Genetics Explained?

Understanding Your Genes

What are we made of?
Humans, animals, plants and all other living things are made up of tiny factory units known as cells. Your cells are designed to carry out specialized tasks depending on where they are located. For example, the cells in our eyes act as a sensitive film that responds to both contrast and color; our kidney cells filter out impurities from the blood and excrete them in urine; our heart contains highly sophisticated cells that respond to electrical signals to keep the heart beating. The inner mechanics of a cell are controlled partly by genes and partly by the environment. A cell is a miniature factory that has a specialized function depending on where it's located within the body. Genetic material, in the form of chromosomes and genes, is stored mainly in the nucleus, the control center of the cell.

 

A cell is a miniature factory that has a specialized function depending on where it is located in the body. Genetic material, in the form of chromosomes and genes made from DNA, is stored mainly in the nucleus, the control center of the cell.

What are genes?
The genetic material of humans and most other organisms is made up of helix-shaped molecules called deoxyribonucleic acid, or DNA. These molecules are packaged in structures known as chromosomes, inside the cell nucleus. DNA contains a series of "information units" called genes. DNA is a collection of chromosomes that can be compared to rows of bookcases at a library. Each gene is like a single book in this library.

As befits a book, a gene contains information written in "letters",
or bases in technical language. The sequence of these bases is
often referred to as the 'genetic code' .

There are only four bases: adenine, thymine, cytosine and
guanine, or A, T, C and G. These bases can be combined to
encode a vast number of different genes. The human genome - all human genes taken together - consists of about 3 billion genetic "letters." Each gene is made up of a sequence of DNA that forms a pattern for the manufacture of proteins in the cell.

Each gene is made up of a sequence of DNA that forms a pattern for the manufacture of proteins in the cell.

Most chromosomes come in pairs, and therefore so do genes. Humans have 23 pairs of chromosomes. In each pair, one chromosome - and therefore one copy of your genes - is inherited from your father, the other from your mother. There are about 30,000-40,000 such gene pairs in each cell.

How do genes work?
Genes determine what we look like - whether we have brown or blue eyes, the color of our hair, our height and so on. They also affect the way our bodies work and how they repair and maintain themselves. How do the genes do this? They accomplish everything by making proteins.

Each gene contains chemically coded instructions for making a particular protein. When a gene is switched on, it sends out these instructions to the cell by issuing a message in a special
language. Cells can understand this language, and they follow the instructions to make proteins that will perform all the critical jobs. Almost every cell contains a complete copy of a person's unique genetic blueprint. Not all genes, however, are switched on, or active, in every cell. For example, the genes that are active in a kidney cell are different from those that are active in a skin cell; as a result, the proteins made by a kidney cell differ from those made by a skin cell.

Proteins are large, complex molecules that perform different tasks within a cell. Some proteins serve as building blocks for various structures within the cell, others act as chemical messengers, or couriers.

These couriers are responsible for communicating important information either within the cell or from one cell to another. Vital functions are performed by highly specialized proteins called enzymes. Enzymes carry out chemical reactions in the body and keep the body's metabolism working correctly.

Why is everyone's DNA different?
With the exception of identical twins, all people have small differences in their DNA resulting from different combinations of the letters in the genetic alphabet (A, T, C and G). These combinations determine a person's genetic code.

Without this diversity, we'd all look the same. The differences account for only about 1% of human DNA - a figure that may sound surprisingly low until you recall that the entire human
genome contains 3 billion genetic "letters"; 1% means 30 million "letters," which is more than enough to make each of us unique.

Some gene variations are fairly common. When a variation appears in at least 1% of the population, scientists call it a "polymorphism" - from poly, meaning "many" and morph, meaning "form." Polymorphisms are not necessarily good or bad, they are just different from the more usual gene version - similar to alternative spellings of the same word, like "gray" and "grey" - and they make slightly different proteins.

A wrongly worded message can make a cell produce a protein with a different ingredient. If this altered protein is an enzyme, for example, it may not work exactly asit should.

A wrongly worded message can make a cell produce a protein with a different ingredient. If this altered protein is an enzyme, for example, it may not work exactly as it should.

A change in only one genetic "letter" can affect the "protein recipe" or instructions that a gene sends to the cell for making a protein.

Scientists refer to gene variations consisting of a difference in one genetic letter as single nucleotide polymorphisms, or SNPs (pronounced "snips"). For example, a SNP might change a DNA sequence from ACG to ATG.

Many polymorphisms, including SNPs, have no effect on how our cells function, but scientists believe that some could be more important. They may have an impact on how we respond to infectious organisms, to environmental chemicals and even to medications. In some cases, gene variations can lead to a faulty message being sent to the cell, and the resulting protein may not work as well as it should.

How did gene variations appear?
No one knows for sure why genetic variations take place. One theory is that among the 40,000 or so genes that make up a human genome, it's inevitable that variations will sometimes happen by chance. Most genetic variations that take place are not passed on. Sometimes however, because the genetic variation gives a person an increased survival advantage in their local environment, they are more likely to survive long enough to produce offspring, thereby passing on their genetic variation to their children. The basis of evolution is survival of the fittest, and many gene variations have persisted because they enabled people to adapt to their environment.

As our diets and lifestyles changed, often moving away from what our bodies were designed to cope with, these gene variations were no longer beneficial and sometimes even harmful.

One example is the blood disease called sickle cell anemia. Having only one copy of the gene for sickle-shaped red blood cells protects the person against malaria, and this gene variation
originally developed in parts of the world with a high risk of malaria. Over time, the number of people carrying the gene variation for sickle cells increased because they were able to survive to have children, so the variant gene was passed down through the generations. In fact, it spread through the population so successfully that descendants of people from these geographical areas still have a high likelihood of having the variation, even if they no longer live in malarial areas. However, individuals who inherit a sickle cell gene from both parents become seriously ill with sickle cell anemia.

What is the role of genes in disease?
Most diseases have some genetic basis. Those that are entirely due to deviations in the genetic material, such as sickle cell anemia or cystic fibrosis, are usually caused by defects in a single gene. The majority of diseases, however, arise from a combination of genetic variations and such environmental influences as diet, lifestyle and exposure to chemicals or other toxins. They are known as complex diseases, and they include such common afflictions as heart disease, most cancers, mental disorders, asthma, arthritis, diabetes and many others. The occurrence of complex diseases is less predictable than that of single-gene diseases. Because they are caused by several factors acting together, no one factor by itself is sufficient to trigger the disease.

A particular gene, for example, can make a person prone to develop cancer, but only in combination with other genes and environmental triggering factors. These potential triggers range from polluted air and radiation to toxic substances such as tobacco smoke or harmful chemicals in the diet. In most cases, unless this trigger is present, the person won't get the disease despite having a genetic risk factor. Often the trigger needs to be present consistently and for a long time before the gene variation becomes actively harmful.

Can complex diseases be prevented?
People with a family history of a complex disease usually know they might be at increased risk because of the likelihood of their having inherited genes that make them more vulnerable. However, genes that predispose a person to disease are not necessarily inherited, they can occur by chance. For people who carry such genes - inherited or not - an obvious preventive approach is to avoid known environmental triggers and modify the diet and lifestyle to minimize the risk of disease.

As more and more information is uncovered about our genetic make-up, we are beginning to understand how variations in our genes can interact with our diet and lifestyle. Identifying such variations can provide us with the opportunity to make informed choices about what we eat and the lives we lead, and to maximize our chances of staying healthy.

The Human Genome
On 26 June 2000, the first draft of the human genome was completed. A genome is the entire genetic material of an organism; completion of the first draft meant that scientists had read and recorded all 3 billion chemical letters of the human genetic code.

The Human Genome Project, launched in 1990, is an international research effort to map human chromosomes and determine the complete chemical sequence of human DNA. After completing the first draft of the genome, the Project set out to refine and process the data. Having a complete picture of the human genome has ushered in a new, genetic era in medicine and healthcare.

We still don't know the function of many genes, and even knowing an individual's entire DNA sequence does not enable scientist to predict everything about his or her appearance, behavior or future health. Moreover, many characteristics depend on complex interactions among several genes, not to mention such contributing factors as diet, lifestyle and the environment. However, knowledge of the human genome is a crucial stepping stone for understanding the way genes work together, explaining genetic susceptibility to disease and diagnosing, preventing and treating various disorders.