Tumor-suppressor genes are genes that normally repair damaged DNA or prevent cells with badly damaged DNA from dividing (Figure A below). If mutations occur in these genes, they may no longer be able to prevent cells with damaged DNA from dividing (Figure B below).
Proto-oncogenes are genes that normally help regulate cell division. Mutations can turn them into oncogenes. Oncogenes are abnormal genes that stimulate the division of cells with damaged DNA.
Figure 24.14
A cell with damaged DNA normally is not allowed to divide, so the damage is not passed on to other cells. If a cell with damaged DNA is allowed to divide, it results in many more damaged cells.
Cells that divide uncontrollably form a tumor. A tumor is an abnormal mass of tissue. Tumors may be benign or malignant. Benign tumors remain localized and generally do not harm health. Malignant tumors are cancer. There are no limits on their growth, so they can invade and damage neighboring tissues. Cells from malignant tumors can also break away from the tumor, enter the circulation, and start growing in another part of the body. This is called metastasis.
Types of Cancer
Cancer is usually classified according to the type of tissue where the cancer begins. Common types of cancer include:
Carcinoma: tumor of epithelial tissues, such as lung tissue.
Sarcoma: tumor of connective tissues, such as bone.
Lymphoma: tumor of lymphatic cells, such as T cells.
Specific cancers are generally named for the organs where the cancers begin. Relatively common cancers include lung, prostate, bladder, and breast cancers. These and several other cancers are listed in the Table below. The figure shows which cancers are most common and which cause the most deaths in U.S. adults.
Common Cancers among Adult Males and Females in the United States Adult Males Adult Females
Most Common Cancers (percent of all cancers) Most Common Causes of Cancer Deaths (percent of all cancer deaths) Most Common Cancers (percent of all cancers) Most Common Causes of Cancer Deaths (percent of all cancer deaths)
Prostate cancer(33%) Lung cancer(31%) Breast cancer(32%) Lung cancer(27%)
Lung cancer(13%) Prostate cancer(10%) Lung cancer(12%) Breast cancer(15%)
Colorectal cancer(10%) Colorectal cancer(10%) Colorectal cancer(11%) Colorectal cancer(10%)
Bladder cancer(7%) Pancreatic cancer(5%) Endometrial cancer(6%) Ovarian cancer(6%)
(Source: http://en.wikipedia.org/wiki/Cancer, License: Creative Commons)
Cancer can also occur in children and teens, but it is rare. Most childhood cancers occur during the first year of life. The most common type of infant cancer is leukemia. It makes up about 30 percent of cancers at this age. With prompt treatment, there is a good chance that an infant with cancer will survive.
Cancer Treatment and Prevention
Most cancers can be treated and some can be cured. The general goal of treatment is to remove the tumor without damaging the rest of the body. Cancer may be treated with a combination of surgery, chemotherapy, and/or radiation. In the past, chemotherapy drugs caused serious side effects. Many of today’s chemotherapy drugs target specific molecules in tumors. This reduces damage to normal body cells and causes fewer side effects.
The outcome of cancer treatment depends on factors such as the type of cancer and its stage. The stage of cancer refers to the extent to which the cancer has developed. Generally, early diagnosis and treatment lead to the best chances of survival. That’s why it’s important for people to be aware of the following warning signs of cancer:
A change in bowel or bladder habits
A sore that does not heal
Unusual bleeding or discharge from any place
A lump in the breast or other parts of the body
Chronic indigestion or difficulty in swallowing
Obvious changes in a wart or mole
Persistent coughing or hoarseness
Having warning signs of cancer does not mean that you have cancer, but you should see a doctor to be sure. Getting recommended tests for particular cancers, such as colonoscopies for colon cancer, can also help detect cancers early, when chances of a cure are greatest.
Many cancers can be prevented, or at least their risk can be reduced. You can help reduce your risk of cancer by avoiding specific carcinogens and maintaining a healthy lifestyle. Carcinogens you can avoid or limit your exposure to include tobacco smoke, sexually transmitted viruses, improperly cooked foods, and UV radiation. Other lifestyle choices you can make to reduce your risk of cancer include being physically active, eating a low-fat diet, and maintaining a normal weight.
Air Pollution and Illness
An estimated 4.6 million people die each year because of air pollution. Worldwide, air pollution causes more deaths than traffic accidents do. Air pollution harms the respiratory and cardiovascular systems. Both outdoor and indoor air can be polluted and contribute to illness and death.
Outdoor Air Pollution
The concentration of pollutants in outdoor air is indicated by the Air Quality Index. The Air Quality Index (AQI) is a measure of certain pollutants in the air in a given location. The health risks associated with different values of the AQI are shown in the Table below . When the AQI is high, you should limit the time you spend outdoors, especially the time you spend exercising. Avoiding exposure to air pollution can help limit its impact on your health. As you can see from Table below, people with certain health problems, including asthma, need to be even more careful about limiting their exposure to air pollution.
Air Quality and Health Risk Air Quality Index(AQI) Quality of Air in Terms of Human Health
0–50 Good
51–100 Moderate
101–150 Unhealthy for sensitive groups1
151–200 Unhealthy for everyone
201–300 Very unhealthy
301–500 Hazardous
1 Sensitive groups include people with asthma, heart disease, or other diseases worsened by air pollution.
(Source: http://en.wikipedia.org/wiki/Air_Quality_Index, License: Creative Commons)
AQI reports to the public generally refer to levels of ground-level ozone and particulates. Ozone is a gas that forms close to the ground when high concentrations of air pollutants are heated by sunlight. Ozone damages both respiratory and cardiovascular systems. For example, it can cause asthma and decrease lung function. It can also convert cholesterol in arteries to plaque, causing cardiovascular disease. In addition, ozone may increase inflammation, which is a symptom of many diseases.
Particulates are tiny particles of solids or liquids suspended in the air. The most concentrated particulate pollution tends to be in the air over densely populated metropolitan areas in developing countries. The primary cause is the burning of fossil fuels by motor vehicles and factories. Particulates settle in airways and lungs and damage the respiratory tract. They can cause asthma and lung cancer. Extremely small particulates may pass through the lungs to the bloodstream and contribute to plaque formation in arteries.
Indoor Air Pollution
Indoor air quality refers to pollutants in the air inside buildings. Indoor air may be more polluted than outdoor air, although with different pollutants. Typical pollutants in indoor air include allergens, mold, bacteria, carbon monoxide, and radon.
Mold and bacteria can be allergens and also cause respiratory system infections. For example, a type of pneumonia, known as Legionnaire’s disease, is caused by bacteria that can spread through air conditioning systems. The disease is not common, but it kills many of the people who contract it.
Carbon monoxide is a gas produced by cars, furnaces, and other devices that burn fuel. It replaces oxygen in the blood and quickly leads to death. Initial symptoms of carbon monoxide poisoning include headache, listlessness, and other flu-like symptoms. Loss of consciousness and death can occur within hours. An estimated 40,000 Americans annually seek medical attention for carbon monoxide poisoning. It is also the most common type of fatal poisoning in the
U.S. Carbon monoxide is colorless and odorless, but it can be detected with carbon monoxide detectors like the one in Figure below.
Figure 24.15
Home carbon monoxide detector
Sick building syndrome (SBS) is a combination of symptoms associated with working in a particular building, typically an office building. It is most common in new and remodeled buildings. It is usually caused by inadequate ventilation. Chemicals released by new building materials may also contribute to the poor air quality. Generally, conditions improve by increasing ventilation. Symptoms of SBS vary widely. They may include headaches, eye irritation, dry cough, dizziness, and asthma.
Bioterrorism
Bioterrorism is terrorism by intentional release or spread of pathogens. As shown in Tables below, below, and below, pathogens used in bioterrorism may include bacteria, viruses, or toxins. Toxins are poisons produced by organisms such as bacteria. The agents may be naturally occurring pathogens or pathogens that have been modified by humans to make them more effective agents of bioterrorism. The agents can spread in a variety of ways, including through air, food, water, direct contact, or cuts in the skin. They have the potential to cause epidemics of deadly human diseases.
Classification of Category A Bioterrorism Agents Based Upon Threat to Public Health Agent Type of Pathogen Mode of Transmission
Anthrax Bacteria Air, food, cuts in skin
Smallpox Virus Air, direct contact
Botulinum Toxin Food, cuts in skin
Classification of Category B Bioterrorism Agents Based Upon Threat to Public Health Agent Type of Pathogen Mode of Transmission
Brucellosis Bacteria Milk, direct contact
Ricin Toxin Air, food, water
Cholera Bacteria Food, water
Classification of Category C Bioterrorism Agents Based Upon Threat to Public Health Agent Type of Pathogen Mode of Transmission
Hantavirus Virus Air
Tuberculosis Bacteria Air
Agents of Bioterrorism
Bioterrorism agents are classified on the basis of their threat to public health, as shown in the tables above. Category A agents (Table below) include anthrax and smallpox. Agents in this category pose the greatest threat. They spread easily and cause serious illness or death. Category B agents (Table below) are considered less of a threat. They do not spread as easily and are less likely to cause death. Category C agents (Table below) are pathogens that are likely to be engineered for bioterrorism in the future. They are easy to produce and have the potential to cause serious illness or death.
Recent Bioterrorism Incidents
Two recent bioterrorism incidents in the U.S. received a great deal of media attention. They heightened public awareness of the threat of bioterrorism. In 2001, letters containing anthrax spores were mailed to several news media offices and two U.S. Senate offices. A total of 22 people were infected, and 5 of them died of anthrax. In 2003, deadly ricin toxin was detected in a letter intended for the White House. The letter was intercepted at a mail-handling facility off White House grounds. Fortunately, the ricin did not cause illness or death.
Lesson Summary
Carcinogens cause cancer by inducing mutations in genes that normally control cell division or other aspects of the cell cycle.
Both indoor and outdoor air may contain pollutants that can cause human illness and death.
In bioterrorism, pathogens are intentionally released or spread and have the potential to cause disease epidemics.
Review Questions
What is a carcinogen?
How do carcinogens cause cancer?
Identify three ways cancer can be treated.
List four warning signs of cancer.
How can you use the Air Quality Index to protect your health from air pollution?
The bacterium that causes plague is classified as a Category A bioterrorism agent. What can you conclude about the bacterium from this classification?
Explain why ozone is usually a worse problem in the summer than in the winter in North America.
Compare and contrast pollutants in indoor and outdoor air, including their effects on human health.
Further Reading / Supplemental Links
Bozzone, Donna M., Cancer Genetics. Chelsea House Publications, 2007.
Panno, Joseph, Ph.D., Cancer: The Role of Genes, Lifestyle, and Environment. Facts on File, 2004.
http://www.uihealthcare.com/topics/cancer/canc4280.html
http://www.bt.cdc.gov/
http://www.cancer.gov/cancertopics/understandingcancer/immunesystem
http://www.cdc.gov/cancer/az/
http://www.cdc.gov/health/airquality.html
http://www.emedicine.com/emerg/topic853.htm
http://www.niaid.nih.gov/publications/immune/the_immune_system.pdf
http://www.sciencedaily.com/releases/2007/08/070801161426.htm
http://www.sciencedaily.com/releases/2007/08/070807101114.htm
http://en.wikipedia.org
Vocabulary
air quality index (AQI)
A measure of certain pollutants in the air in a given location.
bioterrorism
Terrorism by intentional release or spread of pathogens.
cancer
A disease in which abnormal body cells divide of control.
carbon monoxide
A gas produced by cars, furnaces, and other devices that burn fuel; replaces oxygen in the blood and quickly leads to death.
carcinogen
Anything that can cause cancer; may be pathogens, chemical substances, or radiation.
carcinoma
A tumor of epithelial tissues, such as lung tissue.
lymphoma
A tumor of lymphatic cells, such as T cells.
oncogenes
Abnormal genes that stimulate the division of cells with damaged DNA.
ozone
A gas that forms close to the ground when high concentrations of air pollutants are heated by sunlight.
particulates
Tiny particles of solids or liquids suspended in the air; primarily formed by the burning of fossil fuels by motor vehicles and factories.
proto-oncogenes
Genes that normally help regulate cell division.
sarcoma
A tumor of connective tissues, such as bone.
sick building syndrome (SBS)
A combination of symptoms associated with working in a particular building, typically an office building.
tumor
An abnormal mass of tissue.
tumor-suppressor genes
Genes that normally repair damaged DNA or prevent cells with badly damaged DNA from dividing.
Points to Consider
High levels of certain hormones can increase the risk of some types of cancer. For example, high levels of estrogen can increase the risk of breast cancer. Estrogen is a female sex hormone.
What are sex hormones?
How do sex hormones normally affect the body?
How are male and female sex hormones different?
Chapter 25: Reproductive System and Human Development
Lesson 25.1: Male Reproductive System
Lesson Objectives
Explain how the male reproductive system develops before birth and matures during puberty.
Identify structures of the male reproductive system and their functions.
Describe how sperm are produced and how they leave the body.
Introduction
The male reproductive system is a collection of organs and other structures in the pelvic region. Most of the structures are located outside the body. The male reproductive system has two major functions: producing sperm and secreting male sex hormones. Sperm are male gametes, or sex cells, which are necessary for reproduction. During puberty, a boy develops into a sexually mature male, capable of producing sperm and reproducing.
Sexual Development in Males
The main visible differences between boys and girls at birth are
their reproductive organs. Of course, there are other differences between boys and girls at birth, but in this chapter, the focus is on their reproductive systems. As different as the male and female reproductive systems are at birth, they start out relatively similar. Before birth, the expression of genes on the male Y-chromosome brings about the differences.
Development Before Birth
In the first few weeks of life, male and female embryos are essentially the same, except for their chromosomes. Females have two X chromosomes, and males have an X and a Y chromosome. In male embryos, genes on the Y chromosome lead to the synthesis of testosterone. This begins around the sixth week of life.
Testosterone is a masculinizing hormone and the chief sex hormone in males. Testosterone stimulates the embryo’s reproductive organs to develop into male organs. For example, because of testosterone, the embryo develops testes instead of ovaries, which are female organs you will read about in Lesson 25.2.
All the reproductive organs are present by birth. However, they are immature and unable to function. The reproductive organs grow very little during childhood and do not mature until puberty.
Puberty and Its Changes
Puberty is the period during which humans become sexually mature. In the United States, boys generally begin puberty at about age 12 years. Puberty starts when the hypothalamus, a gland in the brain, stimulates the nearby pituitary gland to secrete hormones that target the testes. The main pituitary hormone responsible for puberty in males is luteinizing hormone (LH). It stimulates the testes to produce testosterone. Testosterone promotes protein synthesis and growth. It brings about most of the physical changes of puberty, including the changes outlined in Table below.
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