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

Normal Versus Cancer Cell Division Video

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.

1. Isolate DNA form bacterial and human cells.
2. Cut both DNAs with the same restriction enzyme.
3. DNAs are mixed. Human fragments line up with plasmid by base pairing of exposed single-strand regions.
4. DNA ligase is added to connect human and plasmid DNA together.
5. Plasmids are absorbed by bacteria.
6. 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