Mitosis and Meiosis Lab - DOC by pengxiang

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									                            Mitosis and Meiosis Lab

    The fertilization of a haploid (1N, or one copy of each chromosome) egg by a
haploid sperm produces the diploid (2N, or two copies of each chromosome)
zygote. The nucleus of the zygote contains all of the genetic material necessary
for growth, development and differentiation in an organism. From that single cell,
the process of mitosis produces all of the diploid cells of a multicellular organism,
with each cell containing a copy of the same genetic material. Because cells are
constantly dying and need to be replaced, mitosis produces new diploid cells
throughout the life on an organism. For example, your entire epidermis is
replaced every 15 to 30 days, and you require approximately 200 billion new red
blood cells every day. So you can see that mitosis is of critical importance to us!

Exercise 1: Stages of Mitosis (Figure 1.)

   Using pipe cleaners to represent chromosomes follow the chromosomes on
your lab bench through the process of cell division.
Before you begin, how many chromosomes do you have?
How many homologous pairs of chromosomes?
How can you tell which chromosomes are homologous?

*In your notebook please sketch the chromosomes at each of the following
stages of mitosis. Be sure that the homologous pairs can be distinguished in
your drawings.

Interphase: Before cell division can occur, each chromosome must make an
exact copy of itself in a process called replication. This duplication event occurs
in the S phase of the cell cycle, while the chromosomes are still in their long,
stringy, uncondensed state. Therefore, when interphase is viewed under the
microscope, the chromosomes cannot be identified.
    Using more pipe cleaners make an exact copy of each of your chromosomes.
After the chromosomes have been replicated, the homologous chromosomes are
attached at the centromere, and each strand of the chromosome is called a
chromatid. Because the attached chromatids are identical they are referred to as
sister chromatids. Sketch the replicated chromosomes.
How many chromosomes are there now (assuming that the attached chromatids
represent one chromosome)?
How many homologous pairs of chromosomes?
How many chromatids?
Besides replication of the chromosomes what else should happen during
    After interphase, cell division requires sorting out the cell contents (both
nuclear and cytoplasmic) into two new cells. The process of sorting out the
chromosomes occurs in several phases, collectively known as mitosis. We will
look at the individual phases of mitosis below.

Prophase: During prophase the chromosomes become condensed and are
visible as individual structures. By the end of prophase the nuclear envelope
disintegrates, enabling the chromosomes to be snared by protein fibers that have
grown out of the two centrosomes that formed in the cytoplasm. From our study
of the cytoskeleton, can you predict what class of molecules forms the protein
fibers? These fibers form the spindle which will eventually allow the sister
chromatids to separate from each other.

Metaphase: By metaphase the two centrosomes have moved to opposite ends
of the cell with their protein fibers attached. The other ends of the protein fibers
are attached to the centromere region of the chromosomes. One sister
chromatid of each chromosome is connected to each pole by the spindle
    The distinctive characteristic of metaphase is the alignment of the
chromosomes in a plane through the center of the cell. (Only the centromeres
are aligned.) Sketch you model of chromosomes in metaphase.

Anaphase: During anaphase the centromeres joining the sister chromatids
separate and the chromatids- now called chromosomes- move along the spindle
fibers to opposite poles. The number of chromosomes in the cell is briefly double
as the cell prepares for the final phase of division. Sketch your model of
chromosomes in anaphase.

Telophase and Cytokinesis: Now that the chromatids have separated, the
division of chromosomes is complete. All that remains is to reconstruct the
nuclear envelope and divide the cytoplasm between the two new cells. These
processes will be finished by the end of Telophase, and the chromosomes will
resume their long, stringy form.
How many chromosomes are there in each new cell?
How many pairs of homologous chromosomes in each new cell?
What is the overall result of mitosis

Go to this site:

Click on cell biology
Click on the cell cycle and mitosis
Go through the 4 categories: DNA basics, cell cycle, mitosis, test yourself

Figure 1

 Activity 2:
Using pipe cleaners to represent chromosomes follow the chromosomes on your
lab bench through the process of meiotic cell division.
Before you begin, how many chromosomes do you have?
How many homologous pairs of chromosomes?
How can you tell which chromosomes are homologous?

In your notebook please sketch the chromosomes at each of the following stages
of meiosis. Be sure that the homologous pairs can be distinguished in your
drawings. What is a tetrad? What are the outstanding differences between
mitosis and meiosis?

Figure 2.

Figure 3 (continued from 2)

Meiosis: Figure 2&3
   Meiosis is a process of nuclear division including a reduction in the number of
chromosomes in the cell by half. In many organisms the results of meiosis are
gametes, or the sperm and egg cells. Organisms that reproduce sexually contain

a diploid (2N) complement of chromosomes in their somatic cells. Meiosis
reduces the number of chromosomes to the haploid (1N) number, or a single set
of chromosomes. The events of meiosis are significant for two reasons. First the
1N gametes can fuse at fertilization to create a 2N zygote that will become a new
individual, and secondly, meiosis allows shuffling of the genetic material to insure
that the new individual has a unique set of genes.
     Meiosis, like mitosis, is preceded by replication of each chromosome to form
two chromatids connected at the centromere. However, reduction of the
chromosome number and production of new genetic combinations result from
two events unique to meiosis. Meiosis includes two rounds of chromosome
separation; chromosomes are replicated in the first round, but not in the second,
resulting in a decrease in chromosome number by half by the end of the second
round. Second, during an early phase of meiosis each chromosome pairs along
its length with its homolog. This pairing of homologous chromosomes is called
synapsis, during which the four chromatids exchange various segments of their
genetic material. This exchange of genetic material is called crossing-over and
produces new genetic combinations. During crossing-over there is not gain or
loss of genetic material.

Prophase I: As in mitosis, the chromosomes become visible and the nuclear
envelope disintegrates during this phase. However, in meiosis homologous pairs
of chromosomes associate closely with each other, even in some cases
intertwining their “arms”. Each pair is called a tetrad. While they are in close
contact the chromatids of homologous chromosomes exchange pieces of DNA.
The size of the exchanged sequences varies. Following crossing over in a
homologous pair, none of the chromatids are genetically alike.

Metaphase I: In metaphase of both mitosis and meiosis, chromosomes are
attached to the spindle apparatus and aligned along the center of the cell. In
meiosis, each tetrad stays together as they line up compared to mitosis where
each pair of sister chromatids stay together.

Anaphase I: Homologous pairs of chromosomes are separated from each other
and move to the opposite poles, but the sister chromatids stay attached to each
other. Note that in anaphase of mitosis the sister chromatids detach from each
other as they move to opposite poles.

Telophase I and Cytokinesis: Once the homologous pairs of chromosomes are
at opposite poles, the cytoplasm divides to make the two daughter cells.

Prophase II: The spindle apparatus begins to form again.

Metaphase II: Each chromatid is attached to spindle fibers as the chromosomes
line up in the center of the cell.

Anaphase II: During anaphase II the sister chromatids move to opposite poles...

Telophase II and Cytokinesis: The division of chromatids is complete and the
cytoplasm divides to form daughter cells. How many chromosomes does each
cell have? How many homologous pairs? Are these cells haploid or diploid?

Click on cell biology; meiosis
Go through reproduction, chromosomes…diploid; meiosis I & II; test yourself

The following site has great info and lots of practice. Pay special attention to the
lily anther cells used in the meiosis discussion.

Key Terms, Lecture 1

Acrocentric-Chromosome with the centromere located very close to one end.

Allele- One of the possible mutational states of a gene, distinguished from other
alleles by phenotypic effects.

Apoptosis- Genetically controlled program of cell death, activated as a normal
part of developmental pathways or as a result of cell damage.

Biparental inheritance- Each diploid organism contains two copies of each
chromosome, one inherited from the female and one inherited from the male

Bivalent- Synapsed homologous chromosomes in the first prophase of meiosis.

Cell cycle- Phases of the growth of an individual cell, divided into G1 (gap 1), S
(DNA synthesis), G2 (gap 2) and M (mitosis).

Cell theory- A theory stating that all organisms are composed of basic visible
units called cells.

Centriole- A cytoplasmic organelle composed of nine groups of microtubules,
generally arranged as triplets. Function as the foci for the spindles in cell

Centromere- Specialized region of a chromosome to which sister chromatids
remain attached after replication and the site to which the spindle fibers attach
during cell division. Localization of the centromere determines the shape of the
chromosome during the anaphase portion of cell division.

Centrosome- Region of the cytoplasm containing the centrosome.

Checkpoint- A point in the cell cycle when the cell is monitored before it can
proceed to the next stage of the cell cycle.

Chiasma- The crossed strands of nonsister chromatids seen in diplotene of the
first meiotic division. Provides cytological evidence for chromosomal crossing

Chloroplast- A cytoplasmic self-replicating organelle. The site of photosynthesis.

Chromatid- One of the subunits of a replicated chromosome joined to its sister
chromatid at the centromere.

Chromosome- A DNA molecule complexed with proteins and RNA to form a
threadlike structure containing genetic information arranged in a linear sequence.
Visible during mitosis and meiosis.

Chromosomal theory of inheritance- The idea that the behavior of chromosomes
during meiosis provides support for the Mendelian laws of inheritance.

Crossing over- The exchange of chromosomal material between homologous
chromosomes by breakage and reunion. The exchange of information between
nonsister chromatids during meiosis is the basis of genetic recombination.

Cytokinesis- The division of cytoplasm during mitosis or meiosis.

Daughter chromosomes- A term referring to sister chromatids after they are split
from one another at the initiation of anaphase.

Diakinesis- The final stage of meiotic prophase I in which the chromosomes
become tightly coiled and compacted and move toward the periphery of the

Diploid- The situation where each chromosome exists in pairs in the cell.

Disjunction- The separation of chromosomes at the anaphase stage of cell

Dyad- The products of tetrad separation or disjunction at the first meiotic
prophase. Consists of two sister chromatids joined at the centromere.

Epigenesis- The idea that an organism develops by the appearance and growth
of new structures.

Fixity of species- A doctrine suggesting that animal and plant species have
remained unchanged since the moment of their appearance on earth.

Gamete- A specialized reproductive cell that contains one copy (haploid number)
of each chromosome.

Genetically modified organisms- A plant or animal that has had a gene from
another species transferred to its genome using recombinant DNA technology,
and where the gene is expressed to produce a novel gene product.

Genome- The array of genes carried by an individual.

Homologous chromosomes- Chromosomes that synapse or pair during meiosis.
Chromosomes that are identical with respect to their genetic loci and centromere

Karyokinesis- The process of nuclear division.

Karyotype- The chromosomal compliment of a cell or individual. Often used to
refer to the arrangement of metaphase chromosomes in order according to
length and position of the centromere.

Kinetochore- A fibrous structure within the centromere to which the microtubule
attaches during cell division.

Locus- The site along a chromosome where a particular gene is located.

Metacentric- Placement of the centromere in the middle of a chromosome.

Metaphase plate- The arrangement of meiotic or mitotic chromosomes at the
equator of the cell during metaphase.

Monad- The second division of meiosis, meiosis II, results in the dyad
chromosome splitting into two monads of one chromosome each.

Natural selection- Differential reproductive success of individuals within a
population resulting from variable fitness conferred by genotypic differences.

Nondisjunction- An error during cell division in which the homologous
chromosomes in mitosis or the sister chromatids in meiosis fail to segregate and
migrate to opposite poles. Responsible for genetic defects such as monosomy
and trisomy.

Nucleolus- A nuclear organelle that is the site of ribosome biosynthesis; usually
associated with or formed in association with the NOR (see below).

Nucleolus organizer region- (NOR) - A chromosomal region containing the genes
for rRNA: most often found in physical association with the nucleolus.

P arm- Short arm of a chromosome

Q arm- Long arm of a chromosome.

Reductional division- The chromosomal division that halves the number of
chromosomes contained in a cell during the first meiotic division.

Spindle fibers- Cytoplasmic fibrils formed during cell division that are involved
with the separation of chromatids at anaphase and their movement towards
opposite poles of the cell.

Synapsis- The pairing of homologous chromosomes at meiosis.

Synaptonemal complex- A organelle consisting of a tripartite nucleoprotein ribbon
that forms between the paired homologous chromosomes in the pachytene stage
of the first meiotic division. Only found in cells undergoing meiosis. Crossing over
does not occur in areas where the complex does not exist.

Terminalization- The movement of chiasmata toward the ends of chromosomes
during the diplotene stage of the first meiotic division.

Transcription- Transfer of genetic information from DNA to RNA by the synthesis
of an RNA molecule copied from a DNA template..

Translation- Transfer of genetic information from RNA to protein in association
with a ribosome.

Transmission genetics- The field of genetics concerned with the mechanism by
which genes are transferred from parent to offspring.

Zygote- A diploid cell produced by fusion of two haploid gametes.


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