Nancy M. Esposito
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“No two snowflakes are exactly alike.”
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“Variety is the spice of life.”
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Musical variations on a theme . . .
One of a kind designer gowns . . .
At first glance it would seem that we live in a world which values individuality and uniqueness. To be yourself is to be recognized as being a strong person. Those who dare to do what no one else has done or to be what no one else has been are recognized as trendsetters, heroes and heroines, or adventurers.
In the midst of all of this, however, is the reality that although uniqueness is recognized as a very special quality, there can also be a stigma attached to being “different”. It is almost evident in the terms: a friend might feel flattered if you refer to her as “unique”, but might well take offense at being referred to as “different”. Society tends to feel that Edmund Hillary was unique for his achievements, but the first hippies were “weirdos”’, yet both wanted to do something differently than it had been done before.
Physical and mental differences, whether visible or not, are all too often regarded with suspicion, if not disdain. They are often seen as a reason to laugh at people or as cause for derogatory remarks. Such is often the case with special education students. Although projects such as the Special Olympics, educational campaigns stressing differences in learning styles and research on the brain, and movies such as “Rain Man”’ have helped to inform the general public about just what the terms “mentally retarded”’ or “‘learning disabled”’ mean, many people persist in seeing children placed in Learning Center classes as “‘weird”’, “‘strange”’, or “‘sick”’.
I currently teach a fifth/sixth grade special education class in New Haven where my first challenge each Fall is to talk with students about why they are in a Learning Center class and what the main differences are between learning center and mainstream classes. Often students focus on the fact that they are “‘dumb”’ and mainstream students are “smarter”. From that point we talk about specific reasons a person could need special education, and how it can help him/ her. I make certain to explain to students that if they were “dumb” they could not be classified as learning disabled because that term requires average or above average intelligence for children of their age. Hopefully every child is made to feel that he or she has the same potential to make a positive impact on society as any other person their age.
I feel committed to trying to set an environment where the labels that these children might carry do not inhibit their normal social growth. In the course of this goal. I would like to expose my students to the idea that all individuals have built-in differences, and that these are factors which all people deal with. I feel that the students would benefit a great deal from participating in a basic science unit on heredity and genetics, where my main objective would be for them to see that every one of us is unique and different. My objectives for this unit will include having students begin to understand the basic units of life, including atoms, molecules and cells; and contrasting living organisms with non-living objects. Understanding how atoms combine in an infinite variety of ways to create molecules which are different from each other would begin to demonstrate the concept of differences. Exploring the concept of living versus non-living would further help students to explore differences, and could allow them concrete hands-on activities.
I would begin teaching this portion of my unit by explaining that everything in the world is made up of atoms, which are so tiny that only the most powerful of microscopes can show them. An interesting observation is that if you had one atom for every single person alive today, you could fit them all onto the head of a pin.
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The ancient Greeks were the first people who thought that atoms were the smallest particle of matter that exists in our world. They named these particles atoms because the Greek word “atomos” means “that which cannot be split”. However, many years later, starting in the 1890s scientists found that the atom could be divided into even smaller particles, called protons, neutrons and electrons.
Although they are so small that we cannot see them without an extremely powerful microscope, atoms are considered to be the building blocks of our world. During the eighteenth century, chemists who were experimenting found that they could divide substances like water into two other substances, hydrogen and oxygen. But when they tried to divide the hydrogen and oxygen into other chemicals, they found that they could not do it. The same thing happened when they divided table salt into sodium and chlorine, but could not divide these substances down into further chemicals. Scientists decided from these experiments that most of the things in our world are made up of combinations of substances; these combinations are called chemical compounds. The substances which made up these compounds, which they could not break down were then called elements.
A very important English scientist named John Dalton announced in the early 1800’s that he had a scientific theory concerning atoms. This atomic theory stated that all elements are made up of atoms, and that the atoms in any particular element are always the same. For example, an atom of gold is like any other atom of gold, just as an atom of hydrogen is like any other atom of hydrogen. He further stated that atoms of different elements are different from each other,including having different weights. For example, an atom of oxygen weighs more than an atom of carbon, and an atom of carbon weighs more than an atom of hydrogen.
Scientists went on to discover that atoms join together to form molecules. Molecules are chemical combinations of atoms which can be either the same or different. Atoms of the same element combine to form molecules of that element, while atoms of different elements combine to form molecules of compounds. There are about 100 different kinds of atoms, but there are thousands of different kinds of molecules. One suggestion in understanding this idea is to think of the atoms as letters of the alphabet, and molecules as words which are made from these letters. Any two of the same letter look exactly the same, but the combinations of letters into words yields a tremendous variety of resultant combinations.
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Even though molecules are combinations of atoms, they are still too small for scientists to see under any but the most powerful of microscopes.
The next objective of my unit is to expose students to the concept of cells, giving them an idea of what they are, a general overview of cell parts, and the concept of living organisms versus non-living objects. I would start by explaining to students that all living organisms in our environment, no matter how different they may seem from each other, are made up of cells. Cells are one of the smallest units of matter that are able to grow and reproduce (viruses being another). The cells are highly organized units of molecules and macromolecules in which chemical reactions are carried out to produce the quality we call life.
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Biologists believe that the overall composition of cells is basically the same. Each cell has a nucleus that serves as the control center for the cell. The nucleus also contains the chemical substances which determine the inherited characteristics of the cell. The nucleus contains the plans for the chemicals that are made in the cell and helps to direct the many chemical reactions that go on inside the cell. Chemical messengers constantly pass in and out of the nucleus of the cell bringing in information about what is happening in the cell, what new chemicals are needed, and carrying out the plans for building new materials in the cell. The nucleus is enclosed in a thin covering called a membrane which surrounds it but allows some materials to pass in and out of the nucleus. The nuclear sap, which is somewhat like cytoplasm, contains fine threads called chromatin which will help pass the cell’s characteristics to its daughter cells when it divides.
Cytoplasm is the name given to the grayish, jelly-like material which makes up the cell outside of the nucleus and which is surrounded by the cell membrane. The cytoplasm is almost clear near the outside of the cell, but is denser and appears more granular near the center of the cell. The cytoplasm is about 70% water, but contains a variety of other substances such as dissolved nutrients like proteins, which are the building blocks of the cell, vitamin cofactors and sugars, starches, and fats which provide energy.
The cell membrane is thin and semipermeable to allow gases and fluids to pass through it. This allows cells to get nutrients for growth and oxygen for respiration. It further allows the cell to get rid of carbon dioxide and other waste products. In most cells there is increased room for activity due to the fact that the cell membrane contains many folds and wrinkles .
There are tiny structures in the cell called “little organs” or organelles. Among these are important oval shaped structures called mitochondria, which change their shape as conditions within the cell change. Mitochondria are involved in respiration, which is the process whereby oxygen is taken in and food is burned to generate energy. This process produces waste products including carbon dioxide and water.
There are many chemical reactions taking place constantly within the cell during which chemicals are built up or broken down. There are thousands and thousands of different chemicals within each cell. Lysosomes are small bodies within the cell which contain chemicals called enzymes which aid in the digestion of larger molecules. Lysosomes also remove undigested wastes from the cytoplasm by moving them to the surface of the cell, where they are passed out.
Ribosomes are another important part of the cell. These float freely in the cytoplasm and help in the production of protein. Membranous sacs which are flat and stacked like pancakes are called the Golgi apparatus and are packages of protein made by the ribosomes and transported to wherever they are needed.
All organisms are composed of cells, but not all organisms are considered to be alive. We each assume that we know how to differentiate between those things which are dead and those which are alive. However, there are certain scientific qualities which we use to determine the distinction between these two terms. There are six properties which must all be present in order to qualify a cell as living.
1. Living cells carry on processes which derive energy from their environment to allow them to grow and reproduce. This process is known as metabolism.
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2. Living cells demonstrate growth as an end product of the utilization of energy. They increase in size and weight, although this may be a finite process as with brain cells which do not grow after a point.
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3. Living cells reproduce, giving rise to two identical copies of themselves.
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4. During the process of growth and reproduction, cells occasionally undergo a mutation (permanent change in their genetic information).
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5. Living organisms respond and react to their environment.
6. Living things evolve over time due to mutations and other biological mechanisms.
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Now that my students have hopefully begun to sense the infinite possibilities for grouping things together and the way that almost any two things can be made to seem alike or different, depending upon criteria, my next objective for the unit would be to introduce them to the concept of genetic code. This portion of my unit would begin with a discussion of chromosomes and genes, and DNA.
At this point I would ask them to think about the following questions: Why is it that if you take the seeds from a plant, place them in soil and care for them, you usually get a plant that is close in appearance to the original? Why do puppies seem to resemble the parent dogs as they grow up? Are they exactly the same? Why is it that a human baby grows up to look like other humans instead of like a puppy? After discussion of possibilities, students will usually suggest that there is some kind of a “plan” within the organism that controls what it will turn out to be. The term heredity could be introduced now to refer to all of the traits that are passed on to offspring from parent plants and animals.
In fact, the nucleus of each cell contains the information for all the parts of the cells, the shapes and sizes of the cells of the body, and the jobs they do. There is also information required to keep the cell functioning correctly and plans to repair it should it be damaged. This information is passed in sexually reproducing species through the fertilized egg, and through the daughter cell in asexual reproduction in the form of a set of genes needed for the creation of further structures during the course of development of the organism. The material that directs and coordinates the processes involved in the growth and reproduction of an organism is contained in its genome, or the total amount of genetic information contained in an organism. In complex organisms such as humans and animals, the genome is defined as one complete set of chromosomes. The genetic material carried in the chromosomes is actually contained in the DNA molecule that is threaded from one end of the chromosome to the other.
Before students can understand DNA, we need to return to our description of animal cells and take a closer look into the nucleus. Toward the end of the nineteenth century scientists had shown that the nucleus of a eukaryotic cell (that of a complex unicellular or multicellular organism, which would include all plants and animals) contained a granular region called chromatin. This chromatin was proven to consist of a number of threadlike particles called chromosomes. The name chromosomes came from the Greek words “chroma” meaning color, and “soma” meaning body. They were given the name because they quickly absorb the dyes scientists use on them to be able to study them under microscopes. It should be noted that chromosomes can be clearly seen in most cells only at the time of cell division. Scientists eventually found that these chromosomes contained even smaller units, which were named genes. They realized that within each chromosome the genes could be found to be in a very exact order or pattern which determined traits of the organism.
Before scientists had discovered for certain that chromosomes controlled heredity, they realized that the chromosomes in each cell were made up of protein and a special chemical material called deoxyribonucleic acid. This material was also commonly called DNA. They found that almost all cells within an organism contained the same amount and type of DNA, regardless of the cells’ specific function. Friedrich Miescher, a Swiss chemist, first discovered DNA in 1889 but at that time scientists did not think it was important. However, in 1944 Oswald T. Avery and the Rockefeller Institute in New York City announced that DNA was the sole substance responsible for the transference of hereditary traits.
The structure of DNA was proposed by Francis Crick, an English physicist, and James Watson, an American biologist in 1953. They stated that DNA is composed of two very long strands of molecules twisted together to form a double helix. (A helix resembles the shape of a coil spring or the threads of a screw.) The DNA is made up of a series of genes which are attached to each other and which carry genetic information. A gene is a sequence of nucleotides , with a nucleotide being a unit of nucleic acid made up of three parts: a phosphate, a 5 carbon sugar, and a base. These are arranged so that different units are opposite each other, and it is always the same pair of units which are opposite each other in the same human being and in all other living organisms, even viruses. These units refer to four different chemicals, all belonging to the same family, which are called organic bases. The four bases found in DNA are adenine, thymine, guanine and cytosine (often abbreviated as A, T, G. and C.) In the two chains of the helix, adenine must always pair with thymine, and guanine must always pair with cytosine. These pairs are said to be complementary. The stability of the two long helical chains in DNA is due to the hydrogen bonds that connect the complementary bases along the entire length of the double helix. The backbone of each chain consists of repeating units of sugar molecules (deoxyribose) with attached phosphates.
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Watson and Crick determined that it is the number, arrangement and kinds of units in DNA that determine a person’s genetic code. DNA could be compared to a computer which regulates the activities needed to keep the body healthy, and “tells” various parts of the body when and how much to grow
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Each single strand of the DNA double helix actually contains all of the genetic information in a given organism, since the alignment of the bases on that strand will determine the alignment of the second strand. The replication of genetic information is accomplished by creating two new chains of DNA with bases which pair up to their complementary bases in an existing chain. This would produce two double helical models which are informationally identical to the original because they contain the same sequence of bases as the original.
My objective at this point in the unit would be to give students an overview of cell division, and the passage of hereditary material from one cell to another. Although this can be a very complicated concept, I would try to keep this description as simplified as possible to promote students’ understanding of the material. I would explain to the students at this point that the term “cell cycle” is used to refer to the time between the formation of a particular cell and its eventual division into two daughter cells. The two parts of the cell cycle are : interphase and mitosis or meiosis. Different eukaryotic cells differ in the length of time it takes for them to complete one cell cycle, and in the amount of time spent on any one portion of the stages of the cycle. The four mitotic stages are: prophase, metaphase, anaphase, and telophase.
In higher plants and animals, DNA replication occurs only during the interval of the interphase referred to as the S period ( S referring to synthesis). As stated above, each of the chromosomes in a eukaryotic cell contains a single DNA helix which the cell has to duplicate during the S phase to allow it to pass one copy to each newly created cell. Special enzymes in the cell copy each strand of the DNA helix to make a complementary strand. Following this, two identical copies of the chromosome information are available from the previous single copy. These are referred to as sister chromatids, but are not recognizable during the interphase period. When the cell is ready to divide through meiosis or mitosis, individual chromatids can be stained and identified through a microscope. It is the process of mitosis which ensures that each daughter cell will get one of each of the sister chromatids and therefore end up with a complete set.
During mitosis, the nucleolus becomes undetectable and the chromatid pairs begin the coiling and condensation process. The chromosomes become distinct bodies in the nucleus, with a split along their length, held together in the middle by the centromere. The centriole divides and separates, which creates a radiating system of protein fibers. The two radiating systems which were formed by the splitting of the centriole are now called asters, and they travel to opposite sides of the cell where they become connected by fibers into a system called the spindle. Chromosomes arrange themselves along the equator of the spindle between the asters. Next the chromosomes divide completely, with one set of daughter chromosomes going to each The entire cell divides and two identical daughter cells, each containing copies of the chromosomes of the parent, have been formed.
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Division of special sex cells, or gametes, is more complicated. The gametes produced by women are known as eggs, and those produced by men are sperm. An egg and a sperm unite to form a zygote, which grows into a new individual. The difference in humans which sets sex cells apart from other cells in the body is that most cells contain 46 chromosomes (23 pairs) while sex cells contain only 23 chromosomes. This is because at fertilization each sex cell contributes its 23 chromosomes to the zygote, thus giving the zygote 46 chromosomes.
Sex cells reproduce through meiosis. Each chromosome duplicates itself as in mitosis, and they are held at the middle by centromeres. The centriole divides and each new centriole starts to move to the opposite side of the cell. Double chromosomes that are similar line up next to each other, with some parts overlapping. The double chromosomes separate from each other and line up at the center of the cell. When they separate, the chromosomes exchange complementary pieces.
This is called “crossing over.” The double chromosomes are pulled to opposite ends of the cell and the cell begins to divide in two. The original cell then divides into two new cells which each contain 23 double non-identical chromosomes. These double chromosomes in each cell then split apart and each half is pulled to one side of the cell. The cells divide into two again forming gametes, each containing 23 chromatids. In fertilization, two gametes unite to form a zygote, which contains the full set of 48 chromosomes.
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The next objective I have for this unit is to give students a brief description of Mendel’s experiments and his findings on inherited traits. How is it that physical traits are passed on from parents to their young? This is difficult to tell in humans because there are so many varied characteristics to keep track of; because it takes a long time for children to grow up to allow you to study fully developed characteristics; because most parents do not have a great enough number of children to provide wide study of cases; and, perhaps most importantly, scientists cannot experiment.
Gregor Mendel was an Austrian monk who devoted himself to the study of plants. He decided that the best way to attempt to study inherited physical characteristics was through breeding plants. Plant breeding is relatively easy to control since you can take pollen from one plant and put it on the pistil of another to cross-pollinate plants being studied. Then you can compare characteristics of new plants with the old ones that produced the pollen grains and the ovules. For eight years Mendel pollinated pea plants in different ways and studied the results. He worked with short and tall plants and studied ways that they bred true and situations where they did not breed true. In addition to finding that he could eliminate shortness characteristic in a generation of pea plants, he also found that the shortness characteristic could be hidden for one generation and then appear in the next. Mendel explained this by supposing that every plant had two factors inside itself, one contributed by each parent, that controlled the inheritance of physical traits. If the factor for tallness were T, and the factor that brought about shortness s, then a short plant would be described as ss, and a tall plant as TT. However, if the sperm cell from a short plant were to combine with an egg cell from a tall plant, the result would be sT. Likewise, if a sperm cell from a tall plant combined with the egg cell of a short plant, the result would form a seed that was Ts. Either way the seeds would produce tall plants because the T would drown out the effect of the s. Tallness would be dominant (“master”), and shortness would be recessive (“to draw back”). Mendel died in 1884 not knowing that he would someday become famous. For over thirty years after Mendel’s papers were published, no one paid any attention to them. Today we know that Mendel’s research was the foundation of the study of genetics. His main findings were:
1. Traits are determined by specific factors that are transmitted unchanged from one generation to the next.
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2. These factors can be expressed as either dominant or recessive.
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3. These dominant and recessive factors assort independently from each other.
4. There are predictable ratios for traits to appear in the F2 generation.
The next objective in my unit would be to expose students to the concept of human hereditary diseases, and to briefly discuss at least two of these with them. I would begin by discussing the fact that human hereditary diseases, often referred to as genetic diseases, result from the passage of abnormal chromosomes or mutant genes from parent to child. These inherited diseases exist in the egg or sperm at the moment of fertilization, but are not usually observable until birth or later in the child’s life. It is important for students to understand that there are differences between hereditary diseases or defects and other types of diseases. However, it is often difficult to say whether a disease is caused by genetic factors. About five percent of newborns have an observable physical or anatomical abnormality that can be established as a hereditary defect.
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However, exposure to drugs, infections and environmental factors during pregnancy can also cause observable defects in newborns as can chance events. In general the criteria for classifying a disease as hereditary are: a Mendelian pattern of inheritance (i.e. predictable over several generations), a chromosomal abnormality (loss or gain of a chromosome, or rearrangement of chromosome segments from their normal location), or a biochemical defect that can be assigned to a particular gene.
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Two terms which is important for students to understand at this point are
phenotype
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genotype
. Phenotype refers to visible or otherwise measurable properties of an organism. Genotype refers to the genetic factors responsible for creating the phenotype. Some phenotypic traits are determined by single genes, while others are determined by several.
Parents in a given situation can be classified as heterozygous or homozygous. If homozygous for a given gene, it means that the parent has two identical alleles (AA or BB). An allele is an alternative form of a gene occupying the same locus on homologous chromosomes. If a parent has different alleles for a given gene( Aa or Bb), he is said to be heterozygous for that gene. The phenotype for a homozygote directly reflects the genotype of the allele, but the phenotype of a heterozygote depends upon the relationship between the type of alleles which are present. If one is dominant, and the other is recessive the phenotype will be determined by the dominant allele. Alleles are considered codominant when they contribute equally to the phenotype.
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Genes which are located on the X chromosome are called sex linked because generally only one sex (males) is affected by the mutant alleles. A female may also have sex linked diseases but both of her X chromosomes must carry the mutant allele for her to be affected. (This is assuming that the genes are recessive, as they often are for serious hereditary diseases.) Because chromosomes segregate randomly into egg and sperm cells during meiosis, the probability can be calculated that a daughter will become a carrier of a sex-linked disease or that a son will develop the trait.
In looking at autosomal recessive inheritance, a recessive gene which is abnormal is of little consequence as long as the cell contains a normal allele which can mask it. This explains why heterozygous carriers of harmful recessive genes do not exhibit symptoms of the disease. For offspring to be affected, both parents must be carriers of the recessive gene. Autosomal recessive genes are usually recognized in two ways: the trait is usually present in only one generation; or the parents are heterozygous for the recessive gene, the ratio of nonaffected to affected individuals is roughly three to one (for a large number of children or over many families). In autosomal recessive inheritance , there is more often also some degree of relatedness between the parents, i.e. first cousins. These relatives are more likely to have inherited the same nonfunctional gene from a common ancestor. Diseases which are autosomal recessive and their symptoms include Sickle-cell disease (anemia), Cystic fibrosis (respiratory disorders), some forms of Albinism (lack of skin pigment, vision difficulties) and Phenylketonuria ( mental retardation).
On the other hand, in looking at autosomal dominant inheritance we find that if the gene is expressed as dominant, the condition will exist in the individual regardless of what other allele is present. There is a much smaller recognized number of autosomal dominant defects than there are autosomal recessive defects. This is largely due to the fact that if the dominant gene limits the person’s chance for survival or impairs reproduction, it is less likely that the gene will be passed on to offspring. Some of the dominant genes that have persisted in occurrence are those which do not make themselves evident until later in life, such as Huntington’s disease and those which do not limit reproduction. A rather common autosomal dominant gene that is fully expressed is the one which causes achondroplasia. This is a dwarfism where there is a disproportionate shortening of the arms and legs. This hereditary defect is caused by abnormal growth of cartilage. Other autosomal dominant diseases include Neurofibromatosis (growths in the nervous system and skin) and Polydactyly (extra fingers and toes).
At the conclusion of this unit, students will recognize that there are many contributing factors in deciding what any one person is like, both physically and mentally. If we have achieved our goal in doing this unit together, they will not feel as stigmatized when teased or taunted about being in special education. Hopefully if they do become involved in comparing themselves to others in this world, they will realize how many wonderful things they have in their favor.
