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CHROMOSOMES
Metaphase chromosome
In interphase cells stained for light microscopy, the chromatin usually appears as a diffuse mass within the
nucleus, suggesting that the chromatin is highly extended. As a cell prepares for mitosis, its chromatin coils
and folds up (condenses), eventually forming a characteristic number of short, thick metaphase
chromosomes that are distinguishable from each other with the light microscope. Though interphase
chromatin is generally much less condensed than the chromatin of mitotic chromosomes, it shows several of
the same levels of higher-order packing. Even during interphase, the centromeres and telomeres of
chromosomes, as well as other chromosomal regions in some cells, exist in a highly condensed state similar
to that seen in a metaphase chromosome. This type of interphase chromatin, visible as irregular clumps with
a light microscope, is called heterochromatin, to distinguish it from the less compacted, more dispersed
euchromatin (“true chromatin”). Because of its compaction, heterochromatic DNA is largely inaccessible to
the machinery in the cell responsible for transcribing the genetic information coded in the DNA, a crucial early
step in gene expression. In contrast, the looser packing of euchromatin makes its DNA accessible to this
machinery, so the genes present in euchromatin can be transcribed. The chromosome is a dynamic structure
that is condensed, loosened, modified, and remodeled as necessary for various cell processes, including
mitosis, meiosis, and gene activity.
Figure 1. Classification of chromosomes
with regard to replacement of centromer
Figure 2. Human metaphase chromosomes
Experiment: Examine the metaphase chromosome of mammalian cell in prepared microscope slide.
Polytene chromosomes
Polytene chromosomes (giant chromosomes) are oversized chromosomes which have developed from
standard chromosomes. Specialized cells undergo repeated rounds of DNA replication without cell
division (endomitosis),
to
increase cell volume,
forming
a
giant
polytene
chromosome.
Polytene chromosomes form when multiple rounds of replication produce many sister chromatids that remain
fused together. In addition to increasing the volume of the cells' nuclei and causing cell expansion, polytene
cells may also have a metabolic advantage as multiple copies of genes permits a high level of gene
expression. In Drosophila melanogaster, for example, the chromosomes of the larval salivary glands undergo
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many rounds of endoreduplication, to produce large amounts of glue before pupation. Another example within
the organism itself is the tandem duplication of various polytene bands located near the centromere of the X
chromosome which results in the Bar phenotype of kidney-shaped eyes. Polytene chromosomes have
characteristic light and dark banding patterns that can be used to identify chromosomal rearrangements and
deletions. Dark banding (band) frequently corresponds to inactive chromatin, whereas light banding
(interband) is usually found at areas with higher transcriptional activity. Polytene chromosomes were originally
observed in the larval salivary glands of Chironomus.
Figure 3. Polytene chromosomes from larval salivary gland of Chironomus
Experiment: Observe the Polytene chromosomes of salivary gland of Chironomus. Zoom in band and
interband parts of the chromosomes. Compare the size of regular metaphase chromosomes with polytene
chromosomes.
MITOSIS
When a cell is not dividing, and even as it replicates its DNA in preparation for cell division, each chromosome
is in the form of a long, thin chromatin fiber. After DNA replication, however, the chromosomes condense as
a part of cell division: Each chromatin fiber becomes densely coiled and folded, making the chromosomes
much shorter and so thick that we can see them with a light microscope.
Each duplicated chromosome has two sister chromatids, which are joined copies of the original chromosome.
The two chromatids, each containing an identical DNA molecule, are initially attached all along their lengths
by protein complexes called cohesins; this attachment is known as sister chromatid cohesion. Each sister
chromatid has a centromere, a region of the chromosomal DNA where the chromatid is attached most closely
to its sister chromatid. This attachment is mediated by proteins bound to the centromeric DNA; other bound
proteins condense the DNA, giving the duplicated chromosome a narrow “waist.” The portion of a chromatid
to either side of the centromere is referred to as an arm of the chromatid. (An unduplicated chromosome has
a single centromere, distinguished by the proteins that bind there, and two arms.)
Later in the cell division process, the two sister chromatids of each duplicated chromosome separate and
move into two new nuclei, one forming at each end of the cell. Once the sister chromatids separate, they are
no longer called sister chromatids but are considered individual chromosomes; this step essentially doubles
the number of chromosomes in the cell. Thus, each new nucleus receives a collection of chromosomes
identical to that of the parent cell. Mitosis, the
Division of the genetic material in the nucleus, is usually followed immediately by cytokinesis, the division of
the cytoplasm. One cell has become two, each the genetic equivalent of the parent cell. From a fertilized
egg, mitosis and cytokinesis produced the 200 trillion somatic cells that now make up your body, and the
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same processes continue to generate new cells to replace dead and damaged ones. In contrast, you produce
gametes—eggs or sperm—by a variation of cell division called meiosis, which yields daughter cells with only
one set of chromosomes, half as many chromosomes as the parent cell. Meiosis in humans occurs only in
special cells in the ovaries or testes (the gonads). Generating gametes, meiosis reduces the chromosome
number from 46 (two sets) to 23 (one set). Fertilization fuses two gametes together and returns the
chromosome number to 46 (two sets). Mitosis then conserves that number in every somatic cell nucleus of
the new human individual.
Phases of the Cell Cycle
Mitosis is just one part of the cell cycle. In fact, the mitotic (M) phase, which includes both mitosis and
cytokinesis, is usually the shortest part of the cell cycle. The mitotic phase alternates with a much longer
stage called interphase, which often accounts for about 90% of the cycle. Interphase can be divided into
subphases: the G1 phase (“first gap”), the S phase (“synthesis”), and the G2 phase (“second gap”). The G
phases were misnamed as “gaps” when they were first observed because the cells appeared inactive, but
we now know that intense metabolic activity and growth occur throughout interphase. During all three
subphases of interphase, in fact, a cell grows by producing proteins and cytoplasmic organelles such as
mitochondria and endoplasmic reticulum. Duplication of the chromosomes, crucial for eventual division of the
cell, occurs entirely during the S phase. Thus, a cell grows (G1), continues to grow as it copies its
chromosomes (S), grows more as it completes preparations for cell division (G2), and divides (M). The
daughter cells may then repeat the cycle.
A particular human cell might undergo one division in 24 hours. Of this time, the M phase would occupy less
than 1 hour, while the S phase might occupy about 10–12 hours, or about half the cycle. The rest of the time
would be apportioned between the G1 and G2 phases. The G2 phase usually takes 4–6 hours; in our
example, G1 would occupy about 5–6 hours. G1 is the most variable in length in different types of cells. Some
cells in a multicellular organism divide very infrequently or not at all. These cells spend their time in G1 (or a
related phase called G0) doing their job in the organism—a nerve cell carries impulses, for example. Mitosis
is conventionally broken down into five stages: prophase, prometaphase, metaphase, anaphase, and
telophase. Overlapping with the latter stages of mitosis, cytokinesis completes the mitotic phase. Many of the
events of mitosis depend on the mitotic spindle, which begins to form in the cytoplasm during prophase. This
structure consists of fibers made of microtubules and
Figure 4. Mitosis in a plant cell. These light micrographs show mitosis in cells of an onion root.
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Experiment: Take a small amount of brewers yeast solution by using Pasteur pipette. Put one small drop
of brewers yeast solution onto microscope slide. Examine the amitosis of yeast cells.
Experiment: Observe the mitosis in onion root cells in a prepared slide.
MEIOSIS
Prophase I
Leptoten: Leptotene is of very short duration and progressive condensation and coiling of chromosome fibers
takes place. Individual chromosomes—each consisting of two sister chromatids—become discrete. The two
sister chromatids closely associate and are visually indistinguishable from one another.
Zygoten: Occurs as the chromosomes approximately line up with each other into homologous chromosome
pairs. At this stage, the synapsis (pairing/coming together) of homologous chromosomes takes place,
facilitated by assembly of central element of the synaptonemal complex. The paired chromosomes are called
bivalent or tetrad chromosomes.
Pacytene: At this point a tetrad of the chromosomes has formed known as a bivalent. This is the stage
when chromosomal crossover (crossing over) occurs. Nonsister chromatids of homologous chromosomes
may exchange segments over regions of homology. Sex chromosomes, however, are not wholly identical,
and only exchange information over a small region of homology. At the sites where exchange
happens, chiasmata form. The exchange of information between the non-sister chromatids results in a
recombination of information; each chromosome has the complete set of information it had before, and there
are no gaps formed as a result of the process.
Diplotene: The synaptonemal complex degrades and homologous chromosomes separate from one another
a little. The chromosomes themselves uncoil a bit, allowing some transcription of DNA. However, the
homologous chromosomes of each bivalent remain tightly bound at chiasmata, the regions where crossingover occurred. The chiasmata remain on the chromosomes until they are severed at the transition to
anaphase I.
Diakinesis: Chromosomes condense further during the diakinesis stage. This is the first point in meiosis
where the four parts of the tetrads are actually visible. Sites of crossing over entangle together, effectively
overlapping, making chiasmata clearly visible. The nucleoli disappear, the nuclear membrane disintegrates
into vesicles, and the meiotic spindle begins to form.
Metaphase I

Pairs of homologous chromosomes are now arranged at the metaphase plate, with one chromosome in
each pair facing each pole.

Both chromatids of one homolog are attached to kinetochore microtubules from one pole; those of the
other homolog are attached to microtubules from the opposite pole.
Anaphase I

Breakdown of proteins that are responsible for sister chromatid cohesion along chromatid arms allows
homologs to separate.

The homologs move toward opposite poles, guided by the spindle apparatus.

Sister chromatid cohesion persists at the centromere, causing chromatids to move as a unit toward the
same pole.
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Telophase I and Cytokinesis

When telophase I begins, each half of the cell has a complete haploid set of duplicated chromosomes.
Each chromosome is composed of two sister chromatids; one or both chromatids include regions of
nonsister chromatid DNA.

Cytokinesis (division of the cytoplasm) usually occurs simultaneously with telophase I, forming two
haploid daughter cells.

In animal cells like these, a cleavage furrow forms. (In plant cells, a cell plate forms.)

In some species, chromosomes decondense and nuclear envelopes form.
 No chromosome duplication occurs between meiosis I and meiosis II.
Prophase II
• A spindle apparatus forms.
• In late prophase II, chromosomes, each still composed of two chromatids associated at the centromere,
move toward the metaphase II plate.
Metaphase II
• The chromosomes are positioned at the metaphase plate as in mitosis.
• Because of crossing over in meiosis I, the two sister chromatids of each chromosome are not genetically
identical.
• The kinetochores of sister chromatids are attached to microtubules extending from opposite poles.
Anaphase II
Breakdown of proteins holding the sister chromatids together at the centromere allows the chromatids to
separate. The chromatids move toward opposite poles as individual chromosomes.
Telophase II and Cytokinesis
• Nuclei form, the chromosomes begin decondensing, and cytokinesis occurs.
• The meiotic division of one parent cell produces four daughter cells, each with a haploid set of (unduplicated)
chromosomes.
• The four daughter cells are genetically distinct from one another and from the parent cell.
Experiment: Examine the meiosis in a given slide.
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Figure 5: Meiosis in Lilium candidum