dna

DNA (Deoxyribonucleic acid), is responsible for providing the genetic information necessary for the development and reproduction of all living organisms. It is often compared to a genetic blueprint1 DNA is an enormously long and narrow biological molecule usually packaged in structures called chromosomes (a genetic shoebox), which are located in the nucleus of a cell.

Every living organism has its own DNA code which is organized in the ladderlike structure of the DNA molecule as a series of complementary paired molecules called bases. As a process, DNA functions like a template: information is transferred from the sequence of base pairs within the DNA , by a molecule called messenger RNA (mRNA), to an enzyme called a ribosome (a biochemical assembly machine). The ribosome translates the code by assembling a protein molecule from amino acids (the building blocks of proteins) according to the base pair sequence brought to it by the mRNA. Proteins are very complex molecules used for signaling, regulation, metabolism and structural components.

All cells containing DNA in any one organism have the same identical DNA, but each different cell type will synthesize its own characteristic combination of proteins, some common to most cells, while others are more specialized and help to define the functions of that particular cell type.

It is the complementary structure of DNA which enables it to function as a template for translating the genetic code and the assembly of the proteins. This complementary structure also enables the double stranded DNA to be separated and replicated as two exact duplicates during cell division (replication). 2

In most organisms, DNA is formed as a very long and very narrow double-helix formation of two DNA strands coiled around each other in a head-to-toe "antiparallel" orientation. The strands provide structural support for a complementary pair of bases located inbetween the strands (a base pair is like a letter of a genetic word). A sequence of three base pairs forms a codon ( a DNA word) on the DNA strand that encodes the information for one amino acid residue. A series of codons, and associated start/stop codons, (a DNA sentence or gene) forms the genetic code for the selection of particular amino acids and their specific arrangement necessary for the assembly of a particular protein molecule. The protein molecules, as many as 20,000 different types, are used in the cell, or are transported, often via small containers (vesicles), to other areas of the organism.

Each single strand of DNA is a long biopolymer comprised of repeating units called nucleotides. A nucleotide is a base linked to a sugar and phosphate group which form a sugar/phosphate backbone. Attached to each sugar molecule (deoxyribose) is one of the four bases; Adenine (A), Thymine (T), Guanine (G) or Cytosine (C).

Double stranded DNA is formed by a weak hydrogen bond between the bases holding the nucleotides together. Each base is a structural complement of its opposing base, so both together form the base pair. The complementarity base pairs, A&T, C&G, are identical in size and shape and only one of the four arrangements - TA, AT, GC and CG, will fit between the backbones of double stranded DNA; e.g., adenine always pairs with thymine and guanine always pairs with cytosine. This complementarity, is at work in all DNA functions, and makes it possible for DNA to be copied and repaired relatively easily, while accurately preserving its information content. Thus it forms the basis of semi-conservative DNA replication.


Nuclear DNA is organized and stored as chromosomes within the nucleus. The nucleus is a double membrane separating the DNA from the cytoplasm of the cell enabling certain processing to occur prior to protein synthesis. Each chromosome holds hundreds or thousands of genes. A gene can be described in different ways but in general can be thought of as a whole unit of genetic information.

At conception, the male sperm, (a half stranded DNA with a protein coat), and female ovum, (an unfertilized egg also containing a single strand of DNA), each contribute 23 chromosomes for a total of 46 chromosomes in the fertilized embryo. The total sum of chromosomes is called the kayrotype in eukaryotes (organisms such as plants, yeasts and animals whose cells have a nucleus)

Some organelles in eukaryotic cells (mitochondria and chloroplasts) have their own DNA with a similar organization to bacterial genomes. This DNA encodes for some of the functions in these organelles. Both the mitochondrial DNA (mtDNA) and the chloroplast DNA (cpDNA) originate in the female egg, so the organelle DNA is always inherited from the mother. Some cells, such as blood cells, do not have a nucleus and do not contain DNA. In prokaryote cells (organisms such as common bacteria), DNA is located in a region called the nucleoid. Viruses have a single type of nucleic acid, either DNA or RNA, directly encased in a protein coat called the capsid.

The entire DNA sequence of genes as a whole in any organism is called its genome. The genome provides the necessary genetic instructions to produce the phenotype, the outward physical manifestation of an organism, as well as the necessary processes involved in the replication of nuclear DNA.