The
Body Is Made of CellsThere are approximately 75 trillion cells in the human body. Some of these cells form the ten major systems of the human body: the skeletal, muscular, circulatory, nervous, respiratory, digestive, excretory, endocrine, reproductive, and immune systems. All of these systems are joined together with tissues that are also composed of unique cells. For all of these systems and cells, there are only about 200 different types of human cells, but each of these has a different size, appearance, job, and life span. All 200-or-so cells contain the same complete library of genetic information. It’s called "DNA."
Every cell within an organ has a specific job, and every organ shares some responsibility for the well being of the entire human system. When you cut your skin, the nervous system transmits signals to your other organs telling them how to respond. The muscular system is told to withdraw from the source of pain. The lymphatic system joins ranks with the circulatory system to combat infection. The skeletal system receives a message to create new white blood cells in the bone marrow. The endocrine system creates hormones that tell the body whether to run away or not.
DNA
If you want to understand the human body, you must begin by becoming acquainted with deoxyribonucleic acid, or DNA. Without DNA your cells would not produce the proteins you need and would not know what role to play to keep you going.
If you look closely at a strand of DNA, the first thing you will notice is its special shape. The double-helix structure of DNA reminds many people of a twisted ladder. If you look carefully, you will see that this "ladder" is actually two chains of nucleotides bound together. What is a nucleotide? A nucleotide consists of deoxyribose sugar, a phosphate group, and a set of nitrogen bases. The sugar and the phosphates form the sides of the ladder. The nitrogen bases form the rungs of the ladders. There are four nitrogen bases: adenine (A), thymine (T), cytosine (C), and guanine (G). The bases pair up in specific ways to form the rungs. Adenine always pairs with thymine, and cytosine always pairs with guanine.
The DNA code is communicated by "triplets" or sets of three bases in a row. A triplet indicates one piece of code in the entire code sequence. For instance, the triplet ACC is the code for the amino acid tryptophan. Chains of triplets give instructions for forming the proteins that your body needs to grow and survive. The DNA codes "tell" the cells which proteins to produce.
Genes and Chromosomes
The segment of the DNA strand that codes the protein for specific types of cells is called a "gene." This is a word you have certainly heard on the news during the past several years. A gene is what determines a specific human trait. For instance, some genes determine your hair color, eye color, the length of your bones (height), and the color of your skin. You only have 200 or so different types of cells, but you have millions of genes in you. Your genes are located within each cell. These bundles of DNA strands, coiled around proteins are found in each cell’s nucleus.
Everyone Is Unique
The most incredible thing about genes is that all humans of every race, ethnic group, religion, and gender share 99.9% of the same genes. The remaining 0.1% accounts for all the differences among human beings. Just think. There are six billion people on this planet. Ninety-nine percent of the human recipe is identical. It is a mere one tenth of one percent of a person’s genetic code which makes every single person unlike anyone else.
So how did you receive the genetic coding that makes you unique? You inherited your parents’ genes. Your parents’ sex cells, or gametes, carried the unique genetic blueprint of each parent. Human body cells have 23 pairs of chromosomes, or a total of 46 chromosomes. Gametes, or sex cells, only have 23 chromosomes. This may seem odd a first, but it really is quite logical. A child receives 23 chromosomes from the female and 23 from a male for a total of 46 chromosomes. During conception, the genes of both parents are passed down to the next generation. If gametes contained 46 chromosomes like body cells, the child would have 92 chromosomes in each cell!
A World of Mutations
Mutations can occur in the DNA’s molecular structure, if the molecules become exposed to too much radiation. The nitrogen bases, A, C, T, and G, within the rung of the DNA ladder can be damaged by radiation. This can happen in three different ways. Under the influence of damaging electromagnetic energies such as X-rays or gamma rays or radioactive particles that penetrate the cell, one base pair can replace another pair, one base pair can be removed, or a base pair can be added to the base sequence. In all three cases, the base sequence, the DNA code, is changed. The new information, or "programming," will now produce different proteins in new cells. When a cell reproduces, the new cell will be different than its parent cell. In some cases of mutation, no real damage is done. In others, the results can mean disease.
Cell mutations fall into three categories. The first category is a neutral mutation. In this case, existing cells undergo a mutation, a change in their base pairs that is not passed on to new cells. This happens when the mutation takes place in a mature cell. Red blood cells, for instance, have a life span of only 120 days. They then die and are absorbed by the body. If a mature blood cell, one near the end of its life cycle, dies before it divides, the mutation or damage will not be passed on to new cells.
The second category of mutation is a harmful mutation. The astronauts on the space station are particularly concerned about this type of genetic mutation. Harmful mutations may occur within immature red blood cells exposed to ionizing radiation because they have a long time left to live. If the DNA sequence is changed in an immature cell, the sequence will be replicated in new red blood cells during cell division. The new cells may not perform their jobs in the normal way. Cancer is an example of a disease caused by a harmful mutation in the base pairs of a cell’s DNA.
The third category of cell mutation is a helpful mutation. In some instances, DNA that has mutated actually causes an improvement in the functions of the cell. An example of this was discovered in people living near Milan, Italy. Their heart and artery cells had grown resistant to heart disease caused by eating too much fat. Helpful mutation is also the subject of much science fiction. Think of Peter Parker, a.k.a. "Spiderman™", who was bitten by a spider that had been exposed to radiation. Or the story of the "X-Men™", that revolves around a whole society where children are born with helpful mutations.
Shielding Protects Human Cells from Harmful Radiation
Mutations can occur naturally in the human body or can be the result of something in the environment such as ionizing radiation. This is why astronauts must take precautions to shield themselves from radiation during a solar storm. If they receive too much radiation and a harmful mutation of a cell’s DNA occurs, they may suffer irreparable damage to their bodies.
Certain cells in the human body are more vulnerable to radiation than others. It is very important that these cells be protected from radiation. If you ever have an X-ray, the chances are that parts of your body will be covered with a lead shield during the procedure. The X-ray technician will also take precautions by moving behind a shielding wall. The cells most vulnerable to mutation are found in those organs and systems of the body in which cell division is vital. Because there is a lot of cell replication in these organs, many of the cells are still young and vulnerable to mutation caused by ionizing radiation such as X-rays.
The following is a list of cells that are particularly vulnerable to ionizing radiation:
• Lymphoid cells found in the spleen, liver, and lymph nodes. These cells work, divide, and grow in order to keep the body healthy by fighting disease and infection.
• Sex cells (gametes) if changed in any way can pass along the changes to the offspring.
• Bone marrow contains stem cells that give rise to all blood cells and platelets.
• Epithelial cells line the gastrointestinal tract.
• Epidermal cells form the outer layer of skin that is shed constantly as the body produces new protective layers.
• Hepatic cells are the cells of the liver.
• Epithelium cells form the lining of the lungs.
• Kidney cells serve to remove wastes from the body's fluids.
Auntie’s Recipe
When you copy a recipe from your Auntie’s cookbook and, quite innocently, leave out one, important "base" ingredient, the dish you cook for Thanksgiving dinner will be a mutation of the original. It may taste okay (neutral), it may be simply awful (harmful), it may even be more delicious than you remember (helpful). What are the odds?
The DNA recipe within your cells, composed of bases and sugars and phosphates, must be carefully protected, or the new cooks on the block, your new cells, will not be taking their instructions from the original recipe. It is said that every seven years all of the cells in a body are "new." This does not happen all at once, mind you, and after seven years some cells are many generations removed from their originals, but it happens gradually and inexorably.
We protect our cell recipes by using sunscreen on our skin, by being careful around sources of damaging radiation, by not smoking and damaging the fragile cells in our lungs, and by avoiding drugs that might alter the cells in our nervous system. We can feed our cells the right vitamins and proteins that will keep them cooking. The chemical reactions taking place in our bodies are taking place at the molecular level, amongst atoms, between electrons, and yet they are as real as the atoms that form the ink on this sheet of paper.
