Download Experiment 4: Eukaryotic Cell Divisions

Biol 2281, Spring 2016
E4: Eukaryotic Cell Divisions
Experiment 4: Eukaryotic Cell Divisions
Learning Objectives:
1. Understand the functional importance of mitosis and meiosis.
2. Describe the different phases of the cell cycle and the main characteristics of each phase.
3. Name and describe the phases of mitosis.
4. Identify cells in various steps of mitosis when they are viewed under a microscope.
5. Name and describe the phases of meiosis.
6. Compare the processes and end products of mitotic and meiotic cell divisions
7. Simulate the division process using the modeling kits.
The following material is adapted from the Student Guide that accompanies “Modeling Mitosis and Meiosis Kit”, with
permission by Carolina Biological Supply Company.
Overview
In animal and most plants, body cells are diploid cells (2n) because they contain two sets of
chromosomes in pairs. The two members of a chromosome pair are called homologous chromosomes
and they contain similar genetic information. A cell with only one set of chromosomes is haploid (n).
Each organism contains a characteristic number of chromosomes in its diploid cells. For example, onion
has 16 (8 pairs), and humans have 46 (23 pairs). Gametes of these organisms are always haploid with
half the total number of chromosomes found in body cells.
In eukaryotic cells, mitosis and meiosis are the means by which genetic information, the DNA
encoded in threadlike structures called the chromosomes, is passed from one generation of cells to the
next. In mitosis, the nucleus of a diploid or a haploid cell divides. The result of mitosis is two cells that
are genetically identical, with the same number of chromosomes as the parent cell. In meiosis, the
nucleus of a diploid cell, containing a complete set of chromosomes, divides twice. Genetic information
is exchanged between homologous chromosomes. The result of meiosis is four genetically diverse
haploid cells, called gametes, each with half the number of chromosomes of the parent cell. The terms
mitosis and meiosis are used to describe the orderly process of segregating and distributing the
replicated chromosomes to the new cells. Cytokinesis means the division of the cytoplasm and it occurs
at the end of the cell division.
In terms of the life of a cell, mitosis and meiosis are crucial but relatively brief part of an
ongoing process known as the cell cycle. The longer period between cell divisions is called interphase.
During interphase, the cell grows, replicates its chromosomes, and produces and assembles the cellular
structures needed for cell division. Interphase is a very busy time in the life of the cell and it is
comprised of three parts: the G1, S and G2 phase.
 G1 Phase: occurs immediately following mitosis. It is a growth period. Cells that will not enter
mitosis remain in the G1 stage and carry out their normal functions.
 S phase: is for DNA synthesis stage during which DNA replication occurs.
 G2 phase: is a second growth stage during which structures directly involved in mitosis, such
as spindle fibers, are synthesized.
For most of the cell cycle, chromosomes are dispersed throughout the nucleus of a cell. During
chromosome replication, however, chromosomes thicken and become tightly coiled structures that can
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E4: Eukaryotic Cell Divisions
be observed under a light microscope. Each replicated chromosome consists of two sister-chromatids
joined at the centromere.
Mitotic Cell Division
For unicellular and a few multicellular organisms, mitotic cell division serves as a means of
reproduction. In higher level eukaryotic cells, it serves as a means of growth and repair. Millions
of new cells are formed in the human body each day in this manner.
The process of mitosis is usually viewed in four phases: prophase, metaphase, anaphase and
telophase/cytokinesis.
A. Prophase
During this stage, each chromosome condenses,
rendering the duplicated chromosomes visible as threadlike
structures. A mitotic spindle forms within the cell, outside the
nucleus, but it is not visible during prophase. The mitotic
spindle helps to separate the identical chromatids later in
mitosis. The cell’s nuclear membrane begins to break down as
the cell transition from prophase to metaphase, allowing the
spindle to attach to each of the chromatids. At about the same
time, the nucleoli disappear.
B. Metaphase:
When the nuclear membrane is no longer distinct, the
cell is in metaphase. This brief phase is characterized by the
chromosome lining up in the center of the cell along an
equatorial plane called the metaphase plate. The spindle
fibers (spindle apparatus) made of microtubules have become
attached to the chromosomes at the kinetochores, groups of
proteins that form the outer faces of the centromeres.
C. Anaphase:
Anaphase begins as the microtubules on both sides of
the chromosome pair pull on the attached chromatid. This pull
separates the sister chromatids at the centromere and begins
the movement of the chromatids to opposite ends of the cell.
The chromatids can now be referred to as daughter
chromosomes. At the end of anaphase, each pole of the cell
has an equal and complete collection of daughter
chromosomes
.
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D. Telophase and Cytokinesis:
In telophase, the cell elongates even more. A nuclear
envelope begins to form, from fragments of the parent cell’s
nuclear membrane, around each set of chromosomes at the
poles of the cell. The nucleolus reappears and the
chromosomes become less condensed and less apparent.
Usually cytokinesis begins prior to the end of telophase,
indicating the end of mitosis. In animal cells, a cleavage
furrow forms during cytokinesis. In plant cells, the rigid cell
wall prevents the formation of a cleavage furrow; instead, a
cell plate forms to separate the parent cell into two daughter
cells, and a new cell wall forms along the cell plate.
Materials per student:




one Microscope (Pick up microscope according to your
sign-in number)
one book of lens paper, one bottle of Lens cleaner, one
box of Kim wipes
A box of prepared slides
For today’s activity 3, 4, & 5, obtain one modeling kit
and return the kit back to original container as soon as
you finish each activity.
Lab Activity 1: Mitosis in Onion Root Tip Cells
Cell divisions in plants are, for the most part, localized in specialized regions called meristems. Plants
have two types of meristems: apical and lateral. Found at the tips of stems and roots, apical meristems
contain cells capable of repeated cell divisions and they serve to increase plant organs in length. Lateral
meristems, located beneath the bark of woody plants, increase girth. In this assignment, you will
examine the prepared slides of apical meristem of onion roots.
1. Obtain a slide of a longitudinal section of an Allium (onion) root tip. First focus with the 4X
objective of your compound microscope to get an overall impression of the root morphology.
Then use the 10X objective to scan the apical meristem. Note that most of the nuclei are in the
interphase.
2. Switch to the 40X objective, focusing on a single interphase cell. Note the distinct nucleus, with
one or more nucleoli, and the chromatin dispersed within the bounds of the nuclear envelope.
Draw a typical interphase cell (Part 1, question 1) as it appears on your slide. Use the poster in
lab to help you identify different phases.
3. Find cells in prophase, metaphase, and telophase. Note the cell plate. Draw each phase on your
worksheet/report (Part 1, question 1)
4. Using 40X objective, position your slide so you have 70-100 cells in your field of view. Observe
the phase of each cell and record the totals in the table provided. (Part 1, question 2) This will
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E4: Eukaryotic Cell Divisions
give you an estimate of the percentage of cells in each phase. The frequency of occurrence of a
cell cycle phase is directly proportional to the length of a phase. You can therefore estimate the
amount of time each phase takes. The length of the cell cycle for onion root tip cells is
approximately 24 hours.
Lab Activity 2: Mitosis in Whitefish Blastula
The rapidly dividing cells of whitefish blastula, an early fish embryo, are excellent for studying mitotic
division in animals. With several important exceptions, mitosis in animals is remarkably like that in
plants.
1. Obtain a prepared slide of whitefish blastula. Locate a section for study with the 4X
objective. Then switch to the 10X objective and later to 40X objective to find mitotic phases.
2. Locate a cell in interphase. Note the absence of a cell wall. Note the presence of the nucleus.
Find a prophase cell. The first obvious difference between mitosis in plants and animals is
found in prophase. Whitefish cells contain centrioles (not present in plants). Centrioles
duplicate during interphase. During prophase, one new and one old centriole migrates to each
pole. They may be too small to be seen clearly with your light microscope; however, you can
see a starburst pattern of spindle fibers that appear to radiate from the centrioles.
3. Find a metaphase, an anaphase, and a telophase cell. Also note the cleavage furrow formation
during cytokinesis. Draw prophase and metaphase on your worksheet (Part I, question 3).
Lab Activity 3: Simulate Mitosis Using the Modeling Kit
1. From the chromosome model set labeled with “Mitosis”, obtain four different colored
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3.
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5.
6.
7.
chromosomes and one piece of yarn. The yarn represents the cell membrane. Place the four
chromosomes inside the cell membrane. This model represents a cell with two pairs of
chromosomes.
Modeling chromosome duplication in interphase. Create a duplicate of each chromosome by
attaching a matching chromosome (marked by the same color) to each original chromosome.
The point of attachment represents the centromere that joins the two sister-chromatids.
Modeling metaphase: Line up the replicated chromosome end-to-end in the center of the cell.
Modeling anaphase: disconnect the identical sister-chromatids from one another at the
centromere and move each chromatid away to the opposite sides of the cell.
Modeling cytokinesis: Lift the loop “membrane” and cross it over itself to create a figure
eight. Compare the two sides of the figure eight. Each side should possess the same number
of chromosomes and be identical to the original cell you started with in step 1.
Answer Part I, question 4 on your worksheet.
Answer Part I, question 5, 6, 7 on your worksheet.
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Meiotic Cell Division
The names of the stages of meiosis and some of the processes involved in meiosis are similar to those in
mitosis, but meiosis has a dramatically different purpose. Meiosis occurs in sexually reproducing
organisms to create sex cells that contain half the number of chromosomes of the parent cell.
These sex cells are called gametes in animals and spores in plants. They are haploid cells with increased
genetic variation. When maternal and paternal gametes fuse during fertilization, the offspring of these
organisms then contain a complete set of chromosomes.
Meiotic division consists of two consecutive division, meiosis I and meiosis II, but only one
chromosome replication. Interphase takes place prior to prophase I and is much like the interphase prior
to mitosis: the cell grows, replicates its chromosomes, and prepares itself for cell division. Meiosis I has
five phases: prophase I, metaphase I, anaphase I, telophase I and cytokinesis.
A. Prophase I
During prophase I, the homologous chromosomes come together to form a tetrad, a grouping of
four chromatids. An important step in maintaining genetic variation takes place within the tetrad: along
the lengths of the homologous chromosomes, genes from non-sister chromatids may cross over one
another, exchanging genetic material. This crossing-over mixes maternal and paternal DNA, ensuring
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that daughter cells produced by meiosis I will be genetically different from each other and from the
original parent cell.
B. Metaphase I and anaphase I
During metaphase I, the tetrads line up in the center of the cell along the metaphase plate.
Spindle fibers from each pole of the cell attach to one pair of sister chromatids in each tetrad. In
anaphase I, the homologous chromosomes are separated and pulled to opposite sides of the cells. This is
another important difference between mitosis and meiosis. In mitosis, the spindle pulls one sister
chromatid to opposite side of the cell, but in meiosis I, the spindle pulls a pair of sister chromatids to
opposite sides of the cell.
C. Telophase I and Cytokinesis
These two phases complete the meiosis I resulting in two daughter cells with half the
number of chromosomes in each cell. Each daughter cell is a haploid (n) and is ready to enter
meiosis II.
D. Meiosis II
During meiosis II, the sister-chromatids in each of the two daughter cells move toward the
metaphase plate, and are eventually separated much like what happens in mitosis. Because each cell
entering meiosis II forms two daughter cells, a total of four haploid cells are produced from one diploid
parent cell.
Lab Activity 4: Simulate Meiosis and Crossing-over Using the Modeling Kit
1. Using the chromosome model set labeled with “crossing over”, assemble a cell that has two
autosomes and two sex chromosomes (short) as shown below. The two autosomes have
attached chromosomes on the top, one pink and one orange fragment. Place the small circles
or triangles representing different genes (E and T) on each autosome. The color difference of
the same gene represents different alleles of the gene. (orange circle: E, pink circle: e;
orange triangle: T, pink triangle: t)
Figure 4-1: The parent cell
2. Obtain two pieces of yarn to make loops. These loops represent cell membranes. Place one
loop around the four chromosomes you have assembled in the previous step, and that will be
your starting parent cell. Set aside the other loop for later.
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3. Create a duplicate of each autosome and sex chromosome by connecting matching
chromosome for each at the centromere. Make sure that each replica possesses the same
alleles as the original. Remember to attach fragments to the replicated autosomes. This
simulates which phase of the cell cycle? _________________
4. Prophase I: Line up the homologous chromosomes side-by-side. Each side-by-side pair
forms a tetrad, a paired chromosome structure comprised of four chromatids. Crossing-over
takes place during prophase I of meiosis. Two non-sister chromatids within a tetrad become
close enough to overlap each other and exchange similar pieces of genetic information.
Crossing-over is one of several mechanisms that help ensure genetic variability within a
species. To model crossing-over, exchange the chromosome fragments on the adjacent nonsister chromatids of the autosomal tetrad. The following figure shows you how it should
look like after crossing over.
Figure 4-2: The tetrads after crossing-over
5. Metaphase I through Cytokinesis I: Place the tetrads in the center of the cells. Place the
other loop beside the loop containing the chromosomes. Place one chromosome from each of
the two tetrads in the second loop. This represents the completion of the first cellular
division in meiosis. You should now have a model of two daughter cells.
6. As the two daughter cells enter meiosis II, the chromosomes do not go through replication
again. Instead, the sister-chromatids in each chromosome will separate. Simulate this process
by separating the connected sister-chromatids and moving them into two poles of a dividing
cell. This represents the separation of chromosomes during ______________ of meiosis. At
the same time lift one loop “membrane” and cross it over itself to create a figure eight.
Repeat this procedure with the other loop. This represents the division of two cells into four
cells during telophase II and cytokinesis of meiosis II. You should now have four gametes,
each with one autosome and one sex chromosome as shown in Figure 4-3. Show your TA
what you have modeled.
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E4: Eukaryotic Cell Divisions
Figure 4-3: Four gametes at the end of meiosis
7. Answer Part 2, question 1 in Worksheet/Report.
Lab Activity 5: Simulate Independent Assortment of Genes Located on Different
Chromosomes During Meiosis Using the Modeling Kit.
If two genes are located on two separate pairs of chromosomes, they will segregate
independently of each other during meiosis I. This is due to the random arrangement and separation of
chromosomes during meiosis, giving rise to all possible combinations in equal frequency. Watch the
animation on http://www.sumanasinc.com/webcontent/animations/content/independentassortment.html
1. In the modeling kit labeled with “Independent assortment”, you have two pairs of
chromosomes. Assume that one pair of chromosomes contains the genes (A) for flower color,
and another chromosome pair contains the genes (B) for seed color. Also assume that A is
dominant over an allele, and B allele is dominant over b allele.
2. Assemble a set of chromosomes representing a plant that is heterozygous for both flower
color and for seed color.
3. As this plant is producing pollen (male gametes), what are the possible gene combinations
for flower color and seed color in the gamete populations? Use the modeling kit to simulate
independent assortment during meiosis. Answer Part 2, question 2 in Worksheet/Report.
4. Without using the modeling kit, answer Part 2, question 3 in Worksheet/Report.
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E4: Eukaryotic Cell Divisions
Total Points: 20pts
Note: You do not need a title sheet for this report
Your Name:___________________
Section: __________________
E4: Cell Division Worksheet/Lab Report
Part 1: Mitotic cell division (13 pts)
1. Lab Activity 1: Draw onion root tip cells in the following stages as they appear on your slide.
(2 pts)
Interphase
Prophase
Metaphase
Telophase
2. Lab Activity 1: Time duration of interphase and mitotic phases in onion root-tip cells. (2 pts)
Phase
% of all
cells
counted
Number of cells
Duration of
the Phase
Interphase
Prophase
Metaphase
Anaphase
Telophase
The length of cell cycle in actively
dividing onion root tip is about 24 hours.
number of cells of
all phases
3. Lab Activity 2: Draw whitefish blastula cells in the following stages. (1 pt)
Prophase
Metaphase
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E4: Eukaryotic Cell Divisions
4. Lab Activity 3: Mitosis modeling kit (3 pts)
a. Is this cell a diploid cell or a haploid cell? What is the “n” number?
b. In this modeling process, you used one piece of yarn representing cell membrane. However, the
membrane that actually contained the DNA during interphase is not simulated. Name that
membrane.
c. If the total amount of DNA in G2 phase is 100 picograms, what would be the amount of DNA
in picograms in each of the daughter cell at the end of mitosis?
5. Describe how mitotic cell division in plant cells differs from that in animal cells. (1 pt)
6. Write the term that matches each description: (2 pts)
a) Division of the cytoplasm ____________________________
b) Members of a chromosome pair ____________________________
c) Stage of interphase during which chromosomes replicate______________________
d) The apparatus that pull on the sister chromatids ______________________________
7. Write the mitotic phase that matches each of the following description: (2 pts)
a. Nuclear membrane and nucleolus disappear ____________________________
b. New nuclei are formed ____________________________
c. Sister chromatids are separated ____________________________
d. Chromosomes lined up at the center of the cell ____________________________
Part 2: Meiotic cell division (7 pts)
1. Lab Activity 4:
Suppose that the circular shaped gene has two alleles: “E” (orange colored circle) and “e” (pink
colored circle), and the triangularly-shaped gene has two alleles, “T” (orange colored triangle) and “t”
(pink colored triangle). At the end of your simulated meiosis II in Lab Activity 4, describe the
genotype of the parent cell and the genotypes of each of the four gametes regarding these two genes.
(2 pts)
i. Parent cell: _____________ (refer to Fig.4-1)
ii. Gamete 1: _____________ (refer to Fig. 4-3 for gametes)
iii. Gamete 2: _____________
iv. Gamete 3: _____________
v. Gamete 4: _____________
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E4: Eukaryotic Cell Divisions
2. Lab Activity 5: Independent Assortment (3 pts)
a. Draw the cell containing two chromosome pairs with heterozygous genotype for both the A
(flower color) gene and B (seed color) gene. Draw what you have assembled in step 2. Be sure
to clearly label the alleles.
b. During metaphase I, there are two possible ways for the two tetrads to be aligned at the center of
the cell leading to different assortment of alleles. Draw these two layouts (with metaphase plate).
Possibility 1
Possibility 2
c. Write down the genotypes of all possible gametes. What would be the percentage of each type?
Genotype of gametes
Percentage
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E4: Eukaryotic Cell Divisions
3. Diagram the arrangement of chromosomes in a cell undergoing meiosis as they would appear in the
phases noted below, if the diploid (2n) chromosome number is 8 (2n=8). Note: It is not possible to
use the modeling kit to simulate this question and it is not necessary to label any alleles on the
chromosomes. (2 pts)
Meiosis I: Metaphase I (draw metaphase plate)
Meiosis I: Anaphase I
(For the following, draw what happens in only one of the two daughter cells produced at the end of
meiosis I)
Meiosis II: Metaphase II
Meiosis II: Anaphase II
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Biol 2281: E4 Cell Division
The Eukaryotic Cell Cycle
Experiment 4:
Eukaryotic Cell Divisions
Part I: Mitotic Cell Division
• What is mitosis?
– Process that distributes identical sets of genetic
materials into daughter cells
• Why?
– In unicellular organisms: asexual reproduction
– In multicellular organisms, cell division allows
• growth and development from the zygote
• tissue renewal and repair
• How?
– Interphase (DNA replication)
– Mitotic phase
* Duration of the phase: proportional to the
frequency of occurrence of a cell in that phase
Eukaryotic Cell Cycle
• Interphase: the non-dividing phase; with
intense biochemical activity during which the
cell grows and copies its chromosomes in
preparation for the division
– G1 phase: first growth phase
– S phase: DNA duplication phase
– G2 phase: second growth phase
• Mitotic phase: dividing phase
•
•
•
•
prophase
metaphase
anaphase
Telophase and cytokinesis
Important Terms
• Chromatin
• Chromosome
– Sister-chromatids
– Centromere
– Kinetochore
– Mitotic spindle
Gene Loci
Dominant allele
A
b
D
a
b
D
Homologous
chromosomes
Recessive allele
• Haploid (n) organism
• Diploid (2n) organism
– Homologous chromosomes
Genotype : Aa, bb, DD
Heterozygous:
Homozygous:
Dr. Elizabeth Pickett & Dr. Wen-ju Lin,
Spring 2016
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Biol 2281: E4 Cell Division
Prophase
• Nuclear membrane breaks
down
• Chromosomes condense
• Mitotic spindles begin to
form between centrioles
Metaphase
• Chromosomes lined
up in the center of the
cell (metaphase plate)
• Spindle fibers attached
to kinetochores on the
chromosomes
Anaphase
• Sister-chromatids separate;
• Daughter chromosomes begin to move to
the opposite poles
Cytokinesis in animal and plant cells
Telophase
• The cell elongates
• Chromosomes reach poles of
cell
• Nuclear membrane re-forms
• Nucleolus reappears
• Chromosomes decondense
Part II: Meiotic Division
• What is Meiosis?
– A special type of cell
division that produces
haploid cells (gametes)
– # of chromosomes
reduced by half: one
DNA replication and
two successive
divisions
– Involves gene
recombination: each
gamete is unique
Dr. Elizabeth Pickett & Dr. Wen-ju Lin,
Spring 2016
Overview of Meiosis
• Interphase
• Meiosis I: segregates
homologous chromosomes and
reduces the # of chromosome
by half
–
–
–
–
prophase I
metaphase I
anaphase I
telophase I and cytokinesis
• Meiosis II: separates sister
chromatids of each chromosome
–
–
–
–
prophase II
metaphase II
anaphase II
telophase II and cytokinesis
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Biol 2281: E4 Cell Division
The stages of meiotic cell division: Meiosis I
Crossing Over
• Tetrad Formation
during prophase I
• Exchange of genetic
material between nonsister chromatids:
– Homologous
recombination
• In humans, average of
2 or 3 crossovers per
chromosome pair
How Much Variation Can
Independent Assortment Cause?
• In humans, 2^23 = 8,388,608 possible
combinations of chromosomes for each
gamete.
Animation at:
http://www.sumanasinc.com/webcontent/animations/content/inde
pendentassortment.html
The stages of meiotic cell division: Meiosis II
Unique Features of Meiosis
• Reduction Division
– one DNA replication followed by two cell division
– Maintain a species’ chromosome count
– Significantly contribute to genetic variation among
offspring
• Contribute to Genetic Variation Among
Offspring
– Crossing over
– Independent Assortment
• Meiosis produces four daughter cells
genetically different from the parent cell and
from each other
Dr. Elizabeth Pickett & Dr. Wen-ju Lin,
Spring 2016
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Biol 2281: E4 Cell Division
Chromosome Number
and DNA Amount
Diploid
cell
# of
chromosomes
Amount
of DNA
After
interphase
After
mitosis
After
meiosis I
After
meiosis
II
The Human Example:
Haploid
cell
(gamete)
# of
2n
chromosomes
X
picograms
Diploid
cell
picograms
picograms
picograms
picograms
E4 Activities
•
•
•
•
•
Quiz
Onion root-tip prepared slide
Whitefish blastula prepared slide
Modeling mitosis and meiosis
Modeling crossing-over and
independent assortment
• Practice finding anaphase cells
• Complete worksheet/report
questions. Report must be
submitted before you leave.
Dr. Elizabeth Pickett & Dr. Wen-ju Lin,
Spring 2016
picograms
Amount
of DNA
After
interphase
After
mitosis
After
meiosis I
After
meiosis
II
Haploid
cell
(gamete)
picograms
picograms
picograms
picograms
picograms
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picograms
Sample Quiz Questions
• What is the purpose of mitosis?
• What is the purpose of meiosis?
• Which phase of cell cycle tends to take the longest amount of time?
What occurs during this time?
• What is the difference between sister chromatids and homologous
chromosomes?
• A diploid cell has a total of 24 chromosomes. What is the n number?
• Reduction of chromosome number occurs in ______.
• Name two mechanisms that generate genetic variation during
meiosis.
• How do you calculate the number of possible chromosome
combinations in gametes due to independent assortment?
• Name one difference between animal cell division and plant cell
division.
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