History
Gregor Mendel is often called the father of genetics. In the mid-1800s this young monk worked in the monastery gardens experimenting with pea plants. Mendel began his experiments by studying the stem length of pea plants. He noticed differences. Because pea plants grow and reproduce quickly, Mendel was able to observe many generations. From his experiments, Mendel concluded that traits were passed from the parent plant to their offspring. He hypothesized that certain factors must produce certain traits. Today, Mendel’s factors are called genes. We now know that an organism receives two genes for each trait. One gene comes from the female reproductive cell and the second one comes from the male. The stronger of the two genes is called the dominant gene and the gene that is weaker is called the recessive gene. The dominant gene is the gene that always shows itself. The recessive gene is the gene that is hidden when the dominant gene is present. Mendel observed that the tall gene is dominant in pea plants and the short gene is recessive. That is why plants with one tall gene and one short gene are always tall. The results of Mendel’s experiments were published in 1866. However, it was not until 1900 that his research was analyzed by scientists. Many new discoveries have been made, and yet, Mendel’s original hypothesis still forms the basis of modern genetics.
Modern Genetics
Francis Crick and James Watson became the founding fathers of the new molecular genetics by describing the complex structure of DNA in 1953. Other scientists, building on the knowledge of DNA’s structure, have `cracked’ the genetic code and have begun to identify and locate specific genes. Scientists, working on the Human Genome Project, now hope to map all of the estimated 50,000 to 100,000 human genes and spell out the entire message conveyed by three billion chemical code letters in the human genome. That message, written in the code of DNA, is found in the nucleus of the body’s ten trillion cells (except red blood cells ). Its instructions not only determine the structure, size, coloring, and other physical attributes of each human being, but can also affect intelligence, susceptibility to disease, and behavior. Commenting on the impact of the Human Genome Project, James Watson has said, “We used to think that our fate was in the stars. Now we know that, in large measure, our fate is in our genes.”
Cells
The human body is made up of tiny units called cells. Body cells are microscopic in size, and each cell must perform certain functions in order for the whole body to function. Each cell grows and develops by using the food that is brought to it by the blood. Each cell can dispose of its wastes, respond to heat and light, and reproduce. The adult human body may contain a million billion cells. These cells perform different functions and may look different, but they are all alike in basic structure. The main parts of the cells are the
cell membrane, nucleus,
and the liquid
cytoplasm
between these two. The cell membrane controls substances passing in and out of the cell, the nucleus directs the activity of the cell by containing chromosomes, and the cytoplasm provides a medium for the transportation of substances. The basic living substance of all cells is called
protoplasm
Chromosomes
These are threadlike structures in the nucleus of a cell that control heredity. During cell division, each chromosome makes a copy of itself. A pair of identical chromosomes is formed. Each new daughter cell receives one chromosome from each of the pairs. In both asexual and sexual reproduction, chromosomes are passed from parent to offspring. During asexual reproduction, which occurs in plants, bacteria, and lower organisms, each daughter cell receives its chromosomes from a single parent. During sexual reproduction, the daughter cell receives chromosomes from each parent cell. In this case new organisms contain chromosomes from both of its parents.
Genes
These are parts of a chromosome that control inherited traits. Each gene affects a different trait. Genes determine height, eye color, hair color, and many other characteristics. Each gene is located at a certain place on the chromosome. Genes also control the life processes of cells.
DNA
This is the chemical that makes up chromosomes. It stands for deoxyribonucleic acid. A molecule of DNA looks like a twisted ladder. The sides of the DNA ladder are made up of sugars and phosphates. The steps of the DNA ladder are made up of four kinds of nitrogen bases. A British scientist, Rosalind Franklin took x-ray photographs of DNA crystals. Based on these photographs, Watson, Crick and Wilkins predicted the shape and composition of the DNA molecule. A single DNA molecule, or ladder, can have thousands of steps. The number and arrangement of these steps form a genetic “code.” This code determines the kind of protein that is made. Different genes determine different kinds of inherited traits. During cell division, each chromosome doubles to form a pair of identical chromosomes. Molecules of DNA in the parent chromosome also double. This process by which DNA is duplicated is called replication. The DNA ladder breaks apart between the nitrogen bases in the steps. This is similar to the process of unzipping a zipper. The other nitrogen bases attach to each half of the ladder. The result is two new DNA ladders that are exact copies of the original DNA molecule. Sometimes a DNA molecule does not replicate itself exactly. The chromosomes do not pair correctly. These changes in chromosomes and genes are called mutations. Many mutations are harmful to the organism. However, many also are neutral or occur in non-transcribed regions.
Simplified Concept
DNA makes proteins, proteins perform many different functions in all types of cells, and cells make up the organism