Mitosis / Meiosis / Reproduction: Asexual vs. sexual: Refer to Chapter 8 in the text book
1. Why do cells divide?
•
For unicellular species, cell division is synonymous with reproduction and is essential in order to
continue the species. For multicellular species, (like us) repeated cell division is necessary for growth,
development and repair. (see #2 below).
2. Define cell specialization and explain the pros/ cons associated with it. List the five levels of organization
in multicellular organisms.
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NOTE: Excluding sperm and eggs, every cell in your body contains all the DNA necessary to make
everything that is you. What is not understood is how, or when certain genes are “turned on” to create
one cell type while others are “turned off”. In other words, why don’t you have liver cells in your
bones, or brain cells growing in your stomach?
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Cell growth is followed by specialization – the ability of a cell to do a specific, unique function. The
five levels or organization are listed below.
Cell
•
Tissue
Organ
Organ Systems
Organism.
Single celled organisms must pay attention to all aspects of life. Cell specialization (also known as
differentiation ) results in a “division of labor”. The human body has approximately 200 different cell
types that have distinct roles.
•
The advantage of cell specialization is that it results in a tremendous variety of life forms that can
adapt and evolve as the earth changes. The disadvantage is that cells become “interdependent” on one
another; meaning if one group of cells fail, this can have a domino effect on how the whole organism
functions .
3. Explain what homologous chromosomes are. How are autosomes different from sex chromosomes?
• DNA is arranged into individual packages called chromosomes. See fig. 8-1 and 8-2.
• In organisms that reproduce sexually, chromosomes occur in pairs. See Table 8-1. The chromosome
number differs among species.
• Humans have 23 pairs of chromosomes (a total of 46 individually). They can be arranged from largest
to smallest when viewed as a karyotype. See fig.8-3.
• Of the 23 pairs, 22 are called autosomes and contain about 95% of the genes that spell out you. They
are referred to as homologous pairs because they contain variations of the same genes in the same
location. One member of each homologous pair comes from each parent. Any given trait you have is
the result of how the genes on the homologous pairs are translated.
In the example below, homologous chromosome pair #11 is shown below. The sign on each
chromosome indicates where the hemoglobin gene is located. Notice, the gene is in the same location on
each chromosome. A working copy of the gene (+) is what we want. The mutant form of the gene (-) leads
to a mutation in the protein which causes sickle cell anemia. Look at the following ways that the gene can
express itself in human being:
Person A
+
Person B
+
+
Normal Hemoglobin
Person C
-
Carrier for sickle cell anemia
-
-
Has sickle cell anemia
(Note: is does not matter which parent passed down the mutant gene)
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The 23rd pair is referred to as the sex chromosomes. These are not homologous. They are referred to
as X and Y. They contain genes that are responsible for hormones that determine gender.
If you are XX, (meaning you received one X chromosome from each parent), the stem cells in the
developing gonads will become ovaries and female genitalia will form.
If you are XY, (meaning you received an X chromosome from mom and a Y chromosome from dad),
the stem cells in the developing gonads will become testes and male genitalia will form.
• IMPORTANT NOTE: The whole Y chromosome is not necessary to become male. There is a single
“master-male-gene” called the Sex determining Region or, (SRY). It is the presence or absence of
SRY that determines what direction the premature gonad cells will go.
4.
Explain the difference between haploid vs. diploid cells; somatic vs. sex cells
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The term haploid refers to a cell that has one member of a homologous pair/s of chromosomes. The
symbol (N) is used to represent haploid cells.
•
Sex cells, (sperm and egg) are said to be haploid, or (N) and have 23 chromosomes (for the human
species).
•
The term diploid refers to a cell that has both members of a homologous pair/s of chromosomes. The
symbol (2N) is used to represent diploid cells.
•
Somatic cells ( all cells in your body that are NOT sperm or egg) are said to be diploid, or (2N) and
have 46 chromosomes.
•
Summary:
For the Human Species
Haploid = N = sex cells = 23 chromosomes
Diploid = 2N = somatic cells = 46 chromosomes
Review the above concepts by reading pages 150-153 in your text.
5. Describe the different stages of a cell’s lifecycle (Interphase, Mitosis and Cytokinesis). Compare animal
mitosis to plant mitosis. Read pages 154-159 and study all figures. Also review the completed worksheet
“The Cell Cycle”.
6. Explain what “stem cells” are and how embryonic stem cells differ from adult stem cells . Read pg. 160 in
your text book, and the articles handed out in class.
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Embryonic stem cells are present in the very early stages of development and can become ANY cell
type (the term for this is pluripotent). They can be grown in labs and appear to be “immortal” under
laboratory conditions.
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Adult stem cells are those that have a predestined pathway and are necessary for growth and repair of
tissues throughout one’s life. A skin cell for example will always be a skin cell for the duration of that
person’s life. It has a finite number of divisions before that cell line dies. When enough adult stem cell
lines die, the tissues and organs cannot function and the organism cannot survive.
•
NOTE: Embryonic stem cells are easier to harvest but there is controversy about using human
embryos as “cell factories”. Adult stem cells are harder to find and more difficult to keep alive.
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Recently, scientists have discovered a way to “reboot” normal body cells back to the pluripotent state,
through genetic engineering. We can then guide their development into specific tissues. These are
called induced pluripotent stem cells or iPSC’s. The promise of using stem cells is to transplant
them into damaged or diseased areas of the body and “coax” them to take on the role of the
surrounding tissue or organ. Human stem cells are also being transplanted into other species to create
chimeras; animal / human hybrids that are producing human proteins or perhaps one day human
organs.
7. Explain the role of colchicine in agriculture. Refer to the article on the shared folder, “Polyploidy”
8. Define asexual reproduction, list the different methods of asexual reproduction and provide examples of
organisms that use these methods. See pages 32 & 33 from the LIFE book titled “Asexual Reproduction”.
What are the pros and cons of reproducing sexually vs. asexually? See the bottom of page 164.
9. Explain how cloning is done. Refer to the articles handed out in class and on the shared folder.
10. Define meiosis and explain its importance to organisms that reproduce sexually. How is it different from
mitosis? Explain how male and female sex cells (also called gametes) are formed. Read pages 161-164 and
refer to the double sided handout: “Gamete formation” on one side and “Differences between Mitosis and
Meiosis” on the other side.
11. Define nondisjunction. Describe the following disorders caused by nondisjunction: Down’s syndrome,
Turner’s syndrome, Kleinfelter’s syndrome and Fragile X syndrome . Read the bottom of pg. 239 and Table 1
on the top of pg. 253. Also read the handout titled 12.3 – Changes in Chromosome Numbers.
NOTE: ALL THE ARTICLES THAT WERE READ IN CLASS CAN BE FOUND ON THE SHARED FOLDER