Protein Production: A Simple Summary of Transcription and Translation
Proteins are fundamental to life on Earth. They control all biochemical reactions, provide structure to organisms, and transport vital molecules such as oxygen and carbon dioxide, and even defend the organism as antibodies. The process of decoding the instructions in DNA to make RNA, which in turn is decoded to make a specific protein is known as the central dogma of molecular biology.
This article takes a look at how this central dogma plays out. If you are unfamiliar with the triplet code, or with the structure of proteins take a look at the links.
There are more than 200 different cell types in our bodies. The differences between cells in a multicellular organism arise differences in gene expression, not from differences in the cells' genomes (with the exception of antibody-producing cells).
During development, cells differentiate from each other. During this process, there are a number of regulatory mechanisms that switch genes on and off. As genes code for a specific protein, by switching genes on and off, the organism can control the proteins made by its' different cells. This is very important - you don't want a muscle cell secreting amylase, and you don't want your brain cells to start creating myosin. This regulation of genes is controlled by cell-cell comunications
This analogy may help: Imagine you are painting your house at night - you need lots of light so switch on all of the lights in your house. When you finish painting, you want to watch TV in the lounge. Your purpose has now changed and you want the lighting (gene expression) to suit your purpose. You have two options:
- Switch off the lights using light switches (change the gene expression)
- Shoot out the lights you don't need (deleting genes and mutating DNA)
Which one would you pick? It is safer to turn off the lights, even if you never want to turn it on again. By shooting out the light, you risk damage to the house; by deleting a gene you don't want, you risk damaging genes you do want.
Amino Acid - the building blocks of proteins; there are 20 different types
Codon - a sequence of three organic bases in a nucleic acid that code for a specific amino acid
Exon - Coding region of eukaryotic gene. Parts of the gene that are expressed
Gene- a length of DNA made up of a number of codons; codes for a specific protein
Intron - Non coding region of a gene that separates exons
Polypeptide - a chain of amino acids joined by a peptide bond
Ribosome - a cellular organelle that functions as a protein-making workbench.
RNA - Ribonucleic Acid; a nucleic acid that acts as a messenger, carrying information from the DNA to the Ribosomes
Protein Production faces a number of challenges. Chief amongst these is that proteins are produced in the cytoplasm of the cell, and DNA never leaves the nucleus. To get around this problem, DNA creates a messenger molecule to deliver its information outside of the nucleus: mRNA (messenger RNA). The process of making this messenger molecule is known as transcription, and has a number of steps:
- Initiation: The double helix of the DNA is unwound by RNA Polymerase, which docks on the DNA at a special sequence of bases (promoter)
- Elongation: RNA Polymerase moves downstream unwinding the DNA. As the double helix unwinds, ribonucleotide bases (A, C, G and U) attach themselves to the DNA template strand (the strand being copied) by complementary base pairing.
- RNA Polymerase catalyses the formation of covalent bonds between the nucleotides. In the wake of transcription, DNA strands recoil into the double helix.
- Termination: The RNA transcript is released from the DNA, along with the RNA polymerase.
The next stage in transcription is the addition of a 5' cap and a poly-A tail. These sections of the completed RNA molecule are not translated into protein. Instead they:
- Protect the mRNA from degradation
- Help the mRNA to leave the nucleus
- Anchor the mRNA to the ribosome during Translation
At this point a long RNA molecule has been made, but this is not the end of Transcription. The RNA molecule contains sections that are not needed as part of the protein code that need to be removed. This is like writing every other paragraph of a novel in wingdings - these sections must be removed for the story to make sense! While at first the presence of introns seems incredibly wasteful, a number of genes can give rise to several different proteins, depending on which sections are treated as exons - this is known as alternative RNA splicing. This allows a relatively small number of genes to create a much larger number of different proteins. Humans have just under twice as many genes as a fruit fly, and yet can make many times more protein products.
Sequences not needed to make a protein are called introns; the sequences that are expressed are called exons. The introns are cut out by various enzymes and the exons are spliced together to form a complete RNA molecule.
Once mRNA has left the Nucleus, it is directed to a Ribosome to construct a protein. This process can be broken down into 6 main stages:
- Initiation: Ribosome attaches to the mRNA molecule at the start codon. This sequence (always AUG) signals the start of the gene to be transcribed. The ribosome can enclose two codons at a time
- tRNAs (transfer RNAs) act as couriers. There are many types of tRNA, each one complementary to the 64 possible codon combinations. Each tRNA is bonded to a specific amino acid. As AUG is the start codon, the first amino acid to be 'couriered' is always Methionine.
- Elongation: The stepwise addition of amino acids to the growing polypeptide chain. The next amino acid tRNA attaches to the adjacent mRNA codon.
- The bond holding the tRNA and amino acid together is broken, and a peptide bond is formed between the adjacent amino acids.
- As the Ribosome can only cover two codons at a time, it must now shuffle down to cover a new codon. This releases the first tRNA which is now free to collect another amino acid. Steps 2-5 repeats along the whole length of the mRNA molecule
- Termination: As the polypeptide chain elongates, it peels away from the Ribosome. During this phase, the protein starts to fold into its specific secondary structure. Elongation continues (perhaps for hundreds or thousands of amino acids) until the Ribosome reaches one of three possible Stop codons (UAG, UAA, UGA). At this point, the mRNA dissociates from the ribosome
This seems to be a long, drawn-out process, but as always biology finds a workaround. mRNA molecules can be extremely long - long enough for several Ribosomes to work on the same mRNA strand. This means that a cell can produce lots of copies of the same protein from a single mRNA molecule.
Post Translational Modifications
Sometimes a protein needs some help to fold into its required tertiary structure. Modifications can be made after translation by enzymes such as methylation, phosphorylation and glycosylation. These modifications tend to occur in the Endoplasmic Reticulum, with a few occurring in the Golgi Body.
Post translational modification can also be used to activate or inactivate proteins. This allows a cell to stockpile a particular protein, that only becomes active once it is required. This is particularly important in the case of some hydrolytic enzymes, which would damage the cell if left to run riot. (The alternative to this is packaging within an organelle such as a Lysosome)
Post-Translation Modifications are the domain of Eukaryotes. Prokaryotes (largely) do not need any interference to help their proteins to fold into an active form.
Protein Production in 180 seconds
Where Next? Transcription and Translation
Nobelprize.org, The Official Web Site of the Nobel Prize, explains translation through a series of interactive diagrams
- Translation: DNA to mRNA to Protein | Learn Science at Scitable
Genes encode proteins, and the instructions for making proteins are decoded in two steps. The Scitable team once again provide an amazing resource suitable right up to undergrad level
- DNA Transcription | Learn Science at Scitable
The process of making a ribonucleic acid (RNA) copy of a DNA (deoxyribonucleic acid) molecule, called transcription, is necessary for all forms of life. An in-depth undergrad level exploration of transcription
© 2012 Rhys Baker