Human Biology/Compilation II, Chapters 17-20
Table of Contents
Cell Reproduction and Differentiation
- The cell cycle creates new cells
- Replication, transcription, and translation: An overview
- Cell reproduction: One cell becomes two
- How cell reproduction is regulated
- Environmental factors influence cell differentiation
- Cloning an organism requires an undifferentiated cell
- Therapeutic cloning: Creating tissues and organs
Cancer: Uncontrolled Cell Division and Differentiation
- Tumors can be benign or cancerous
- Cancerous cells lose control of their functions and structures
- How cancer develops
- Advances in diagnosis enable early detection
- Cancer treatments
- The 10 most common cancers
- Most cancers can be prevented
Genetics and Inheritance
- Your genotype is the genetic basis of your phenotype
- Genetic inheritance follows certain patterns
- Other dominance patterns
- Other factors influence inheritance patterns and phenotype
- Sex-linked inheritance: X and Y chromosomes carry different genes
- Chromosomes may be altered in number or structure
- Many inherited genetic disorders involve recessive alleles
- Genes code for proteins, not for specific behaviors
DNA Technology and Genetic Engineering
- DNA sequencing reveals structure of DNA
- DNA can be cloned in the laboratory
- Genetic engineering creates transgenic organisms
- Gene therapy: The hope of the future?
Cell Reproduction and Differentiation
In multicellular organisms, cell division and growth are what allows that organism to grow. Amazingly, a human life starts out as a single cell and by the time you are born, you consist of over 10 trillion cells! Cell division continues throughout your life, especially during childhood and adolescence. Red blood cells alone are replaced every 120 days, which works out to 175 million cell divisions every minute!
 |
| Cell Division / visualphotos.com / accessed 3/8/12 |
The Cell Cycle Creates New Cells
The cell cycle is defined as the creation of new cells from existing cells via a repetitive sequence of events. There are two phases: interphase and the mitotic phase.
Interphase is the longer of the two phases, during which cells grow and DNA is duplicated. Interphase is divided into three subphases: G1, S and G2. G1 stands for "first gap", which is the period between the last cell division and DNA synthesis. S phase stands for "synthesis". During this phase chromosomes are duplicated. And during G2 or "second gap", the cell continues growth in preparation for cell division.
In the mitotic phase there are two subphases: mitosis and cytokinesis. In mitosis, the DNA is divided into two sets and the nucleus divides. In cytokinesis, the cytoplasm divides and two new "daughter" cells are formed.
Replication, Transcription, and Translation: An Overview
There are three billion pairs of DNA on 46 chromosomes in a human. Chromosomes organize and arrange the DNA inside the nucleus. Because DNA represents all instructions for life, when a cell divides, it must have the exact same DNA
every time it splits or divides. DNA replication, then, is the process of copying the DNA before the cell divides. Transcription is the process by which the DNA code of a single gene is converted to a single strand of messenger RNA (a gene being the smallest unit of a chromosome). DNA/chromosomes are too big to pass through the nucleus, so it must convert to a smaller form to pass into the cytoplasm. And translation is the process of converting mRNA into proteins.
 |
| DNA Strand / getfreeimage.com / accessed 3/8/12 |
Replication: Copying DNA before cell division
Remember that DNA is a double stranded helix. When it replicates, enzymes at various points along the helix unwind a portion of the DNA. This creates a bubble in the two strands and new strands begin to form in the bubble. This proceeds outward in opposite directions until until the replication bubbles join. This happens at thousands of sites at once, making the process very quick, 7-8 hours, to be exact.
Mutations are alterations in DNA
Any alteration in the DNA is a mutation. It can be caused by a mistake in the replication process as well as by chemical or physical forces that damage a segment. If these errors are not corrected before duplication, cancer may result. It can also be passed along to future generations, depending on where the mutation is located.
Mechanisms of DNA repair
There are numerous enzymes that actually repair DNA damage. They can cut out damaged sections as well as reconnect its backbone. This process is most active between the replication process and the beginning of mitosis. This way, the best possible copy is passed on to the daughter cell. If the damaged genes are the ones that actually control the repair process, then mutated DNA may accumulate faster than normal. This has been connected to an increased risk of colon and breast cancer.
Transcription: Converting a gene's code into mRNA
Transcription, again, converts a single gene code into mRNA. It is very similar to the replication process except that only one segment of DNA and one single gene unwind, instead of the entire molecule. Because RNA is a single strand, only one of the two strands has the genetic code that specifies the synthesis of RNA. Also, RNA carries a base of uracil instead of thymine, like DNA. And the sugar of RNA is ribose, rather than deoxyribose. The mRNA attaches to a ribosome, which is the template for protein synthesis.
Translation: Making a protein from RNA
There are three steps to the translation process, which converts RNA to protein.
- Initiation: a tRNA (transfer molecule), binds to two ribosome subunits and the mRNA molecule. This moves along the mRNA until they find a "start" codon. Then they are joined by a larger ribosomal subunit to form an intact ribosome and this holds the mRNA in place while the tRNA tranports amino acids to it.
- Elongation: The tRNA captures amino acids and brings it to mRNA. As mRNA passes between two ribosomal subunits, the ribosome binds to the tRNA, which basically glues the the bond between the new and "old" amino acid. The tRNA is then freed to find more amino acids. This forms the long chain one amino acid at a time.
Cell Reproduction: One Cell Becomes Two
Mitosis: Daughter cells are identical to the parent cell
Mitosis, again, is the process of nuclear division, where
the sister chromatids of the duplicated chromosomes separate. After mitosis,
each daughter cell has an identical set of DNA to the parent cell. And although
there are very defined phases for mitosis, it is a seamless process.
Prophase -
begins when you can first see the duplicated chromosomes. In this phase, the
tubular elements of the cytoskeleton come apart and reassemble between pairs of
centrioles.
Metaphase - could be described as a tug-of-war, where
molecules are pulled in opposite directions, but do not separate yet.
Anaphase - here, the molecules separate abruptly and move to
opposite cell sides. ATP energy is
required for this step.
Telophase -
Once two sets of chromosomes have reached opposite polar ends, telophase
begins. In this phase, new nuclear membranes form around the chromosomes after
the mitotic spindle comes apart.
Cytokinesis divides one cell into two identical cells
Cytokinesis was the forerunner to our delivery and recycle
systems. Living cells had it down way before we did. This is the process used
to divide a cell into two daughter cells. A contractile ring is assembled just
before it is needed from the remnants of the cytoskeleton. This ring tightens
and pinches the cell in two. Then the ring disassembles to form new
cytoskeletons. Quite amazing, actually.
 |
| Mitosis / shutterstock.com / accessed 3/8/12 |
Mitosis produces diploid cells, and meiosis produces
haploid cells
Mitosis Dance Video (It takes a few minutes to watch, but is awesome and very creative).
Diploid cells are human cells that have 46 chromosomes (23
pairs). They reproduce by undergoing mitosis. Sperm and egg cells are haploid
cell, which only have one set of 23 chromosomes. These cells are created by
meiosis.
Meiosis: Preparing for sexual reproduction
Meiosis is the sequence of two nuclear divisions where the
human genes are mixed, reshuffled and reduced by half. Once fertilization
occurs, this egg and all subsequent cells become diploid. Meiosis I and II have
four stages: prophase, metaphase, anaphase, and telophase.
 |
| Plant Cell Meosis / nature.com / accessed 3/8/12 |
Sex differences in meiosis: Four sperm versus one egg
In a male, meiosis creates four sperm, because the chances
are very slim that any one will reach the egg. In females, as much of the
cytoplasm as possible is reserved for one daughter cell at each cell division.
Meiosis II in a female is not complete until the egg is fertilized.
How Cell Reproduction is
Regulated
Not all cells in the body divide at the same rate. There are
some cells that stop dividing after adolescence and some that divide rapidly
throughout your life span. And
then, there are variable speeds at which they divide. There are internal
controls inside the cell and there are also checkpoints along the way that may
stop cell division if that cell is not ready. Another factor that may stop cell
division comes from without the cell. For example, if certain nutrients and/or
hormones are not available. And finally, other cells can also affect the cycle.
When they come into contact with one another, they release a substance that
stops cell division. This is an example of a negative feedback system working
to control tissue growth and organ size.
Environmental Factors
Influence Cell Differentiation
How is it, that even though every cell starts out with the
same DNA, there are many different kinds of specialized cells? Differentiation
is the process that develops a cell differently than the parent or sister
cell. This happens because
different genes are expressed.
Differentiation during early development
It is the environment surrounding the cells that affects
what genes are developed in each cell. For example, some may be exposed to more
O2 content or a different pH. So,
what is happening internally is very much mimicked by what happens externally
as a child develops. Our surroundings or environment have a great deal to do
with how we develop psychologically, emotionally and socially.
Differentiation later in development
In later development there are two factors that influence
differentiation, one is it’s environment and two is the history of the cells
that came before it. For instance, a cell that begins to be a muscle cell also
differentiates into either smooth, cardiac or skeletal muscle.
Cloning an Organism
Requires an Undifferentiated Cell
There are two techniques for accomplishing reproductive
cloning: embryo splitting and somatic cell nuclear transfer.
Embryo splitting: producing identical offspring
Cloning by embryo splitting produces eight identical clones
to each other, but not exact copies of the parent. This is done in vitro (a
fertilized egg is implanted into a surrogate mother).
Somatic cell nuclear transfer: cloning an adult
A somatic cell is any cell in your body except a sex cell
and it has a full set of DNA. This process develops an identical clone to the
parent. Dolly the sheep was the
first successful result of this process.
Therapeutic Cloning:
Creating Tissues and Organs
Therapeutic cloning is for the purpose of treating disease
in humans with the ultimate goal being to create cells, tissues and possibly
even whole organs for patients. However, as it becomes more successful, it
brings up the issues of human cloning. It is a social and ethical debate that
must have answers before we get there.
Cancer: Uncontrolled Cell
Division and Differentiation
Cancer is an ugly word to all of us and one that is greatly
feared. Most of us have been touched by this disease in one way or another. One
way to mitigate this fear is to begin to understand what cancer is. In doing
so, we need to understand a couple of things about normal cell development:
One, normal cells have mechanisms that regulate the rate of cell division. It
is carefully controlled by an internal clock, hormones, and from nearby cells.
Two, normal cells, for the most part, remain in one place. One exception to
that being blood cells.
 |
| Tumor / google.com / accessed 3/8/12 |
Tumors can be Benign or
Cancerous
Sometimes normal cells increase their rate of division as
part of the their normal function, which is called hyperplasia. But when this
goes out of control, a mass forms. This is called a tumor or neoplasm. But not
all tumors are cancerous. Benign tumors are defined as a mass that remains in
one place and is well-defined. It
still has most of the structural features of the original cells and may be
surrounded by connective tissue. Benign tumors usually only threaten your
health if they become so large that they start to crowd out normal cells. Most
of the time, these tumors can be removed easily via surgery. Moles are
sometimes a form of a benign mass, which is why it is good to monitor them for
any changes, in case they start to turn cancerous.
Cancerous Cells Lose
Control of their Functions and Structures
 |
| Lung Cancer Cell Division / bayer.com / accessed 3/8/12 |
In addition to rapidly increased cell division, cancerous
cells also lose structure and function. The nucleus may become larger and there
is less cytoplasm. This is called dysplasia. Dysplasia often is a sign of
precancerous cells forming. When some of the cells in a tumor start to lose any
semblance of structure, organization and regulatory control, it is defined as
cancer. In situ cancer remains in one place, and if caught early enough, can be
removed. If the cancer cells begin
to separate, then metastasis may result, spreading to other parts of the body.
They spread through the blood or lymph and begin to develop new colonies,
sometimes totally overrunning entire tissues, organs and organ systems. It
consumes everything just like a forest fire. One in three people in the U.S.
will experience cancer in their lifetime and one in four people will die from
it. It is the number two cause of death in the United States, second only to
heart disease.
 |
| Metastasis Colonies / fmp-berlin.info.com / accessed 3/8/12 |
How Cancer Develops
Mutant forms of proto-oncogenes, tumor suppressor genes,
and mutator genes contribute to cancer
There are three types of genes that have been identified
that help to control various cell activity. Proto-oncogenes promote cell
growth, differentiation, division or adhesion. Tumor suppressor genes normally
inhibit unchecked cell growth, division, differentiation or adhesion. And mutator
genes help in DNA repair during replication. If any of these particular genes
become damaged themselves, they mutate and a variety of cancers may begin to develop more readily.
 |
| Air Pollution / toxipedia.org / accessed 3/8/12 |
A variety of factors can lead to cancer
No single defect is enough to cause cancer. It is when
numerous factors are present, that allows cancer to form. Possibly the number
one factor is age. Cells begin to wear out and repair mechanisms begin to fail
more frequently. Factors that cause cancer, or carcinogens, are listed below.
 |
| Sunburn / lifehacker.com / accessed 3/8/12 |
- Viruses
and Bacteria
- Chemicals
in the Environment
- Tobacco
- Radiation
- Diet
- Internal
Factors
Tobacco accounts for over 30% off all cancer deaths. Diet
also may be up to 30%, rivaling tobacco. The radiation from sunlight causes up to
80% of all skin cancers, including melanoma.
The immune system plays an important role in cancer prevention
Suppression of the immune system is involved in many
cancers. Some cancers can suppress the immune system, while others can disguise
themselves from attack by the immune system. Other factors that can suppress
the immune system are drugs, viruses (including HIV), states of anxiety,
stress, and depression, which allow cancers to develop more easily.
Advances in Diagnosis
Enable Early Detection
Early detection of cancer plays a key role in the success of
beating it. The sooner it can be treated, the less likely it is to metastasize.
Prompt treatment, in some cases, can cure it.
Tumor imaging: X-rays, PET, and MRI
X-rays are the traditional way of detecting tumors. But
advances in technology can now detect changes that traditional x-rays may miss
by using either the PET, positron-emission tomography, or MRI, magnetic
resonance imaging.
Genetic testing can identify mutated genes
Hundreds of genes and their mutated counterparts have been
identified and tests are being devised to detect them. However, if a mutated
gene is found, this is just a risk factor, not a be-all, end-all for developing
cancer. And there may not be a cure, either.
Enzyme tests may detect cancer markers
Telomerase is an enzyme that is rarely found in normal
cells, but is almost always present in cancer cells. This and other cancer
markers are being explored for early detection of cancer.
Cancer Treatments
While some cancers are more treatable than others, when
detected early enough, current treatments cure approximately 50% of all cancer
cases.
Conventional cancer treatments: surgery, radiation, and chemotherapy
Surgery, Radiation and Chemotherapy are the traditional
forms of treating cancer, many times combining two or more of these. The
drawbacks being that surgery may miss some metastasized cells, allowing the
cancer to reappear later. And radiation can also miss these as well as destroy
healthy cells in the process of destroying cancerous cells. Chemotherapy is the
administration of chemicals and addresses some of the limitations of surgery
and radiation. However, it can damage normal cells, especially in the bone
marrow and digestive tract. Many times the tumors become resistant to the
chemical, just like bacteria to antibiotics. Combinations of these therapies
sometimes work best.
 |
| Treatment of Cancer / nanobiotechnews.com / accessed 3/8/12 |
Magnetism and photodynamic therapy target malignant cells
Researching have been developing techniques to target
malignant cells more precisely and avoid killing off normal healthy cells. Two
of the more promising techniques are magnetism and photodynamic therapy.
Magnetism is currently in clinical trials with liver cancer
patients. In this technique, a powerful magnet is positioned at the tumor site.
Tiny metallic beads, coated with a chemotherapy drug, are injected into the
bloodstream. The magnet pulls the beads into the tumor and the chemo drug then
kills the cancer.
Photodynamic therapy has been approved and in use for
several years to treat tumors of the esophagus and lungs. And it looks
promising for other cancers as well, as they refine laser techniques. In this
therapy, the patient takes a light-sensitive drug that is drawn into the cancer
cells. Then laser light, at a particular frequency, is focused on the tumor.
This triggers a series of chemical reactions that kill malignant cells.
Immunotherapy promotes immune response
In immunotherapy, they attempt to boost the responsiveness
of the immune system so that it may fight cancer more effectively. Research has
been focused on finding specific antigen molecules that are present in cancer
cells, but not normal cells. Then they are used to produce antibodies that
target cancer. In one step further, researchers are trying to attach
radioactive molecules or chemotherapy drugs to the antibodies to deliver treatment
to the cancer cells and sparing the normal cells. They are also testing
vaccines made from a patient’s own cancer cells. Once injected, the modified
cancer cells seem to stimulate the person’s immune system to combat the
abnormal cells.
“Starving” cancer by inhibiting angiogenesis
Because tumors grow and divide rapidly, they need a great
deal of energy. Something within the tumor is actually promoting angiogenesis
or the growth of new blood vessels, for energy. There are proteins that promote
angiogenesis and proteins that inhibit it. So the theory is to starve the tumor
by limiting its blood supply. Several anti-angiogenic drugs are now on the
market.
Molecular treatments target defective genes
Research is being done for gene therapy, attempting to inactivate
specific genes, or the proteins they encode, to slow cell division. Gene
therapy would either repair or replace defective genes with normal genes. One
key target is the p53 tumor suppressor gene, that when defective, contributes
to numerous cancers.
The 10 Most Common Cancers
The following is a list of the 10 most common cancers.
- Skin
cancer – Look for changes in your
skin
- Lung
cancer - Smoking is leading risk factor
- Breast
cancer – Early detection pays off
- Prostate
cancer - Most common after age 50
- Cancers
of colon and rectum – Tests can
detect them early
- Lymphoma – Cancers of lymphoid tissues
- Urinary
bladder cancer – Surgery is often
successful if done early
- Kidney
cancer – Detected during examination
for a renal-related problem
- Cancer
of the uterus – Unusual uterine
bleeding is major symptom
- Leukemia – Chemotherapy is often effective
Each of these has its own risk factors, warning signs,
methods of detection, treatments, and survival rates. The most frequent cancers
are skin, lung, colon and rectum cancers. Prostate cancer is most common in men
and breast cancer in women. The most deadly cancers are lung, colon, rectum,
and breast cancers.
 |
| Healthy Food / i.istockimg.com / accessed 3/8/12 |
Most Cancers Can Be
Prevented
Despite heritable cancers, most incidences of cancer are
thought to be preventable. (And early detection plays a vital role in
successfully treating heritable cancers.) At least 60% of all cancer cases,
other than non-melanoma skin cancers, are thought to be caused by only two
factors – smoking and poor diet. Skin cancers could be prevented by limiting
our exposure to the sun. Public
education is vital to decreasing the death rate of cancer. The following are
tips to reducing your own risk:
- Know
your family history.
- Know
your own body.
- Get
regular medical screenings for cancer.
- Avoid
direct sunlight between 10 am - 4 pm. (where sunscreen and a broad-brimmed
hat).
- Watch
your diet and your weight.
- Don’t
smoke. (And limit your exposure to second-hand smoke).
- If you
consume alcohol, drink in moderation.
- Stay
informed.
Genetics and Inheritance
Genetics is the study of genes and their transmission from
one generation to the next. The
development of a human being is contained in the DNA within the nucleus of the
fertilized egg. This information is expressed in the form of genes, which are
DNA sequencing codes for one or more proteins. Each of us inherits one complete
set of genes from both mother and father. Though each of has a complete set,
they vary from individual to individual, which accounts for our differences.
Genes affect our traits, features, health and possibly even our thoughts and
actions.
Your Genotype is the Genetic Basis of your Phenotype
We each possess 23 pairs of chromosomes, 22 are autosomes,
and one is a pair of sex chromosomes. Humans inherit one of each pair, giving
us two copies of each gene. But when you look closer, there are small
differences in the DNA sequence. These differences produce alleles, which are
alternative versions of the gene.
Your complete set of alleles is your genotype. Your genotype influences
you phenotype, which is the observable physical and functional traits that
characterize you.
Genetic Inheritance Follows
Certain Patterns
Punnett square analysis predicts patterns of inheritance
People with the same two alleles of a gene (AA or aa) are
homozygous. And people with different alleles (Aa) are heterozygous. A Punnett
square helps to provide possible combinations of patterns of inheritance for
that particular genotype. You place the male gametes on one axis and the female
on the other axis. Then you place the possible combinations in each square. But
keep in mind that each parent will only donate one of each chromosome.
 |
| Mendels Flowers / upload.wikimedia.org / accessed 3/8/12 |
Mendel established the basic principles of genetics
Gregory Mendel was a university-educated Austrian monk who
lived in the 1850’s. He specialized in natural history. Over a seven year
period, he studied and experimented
with garden peas, noting that there were discrete factors of heredity
that unite during fertilization and then separate again with sperm and egg. The
law of segregation is now known as Mendel’s first rule of inheritance: when
gametes are formed in the parents, the alleles separate from each other so that
only one gamete gets only one allele of each gene.
Dominant alleles are expressed over recessive alleles
 |
| Dominant Allele Trait / dreamstime.com / accessed 3/8/12 |
A dominant allele is expressed as (A) and a recessive allele
as (a). Mendel noted that some peas skipped a generation in regards to color.
He determined that some colors had dominance over others. In this case, yellow
was dominant and green, recessive.
This holds true for humans as well. There are some recessive alleles,
that when present, result in the absence of a functionally important protein.
An example of this is cystic fibrosis, which afflicts only homozygous recessive
people. The term dominant only refers to how an allele behaves in combination
with a recessive allele in a heterozygote.
Two-trait crosses: independent assortment of genes for different traits
Mendel formulated his law of independent assortment when
performing a two-trait crossing. This states that the alleles of different
genes are distributed to egg and sperm cells independently of each other during
meiosis. But he was only partially right. Only genes located on different
chromosomes always assort independently. Alleles located on the same chromosome
(linked alleles), may or may not be inherited together.
Other Dominance Patterns
Incomplete dominance: heterozygotes have an intermediate phenotype
In incomplete dominance, the alleles do not follow the
dominant/recessive pattern. Instead, the heterozygous genotype results in a
phenotype that is intermediate between the two homozygous phenotypes. An example of this is the trait in
Caucasians of having either straight, wavy, or curly hair.
Codominance: both gene products are equally expressed
In codominance the products of the two alleles are expressed
equally. One example of this is
blood type. There are 3 alleles for this gene: O, A and B. O is recessive and
when combined with A or B, the person will have either A or B. But But the A
and B alleles are codominant. If a person is heterozygous with an A and a B
allele, they will have AB antigens in their red blood cells. Sickle-cell anemia
is another example.
 |
| Polygenic Inheritance / static.guin.co.ak / accessed 3/8/12 |
Other Factors Influence
Inheritance Patterns and Phenotype
Polygenic inheritance: phenotype is influenced by many genes
Polygenic inheritance is when phenotypic traits depend on
numerous genes, all acting simultaneously. For example, eye color is controlled by at least three
genes, with a range of color phenotypes from nearly black to light blue. Other
examples include height, body size, and shape.
Both genotype and the environment affect phenotype
Phenotype is influenced in part by our genotype and in part
by our environment. An example being our body size and height and the effect of
diet. In developed countries especially, there is a trend towards increased
height and weight in certain populations. Sometimes those changes occur within
one generation, which is too short a time period to be due to the gene pool. It
is mainly due to improvements in diet and nutrition, especially in the
young. Another example is our
inheritable risk factors (genes). If
we know that something is a risk, say heart disease, by dieting, exercise, and
reducing environmental risks, you may eliminate or at least, much improve your
chances of never developing thos problem.
Linked alleles may or may not be inherited together
Many alleles for different traits are sometimes inherited
together because they are physically joined on the same chromosome. These are linked alleles and the closer
they are on the chromosome, the more likely you are to inherit both
traits.
 |
| Human Karyotype / sciencephoto.com / accessed 3/8/12 |
Sex-Linked Inheritance: X
and Y Chromosomes Carry Different Genes
A karyotype is a composite display of all 23 chromosomes of
an organism. They are only identifiable right before cell division. Every pair
is matched up (autosomes), except for the last pair, which are the sex
chromosomes, X and Y. Females have two XX chromosomes and males have an X and a
Y chromosome. It is the male sperm that dictates the sex of the offspring.
Females will of course donate an X, whereas males can either donate an X or a Y
– so, a Y will determine a male and an X a female.
Sex-linked inheritance depends on genes located on sex chromosomes
 |
| X Chromosomes / cdn.physorg.com / accessed 3/8/12 |
Sex-linked inheritance is X-linked if the gene is only
located on the X chromosome and Y-linked if it is only on the Y
chromosome. Genes on the Y chromosome
are mainly related to “maleness”. Meaning the male sex organs, production of
sperm, and the development of secondary sex characteristics. But genes on the X
chromosome are numerous and not related to sex determination. In females,
because they have two X chromosomes, there is a backup if one is abnormal, just
like the autosomes. But in males, they only have one X and because of this,
they are more susceptible to diseases associated with recessive alleles on the
sex chromosomes. The best example of an X-linked disease is hemophilia.
Muscular dystrophy and red-green color blindness are two more examples.
Sex-influenced traits are affected by actions of sex genes
An example of sex-influenced traits that are affected by the
actions of genes on the sex chromosome is baldness. This allele is recessive in
females and so much rarer. But in males, with the same gene, their hair loss is
significant. This is due to testosterone, which stimulates this allele,
changing it from recessive to dominant.
Chromosomes may be Altered
in Number or Structure
Nondisjunction is the failure of homologous chromosomes or
sister chromatids to separate properly. If this happens during mitosis, it is
not as serious as when it happens during meiosis. During meiosis, if this goes awry, it has the potential to
alter the development of an entire organism. Most of the time, this is taken
care of before we even know there was a problem. The embryos do not survive.
Down syndrome: three copies of chromosome 21
Of the embryos that do survive the alteration of autosomal
chromosome numbers, the most common is Down syndrome. There are three types of
Down syndrome, but the most common is caused by having three copies of
chromosome 21 (trisomy 21). Age of the mother is a determining factor in the
risk of having a child with Down syndrome. Under age 30, the risk factor is one
in every 1300. By age 40, it jumps to one in 100 and by age 45, one in 25. And
recent studies show that paternal age may also play a part if both parents are
over age 35.
Alterations of the number of sex chromosomes
There are a variety of combinations that nondisjunction of
the sex chromosomes can produce. The following are the most common:
- XYY –
Jacob syndrome.
- XXY –
Klinefelter syndrome.
- XXX –
Trisomy-X syndrome.
- XO –
Turner syndrome.
Deletions and translocations alter chromosome structure
Deletions occur when a piece of chromosome breaks off and is
lost. Most of the time, the lose of a gene kills the sperm, egg, or embryo. But
on rare occasions, there is a live birth. An example is cri-du-chat syndrome.
These babies are usually mentally and physically retarded, and have kitten-like
cry’s due to a small larynx.
When a piece of chromosome breaks away and reattaches to
another site it is called translocation. Translocation can have a subtle change
in gene expression and may affect their ability to function. This can increase
the risk of certain cancers, including one type of leukemia.
 |
| Recessive Alleles / junglemagazine.com/ accessed 3/8/12 |
Many Inherited Genetic
Disorders Involve Recessive Alleles
Most genetic disorders are caused by two defective,
recessive alleles. If you only inherit one, you can pass it to your children,
but do not have the disease yourself because the good allele is dominant.
Phenylketonuria is caused by a missing enzyme
PKU occurs one in every 12,000 births in Caucasians. It is
caused by mutation of the gene on chromosome 1. It can lead to toxic levels of
phenylpyruvic acid which can cause mental retardation, slow growth rate, and
early death. Because of the serious mental retardation factor, all states now
require that newborns be tested for PKU (the Guthrie test). It can be treated
but requires limiting diet intake of phenylalanine, which is in most protein.
Not an easy task and expensive to boot.
Tay-Sachs disease leads to brain dysfunction
Another enzyme deficiency is Tay-Sachs disease. This is
caused by a recessive gene on chromosome 15. Tay-Sachs is rare in the general
population. But it is fairly common among Ashkenazi Jews of Central European
descent. Approximately one in 3500 have this disease. It does not manifest at
first, but by 4-8 months, motor and brain function begin to decline. They
gradually develop seizures, become blind and paralyzed and usually die by age 3
or 4. At this time, there is no known cure or treatment. Tests are available to
determine if either parent is a carrier.
Huntington disease is caused by a dominant-lethal allele
Dominant-lethal alleles are not common and tend to eliminate
themselves from the population. Huntington’s disease is unusual. It remains in
the population because it’s symptoms do not appear until your 30’s. By this
point, you have already had children and passed along the disease. It is a
progressive nerve degeneration that leads to physical and mental disability and
death. There is a test for the HD
allele. This is one disease that would benefit greatly from genetic testing.
 |
| Human Behaviors / socialwork.vcu.edu / accessed 3/8/12 |
Genes Code for Proteins,
Not for Specific Behaviors
There has been some debate as to whether or not genes cause
depression, happiness, etc… The truth of the matter is that genes code for
specific proteins. They may influence patterns of behavior. For instance,
proteins may act as a hormone and hormones can affect your emotions. However,
they do not cause specific behaviors.
 |
| Genome DNA / ramshrestha.wordpress.com / accessed 3/8/12 |
|
DNA Technology and Genetic
Engineering
Recombinant DNA technology is a fairly new field of
biotechnology, or the technology application of biological knowledge for human
purposes. We can now take DNA apart, analyze it and reconstitute it, which
produces molecules that have never existed before. This process is called
genetic engineering. Genetic
engineering is still in its infancy, but has incredible potential. But with
that said, it also holds great risks. We need to have a better understanding of
how it all works, in order to make better judgments in regards to how it should
be applied. To read more about the genome project, click here:
http://www.genome.gov/
DNA Sequencing Reveals
Structure of DNA
DNA sequencing involves synthesis of a new strand of DNA to
a single strand and then sequencing that strand. They add four nucleotides and then DNA polymerase (an
enzyme) to the mixture, which begins synthesis. Then gel electrophoresis is implemented which creates an
electrical field that causes the DNA to migrate through. It is after this
process that sequence of DNA is calculated.
DNA can be Cloned in the
Laboratory
Because we have now learned enough about DNA to be able to
manipulate it, we now have the ability to develop organisms that have never
existed as well as modify defective human genes.
 |
| Recombinant DNA / nanotechweb.org / | accessed 3/8/12 |
Recombinant DNA technology: isolating and cloning genes
The goal of recombinant DNA technology is to transfer pieces
of DNA and the genes it contains, from one organism to another. It is commonly used to produce protein
products from bacteria. This
process uses restriction enzymes, DNA ligases, plasmids, and bacteria. The
process for cloning a gene or a protein product of a gene is listed below.
- Isolate
DNA form bacterial and human cells.
- Cut
both DNAs with the same restriction enzyme.
- DNAs
are mixed. Human fragments line up with plasmid by base pairing of exposed
single-strand regions.
- DNA
ligase is added to connect human and plasmid DNA together.
- Plasmids
are absorbed by bacteria.
- Bacteria
containing the recombinant plasmids of interest are selected and cloned.
Cloning DNA fragments: the polymerase chain reaction
Polymerase chain reaction (PCR) is a technique used to make
millions of copies of a small fragment of DNA quickly. But it does not work for
cloning whole genes or the proteins they produce because they lack the
regulatory genes and proteins that are required to activate the genes.
Identifying the source of DNA: DNA fingerprinting
Identifying the source of a fragment of DNA after it has
been copied or cloned is termed DNA fingerprinting. We use this technique to
identify criminals, unknown deceased individuals, paternity, and tracing of
ancestral relationships. Law enforcement has use this technique to trace
illegal trade of endangered live animals, animal meat, and ivory. In
evolutionary studies it has been used to establish relationships between
fossils. And in biology, it is used to study mating relationships between
animals.
Genetic Engineering
Creates Transgenic Organisms
Transgenic organisms are organisms that have been
genetically engineered. They have one or more foreign genes from a different
species. This has created a whole new field in science.
Transgenic bacteria have many uses
One of the first applications used was the production of
hormones from bacteria, as well as a few nonhormone proteins. We produce
insulin now with this process as well as tissue plasminogen activator (tPA) and
human blood clotting factor VIII. Bacteria is also used to produce vaccines,
enzymes, citric acid and ethanol, and producing drugs for human use. We also use
transgenic bacteria to clean up toxic wastes and oil pollutants, remove sulfur
from coal, and monitor hazardous waste sites.
 |
| Cornpharma / wa.greens.org.au / accessed 3/8/12 |
Transgenic plants: more vitamins and better pest resistance
Genetic engineering has produced tomato plants that resist
freezing, crops that are resistant to insects, and herbicides. Transgenic
plants are being used to produce edible vaccines against infectious disease and
they can even be made to produce human proteins. There are some concerns
however in regards to safety as well as concerns about crop failures.
 |
| Genetic Engineering / watermarked.cutcaster.com / | accessed 3/8/12 |
Transgenic animals: a bigger challenge
Producing transgenic animals has proven to be the more
difficult of all the processes. The number of animals that can be produced at
one time is very limited. But despite the drawbacks, production is moving
forward for use in food production and to study human disease. Pharmaceutical
companies are also inserting genes into goats, sheep and cows for human protein
production. This
process of producing proteins from farm animals is termed gene
farming.
Gene Therapy: The Hope of
the Future?
Gene therapy must overcome many obstacles
 |
| Gene Therapy / en.wikipedia.com / accessed 3/8/12 |
The first obstacle we are faced with is getting enough of
reombinant DNA containing the gene of interest into the right cells. What is
needed is a delivery system that delivers recombinant DNA to all body cells or
to specfic tissues or cell types efficiently. This, we do not have yet. The
second challenge is that even if we could correct a specific genetic disorder
in an individual, we still do not have a way to stop the disease from being
passed on to the next generation.
Vectors transfer genes into human cells
The strategy for correcting a disease is to get enough of
the gene into enough living cells to produce enough of the missing protein to
prevent that particular disease. There are two methods for accomplishing this
and both use vectors, or transporters, that are capable of delivering genes
into human cells. Retroviruses are the best known vectors. But they have
drawbacks also. Research is currently being done to get around some of the
issues, but are currently only in an experimental phase.
Success with SCID gives hope
There was limited success in treating SCID, severe
immunodeficiency disease, back in 1990. Genetically engineered T cells were
reintroduced back into a human body and ADA was produced. Ashanthi DeSilva’s
condition improved, but it was short lived and her treatments had to be
supplemented with regular doses of ADA. Approximately 10 years later, there are
finally SCID patients that have been treated successfully with gene therapy,
without any other treatment required.
Research targets cystic fibrosis and cancer
There has been limited success in treating cystic fibrosis
with a gene therapy administered via a nasal spray, but not enough to prevent
the disease. Research is being done to find better delivery vectors. They are
also working on targeting certain types of cancer and we may have several
promising approaches very soon. It is one thing to have gene therapy of somatic
cells that target specific disease in specific individuals. It will be another
matter entirely to have gene therapy that targets germ cells that lead to sperm
and egg. This would drastically affect future generations and brings up legal,
moral, and ethical issues that we need to be dealing with now. To read more on the controversy go to
click on the link:
http://www.ucsusa.org/food_and_agriculture/science_and_impacts/science/pharma-and-industrial-crops.html
