DNA (deoxyribonucleic acid) is the genetic material of all known life from the simplest one-celled bacteria to the most magnificent five-ton elephant on the African plain. "Genetic material" refers to the molecules that contain two important sets of instructions: one for making proteins for the cell's current needs, and the other for making copies of themselves, or replicating, so that the exact same genetic code can by used by future generations of cells.
Keeping the cell alive long enough to reproduce requires a great many of these protein products, which DNA orders via the mRNA (messenger ribonucleic acid) it creates as an envoy to the ribosomes, where proteins are actually synthesized.
The encoding of genetic information by DNA into messenger RNA is called transcription, while the making of proteins on the basis of directions from mRNA is called translation.
Translation involves the cobbling together of proteins via peptide bonds to form long chains of amino acids or the monomers in this scheme. 20 different amino acids exist, and the human body needs some of each one of these to survive.
The protein synthesis in translation involves a coordinated meeting of mRNA, aminoacyl-tRNA complexes and a pair of ribosomal subunits, among other players.
Nucleic Acids: An Overview
Nucleic acids consist of repeating subunits, or monomers, called nucleotides. Each nucleotide consists of three distinct components of its own: a ribose (five-carbon) sugar, one to three phosphate groups and a nitrogenous base.
Each nucleic acid has one of four possible bases in each nucleotide, two of which are purines and two of which are pyrimidines. The differences in the bases between nucleotides is what gives different nucleotides their essential character.
Nucleotides can exist outside of nucleic acids, and in fact, some of these nucleotides are central to all of metabolism. The nucleotides adenosine diphosphate (ADP) and adenosine triphosphate (ATP) are at the heart of the equations in which energy for cellular use is extracted from the chemical bonds of nutrients.
The nucleotides in nucleic acids, however, have only one phosphate, which is shared with the next nucleotide in the nucleic acid strand.
Basic Differences Between DNA and RNA
At the molecular level, DNA differs from RNA in two ways. One is that the sugar in DNA is deoxyribose, whereas in RNA it is ribose (hence their respective names). Deoxyribose differs from ribose in that, instead of having a hydroxyl (-OH) group at the number-2 carbon position, it has a hydrogen atom (-H). Thus deoxyribose is one oxygen atom short of ribose, hence "deoxy."
The second structural difference between the nucleic acids lies in the composition of their nitrogenous bases. DNA and RNA both contain the two purine bases adenine (A) and guanine (G) as well as the pyrimidine base cytosine (C). But while the second pyrimidine base in DNA is thymine (T) in RNA this base is uracil (U).
As it happens, in nucleic acids, A binds to and only to T (or U, if the molecule is RNA), and C binds to and only to G. This specific and unique complementary base pairing arrangement is required for the proper transmission of DNA information to mRNA information in transcription and mRNA information to tRNA information during translation.
Other Differences Between DNA and RNA
At a more macro level, DNA is double-stranded while RNA is single-stranded. Specifically, DNA takes the form of a double helix, which is like a ladder twisted in different directions at both ends.
The strands are bonded at each nucleotide by their respective nitrogenous bases. This means that an "A"-bearing nucleotide can only have a "T"-bearing nucleotide on its "partner" nucleotide. This means that in sum, the two DNA strands are complementary to each other.
DNA molecules can be thousands of bases (or more properly, base pairs) long. In fact, a human chromosome is nothing more than a single very long strand of DNA coupled with a good deal of protein. RNA molecules of all types, on the other hand, tend to be comparatively small.
Also, DNA is found primarily in the nuclei of eukaryotes but also in mitochondria and chloroplasts. Most RNA, on the other hand, is found in the nucleus and the cytoplasm. Also, as you'll soon see, RNA comes in various types.
Types of RNA
RNA comes in three primary types. The first is mRNA, which is made from a DNA template during transcription in the nucleus. Once complete, the mRNA strand makes its way out of the nucleus via a pore in the nuclear envelope and winds up directing the show at the ribosome, the site of protein translation.
The second type of RNA is transfer RNA (tRNA). This is a smaller nucleic acid molecule and comes in 20 subtypes, one for each amino acid. Its purpose it to shuttle its "assigned" amino acid to the site of translation on the ribosome so that it can be added to the growing polypeptide (small protein, often in progress) chain.
The third type of RNA is ribosomal RNA (rRNA). This type of RNA makes up a significant fraction of the mass of ribosomes with proteins specific to ribosomes making up the rest of the mass.
Before Translation: Creating an mRNA template
The oft-quoted "central dogma" of molecular biology is DNA to RNA to protein. Phrased even more succinctly, it might be put transcription to translation. Transcription is the first definitive step toward protein synthesis and is one of the ongoing necessities of any cell.
This process begins with the unwinding of the DNA molecule into single strands so that the enzymes and nucleotides participating in transcription have room to move to the scene.
Then, along one of the DNA strands, a strand of mRNA is assembled with the help of the enzyme RNA polymerase. This mRNA strand has a base sequence complementary to that of the template strand, save for the fact that U appears wherever T would appear in DNA.
- For example, if the DNA sequence undergoing transcription is ATTCGCGGTATGTC, then the resulting strand of mRNA would feature the sequence UAAGCGCCAUACAG.
When an mRNA strand is being synthesized, certain lengths of DNA, called introns, are eventually spliced out of the mRNA sequence because they do not code for any protein products. Only the portions of the DNA strand that actually code for something, called exons, contribute to the final mRNA molecule.
What’s Involved in Translation
Various structures are needed at the site of protein synthesis for successful translation.
The ribosome: Each ribosome is made of a small ribosomal subunit and a large ribosomal subunit. These only exist as a pair once translation is beginning. They contain a large amount of rRNA as well as protein. These are one of the few cell components that exist in both prokaryotes and eukaryotes.
mRNA: This molecule carries direct instructions from the cell's DNA to manufacture a specific protein. If DNA can be thought of as the blueprint of the entire organism, a strand of mRNA contains just enough information to make one decisive component of that organism.
tRNA: This nucleic acid forms bonds with amino acids on a one-to-one basis to form what are called aminoacyl-tRNA complexes. This just means that the taxi (the tRNA) is currently carrying its intended and sole kind of passenger (the specific amino acid) from among the 20 "types" of people in the vicinity.
Amino acids: These are small acids with an amino (-NH2) group, a carboxylic acid (-COOH) group, and a side chain bound to a central carbon atom along with a hydrogen atom. Importantly, codes for each one of the 20 amino acids are carried in groups of three mRNA bases called triplet codons.
How Does Translation Work?
Translation is based on a relatively simple triplet code. Consider that any group of three consecutive bases can include one of 64 possible combinations (for example, AAG, CGU, etc.), because four raised to the third power is 64.
This means that there are more than enough combinations to generate 20 amino acids. In fact, it would be possible for more than one codon to code for the same amino acid.
This is, in fact, the case. Some amino acids are synthesized from more than one codon. For example, leucine is associated with six distinct codon sequences. The triplet code is this "degenerate."
Importantly, however, it is not redundant. That is, the same mRNA codon cannot code for more than one amino acid.
Mechanics of Translation
The physical site of translation in all organisms is the ribosome. Some portions of the ribosome also have enzymatic properties.
Translation in prokaryotes begins with initiation via an initiation factor signal from a codon appropriately called the START codon. This is absent in eukaryotes, and instead, the first amino acid selected is methionine, coded for by AUG, which functions as sort of a START codon.
As each additional three-segment strip of mRNA is exposed on the surface of the ribosome, a tRNA bearing the called-for amino acid wanders into the scene and drops off its passenger. This binding site is called the "A" site of the ribosome.
This interaction happens at the molecular level because these tRNA molecules have base sequences complementary to the incoming mRNA and hence bind to the mRNA readily.
Building the Polypeptide Chain
In the elongation phase of translation, the ribosome moves by three bases, a process called translation. This exposes the "A" site anew and leads to the polypeptide, whatever its length in this thought experiment, being shifted to the "P" site.
When a new aminoacyl-tRNA complex arrives at the "A" site, the entire polypeptide chain is removed from the "P" site and attached to the amino acid that has just been deposited at the "A" site, via a peptide bond. Thus when the translocation of the ribosome down the "track" of the mRNA molecule occurs again, a cycle will have been completed, and the growing polypeptide chain is now longer by one amino acid.
In the termination phase, the ribosome encounters one of three termination codons, or STOP codons, that are incorporated into mRNA (UAG, UGA and UAA). This causes not tRNA but substances called release factors to flock to the site, and this leads to the release of the polypeptide chain. The ribosomes separate into their constituent subunits, and translation is complete.
What Happens After Translation
The process of translation creates a polypeptide chain that still needs to be modified before it can work properly as a new protein. The primary structure of a protein, its amino acid sequence, represents only a small part of its eventual function.
The protein is modified after translation by folding it into specific shapes, a process that often occurs spontaneously owing to electrostatic interactions between amino acids in non-neighboring spots along the polypeptide chain.
How Genetic Mutations Affect Translation
Ribosomes are great workers, but they are not quality-control engineers. They can only create proteins from the mRNA template they are given. They are unable to detect errors in that template. Therefore, errors in translation would be inevitable even in a world of perfectly functioning ribosomes.
Mutations that change a single amino can disrupt protein function, such as the mutation that causes sickle cell anemia. Mutations that add or delete a base pair can throw entire triplet code off so that most or all subsequent amino acids will also be wrong.
Mutations could create an early STOP codon, meaning that only part of the protein gets synthesized. All of these conditions can be debilitating to various degrees, and attempting to conquer inborn errors such as these represents an ongoing and complex challenge for medical researchers.
- LibreTexts Chemistry: Protein Synthesis and the Genetic Code
- Carleton College Microbial Life Educational Resources: Translation
- Scitable By Nature Education: Ribosomes, Transcription, and Translation
- LibreTexts Chemistry: Nucleotides and Nucleic Acids
- Scitable By Nature Education: Translation: DNA to mRNA to Protein
- Scitable By Nature Education: DNA Replication and Causes of Mutation
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.