Last updated: 10/24/01
GST 2020 -
Changing Life on Earth
Supplement for Agenda 7
Contents:
Energy priorities for different metabolic pathways
The priorities for energy use are listed below. The order is that the first listed is
used first; if insufficient energy is available, then go to the next in the list. Note
that each source is a different metabolic pathway.
- glucose in bloodstream
- stored glucose
- other carbohydrates
- fats
- proteins (This damages the body if it goes far enough.)
The priorities for using or storing energy from food are listed below. The order is
that energy from food first goes to the first on the list. If there is an excess supply of
energy from food, then the excess goes to the next on the list. Note that each use is a
different metabolic pathway.
- carbohydrates converted directly to energy
- excess energy goes to energy storage in the form of carbohydrates
- excess energy goes to energy storage in the form of fats
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Comparison of Types of Cell Division: Mitosis
and Meiosis
| Characteristic |
Mitosis |
Meiosis |
| When is it used? |
1. Asexual reproduction
2. "Front end" of sexual
reproduction
3. Normal growth for
multicellular organisms |
Making gametes or sex cells (sperm, ovum) for sexual
reproduction |
| Number of chromosomes |
Same as original (diploid) |
Half of original (haploid) |
| Number of cells |
1 divides into 2 |
1 divides twice into 4 |
| Level of diversity in daughter cells |
No diversity, daughter cells are clones (except for
mutations) |
Diversity from:
1. Crossing over during
chromosome separation
2. Independent assortment
of chromosomes
3. Sexual recombination at
fertilization |
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A worked-out Punnett Square problem:
Two organisms reproduce, one with the genotype Aa and the other with the genotype aa.
Find (a) the phenotypes of the parents, (b) the genotypes of the offspring, with
percentages of each, and (c) the phenotypes of the offspring, with percentages of each.
- Parental phenotypes. The Aa parent has the dominant phenotype while the aa parent has
the recessive phenotype.
- Genotypes of the offspring. Use the Punnett Square method. The first step is to draw the
square. Then write the possible genotypes of the gametes (sex cells, or sperm and ovum
cells) along the top and left-hand edges, as shown below. It does not matter which parent
goes where; here, the first parent (Aa) goes along the top.
The second step is to fill in the interior cells. Each interior cell gets one gene from
the top, the gene in the same column it is in, and one gene from the left, the gene in the
same row it is in, as shown below.
The genotypes are then: Aa (one of the four cells) 25% and aa (three of the four cells)
75%.
- Using the genotypes, the phenotypes are: dominant (Aa) 25% and recessive (aa) 75%.
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Anthrax in the news and in this course:
Anthrax is a disease following from infection with the bacterium Bacillus anthracis.
In nature, the life cycle of this bacterium is:
- Existence outside of a host (infected organism) in spore form. The spore is a bacterium
inside a tough coat (I haven't been able to find out yet what the coat is made of -
protein, cellulose, etc.). The cellular metabolism (chemical reactions) are shut down, so
it needs no nutrients and generates no waste in this state. It apparently can exist up to
a century in this form, in soil.
- An animal becomes infected (the three routes are (a) skin infection through an open sore
or wound (most common but not the most dangerous), (b) lung infection through deep
inhalation (not common in nature but the most deadly) and (c) infection through eating
uncooked infected meat (rare). The infected animal, whether human or non-human, is called
a "host" to the bacteria. The skin infection form for people is relatively
common in rural areas, through human contact with infected animals.
- The bacterium itself is not harmful, but it releases toxins (harmful chemicals) that are
poisonous. The bacterium divides rapidly, so the level of toxins increases rapidly. The
bacteria can reproduce (divide) as rapidly as every half hour, with the result that the
number of bacteria doubles every half hour. (If this incredible rate of cell division
continued for 23 hours, the number of bacteria would equal the total number of all cells
in the human body, about 65 trillion.) Treatment with antibiotics (Cipro, penicillin,
etc.) kills the bacteria but leaves the toxins, so if the infection has become advanced
(many bacteria, high level of released toxins) the infection is lethal anyway. Without
treatment, or with treatment that is too late, the host dies from the toxins. The bacteria
continue to multiply inside the cells of the now-dead host, using the existing nutrients
in the cells.
- When they run out of nutrients, the bacteria return to the spore state. They return to
the cell as the host decomposes, or they flow out in the ooze from the host's infections.
How does the infection work inside the host? Living animals have a complex immune
system, which normally protects the animal from infections. The immune system, for
example, is why colds and flue normally go away. The infection actually causes the immune
system to overreact, and this overreaction is what actually kills the host . (Something
similar, but not deadly, happens with allergies; it is the overreaction of the immune
system that causes the allergic reaction.). Here are the stages of the infection:
- When the bacterial spores enter the body, they are engulfed by "macrophages,"
cells within the immune system that isolate foreign objects, essentially by swallowing
them whole. The spores become normal anthrax bacteria, and multiply within the
macrophages, and burst the macrophages open, at which point the bacteria enter the
bloodstream. (Apparently even in the bloodstream, the anthrax bacteria cannot travel very
far from the site of the infection, which is something that I don't understand yet.)
- The anthrax bacteria generate and release their toxins. There are three toxins that work
together to produce the lethal effect:
- "Protective antigen" (named at a time when biologists thought it actually
protected us!). Once released from the anthrax bacteria, these proteins self-assemble in
clusters of seven. The cluster has a "docking space" which can accept one
molecule of either of the other two toxins (see below), which are both enzymes. This
completed assembly of seven protective antigen proteins and toxic enzymes enters a host
cell, crossing the cell membrane by the vesicle method.
- "EF" or "edema factor" (edema means swelling and oozing). Once
inside the host cell, this enzyme stimulates production of a normal cellular hormone, so
much so that the hormone levels immobilize the cellular immune system. The cell is now
defenseless.
- The "LF" or "lethal factor" enzyme stimulates macrophage cells to
cause inflammation, which is normally a protective mechanism. But the LF enzyme is so
active that the hormone and inflammation become lethal to the cells.
So, the infection turns the body's normal defense mechanism into weapons against the
cell, by causing it to produce such high levels of chemicals which are normally
protective, but which become lethal at these high levels.
Anthrax is apparently a relatively recent disease in evolutionary terms; genetic
evidence suggests it developed approximately 10,000 years ago (modern humans developed
perhaps 50,000 years ago, and life on earth began about 4 billion years ago).
At present, we have no means of treating for the toxins once they are released at high
levels, so treatment is to use antibiotics to kill off the bacteria before they release
high levels of toxins. Knowing the amino acid sequence of the toxic proteins, and how they
work, biologists expect that we will be able to develop medicines that will work against
the toxins.
Anthrax Spores: Alive or Dead?
According to the list of characteristics of life, is Anthrax alive or dead when it is
in the spore state? It does not use energy, it does not reproduce and it does not evolve.
It only senses and responds to the environment if the environment is the interior of a
potential host. Remember that generally, life shows all four characteristics. This
in-between case is the norm in Biology. This is similar to viruses, which have some
characteristics of life, but not all. Viruses cannot reproduce themselves, for example,
but depend upon normal living cells to make copies of the virus.
Anthrax: Reproduction and Evolution
Anthrax reproduces (divides) asexually. The only source of variation is mutations. If a
particular anthrax cell has a mutation, all descendants of that cell will have the same
mutation. This means that the family tree of anthrax is like a pitchfork: for each
mutation there is a new fork that continues the same, like the "Evolution theme"
diagram for this course, at the top of each web page. These forks or branches
("tines" in the fork analogy) are called "strains." Each of the
mutations is at least somewhat advantageous; otherwise it would lose out in the
competition for resources. Suppose that one trait carries a mutation "A" while
another carries "B". There is no way for these traits to mix so that there could
be a trait with both A and B. The only way that this could happen would be if one of these
two strains acquired the other mutation. This would be a slow process, so the evolution of
anthrax is a slow process.
With sexual reproduction, as soon as the A strain and the B strain mated, there would
be a good chance of an AB bacterium being produced. The pace of evolution is much faster
with sexual reproduction.
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Changing
Life on Earth, GST 2020, 4 cr
