DNA - DeoxyriboNucleic Acid

  
   

DNA Sequence Variation

Each DNA molecule contains many genes--the basic physical and functional units of heredity. A gene is a specific sequence of nucleotide bases, whose sequences carry the information required for constructing proteins, which provide the structural components of cells and tissues as well as enzymes for essential biochemical reactions. The human genome is estimated to comprise more than 30,000 genes.

All living organisms are composed largely of proteins--which are coded for by genes. Proteins are large, complex molecules made up of long chains of subunits called amino acids. Twenty different kinds of amino acids are usually found in proteins. Within the gene, each specific sequence of three DNA bases (codons) directs the cells protein-synthesizing machinery to add specific amino acids. For example, the base sequence ATG codes for the amino acid methionine. Since 3 bases code for 1 amino acid, the protein coded by an average-sized gene (3000 bp) will contain 1000 amino acids. The DNA code is thus a series of codons that specify which amino acids are required to make up specific proteins.

Some variations in a person's genetic code will have no effect on the protein that is produced, others can lead to disease or an increased susceptibility to a disease.

   

Gene in Detail

Apart from reproductive gametes, each cell of the human body contains 23 pairs of chromosomes, each a packet of compressed and entwined DNA. Every strand of the DNA is a huge natural polymer of repeating nucleotide units, each of which comprises a phosphate group, a sugar (deoxyribose), and a base (either adenine, thymine, cytosine, or guanine). Every strand thus embodies a code of four characters (A's, T's, C's, and G's), the recipe for the machinery of human life. In its normal state, DNA takes the form of a highly regular double-stranded helix, the strands of which are linked by hydrogen bonds between adenine and thymine (A,T) and between cytosine and guanine (C, G). Each such linkage is said to constitute a base pair; some three billion base pairs constitute the human genome. It is the specificity of these base-pair linkages that underlies the mechanism of DNA replication illustrated here. Each strand of the double helix serves as a template for the synthesis of a new strand, the nucleotide sequence of which is strictly determined. Replication thus produces twin daughter helices, each an exact replica of its sole parent.


Mutations-  A permanent structural alteration in DNA. In most cases, DNA changes either have no effect or cause harm, but occasionally a mutation can improve an organism's chance of surviving and passing the beneficial change on to its descendants.



   

Cutting DNA with Restriction Enzymes

Isolated from various bacteria, restriction enzymes serve as microscopic scalpels that cut DNA molecules at specific sites. The enzyme EcoRI, for example, cuts double-stranded DNA only where it finds the sequence GAATTC. The resulting fragments can then be separated by gel electrophoresis. The electrophoresis pattern itself can be of interest, since variations in the pattern from a given chromosomal region can sometimes be associated with variations in genetic traits, including susceptibilities to certain diseases. Knowledge of the cutting sites also yields a kind of physical map known as a restriction map.

   

DNA Trace

   

Actual mtDNA Haplotype Sequence 

Ejemplo Actual de Secuencia ADN  mitocondrial

093C,129A,187T,189C,223T,263C,278T,293G,294T,311C,360T,368C,519C 73G,151T,152C,182T,186A,189C,195C,198T,247A,263G,297G,315.1C,316A,523-,524-

Back to DNA 101  in Graphics

All Graphics courtesy of  the
U.S. Department of Energy Human Genome Program
and the Human Genome Research Institute

Page Design by Dra. Ana Oquendo Pabón, MD

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