Introduction to Protein Synthesis
From MyMCAT
Contents |
Introduction
In the previous section, Introduction to DNA, the concept of DNA was discussed. DNA holds all the information necessary to replicate and survive, yet DNA alone does not have any significant function beyond information storage. It is predominately the proteins in a cell which act as enzymes or which have unique properties which the cell uses to carry out its autonomous functions. But how does the cell generate these proteins?
The Central Dogma of Molecular Biology
The central dogma of molecular biology states that there are specific associations and order in a cell's DNA, RNA, and proteins. Specifically, DNA can be completely replicated to produce more DNA or fragments can be replicated through transcription into RNA. The RNA can then in turn be converted into amino acids through translation. Thus all the information required to produce a protein is in the RNA and all the information required to produce RNA is in the DNA.
As a result of this lemma, all cells require DNA and must have a means of replicating, transcribing, and translating genetic material. The only exception to this rule is viruses. Some viruses skip DNA all together and use RNA for both genetic storage and protein transcription. In this case, the viruses must have a way of replicating RNA if it is to ever produce progeny. A second class of viruses, known as retroviruses, have the unique feature of going against the central dogma's flow of information. Retroviruses are named so because they contain an enzyme known as reverse transcriptase, which as its name implies can do the opposite of transcription, turning RNA back into DNA. These viruses often use this complicated process to convert their genetic material which is stored in RNA to a DNA fragment which they then integrate into the host's DNA so that it may hide and persist there as long as the cell is alive. HIV is the classic example of this type of virus.
Transcription
The information carried inside genes is used to produce RNA from its DNA template. While DNA and RNA may seem extremely similar at first glance there are many minor and major structural and functional properties. Structurally, there is a 2'-OH in DNA while in RNA it is replaced by just a hydrogen and thymine is never found in DNA- instead, it is replaced by uracil. Functionally, RNA carries out most functions as a single strand or as a strand that folds back on itself and it has the potential for a much greater diversity of structures than DNA. RNA is also unstable and degrades much more rapidly than DNA. These differences both explain why DNA is used to store genetic material instead of RNA, and why RNA works well as a transient intermediate in the final goal of protein production.
Transcription occurs with many respects like DNA replication except that only a fragment of one strand is copied, not the entire genome. In fact, where DNA used the enzyme DNA-Dependent DNA Polymerase to copy DNA from DNA, RNA is produce from a similar protein complex known as DNA-Dependent RNA Polymerase.
Like DNA replication, transcription resembles replication in its chemical elements (nucleic acids), direction of synthesis (5'->3'), its use of a template, and even in its major steps of initiation, elongation, and termination phases. Unlike DNA replication however, only one strand is used as a template (and thus only one RNA is produce) and it does not require a "primer" to begin.
Prokaryotes vs Eukaryotes in Transcription
In prokaryotes, transcription is a relatively simple process. The DNA, the RNA machinery (RNA Polymerase), and all the necessary ingredients are all found mixed together in the cytosol and once RNA has been made it can be directly used in later processes like translation (to make proteins). In fact, translation often occurs before transcription has even finished with mRNA fragments being produced from RNA polymerase and ribosomes following directly behind it reading the newly synthesized mRNA building peptides chains.
In eukaryotes on the otherhand, DNA is found in the nucleus while the machinery to produce proteins is found exclusively outside of the nucleus. As such, RNA destined to produce proteins must be exported out of the nucleus prior to ribosome processing. Furthermore, eukaryotic cells also post-transcriptionally modify their mRNA to protect them against degradation. Firstly, a modified guanine nucleotide is attached to the 5' end of the pre-mRNA in a process known as 5' capping. Secondly, a long chain of Adenines are attached to the 3' end forming what is known as a polyA tail. The purpose of both of these modifications is to protect the mRNA from degredation in the cytosol and allow it more time to be read by ribosomes before it is discarded.
Another modification unique to eukaryotic mRNA is splicing. Unlike prokaryotic mRNA which codes directly from RNA to amino acids, eukaryotic mRNA often has long stretches of nucleotides which are not meant to be read by the ribosomes (and consequently do not code for any meaningful amino acids). As such, before an mRNA exists the nucleus, these non-translatable regions, known as introns, are removed by a complex called the splicosome.
Roles of RNA products
As previously discussed, RNA is generally used as an intermediate in the production of proteins. This type of RNA is generally known as messenger RNA or mRNA however there are other types of RNAs which get produced in the same way but do not get translated into proteins. Ribosomal RNAs (rRNA) are used as part of the framework of ribosomes, the macromolecular factories that produce proteins from mRNA. Transfer RNA (tRNA) are used as the stepping stone to convert RNA to one amino acid. As one would expect, there is a different tRNA for each possible amino acid and the ribosome is capable of selecting all the necessary tRNAs required to produce a protein in the correct amino acid order. There are also RNAs which are used purely for regulation of genes and RNAs known has ribozymes, which when folded into a unique shape, have the ability to catalyze reactions just like enzymes.
