Chromosomes and Cell
Reproduction
• I. Cell Growth
– Most living cells are between 2 and 20 µm in diameter.
– How big a cell can grow is determined by physical factors
below:
1. Diffusion rate of materials into and out of the cell.
(Remember diffusion only works well over short
distance)
2. The amount of DNA available to program the cell’s
metabolism.
3. The cell’s surface area-to-volume ratio.
– So, mitosis is important for cell survival by taking a large
cell and dividing it into two identical smaller cells with
identical diploid (2n) genetic material.
• II. Cell Division
– Cell division, also called cell reproduction occurs in humans.
1. Eukaryotic organisms undergoing growth, development, and
repair divide their cells by Mitosis.
2. Bacteria cells undergoing reproduction divide by binary fission.
– Binary fission is a form of asexual reproduction that produces
identical offspring.
– In asexual reproduction, a single parent passes exact copies of all its
DNA to its offspring (clones).
3. The formation of gametes involves a third type of cell division.
– Gametes, an organism’s reproductive cells, such as sperm or egg
cells. (Meiosis)
– Regardless of the type of cell division that occurs, all of the DNA
(deoxyribonucleic acid) must be present in each of the resulting cells.
III. Chromosomes: The cell structure in the nucleus that carries the genetic
material—DNA.
Chromosomes are the structures that carry MOST genetic information.
A.
Chromosome Structure:
A chromosome is a very long DNA molecule that contains many genes.
Chromatin- It is the DNA and proteins in the nucleus of a non- dividing
cell.
During most of a cell’s life cycle (interphase) chromosomes exist as
chromatin, which are strands of DNA wrapped around protein molecules
(uncoiled like tangled-up spaghetti). Chromatin is not visible during this
stage. DNA can not be seen under a compound light microscope. DNA
can be seen under an electron microscope.
Chromatin
Chromosome
Chromosome Structure Cont.
B. Two Parts of the Chromosome:
1. Chromatid- Before the DNA coils up, DNA is copied.
– The chromatin coils up into two identical parts of a
chromosome, which are called chromatids.
–
AKA—sister chromatids.
2. Centromere The point at which
each pair of chromatids are attached.
Holds chromatid (duplicated chromosomes) together.
A chromatid is attached to a spindle
fiber by the centromere.
DRAW sister chromatids:
•
IV. The Cell Cycle
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The life of a eukaryotic cell is the cell cycle which includes
interphase, mitosis, and cytokinesis.
A cell can only grow so large, when it reaches a certain point it will
divide into two cells.
So, the cell is as large as it can get—then it divides and the two cells
are small. This process is called mitosis.
The cell cycle is a repeating sequence of cellular growth and division
during the life of an organism.
A cell spends 90% of its time in the first three phases of the cycle.
What phase does the cell spend the most time? Interphase
A. Interphase:
• The busiest and longest part of the cell cycle.
• The cell is growing and the chromosomes are replicating
(copying).
Before mitosis begins, the chromosomes are copied. Each
chromosome now consists of two chromatids.
*Humans have 46 chromosomes (23 pairs), so now 92
chromosomes are ready for cell division. 46 will end up in 1
cell and the other 46 will end up in the 2nd cell.
• DNA is in the form of chromatin. Since DNA is in the form
of chromatin, it is not visible
Interphase Has Three distinct Phases:
1. G1 phase— Cellular growth.
2. S phase— Cellular growth and DNA synthesis— DNA
(chromatin) is replicated.
3. G2 phase— Cellular growth and preparation for
cell
division mitosis.
B. Mitosis
• Mitosis is the division of all somatic (body cells/non-sex cells). A
cut on your hand will repair itself using the process of mitosis.
• The first step of mitosis is the chromatin coiling up into
chromosomes.
• The first three steps are all involved with splitting of the nucleus.
• The last step involves the splitting of the cytoplasm, known as
cytokinesis.
• The end result of mitosis is the formation of two identical daughter
cells that are diploid (have a full set of chromosomes (2n).
• Mitotic cell division: The parent cell divides to form 2 daughter
cells with the same number of chromosomes 2n=46.
• For example: A human cell has 46 chromosomes. How many
chromosomes will it have after mitotic cell division? 46
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Here are the four stages of Mitosis:
1. Prophase 2. Metaphase 3. Anaphase
4. Telophase
1.
Prophase (P)
a. Stringy chromatin coils up into chromosomes.
Chromosomes are most easily seen during this phase because the
chromatin shortens and thickens.
Each duplicated chromosome is made of the two halves called sister
chromatids.
Sister chromatids and the DNA they contain are exact copies of
each
other and are formed when DNA is copied during Interphase.
b. Nuclear membrane disintegrates (disappears)
Spindle forms between the pair of centrioles which begin to move
to the opposite end (poles).
Spindles are cell structures made up of
both centrioles and individual microtubule
fibers that are involved in moving
chromosomes during cell division.
Centrioles help anchor spindles for movement
of chromosomes. The spindles pull the
chromosomes to opposite sides of the cell.
DRAW the cell in Prophase:
2. Metaphase


Chromosomes line up on the equator. (Centromeres
are attached to the spindle fibers.)
DRAW the cell in Metaphase:
3. Anaphase

a) Centomeres split separating chromatids.
Chromatids are now called chromosomes.

b) Chromosomes move toward opposite poles.

Draw the cell in Anaphase
4. Telophase
a) The identical chromatids, now chromosomes, reach
opposite poles.
b) Nuclear membrane reappears
c) Spindles disappear
d) Chromosomes uncoil into threadlike pieces
DNA is now called chromatin
Draw cell in Telophase
As mitosis ends, cytokinesis begins.
C. Cytokinesis
Divides the cytoplasm and organelles
between the two new cells.
**New daughter cells
will enter into
Interphase to begin
the cycle over again.
What are the differences between mitosis in animal cells and in
plant cells?
1. Plant cells do NOT have centrioles
2. Plant cells divide their cytoplasm with a cell plate.
Cell Plate
Centrioles
V The Cell Cycle
Factors that control the cell cycle:
•
The cell cycle is carefully controlled. Failure of cellular control can result
in cancer.
Cancer is the uncontrolled growth of cells. Cancer is a disorder of cell
division. Cancer is a mistake in the cell cycle.
Cancer is one result of uncontrolled cell division.
•
The loss of control may be caused by environmental
•
factors or by changes in enzyme production.
Cancerous cells affect normal cells, forming
masses of tissue called tumors that deprive
normal cells of nutrients.
In the final stages,cancer cells enter the
circulatory system and
spread throughout the body forming new tumors
that disrupt the functioning of organs.
Lung Cancer Cells