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Genetics 101
  Overview of Genetics

By Amy Adams, MS

Reviewed By Amanda Toland, PhD and Adam Lowe
Last updated: October 19, 2011



The field of genetics is in the news daily, with researchers mapping the human genome, cloning animals, and identifying new disease genes. For many people, the problem is putting this information into context and understanding what recent genetic "breakthroughs" really mean in terms of our health. To help answer these questions, we've created a primer to help you understand how advances in genetics can help in the treatment of common diseases.

 
 
 

What is DNA?

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DNA (deoxyribonucleic acid) is a long molecule that is contained within almost all of our cells in a compartment called the nucleus. It is composed of individual units called bases. There are four types of bases, designated A (adenine), T (thymine), G (guanine), and C (cytosine).

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Each DNA molecule is made of two individual strands paired together. Each strand consists of a series of the four bases. When the two strands pair up, an A on one strand is always across from a T on the other strand, and a C always pairs with a G. These A/T and G/C combination are called base pairs. The double-stranded molecule then twists like a coiled ribbon into a shape called a double helix. A piece of DNA millions of base pairs long — in conjunction with some proteins — is a chromosome.

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What are genes?

The sequence of bases in each gene contains instructions for making a single protein.
Each chromosome contains thousands of genes, each of which is several thousand bases long. The sequence of bases in each gene contains instructions for making a single protein. Each protein serves a particular function in the body. For example, enzymes help us digest food, structural elements give our cells shape, and signaling molecules help the cells communicate with each other.

Additional bases that come before the genes on a chromosome tell cells when each gene should be used. For example, these sequences might contain instructions that a protein for making hair should only be made in certain skin cells, and not by other cells of the body.

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How do we inherit our genes?

Humans inherit 23 chromosomes from each of their parents for a total of 46 chromosomes. Of these, 44 are identical in men and women — these are called autosomes. The remaining two chromosomes are called sex chromosomes, which are designated X and Y. Women inherit two X chromosomes, whereas men inherit one X chromosome from their mother and one Y chromosome from their father. Because of the way we inherit our chromosomes, we all have two copies of every gene that is contained on the autosomes. Depending on the combination of the genes we inherit, we end up with some traits that resemble our mother and others that resemble our father. Women have two copies of each gene on the X chromosome, while men have only the genes that they inherit from their mother on the X chromosome and only genes that they inherit from their father on the Y chromosome.

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What is a mutation?

Each gene is made up of a series of bases, and those bases provide instructions for making a single protein. Any change in the sequence of bases — and therefore in the protein instructions — is a mutation. Just like changing a letter in a sentence can change the sentence's meaning, a mutation can change the instruction contained in the gene. Some mutations have little or no effect on the protein, while others cause the protein not to function at all.

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What kind of problems can mutations cause?

Some mutations result in proteins that do not function normally, and may end up causing disease. There are several ways that gene mutation can change the way a protein functions, including:

  • Altered function: Some mutations result in a protein that cannot carry out its normal function in the cell, or cannot carry out that function very well. One example of this type of mutation is sickle cell anemia. In this disorder, an altered protein in red blood cells alters the shape of the red blood cell, which causes the cell to become stuck in blood vessels. This prevents cells from carrying sufficient oxygen to the rest of the body.
  • Lack of protein: Some mutations prevent the protein from being made. One example of this type of mutation is hemophilia. In this condition, a mutation results in the absence of a protein that causes blood to clot. The result is uncontrolled bleeding in response to injury.
  • Change in how much protein is made: Some mutations cause too much or too little of a normal protein to be made. Although the protein itself functions properly, it is not present in quantities that are appropriate. One example of this is in the development of some cancers. In this case, a protein that prevents additional mutations from building up can become turned off. Without this protein, the cell accumulates mutations and becomes increasingly cancerous.

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Can we overcome our genetics?

Our risk for almost any medical condition is a function of both our genes and our environment. While we can't change our genes, we can apply our knowledge of our family medical history to predict our risk for specific problems. This, in turn, allows us to focus on the things we can change — diet, lifestyle, screening, treatment — to ensure a long, healthy life.

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