Deoxyribonucleic Acid (DNA): Structure, Function & Importance

DNA, or deoxyribonucleic acid, is a nucleic acid (one of two such acids found in nature) that serves to store genetic information about an organism in a way that can be transmitted to subsequent generations. The other nucleic acid is RNA, or ribonucleic acid.

DNA carries the genetic code for every single protein your body makes and thus acts as a template for the entirety of you. A string of DNA that codes for a single protein product is called a gene.

DNA consists of very long polymers of monomeric units called nucleotides, which contain three distinct regions and come in four distinct flavors in DNA, thanks to variances in the structure of one of these three regions.

In living things, DNA is bundled along with proteins called histones to create a substance called chromatin. The chromatin in eukaryotic organisms is broken into a number of distinct chunks, called chromosomes. DNA is passed from parents to their offspring, but some of your DNA was passed down from your mother exclusively, as you'll see.

The Structure of DNA

DNA is made up of nucleotides, and each nucleotide includes a nitrogenous base, one to three phosphate groups (in DNA, there is only one) and a five-carbon sugar molecule called deoxyribose. (The corresponding sugar in RNA is ribose.)

In nature, DNA exists as a paired molecule with two complementary strands. These two strands are joined at every nucleotide across the middle, and the resulting "ladder" is twisted into the form of a double helix, or pair of offset spirals.

The nitrogenous bases come in one of four varieties: adenine (A), cytosine (C), guanine (G) and thymine (T). Adenine and guanine are in a class of molecules called purines, which contain two joined chemical rings, whereas cytosine and thymine belong to the class of molecules known as pyrimidines, which are smaller and contain only one ring.

Specific Base-Pair Bonding

It is the bonding of bases between nucleotides in adjacent strands that creates the "rungs" of the DNA "ladder." As it happens, a purine can only bind with a pyrimidine in this setting, and it is even more specific than that: A binds to and only to T, whereas C binds to and only to G.

This one-to-one base pairing means that if the sequence of nucleotides (synonymous with "sequence of bases" for practical purposes) for one DNA strand is known, the sequence of bases in the other, complementary strand can easily be determined.

Bonding between adjacent nucleotides in the same DNA strand is brought about by the formation of hydrogen bonds between the sugar of one nucleotide and the phosphate group of the next.

Where Is DNA Found?

In prokaryotic organisms, the DNA sits in the cytoplasm of the cell, as prokaryotes lack nuclei. In eukaryotic cells, DNA sits in the nucleus. Here, it is broken into chromosomes. Humans have 46 distinct chromosomes with 23 from each parent.

These 23 different chromosomes are all distinct on physical appearance under a microscope, so they can be numbered 1 through 22 and then X or Y for the sex chromosome. Corresponding chromosomes from different parents (e.g., chromosome 11 from your mother and chromosome 11 from your father) are called homologous chromosomes.

DNA is also found in the mitochondria of eukaryotes generally as well as in the chloroplasts of plant cells specifically. This by itself supports the prevailing idea that both of these organelles existed as free-standing bacteria before being engulfed by early eukaryotes over two billion years ago.

The fact that the DNA in mitochondria and chloroplasts code for protein products that nuclear DNA does not lends even more credence to the theory.

Because the DNA that makes its way into mitochondria only gets there from the mother's egg cell, thanks to the way sperm and egg are generated and combine, all mitochondrial DNA comes through the maternal line, or the mothers of whatever organism's DNA is being examined.

DNA Replication

Before every cell division, all of the DNA in the cell nucleus has to be copied, or replicated, so that each new cell created in the division that is soon to come can have a copy. Because DNA is double-stranded, it needs to be unwound before replication can start, so that the enzymes and other molecules that participate in replication have room along the strands to do their work.

When a single DNA strand is copied, the product is actually a new strand complementary to the template (copied) strand. It thus has the same base DNA sequence as the strand that was bound to the template before replication started.

Thus each old DNA strand is paired with one new DNA strand in every new replicated double-stranded DNA molecule. This is referred to as semiconservative replication.

Introns and Exons

DNA consists of introns, or sections of DNA that do not code for any protein products and exons, which are coding regions that do make protein products.

The way that exons pass along information about proteins is through transcription or the making of messenger RNA (mRNA) from DNA.

When a DNA strand is transcribed, the resulting strand of mRNA has the same base sequence as the template strand's DNA complement, except for one difference: where thymine occurs in DNA, uracil (U) occurs in RNA.

Before the mRNA can be sent to be translated into a protein, the introns (the non-coding portion of genes) needs to be taken out of the strand. Enzymes "splice" or "cut" the introns out of the strands and attach all of the exons together to form the final coding strand of mRNA.

This is called RNA post-transcriptional processing.

RNA Transcription

During RNA transcription, ribonucleic acid is created from a strand of DNA that has been separated from its complementary partner. The DNA strand being thus used is known as the template strand. Transcription itself is dependent on a number of factors, including enzymes (e.g., RNA polymerase).

Transcription occurs in the nucleus. When the mRNA strand is complete, it leaves the nucleus through the nuclear envelope until it attaches to a ribosome, where translation and protein synthesis unfold. Thus transcription and translation are physically segregated from one another.

How Was the Structure of DNA Discovered?

James Watson and Francis Crick are known for being the co-discoverers of one of the deepest mysteries in molecular biology: the double helix DNA structure and shape, the molecule responsible for the unique genetic code carried by everyone.

While the duo earned their place in the pantheon of great scientists, their work was contingent on the findings of a variety of other scientists and researchers, both past and operating in Watson's and Crick's own time.

In the middle of the 20th century, in 1950, the Austrian Erwin Chargaff discovered that the amount of adenine in DNA strands and the amount of thymine present were always identical, and that a similar relationship held for cytosine and guanine. Thus the amount of purines present (A + G) was equal to the amount of pyrimidines present.

Also, British scientist Rosalind Franklin used X-ray crystallography to speculate that DNA strands form phosphate-containing complexes located along the outside of the strand.

This was consistent with a double helix model, but Franklin did not recognize this since no one had any good reason to suspect this DNA shape. But by 1953, Watson and Crick had managed to put it all together using Franklin's research. They were helped by the fact that chemical-molecule model-building was itself a rapidly improving endeavor at the time

References

About the Author

Kevin Beck holds a bachelor's degree in physics with minors in math and chemistry from the University of Vermont. Formerly with ScienceBlogs.com and the editor of "Run Strong," he has written for Runner's World, Men's Fitness, Competitor, and a variety of other publications. More about Kevin and links to his professional work can be found at www.kemibe.com.

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