolus disappears, the nuclear envelope disintegrates, and the MITOTIC APPARATUS (SPINDLE FIBERS, ASTERS, and CENTRIOLES) appear. By METAPHASE the chromosomes have lined up single file along the METAPHASE PLATE (or EQUATORIAL PLANE). They then move to opposite poles of the cell (ANAPHASE); finally terminating with TELOPHASE (characterized by CYTOKINESIS- cell division). Several differences exist between plant and animal cells- consult your text for details.
ANAP
HASE
ME
TAP
HA
SE
ERPH
A SYNTHES
GG 1 2
Figure 2. The cell cycle. Interphase is divided into a three sub-stages: G1 ("growth 1"), SYNTHESIS, and G2 ("growth 2"). The remaining phases of mitosis and their relative timing are also indicated in this diagram.
PROCEDURE:1- Examine the demonstration models and wall charts and identify each of the stages of mitosis using figures 1 and 2 as a guide. Make sure you can recognize the NUCLEUS, NUCLEOLUS, CONDENSED CHROMOSOMES, CENTROMERES, CENTRIOLES, SPINDLE FIBERS, ASTERS, and CLEAVAGE FURROW or CELL PLATE as appropriate for the phase of mitosis and type of cell (plant or animal). Memorize these stages in order and the structures that are visible in a plant and animal cell. Familiarity with this process is absolutely necessary for your understanding of meiosis and genetics. 2- Mitosis Animation. Remove the "flip book" diagram from your laboratory manual. Each picture depicts a portion of the mitotic process in an animal cell. When assembled in order you can flip through the pages and observe a mitosis animation. The reverse side of the page depicts chromosomes (used in later parts of this laboratory). Cut along the dashed lines to remove the frames from the page. Stack them with frame number 1 on the top of
A
B
C D
E
X'OVER X'OVER
X'OVER X'OVER
X'OVER X'OVER
G2
G2
G2
G1
G1
G 1
IIII
II
II
IIIIII
III
III
II II II II
III III III III
II II II II
II II
III
1G
X'OVER X'OVER
III III III III
G2
II
G 2
II II
G 2
III III
G 2
III III
Figure 3. Procedure for modeling mitosis and meiosis. Only chromosome pairs numbers II and III are shown for clarity. (A) Simulating interphase at G1. (B) Interphase following synthesis (during G2). (C) One possible arrangement of chromosomes during a mitotic metaphase. (D) Crossing over during prophase I and/or metaphase I of meiosis. (E) Results of crossing over depicted in "D". See the simulation sections of the laboratory for more information.
the deck. Straighten the pile (make sure that the side opposite the numbers is even) and then hold together at the numbered side (do NOT staple them together or throw the pieces away when you are finished). Use your thumb to flip through the pages. With a little straightening and practice you will see an animated mitotic sequence. 3- Most students have difficulty identifying the various phases of mitosis from prepared slides. This is because few, if any, of the cell stages on a slide look exactly like the diagrams in a text (since mitosis is a continuous process). Before you continue with the slides, practice by identifying the beginning and end of the mitotic stages shown in your flip book. Use the frame numbers at the bottom of the pictures. Also indicate the structures that can be observed at each stage. Enter these answers in the Results Section. This practice will help you identify the mitotic sequences in another exercise.
4- Mitosis Simulation. Work in groups of two. Turn over the flip book and remove the chromosome pictures labeled "G1" and "G2". The "G1" chromosomes depict diffuse chromosomes (before DNA synthesis). Those labeled "G2" show the state of the chromosomes following DNA replication. Compare the largest of the "G1" and G2" chromosomes. Note that the G2chromosomes are composed of two CHROMATIDS joined together at the CENTROMERE. The chromatids are exact copies (as indicated by the ame banding pattern). If one of the bands includes a gene affecting eye color (brown, for example), then both chromatids will carry the brown-eyed gene. The chromatids will later separate insuring that each of the daughter cells receives their own copy of the brown-eyed gene. In this simulation the
chromatids are identified by a Roman Numeral with a male ( ) or female ( ) subscript. I have used the convention of starting the numbering with the largest of the chromosomes. The male and female subscripts are included only to remind you that one of the chromosome pair came from the organism's father, the other from the mother. 5- On a separate sheet of paper draw a circle 15-18 cm (7-8 inches) in diameter (this is your cell). At the center of the circle lightly draw another circle to represent the nucleus. Place your "G1" chromosome cards on top of one another within the nucleus (this represents the mitotic interphase before replication; Fig 3A). Replicate your chromosomes by replacing the "G1" cards with the proper "G2" card (Fig 3B). Pretend (or hallucinate) that the nucleus has disintegrated and the chromosomes have condensed (you are now in prophase). Move the cards into a metaphase configuration and record the order of your chromosomes from top to bottom in the results section. Continue with the remainder of the simulation by moving the chromosomes cards through the various phases of mitosis using figures 2 and 3 as guides. When you reach telophase, separate the chromatids at the centromere (cut them along the dotted line) and then move them to opposite corners of your cell. Draw a line down the center of your cell (cytokinesis) and record the results of your simulation in the Results Section. 6- Re-run the simulation using your partner's set of diagrams but scramble the order of the chromosomes at metaphase (as compared to your first simulation). Record the new metaphase order and the outcome of the simulation in the results section. Don't throw the remaining cards away; you'll need them later.
____________________________________________________________________________ EXERCISE Mitosis in Animal Cells.
PURPOSE: To observe representative stages of mitosis in both plant and animal cells. In addition, the student will sample a population of animal cells to determine the relative timing of the stages of mitosis.MATERIALS NEEDED:
Prepared Whitefish Blastula Slides. Each stage of mitosis does not proceed at the same rate. Interphase, for example, is a much more time-consuming phase of mitosis than any of the others (Fig 2). It is possible to estimate the relative amount of time a cell spends in each stage by sampling a population of dividing cells. If a stage takes a long time, then a large proportion of the cells will be seen in that phase. Alternately, if cells quickly race through a phase, few representatives of that stage will be seen on the slide. PROCEDURE: 1- Mitosis in an Animal Cell. Obtain a prepared slide of whitefish blastula mitosis. The blastula is a stage of embryonic development in animals. It is spherical in shape and, since developmental stages are characterized by accelerated cell division, it's a good place to observe the various mitotic phases. Several thin slices of the blastula will be found on your slide. 2- Scan the slide and locate a representative cell for each stage of the mitotic sequence (Fig 1). You will probably have to scan several of the blastula sections and may have to obtain a second slide to see all the stages. Make a sketch of each phase and label the chromosomes, spindles, asters, and other structures, if present. 3- Timing of Mitotic Stages in an Animal Cell. Enlist the aid of your neighbor for this part of the laboratory. While one of you scans the slide and calls out the mitotic stage for the cells, the other should keep a tally of the number of cells in each phase. Sample 100 or more cells and then enter your counts in the appropriate table in the results section (you will have to look at more than one section; make sure that you don't recount a blastula slice twice). Were some phases under-represented? Which stage occurs most quickly? ____________________________________________________________________________
TELOPHASE I
ANAPHASE I
METAPHASE I
PROPHASE I
MEIOTIC INTERPHASE
PROPHASE II
METAPHASE II
ANAPHASE II
TELOPHASE II
MEIOTIC INTERPHASE
PROPHASE II
ANAPHASE II
TELOPHASE II
METAPHASE II
PREMEIOTIC INTERPHASE
Figure 4. Meiosis in a typical animal cell. Phases of meiosis I are shown at the center of the diagram, starting with the premeiotic interphase and connected by hatched arrows. Bold arrows join the stages of the second part of meiosis (on the left and right sides of the diagram. Note that four cells are potentially produced by meiosis; each with half the original number of chromosomesEXERCISE Meiosis.PURPOSE: To simulate meiosis. MATERIALS NEEDED:
Chromosome Pictures Labeled "MEIOSIS" (From the back of the flip sheets).
Models and Wall Charts Depicting Meiosis.
As previously mentioned, meiosis reduces the diploid number of a cell in half to produce a haploid cells. Among animals these haploid cells are used as gametes (eggs and sperm). For most of the plants with which you are familiar, entire haploid individuals are built. You will find that meiosis is not much more difficult to learn than was mitosis since it is essentially mitotic cell division run through twice. Meiosis therefore uses the same names for the various phases (followed by a "I" or "II" to indicate the first or second division; Fig 3). There are, however, several critical events to keep in mind for meiosis that differentiate it from meiosis: During prophase I homologous chromosomes pair up (SYNAPSIS) and remain in close contact through the first metaphase. This close approximation of two chromosomes (four chromatids) is called a "TETRAD". Tetrads can be seen during prophase I and metaphase I. Compare the position of the chromosomes in metaphase of mitosis with those of metaphase I in meiosis (Fig 2 vs. Fig 3). While in the tetrad configuration the arms of homologous chromosomes may overlap and produce CHIASMATA (singular, CHIASMA). At this time the genes of one chromatid can be exchanged with those on a sister chromatid. This process of CROSSING OVER produces new genetic combinations that were not present in the original parental chromosomes. The effects of this GENETIC RECOMBINATION will be looked at in greater detail in the next section. Following crossing over, chromosomes separate and reduce the number of chromosomes by half. This is the REDUCTION DIVISION. The second part of meiosis has no surprises and continues as a haploid mitosis. These events are shown in greater detail in figure 4.
PROCEDURE:1- Simulation of Meiosis. Work in groups of two. Separate the chromosomes labeled "MEIOSIS" from the flip sheets. As before, draw a circle on a sheet of paper and pile the chromosome cards in the middle to represent the premeiotic interphase of the cell. Start with the "G1 cards, and then replace them with the chromosomes labeled "MEIOSIS" (Fig 3 A,B). 2- Run through the remainder of the simulation using figure 4 as a guide. Pay particular attention to the arrangement of your chromosomes during metaphase I and the following anaphase (record the positions of the chromosomes as you did for the mitosis simulation). This step is the "key" to the meiosis since it affects the distribution of chromatids during the second division (Fig 4). Record the results of your simulation in the Results Section. Re-run the simulation using your partners cards with a different metaphase I order and record these results..
REPORT SECTIONby __________________________________To complete your assignment for this exercise, fill in all the information requested in the RESULTS and DISCUSSION sections, tear them out at the perforations and hand them in. These two completed sections constitute your Laboratory Report for the Experiment
EXERCISE -An Introduction to Mitosis.Mitosis Animation. Fill in the following table by entering the frame number range that corresponds to each of the phases of mitosis. Include a list of structures that can be seen in cells at each phase. If a structure can only be observed in a plant or animal cell, indicate so. Mitotic Stage
Frame Numbers that can be seen (range)
Structures that can be seen in living cells
InterphaseProphaseMetaphaseAnaphaseTelophase Mitosis Simulation. Record the sequence of chromosomes you used for metaphase during the first and second simulations in the following space. Use the number and male/female codes printed on the cards to identify them from top to bottom. Simulation Chromosome OrderFirstSecond
Now check off the set of chromosomes found in each of the daughter cells produced during your two simulations:
Chromosomes Present After Mitosis Simulation I I II II III III IV IV1st (cell 1)1st (cell 2)2nd (cell 1)2nd (cell 2)
Were the simulations identical regardless of the starting chromosome order? Of what importance would this result be to a real cell? _____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
____________________________________________________________________________ EXERCISE - Mitosis in Animal Cells.
Mitosis in an Animal Cell. In the spaces provided below, make a sketch of some representative Whitefish blastula cells for each phase of mitosis. Label any cellular structures that may be visible.
INTERPHASE PROPHASE METAPHASE
ANAPHASE TELOPHASE
Timing of Mitotic Stages in an Animal Cell. Record your cell counts for each of the phases of mitosis in the following table:
Interphase
Prophase
Metaphase
Anaphase
Telophase
Number of cells in each phase:
If each cell cycle took an average of 60 min to run from interphase through completion, how long would each phase take (show your calculations):
INTERPHASE: _________________________________________________________________
PROPHASE: _________________________________________________________________
METAPHASE: _________________________________________________________________
ANAPHASE: _________________________________________________________________
TELOPHASE: _________________________________________________________________
___________________________________________________________________________ EXERCISE Meiosis. Simulation of Meiosis. Record the sequence of chromosomes you used for metaphase I during the first and second simulations in the following space. Use the number and male/female codes printed on the cards to identify them from left to right and top to bottom. Chromosome OrderSimulation
Left Right Left Right Left Right Left Right
First:Second: Now check off the set of chromosomes found in each of the daughter cells produced during your two simulations:
Simulation
I I II II III III IV IV
1st (cell 1)1st (cell 2)1st (cell 3)1st (cell 4)2nd (cell 1)2nd (cell 2)2nd (cell 3)2nd (cell 4)
Were the simulations identical regardless of the starting chromosome order? Of what importance would this result be to a real cell? _________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
12
34
56
78
9
10
11
11
11
11
12
22
22
22
22
12
34
56
78
920
12
34
56
7
89
30
33
33
33
12
34
56
ME
IOS
ISM
EIO
SIS
ME
IOS
ISM
EIO
SIS
ME
IOS
ISM
EIO
SIS
ME
IOS
ISM
EIO
SIS
X'O
VER
X'O
VER
X'O
VER
X'O
VER
X'O
VER
X'O
VER
X'O
VER
X'O
VER
X'O
VER
X'O
VER
X'O
VER
X'O
VER
G2
G2
G2
G2
G2
G2
G2
G2
G1
G1
G1
G1
G1
G1
G1
G1
II
II
IIII
IIII
IIIIII
IIIIII
IVIV
IVIV
II
II
IIII
IIII
IIIIII
IIIIII
IVIV
IVIV
II
II
IIII
IIII
IIIIII
IIIIII
IVIV
IVIV
II
II
IIII
IIII
II
IIII
IIIIIIIVIV
