Eukaryotic Cell by mikesanye


									                           Eukaryotic Cell Division
   Cells must continually grow and divide in order for an organism to grow, maintain its
structure, and reproduce. Cell division involves the replication, or copying, of the complete set
of hereditary information. It also involves the equal distribution of the genetic material in the
resulting cells. The hereditary information of organisms is contained in large molecules called
deoxyribonucleic acid, DNA.

        Upon completion of this laboratory you will be able to:
        1. list the similarities and differences between mitosis and meiosis.
        2. describe and recognize the stages of mitosis under a microscope or on models.
        3. know the events that occur during each stage of mitosis and meiosis.
        4. define the boldface terms.

   Chromosomes (Figure 1) are tiny rod-shaped structures in the cell’s nucleus that carry the
genetic message. A single complete set of chromosomes for an organism is referred to as the
haploid number of chromosomes or 1n. Most organisms are diploid, having two complete sets
of chromosomes, 2n. All species contain a specific number of chromosomes; for example
humans have 23 pairs of chromosomes (for a total of 46). The matched chromosomes are called
homologous chromosomes. They are alike in their size, structure, and the genes that they carry.
Thousands of genes may be located on a single chromosome. A gene is the set of instructions
for one protein product. It is estimated that humans have approximately 30,000 genes.

    Electron micrograph      Artist’s rendition     Chromosome under increasing magnification

                  Figure 1. Several renditions of the structure of a chromosome.

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Genetic material doesn’t always appear as it does in Figure 1. Usually it exists as loose strands
of DNA and protein. A completely uncoiled human DNA strand in a single cell can be up to
three feet long! Just before cell division the DNA strand coils, condensing 100X to form a
chromosome. This structure helps prevent tangling and breakage of the genetic material during
cell division.

Two Cells From One – Mitosis
Even after you are completely grown your somatic cells continue to grow and divide. Somatic
cells are those cells that make up the structure of the body and include all the cells in the body
except reproductive cells. Thus somatic cells are not involved in passing the genetic information
from one generation to the next. A somatic cell’s nucleus divides through a process called
mitosis. Following a mitotic cell division the two resulting daughter cells are genetically
identical to each other and to the parent cell. The original parental cell and the two resulting
daughter cells contain the same number of chromosomes and, barring mutations, possess
identical copies of the same genetic blueprint.
    1. Watch the animations on mitosis:
               Plant cell mitosis:
                         Animal cell mitosis:
    2. Obtain a prepared slide of Allium (onion) root tip. Root tips in plants contain cells that
         are constantly undergoing mitotic divisions.
    3. Using the 430x objective select a region of the root tip that is one cell thick from the area
         designated in Figure 2; the mitotic stages will be clear here. Locate a region that contains
         the mitotic stages shown in Figure 3.
    4. Work with your partner to quiz each other on your ability to identify the different mitotic

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                                Figure 2. Allium (onion) root tip.

       Interphase        Prophase          Metaphase         Anaphase          Telophase
                              Figure 3. Stages of Plant Cell Mitosis
   5. Draw in the labeled space provided the appropriate mitotic stage (prophase, metaphase,
        anaphase, and telophase) as it appears to you while observing it with the microscope.

 The Stages of Mitosis
 The genetic material replicates, or copies,
 itself. Instead of having the usual 2 copies of
 genetic material the cell now has 4 (= 4n).
 The genetic material is dispersed throughout
 the nucleus. The nucleolus is usually visible,
 looking like a nucleus within the nucleus.
 The cell is now prepared to proceed with
 division. The pictures you draw for the
 following stages should resemble the one on
 the right. Be sure to label your drawings!

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     The genetic material in the nucleus loops
and coils forming visible thread-like
structures called chromosomes. In animal
cells the spindle apparatus forms and
functions in separating the chromosomes
during division. It consists of a star-like
formation of fibers running from one end of
the cell to the other. This structure is not
visible in plant cells like the ones you are
observing. The nuclear envelope and
nucleolus start to disappear.

     Fibers from the fully formed spindle
apparatus are attached to the constricted
region in each chromosome called a
centromere. The replicated chromosomes
are lined up midway between the two ends of
the cell. This imaginary midline is called the
spindle equator. The nuclear envelope has
completely disappeared.

The replicated chromosomes begin to
separate. The division of the centromeres and
shortening of spindle fibers accomplish this.
The centromeres divide separating the
replicated chromosomes. As the two
chromosomes divide each one goes to
opposite ends of the cell. Note: The cell is
now in the process of restoring its normal
diploid (2n) state.

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    The chromosomes are now clustered
 together at opposite ends of the cell. The
 chromosomes start to unwind and may no
 longer be clearly visible. The spindle
 apparatus disappears and the nuclear envelope

 Division of the cytoplasm usually follows
 nuclear division. In plants the onset of
 cytokinesis is marked by the formation of a
 cell plate between the two nuclei in late
 telophase. In animals cleavage occurs when
 the cell membrane pinches in, or furrows,
 between the two telophase nuclei and starts to
 separate the cytoplasm. It may be difficult to
 differentiate between late telophase and

Time spent in mitotic stages.
   Now you are going to calculate how long a specific stage of mitosis lasts in comparison to the
other stages. This exercise assumes that the more cells there are in a specific stage, the longer
that stage lasts. A nucleus undergoing division is called a mitotic figure. For this exercise find
an area on the slide which has as many mitotic figures as possible.
        1. At 430x count the total number of cells in one field of view. Include all cells, both
              mitotic and non-mitotic. Record this number in Table 1 next to “Total # of Cells”.
        2. Count the number of cells in prophase and record in the appropriate column in

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        3. Do the same with metaphase, anaphase, and telophase nuclei.
        4. Add together the number of cells undergoing mitotic division:
         (# in prophase) + (# metaphase) + (# anaphase) + (# telophase) = (total # of mitotic cells)
                        Total # of mitotic cells = _______________________
        5. Total number of cells – total mitotic cells = _____cells in interphase. Record this in
              the appropriate column in Table 1.
        6. With the following equation, calculate the duration of each mitotic stage using the
              data from the entire class:
               duration of mitotic stage (in hours) = number of cells in a stage x 24 hr.
                                                        total number of cells

                     Table 1. Duration of mitotic stages in onion root tip cells.
                            # of cells you counted   Class total # of cells   Duration of stage (hrs.)

 Total number of cells                                                              ……………
 Number in prophase
 Number in metaphase

 Number in anaphase

 Number in telophase
 Number in interphase

Two Halves Make a Whole: Meiosis
   In sexually reproducing organisms, egg and sperm cells combine their genetic information to
produce a new, unique organism. Gametes, or sex cells, are different from other body cells
because each contains only one copy of the genetic blueprint instead of the usual two. In other
words, they are haploid (1n).
   If gametes contained the usual two sets of chromosomes, each new generation would have
twice the number of chromosomes. Just imagine! If this really happened, the 46 chromosomes in
your cells would be increased to 368 in your great-grandchildren and 924 chromosomes in their
great-grandchildren! Meiosis is the process of cell division that prevents this wild escalation in
the number of chromosomes contained in our cells. Remember that meiosis occurs only in the
germ cells, the reproductive cells that carry the genetic information from one generation to the
next. In female animals meiotic cell division results in one functional egg and three

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nonfunctional cells called polar bodies. In males each full meiotic division produces four
functional sperm. Refer to your textbook for help in the study of the stages of meiosis.
Important Differences Between Meiosis and Mitosis:
   1. In prophase I of meiosis there is a pairing of homologous chromosomes called synapsis.
        These chromosomes have already replicated, so four chromosomes are joined together
        forming a tetrad. This does not occur during mitosis.
   2. In prophase of meiosis crossing-over occurs. At this time the matched homologous
        chromosomes break at exactly the same places and exchange similar segments. This
        process effectively exchanges similar genes between the matched and paired
        chromosomes. Genes come in alternative forms called alleles. For example the gene for
        eye color may have several alternative forms (blue or brown). There is no gain or loss of
        genetic material during crossing-over but rather an exchange of the same genes, (but
        sometimes different alleles), one for one. Thus, new genetic combinations on a
        chromosome may be formed. This is recombination.
   3. The cells in meiosis go through a second successive division without first doubling their
        genetic material. This result in four cells called gametes. Each of these cells contains
        only one copy (1n) of the hereditary blueprint for the organism.

Meiosis-demonstrating the process.
Procedure: As directed by your instructor, demonstrate meiosis using the following materials:
        4 chromosomes (chains of pop beads) 2 centrioles
        4 centromeres                           2 nuclear envelopes
        spindle fibers
The beads on two chains should be yellow while those on the other two chains red. A magnet
would be placed in the center of each chain. These chains of beads represent homologous
chromosomes (one contributed by Mom and one contributed by Dad) with each bead being a
gene and the magnet being the centromere. Mark a “gene” as shown by your instructor at the
same site (locus) on each strand as shown in Figure 4.

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                       Figure 4. Pop bead chromosomes with marked genes

Interphase: The “chromosomes” you obtained have already replicated and are in the interphase
stage. The two sets of replicated chromosomes are dispersed in the nucleus and the nuclear
envelope is present as represented by the string. In animal cells centrioles, from which the
spindle apparatus will arise, are near the nucleus and at right angles to each other.

Prophase I: The two homologous chromosomes pair with each other. This is called synapsis.
To simulate synapsis slide the homologues together and entwine them (two like strands entwined
with the other two like strands). See Figure 5A. Crossing-over, the exchange of matched
genetic material, occurs now. For crossing-over detach a small section in one strand of each
replicated chromosome and exchange them. See Figure 5B. The nuclear envelope is still present,
but it is beginning to disappear.

                                5A. Synapsis                   5B. Crossing-over
                 Figure 5. Synapsis and crossing-over of chromosomes during Prophase.

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Metaphase I: Line the chromosomes up at the imaginary equator of the cell. The centromeres
of each homologue lie on the equator. Move the centrioles so that they are at opposite ends of
the cell (see Figure 6).

                             Figure 6. Chromosomes at metaphase.

Anaphase I: The homologous chromosomes separate with each homologue moving to opposite
poles. Note that each homologue is still in its replicated condition. Separate the homologues.
(Figure 7.)

               Figure 7. Separation of homologous chromosomes during Anaphase I.

Telophase I: Pile the chromosomes up near their respective poles. A new nuclear envelope will
surround each set of chromosomes. Furrowing of the membrane occurs to form two new
daughter cells. There are now two daughter cells with a cell membrane between the two new

Draw your pop bead chromosomes as they appear after meiosis I in the two nuclei labeled “after
meiosis I”

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          Daughter Cell #1 after Meiosis I              Daughter Cell #2 after Meiosis I

Interkinesis: The first meiotic division is now complete and two cells are formed. Cytokinesis
has occurred and the nuclear envelope has reformed. It is important to note that DNA replication
does not occur prior to the second division. The spindle reforms to provide a complete spindle
apparatus for each daughter cell.

Meiosis II
Prophase II: The replicated chromosomes are still attached by their centromeres. During
prophase II the nuclear envelope disorganizes and the chromosomes recondense. Remember this
is occurring in two cells simultaneously.
Metaphase II: Align the replicated chromosomes at the equator of the cell.
Anaphase II: Separate the replicated homologues and their centromeres. The individual
daughter chromosomes should now be heading towards opposite poles of the cell.
Telophase II: Pile up the daughter chromosomes once they are at opposite poles of the cell.
The nuclear envelope reforms and the cytoplasm divides. Four daughter nuclei now exist. Each
nucleus contains one individual chromosome, one-half the number present in the original cell.
Draw your pop bead chromosomes as they appear after meiosis II in the space provided labeled
“Gamete nuclei”. (Don’t forget to include the crossover that occurred in Meiosis I.)

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                                Four new daughter cells after meiosis II
In this meiosis exercise you simulated spermatogenesis, the production of four sperm cells from
one parent cell. The female process of oogenesis (see below) is somewhat different. Due to
unequal division of cytoplasm, one functional egg and three “runts” called polar bodies are
formed. Polar bodies are almost always non-functional.

Fill in Table 2, Mitosis vs. Meiosis

    Table 2. Mitosis vs. Meiosis: a comparison of characteristics.

                                                                           Mitosis   Meiosis

    Number of divisions

    Number of daughter cells

    Chromosome number in daughter cells (haploid or diploid?)

    Daughter cells identical to each other and parent cell? (yes or no)

    Produces gametes (yes or no)?

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