Anita G. Santora
All matter is composed of atoms. Atoms combine (join) to form molecules of elements (made up of just one kind of atom or compounds (made up of more than one kind of atom. Because of the unique structure of each type of molecule it can only join to certain other molecules and will only bond at specific points in its structure.
The basic structure for all living things is the cell. The simplest living things are composed of just one cell or one type of cell. In more complicated living things the cells are specialized according to function and band together to form organs and systems within the body. Microscopic examination of a human cell will readily identify it as a skin cell, muscle cell, nerve cell, etc.
All living things grow and develop by means of cell replication and division; the process of mitosis. Through mitosis the cell’s growth is culminated by its splitting into two identical cells, which in turn divide into two identical cells, and so on. The additional cells account for the growth of the organism; however, cells are constantly being shed from the body and must be replaced as well.
Within the nucleus, or control center, of each human cell (with the exception of the mature sex cells) are twenty-three pairs of chromosomes. These forty-six chromosomes are made up of protein structures supporting DNA, deoxyribonucleic acid; complicated chains of molecules whose structure has been described as a double helix. Dr. Margretta Seashore beautifully (in its simplicity) demonstrated this structure during one of our seminars by suspending an elastic band between two pencils and twisting the pencils in opposite directions. If we compare this structure to a twisting zipper, the outer ribbons are composed of molecules of sugar and phosphates with each interlocking tooth representing a molecule of one of four nitrogenous bases: adenine (A), thymine (T), guanine (G), or cytosine (C). The structure of the adenine (A) molecule will only allow it to bond with the thymine (T) molecule and the guanine (G) molecule similarly can only bond with the cytosine (C) molecule.
The order of these bases forms a code to be passed from one cell to the next generation of cells. This code is arranged in sets of chemical sentences referred to as genes. Genes carry the traits, or characteristics, passed on by the parents.
Traits passed on by the parents are not necessarily the traits that they themselves exhibit. When the sex cells are produced, the ultimate result of cell division is a reduction in the number of chromosomes within the cell; so that the offspring will receive twenty-three single chromosomes from each parent. These haploid cells (cells with half the usual number of chromosomes) will fuse at fertilization forming a new cell with the full number of chromosomes (diploid cell).
This process of cell division and reduction of the number of chromosomes in the production of sex cells is called meiosis. In the first phase of meiosis the full set of forty-six chromosomes duplicate and line up, pairing with their counterparts, and exchange DNA (breaking off, crossing over, and exchanging genetic material). The forty-six duplicated chromosomes now within the cell contain all the original genetic material recombined so that they are no longer identical to the original paternal and maternal chromosomes; they are a mixture of both. In the crossing over process the DNA may break in such a way as to change some of the sentence structure of the chemical code specifying one or more genes. These changes contribute to the diversity found within a species.
In the male, the original germ cell then divides into two cells with twenty-three duplicated chromosomes which in turn divide into two cells with twenty-three single chromosomes; thereby producing four sperm cells, each containing twenty-three chromosomes carrying the newly recombined genetic material from each completed meiotic division.
In the female, the process is slightly different. The female produces only one egg cell containing twenty-three recombined single chromosomes from each completed meiotic division. While most of the cytoplasm is retained by the developing ovum after each division, the rest of the genetic material is sloughed off in the form of polar bodies.
When the resultant ovum is penetrated by a sperm, the twenty-three chromosomes carried by each now pair up, again chromosome by chromosome and gene by gene, forming the first complete cell of the new life. This cell will now undergo mitosis, the replicating and splitting that forms new cells, following the orders contained in its own chromosome pairs.
The male and female will each contribute one sex chromosome; the ovum always contributes what is designated as the X chromosome and the sperm contributes either an X chromosome or a Y chromosome. Therefore, the sex of the offspring is determined by the sperm. A male offspring is the result of an egg being fertilized by a sperm carrying a Y chromosome. A female offspring if the result of an egg being fertilized by a sperm carrying an X chromosome.
Sample lesson plan Passing on Genes to the Next Generation
Objectives
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To review the process of meiosis in the production of sex cells.
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To visually simulate the variations in genetic material passed on by the same parent.
Materials needed One copy of the following two pages for each student. Pencils, sets of crayons.
Method Pass worksheet one and the rest of the material. Go over the instructions with the class. They should complete the first worksheet before you pass worksheet two.
Passing on Genes to the Next Generation
Worksheet one: Replication
Every human cell (except for mature sex cells) has twenty-three pairs of chromosomes. This drawing represents one pair of imaginary duplicated chromosomes. They are drawn in a shape (Tetra) they assume after replication. Each numbered section represents an imaginary gene.
Using the colors indicated by the letters, lightly color in each section that has a capital letter. Use your crayon to outline the sections with small letters.
(figure available in print form)
CHROMOSOME FROM FATHER
(figure available in print form)
CHROMOSOME FROM MOTHER
New copy the letters and the coloring from the left side of the chromosome diagram to the newly replicated right side.
During meiosis the chromosomes cross over and exchange genetic material before making the sex cells that are passed on to the next generation.
Passing on Genes to the Next Generation
Worksheet two: New Chromosomes
After two cell divisions, a single chromosome (for each of the twenty-three original pairs) is passed to the newly produced sex cell.
To form a new chromosome that may be the final result of this exchange, choose any letter from each numbered section to complete the new single chromosome below. You may choose to copy an entire strand or one complete chromosome (right or left side of either) instead of selecting letters section by section. You cannot replace a capital letter with a small letter if the original chromosomes both have a capital letter in that section. You cannot replace a small letter with a capital letter if the original chromosomes both have a small letter in that section.
Using the indicated color, lightly fill in all sections that contain a capital letter.Outline all sections that contain a small letter.
(figure available in print form)
Using the indicated color, lightly fill in all sections that contain a capital letter. Outline all sections that contain a small letter.
Comparing our new chromosomes with others in the class will illustrate why offspring, even with the same parents, are not exactly alike.