DNA

DNA (deoxyribonucleic acid) is the information storage molecule for the cell. Each cell contains within the DNA the entire instruction set to produce and run an entire organism (including you). In multicellular organisms, most of the DNA is stored in a central location within the cell, called the nucleus.

Bases

DNA is composed of only four bases, which can be thought of as letters of the DNAalphabet. The structure of these four letters is shown in the figure to the right. There are two purine bases (adenine and guanine) and two pyrimidine bases (thymine andcytosine). The bases are commonly abbreviated with the first letter of their names:

Nucleotides

When a base, such as adenine,is linked to a deoxyribose sugar through a glycosidic bond, the structure is called a nucleoside. When the deoxyribose sugar isphosphorylated, on either the 3' or the 5' position (or both), the structure is a called a nucleotide. The precursors of DNA synthesis are deoxynucleoside-5'-triphosphates or dNTPs.

DNA structure

DNA is composed of a polynucleotide (multiple nucleotides) chain that is formed by linking nucleotides through 3',5'- phosphodiester bonds. In this way, DNAforms a long chain of sequential nucleotides.

DNA chains are not usually just single strands of nucleotides.

Usually, nucleotides of two polynucleotide strands are base-paired to each other. The nucleotide base adenine pairs through hydrogen bonds with the nucleotide thymine forming an adenine-thymine (A-T) base pair. Guaninebinds with cytosine forming a guanine-cytosine (G-C) base pair. Because of this base pairing, polynucleotide chains in double-stranded DNA are always complementary to each other.

DNA Double Helix

The nature of the bonds between nucleotide bases results in the famous "double helix" DNA structure. The hydrogen bonding between strands of DNA holds the strands together until they are separated for DNA replication or RNA transcription.

DNA Replication

DNA is replicated (reproduced) through an extremely complicated process. This is the simplified version. The two strands of DNAare separated and a new complementary strand is synthesized for each old strand by adding complementary base pairs to the newly-synthesized strand (see figure at right). The DNA polymerase (the enzyme that makes new DNA) reads the base on the original strand and adds the complementary base to the newly synthesized strand. DNA is always synthesized in a 5' to 3' direction. Since one strand runs in the opposite direction, it must be synthesized in pieces that are later connected with DNA ligase. In reality, both strands are synthesized in pieces, since the DNA is unwound at numerous sites simultaneously on each chromosome in order to produce a complete copy in a short period of time (hours instead of weeks).

RNA

How does DNA turn in you ?

Since DNA just stores information, there needs to be a way to turn the information into physical bodies. This feat is accomplished through the synthesis of an intermediate information-carrying molecule that carries the information from the nucleus to the cytoplasm, where it is processed into proteins. The intermediate molecule is called RNA (ribonucleic acid). As you can tell from the name, it differs from DNA in that part of it (the ribosesugar) has an extra oxygen compared to DNA (the "deoxy" version). In addition, RNA does not use thymine as a base, but another nucleotideknown as uracil (see figure right). Uracil forms base pairs with adenine, as thymine does in DNA. RNA is transcribed from DNA through an enzyme called RNA polymerase, in a process that is known as transcription. There are several kinds of RNA, which have different functions. However, the RNA that eventually codes for proteinsis called messenger RNA (mRNA).

Translation

Once mRNA is synthesized, it is processed (usually through splicing) in the nucleus before moving to the cytoplasm. In many instances, alternate splicing of an RNA transcript can produce multiple proteins. It is one of the chief ways that 25,000 genes in the human genome can make ~100,000 proteins. Proteins are produced from an mRNA template using a process known as translation. The translation process is quite complicated and requires numerous molecular machines and co-factors. This description is a simplified version of what happens. A large molecular machine, called the ribosome, binds to the mRNA. Ribosomes are composed of two subunits. The large subunit is composed of three forms of ribosomal RNA (rRNA) and 49 different proteins. The smaller subunit is composed of one rRNA and 33 different proteins. A third kind of RNA, transfer RNA (tRNA), carries amino acids that are used to make proteins according to the mRNAtemplate. One end of tRNA, consisting of three bases, called the anticodon, binds to a series of three complementary bases, called the codon on the mRNA strand. The other end of the tRNAbinds to a specific amino acid. Specific sequence on the RNA are translated into proteinsaccording to the genetic code. Each set of three nucleotide bases codes for one amino acid. This code is nearly universal for all organisms (with a few exceptions). The code is reproduced below:

The 30s subunit of the ribosome binds to the mRNA. A specific tRNA (usually the first codon codes for methionine) binds to the codon of the mRNA The 50s ribosomal subunit binds to the 30s subunit. The next tRNA binds to the next codon on the mRNA and numerous initiation factors and the enzyme peptidyl transferase link the two amino acids together through a peptide bond. As the ribosomal complex moves along the mRNA, the peptide chain gets longer until the entire protein is formed. Other molecular machines are responsible for making sure that the protein folds into the correct three dimensional shape.

followed by:

Ribosomes are small organelles found in the cells of all life forms. They are quite small, only a few hundred nanometers across. It has been theorized that ribosomes were once independent life forms, reproducing on their own, until other chemicals such as carbohydrates and DNA were brought into the fold at a later time. Ribosomes are composed of rRNA (ribosomal RNA, or ribonucleic acid) and other catalytic proteins. Their main function is to produce a variety of proteins from simple genetic instructions which propagate outwards from the cellular nucleolus in the form of messenger RNA (mRNA). Ribosomes float in the cytoplasm of a cell or bind to the endoplasmic reticulum, ribbon-like structures found within the cell.

Ribosomes are sometimes referred to as simply RNA. Like DNA, they are long chains of amino acids, but their base pairs are different and they are usually not as long as DNA. Ribosomes play a key part in protein synthesis, the process that generates organic tissue. Genetic instructions for the creation of new proteins come from mRNA. Ribosomes always have two subunits which interlock and behave as a single entity.

The exact type of ribosome found within a cell can vary based on the kind of organism that the cell is a part of. Eukaryotes (multi-celled organisms) have their own kind of ribosome, whereas prokaryotes (single-celled organisms) have another. Certain organelles within the cell, chloroplast and mitochondria, have their own distinct ribosomes as well. Ribosomes make up the majority of RNA content within a cell, being responsible for about 95%.