This is a graded assignment. The grade weight of this assignment is "2."
A grade weight of 2 counts twice as much as a grade weight of 1, a
grade weight of 4 counts twice as much as a grade weight of 2 and 4
times as much as a grade weight of 1, and so on.
THE
CONTENTS OF THE NUCLEUS
The
nucleus of any living active cell is bounded by a nuclear membrane.
Inside the nucleus at certain times you can see, under high
magnification, a number of ribbon-like molecules. These elongated
molecules in the nucleus are called chromosomes.
Let us suppose that we are looking at the nucleus of an egg before it
has been fertilized. Let us suppose, too, that this particular kind of
egg has two chromosomes in its nucleus. Each chromosome is slightly
different in size and chemical composition from the other. Our highest
magnifications show us that a chromosome consists of an extremely
slender thread coiled into so tight a spiral that it appears solid. A
long coiled spring of fine wire might be used to represent a chromosome,
if you were building a model of a nucleus. However, a wire is made
of the same substance from end to end. Not so with a chromosome.
Arranged along and amongst its coils there are proteins which help wind
up the chromatin to prevent it getting tangled. If tiny beads were
strung on your fine wire and then coiled into a tight spring, it might
serve as a very imperfect model of a chromosome.
Each chromosome contains hundreds or even thousands of distinctly
separable genes arranged in single file along its whole length. Each
gene differs markedly (in chemical composition and function, for
instance) from every other gene. Each gene has its own place in line on
the chromosome, too.
Two chromosomes with their serially arranged genes are found within the
nucleus of the egg cell we are examining. In the nucleus of a sperm of
the same species there are also two chromosomes. When the sperm
fertilizes the egg and the two nuclei fuse, the fused nucleus obviously
contains four chromosomes. Before a fertilized egg starts to divide, the
nucleus must pass through what is called the resting stage, or
interphase. All four of the chromosomes uncoil within the nucleus in
somewhat the way your coiled spring of fine wire might be pulled out of
its spiral coil. The uncoiled chromosomes are so long and so slender and
are so twisted about in crisscross fashion within the nucleus that you
can see only a mass of tangled threads or perhaps only granules
scattered about with no threads visible. When the nucleus is in this
condition, the cell is usually said to be in interphase. However, the
cell is not resting: it is alive and actively working. The interphase is
merely the term applied to the stage when a cell is not dividing. A
fertilized egg goes through a resting stage, then starts to divide. The
division is called mitosis.
CELL DIVISION
Plants
and animals grow by adding new cells to their bodies. Older cells divide
to form new cells. This is one of the many ways God has enabled His
creation to regenerate and live. In most plants, this dividing and adding of new cells
continues as long as the plant is alive. In animals, the total number of
cells does not usually increase after adulthood is reached, but cell division
continues because new cells are needed to take the places of those that
are destroyed or worn out.
The cell must go through a series of gradual changes before a single
cell can become two cells. The process is known as mitosis, which is
followed by cytokinesis. Together these are called cell division.
Prior to mitosis, significant changes take place within the nucleus.
Each gene along the entire length of the chromosome duplicates itself;
in other words, it builds another gene exactly like itself, with the
result that there are now two identical chromosomes lying side by side,
attached by the kinetochore. This chromosome duplication (DNA
replication) takes place during the synthesis phase.
Click here to watch a video!
Learn about DNA in the cell's
nucleus and how it is replicated and its role in cellular
reproduction.
MULTIPLE
CHOICE
Please
choose the best answer for each of the following multiple choice
questions below. (If necessary, open another window in Study Hall
while you complete this assignment, so you are not timed out of
the Academy system.) When you have completed your work, click on
the “submit” button at the bottom of the page to turn in your
assignment.
HOW MUCH HAVE YOU LEARNED?
Play the
CELL DIVISION GAME with the chromosomes. Try both Mitosis and Meiosis. How
many chromosomes or chromatids do you end up with at the end of
each cell division?
McRel
Biology
Benchmark 4th Ed.
Understands the processes of
cell division and differentiation (e.g., meiosis, mitosis,
embryo formation, cellular replication and differentiation into
the many specialized cells, tissues, and organs that comprise
the final organism; each cell retains the basic information
needed to reproduce itself)
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