Genetic information is passed from each parent to offspring through sex cells, and the process of fertilization and division of cells.
All humans are made of cells; no cell of the human body is self sufficient. Every human originates as a single fertilized cell that divides over and over to make billions of cells in a mature human body or being. The process of cellular reproduction holds the key to heredity in living things.
Cells contain chromosomes consisting of proteins and a giant molecule called DNA. Cells are made up of thousands of genes encoded or patterned in the DNA (deoxyribonucleic) acid molecule. These genes contain information necessary for human physical development and are sometimes thought of as blueprints for a human’s structure and form. Sometimes the DNA can be thought of as a computer tape with instructions. The forty six threads of DNA have a combined length of six or more feet locked into a nucleus less than four ten thousandths of an inch in diameter.
Human cells contain 23 pairs of chromosomes (a total of 46). Half of each pair is contributed by the sperm cell from the father on fertilization, and the other half by the egg cell from the mother during the fertilization process.
There are two kinds of cells:
1. Sex cells consisting of sperm and egg or ovum cell.
2. Somatic cells which are body cells, or all of the cells except those that are sex cells. Somatic cells have the same chromosome number in each cell, 46. During reproduction a mirror image is produced, and two cells result.
There are two kinds of cellular reproduction.
1. Sex cells (Sperm and the Egg)
The process of meiosis occurs before fertilization. The Chromosome number is cut to one-half; and this is called a reduction process. Then a mitosis-like step occurs, and each of the cells split in two. The first step in meiosis is the dividing of the cells into identical cells, but replication of DNA occurs, so the number of chromosomes and genes are doubled. During the second step each of the two cells split in two and each sex cell now has only half the number of genes and chromosomes of the original parent cell. One chromosome of each pair goes into one new cell, and the other chromosome of that pair goes into the other new cell. So egg cells and sperm cells have half the number of chromosomes as body cells, as a result of the split. Each sperm has twenty three chromosomes, and each egg has twenty three chromosomes.
Meiosis in the female doesn’t take place continually throughout the age of reproductive maturity as it does in the male. A primary oocyte remains in the first stage for many years, and doesn’t progress until the egg is about to mature in the follicle at sexual maturity. Then it does divide, and two cells unequal in size result. The larger one receives cytoplasm. The smaller cell may or may not divide, and disintegrates. It is called a polar body.
2. Somatic (body) cells may undergo mitosis every three or four hours. Mitosis is a form of cell reproduction in which each of two daughter cells receives a set of chromosomes of the same kind and of the same number as the mother cell. The cell division occurs after the cell has reached a certain size. Growth involves increases in number of cells. Each daughter receives the same number of chromosomes.
During reproduction each new cell formed will contain combined information. When the sperm goes to the ovum it arrives with twenty-three chromosomes. The egg has twenty-three chromosomes also and so again the diploid number is restored to the zygote.
During reproduction each new cell formed will contain combined information from each parent.
Since numbers of genes from each parent are in the tens of thousands, the combinations possible are almost infinite, because of random segregation of chromosomes during meiosis and because of crossing over during meiosis.