Chromosomes, Mitosis, and Meiosis
Vocabulary
Chromatid- each of the two threadlike strands into which a chromosome divides longitudinally
during cell division. Each contains a double helix of DNA
Chromosome- a threadlike structure of nucleic acids and protein found in the nucleus of most
living cells, carrying genetic information in the form of genes
Centromere- the point on a chromosome by which it is attached to a spindle fiber during cell
division
Chromatin- the material of which the chromosomes of organisms other than bacteria (i.e.,
eukaryotes) are composed. It consists of protein, RNA, and DNA
Karyotype- the number and visual appearance of the chromosomes in the cell nuclei of an
organism or species
Cytokinesis- the cytoplasmic division of a cell at the end of mitosis or meiosis, bringing about
the separation into two daughter cells
Diploid- (of a cell or nucleus) containing two complete sets of chromosomes, one from each
parent
Haploid- (of a cell or nucleus) having a single set of unpaired chromosomes
Polyploidy- those containing more than two paired (homologous) sets of chromosomes
Homologous Chromosomes- a set of one maternal chromosome and one
paternal chromosome that pair up with each other inside a cell during meiosis. These copies have
the same genes in the same locations
Maternal Homologue- ones received from your mother
Paternal Homologue- ones received from your father
Sister Chromatids- pieces of identical DNA that are crucial in the process of cell replication and
division
Tetrad- A group of four closely associated chromatids of a homologous pair formed by synapsis
Dyad- A chromosome consisting of two sister chromatids joined at a centromere
1. Independent Assortment: Since each homologous pair assorts independently from all the
others, the process produces 2n possible combinations of maternal and paternal
chromosomes in gametes, where n is the haploid number. In humans, the possible
combinations would be 223, or about eight million. Thus, each human gamete contains
one of eight million possible assortments of chromosomes inherited from that person's
mother and father. Genetic variation results from this reshuffling of chromosomes,
because the maternal and paternal homologues will carry different genetic information at
many of their corresponding loci.
Crossing Over: The exchange of genetic material between homologues; occurs during
prophase of meiosis. Occurs when homologous portions of two nonsister chromatids
trade places. During prophase I, X-shaped chiasmata become visible at places where this
2.
3.
4.
5.
homologous strand exchange occurs. Produces chromosomes that contain genes from
both parents. In humans, there is an average of two or three crossovers per chromosome
pair.
Genetic variability allows differing genes to express themselves through the generations,
which is a means for evolution to occur.
Colchicine is a substance that disallows the formation of microtubules and thus of the
spindle fibers in cell division. Under the action of this drug the cells interrupt division at
metaphase and the anaphase does not occur. Therefore the use of colchicine in the study
of karyotypes is interesting because chromosomes will be seen replicated and condensed.
You could locate tumor suppressor or oncogenes genes that have mutated to combat them
using specially created antibodies. Maybe there would be a way to freeze cancer cells in
the G1 stage, to keep them from ever replicating, and trigger apoptosis.
Despite differences between prokaryotes and eukaryotes, there are several common
features in their cell division processes. Replication of the DNA must occur. Segregation
of the "original" and its "replica" follow. Cytokinesis ends the cell division process.
Whether the cell was eukaryotic or prokaryotic, these basic events must occur.
Prokaryotes are much simpler in their organization than are eukaryotes. There are
a great many more organelles in eukaryotes, also more chromosomes. The usual method
of prokaryote cell division is termed binary fission (The method by which bacteria
reproduce. The circular DNA molecule is replicated; then the cell splits into two identical
cells, each containing an exact copy of the original cell's DNA.). The prokaryotic
chromosome is a single DNA molecule that first replicates, then attaches each copy to a
different part of the cell membrane. When the cell begins to pull apart, the replicate and
original chromosomes are separated. Following cell splitting (cytokinesis), there are then
two cells of identical genetic composition.
Due to their increased numbers of chromosomes, organelles and complexity,
eukaryote cell division is more complicated, although the same processes of replication,
segregation, and cytokinesis still occur. Eukaryotic chromosomes occur in the cell in
greater numbers than prokaryotic chromosomes.
Mitosis Web Quest
Chromatid- Each of a pair of identical DNA molecules after DNA replication, joined at the
centromere.
Chromatin- Protein/DNA complex making the chromosome.
Chromosomes- Molecules of DNA complexed with specific proteins responsible in eukaryotes
for storage and transmission of genetic information.
Histones- Five kinds of proteins forming complexes with Eukaryotic DNA.
Kinetochore- Structure forming at centromere during Mitosis for binding microtubules.
Nucleosomes- Basic structural unit of eukaryotic chromosome forming "beads on a string."
Cell Cycle:
Interphase. The cell is engaged in metabolic activity and performing its duty as
part of a tissue. The DNA duplicates during interphase to prepare for mitosis (the
next four phases that lead up to and include nuclear division). Chromosomes are
not clearly discerned in the nucleus, although a dark spot called the nucleolus
may be visible.
Prophase. Chromatin in the nucleus begins to condense and becomes visible in
the light microscope as chromosomes. The nuclear membrane dissolves,
marking the beginning of prometaphase. Proteins attach to the centromeres
creating the kinetochores. Microtubules attach at the kinetochores and the
chromosomes begin moving.
Metaphase. Spindle fibers align the chromosomes along the middle of the cell
nucleus. This line is referred to as the metaphase plate. This organization helps
to ensure that in the next phase, when the chromosomes are separated, each new
nucleus will receive one copy of each chromosome.
Anaphase. The paired chromosomes separate at the kinetochores and move to
opposite sides of the cell. Motion results from a combination of kinetochore
movement along the spindle microtubules and through the physical interaction of
polar microtubules.
Telophase. New membranes form around the daughter nuclei while the
chromosomes disperse and are no longer visible under the light microscope.
Cytokinesis or the partitioning of the cell may also begin during this stage.
Number
Interphase
Prophase
Metaphase
Anaphase
Telophase
Total
20
10
3
2
1
36
55.56%
27.78%
8.33%
5.56%
2.78%
100%
13.33 hrs.
6.67 hrs.
2.00 hrs.
1.33 hrs.
0.67 hrs.
24 hrs.
of Cells
Percent of
Cells
Number
of Hrs
spent in
phase
Questions:
1. Longest amount of time: Interphase
Shortest amount of time: Telophase
2. Chromatin in the nucleus begins to condense and becomes visible in the light
microscope as chromosomes. The nuclear membrane also dissolves.
3. Metaphase was easiest to tell apart since the chromosomes lined up in a straight line.
4. New membranes form around the daughter nuclei while the chromosomes disperse
and are no longer visible under the light microscope.
5. I did not notice cytokinesis in the cells. Partitioning of the cell would occur during
cytokinesis.
6. Division of the Cytoplasm (Cytokinesis) in Plants vs Animals: Cell Plate Formation
Occurs in Plants: Plant cells form a cell plate (cellulose) that separates the two new
cells. Vesicles containing building materials fuse with one another to form the disklike cell plate between the two new cells.
7. The cell membranes on opposite sides of the cell become pinched-in allowing for the
cell to divide. The initial structure that forms is called a cleavage furrow. The
cleavage furrow continues to pinch in, until the two sides are touching.
8. Cell Replacement. Cells are constantly sloughed off, dying and being replaced by
new ones in the skin and digestive tract. When damaged tissues are repaired, the new
cells must be exact copies of the cells being replaced so as to retain normal function
of cells.
9. I learned how to tell the difference between interphase and prophase: the presence of
a nuclear membrane and condensed chromatin. I learned the difference between
anaphase and telophase: the presence of new membranes and the chromosomes
dissolve. I also learned the distinction between chromatin and chromosomes.
Part Two of Web Quest
1. 300 million cells are replaced in our bodies every second.
2. Skin cells divide often, as they often need to be replaced.
3. Most cells have a relatively short lifespan.
4. When a cell dies, it breaks up and leaves an empty space in the tissue. This signals cells
nearby to start dividing in order to fill the space.
5. G1- Gap 1 Phase- cell growth begins
S- Synthesis Phase- chromosomes divide and duplicate, cell growth continues
G2- Gap 2 Phase- cell reaches proper size
6. The cell must be big enough to continue the cell cycle process.
7. The chromosomes must be duplicated so that there is enough genetic material to create
new cells.
8. Checkpoint 1 checks to make sure the cell has duplicated its genetic material and that the
cell is large enough to proceed. Checkpoint 2 checks that all genetic material is
duplicated, that the cell size is appropriate, and that all genetic material is intact (if not, it
will repair the chromosome). Checkpoint 3 makes sure the chromosomes are attached to
spindles.
9. These checks are important because the genetic material cannot be damaged, and the cell
must be able to correctly make it through mitosis without needless errors.
Part Three of Web Quest:
1. Meiosis is a process to convert a diploid cell to a haploid gamete, and cause a change in
the genetic information to increase diversity in the offspring. It is when a gamete divides
to produce two new identical cells.
2. Asexual reproduction is a method of reproduction that is rapid and effective allowing the
spread of an organism; however since the offspring are identical, there is no mechanism
for introducing diversity.
3. Multicellular organisms can include both diploid and haploid eukaryotic cells. Most of
the cells of the organism's body are diploid. The gametes--eggs and sperm-are haploid
eukaryotes. During fertilization two haploid gametes fuse, thus restoring the diploid
condition to the resulting embryo. Having two sets of genetic information offers the
eukaryote certain advantages over prokaryotes, and the emergence of the diploid state
was an important milestone in evolution.
4. A karyotype analysis usually involves blocking cells in mitosis and staining the
condensed chromosomes with Giemsa dye. The dye stains regions of chromosomes that
are rich in the base pairs Adenine (A) and Thymine (T) producing a dark band. A
common misconception is that bands represent single genes, but in fact the thinnest bands
contain over a million base pairs and potentially hundreds of genes.
5.
Regular Karyotype
Traditional karyotyping
allows scientists to view the
full set of human
chromosomes in black and
white, a technique that is
useful for observing the
number, size and shape of
the chromosomes. Hard to
interpret.
Both
To detect the number and
visual appearance of the
chromosomes in the cell
nuclei of an organism or
species
Spectral Karyotype
Spectral karyotyping (SKY)
is a laboratory technique
that allows scientists to
visualize all of the human
chromosomes at one time
by "painting" each pair of
chromosomes in a different
fluorescent color.
6.
7. Translocation- a chromosome abnormality caused by rearrangement of parts between
nonhomologous chromosomes. A gene fusion may be created when the translocation
joins two otherwise-separated genes, the occurrence of which is common in cancer.
Nondisjunction- the failure of one or more pairs of homologous chromosomes or sister
chromatids to separate normally during nuclear division, usually resulting in an abnormal
distribution of chromosomes in the daughter nuclei.
Deletion- the loss or absence of a section from a nucleic acid molecule or chromosome.
1. Meiosis II is very similar to Mitosis. In both cases chromosomes line up and sister
chromatids are separated by the action of the spindle fibers. The daughter cells are
genetically identical to one another.
2. Meiosis generates genetic diversity in two ways. First, in prophase I of meiosis,
homologous chromosomes pair with each other and, in the process called crossing over or
recombination, exchange reciprocal chromosomal segments with one another.
3. Transformation is genetic change via the uptake of exogenous DNA from the
environment. This is like Griffith's experiments discussed in a previous chapter.
Conjugation is a way of transferring DNA involving the direct coupling of two bacterial
cells and the one way transfer of sequential DNA.
Sexduction is related to conjugation, but specialized around a certain type of
phenomenon.
Transduction is genetic exchange mediated by viruses, but involves the exchange of
genetic materials between different bacteria.
4. An advantage of asexual reproduction, binary fission, is that no mate is needed. Many
offspring are produced quickly.
5. When two genes are close together on the same chromosome, they do not assort
independently and are said to be linked. Whereas genes located on different
chromosomes assort independently and have a recombination frequency of 50%, linked
genes have a recombination frequency that is less than 50%.
Free Response Questions:
1.
A. Asexually- Ameoba, Paramecium- through binary fission
Sexually- Mammals, bees
Both Asexually and Sexually- jellyfish, mushroom (spores)
B. Sexual reproduction takes advantage of desirable environmental conditions. Asexual
reproduction is easier, less costly in energy, and is less likely to make a genetic mistake
(mutation) in the reproduction of a new organism. Sexual reproduction is more energy costly, but
it adds more genetic variation to the population because a new organism gets some traits from
mom and some from dad.
2.
A. Trisomy 21 is due to an extra copy of chromosome number 21. Instead of having the normal 2
copies of chromosome number 21, the person with Down syndrome has 3 copies of chromosome
number 21. Monosmy 21 never occurs because carriers do not have visible health problems.
B. Autistic-like behaviors can be partially remedied by normalizing excessive levels of protein
synthesis in the brain. Additional investigation revealed that these changes were likely due to a
reduction in protein production—the levels of newly synthesized proteins in the brains. Autism is
what is considered a spectrum disorder. This means every child with DS-ASD will be different in
one way or another. Some will have speech, some will not. Some will rely heavily on routine and
order, and others will be more easy-going. Combined with the wide range of abilities seen in
Down syndrome alone, it can feel mystifying. It is easier if you have an understanding of ASD
disorders separate from Down syndrome. Autism, autistic-like condition, autistic-spectrum
disorder (ASD) and pervasive developmental disorder (PDD) are terms that mean the same thing,
more or less. They all refer to a neurobehavioral syndrome diagnosed by the appearance of
specific symptoms and developmental delays early in life. These symptoms result from an
underlying disorder of the brain, which may have multiple causes, including Down syndrome.