You have often heard that our genotype (genes) determine our phenotypes (physical features) or in other words gene expression but how does it work? It can be summarized by the central dogma of molecular biology:
DNA→RNA→Protein
Although this is an extreme simplification of the process, it accurately sums up the process of gene expression; let’s delve a little deeper.
TRANSCRIPTION (DNA→RNA):
In the process of gene expression, the first step is transcription which occurs in the nucleus for eukaryotes and in the nucleoid region for prokaryotes. Transcription refers to creation of an RNA strand known as messenger RNA (mRNA.) DNA is a double-stranded helical figure and each strand is complementary to the other thus an RNA polymerase (enzyme that synthesizes an mRNA using ribonucleic tri-phosphates (rNTs) will use only one of the strands as a template to create the complementary strand of that DNA strand out of RNA. Main difference between RNA and DNA is that DNA contains the bases Adenine, Thymine, Guanine and cytosine; Adenine which pairs with Thymine, and Guanine which pairs with Cytosine. Whereas, an RNA contains all similar bases to DNA but instead of Thymine it contains Uracil that binds with Adenine. In a selected gene in a eukaryote or prokaryote there is a promoter gene sequence upstream of its action protein coding region. Promoter region is where the RNA polymerase will eventually bind. In eukaryotes transcription factors will bind to the promoter region which than cause the RNA polymerase to bind. In either case the RNA polymerase binds at the promoter region of a single strand and reads the template strand in a 3’ to 5’ end to synthesize a complementary RNA strand from 5’ to 3’ end.
mRNA Processing (eukaryotes only):
Prokaryotes such as bacteria produce mRNA and without any further process to move on to the next step of translation into a protein. Eukaryotes however have an intermediate step known as mRNA processing where the transcribed mRNA is altered before it leaves the nucleus and goes into the cytoplasm to be translated. The non-processed mRNA is called the primary RNA. In mRNA processing, the transcribed mRNA is spliced apart and spliced together back by RNA and protein complexes known as spliceosomes. In primary RNA, there are non-protein coding regions called introns whereas the protein coding regions are called exons; the ratio of introns is usually greater than the exons. The introns are spliced out by the spliceosome and the exons are spliced together. Primary RNA which is processed by adding the 5’ cap on the 5’ end and the Poly A tail on the 3’ end. These tails help the mRNA in transport out of the nucleus, slow the degradation process of the mRNA and assist the mRNA binding to the ribosome. The same primary RNA can produce multiple different final mRNAs based on which areas are considered exons; this phenomenon is known as alternative RNA splicing. Once mRNA processing is dealt with, the mRNA can move out of the nucleus into the cytoplasmic region.
TRANSLATION (RNA→protein): Second focal step in gene expression is translation. In translation, the mRNA binds to a ribosome which in turn reads the mRNA in sets of codon triplets, reads three mRNA nucleotides at once. In prokaryotes translation can happen simultaneously as transcription as there are no membrane-bound organelles limiting the content of mRNA to the ribosome however in the eukaryotes there is the clear barrier of the nucleus and cytoplasm region where the translation occurs. A ribosome is a complex of ribosomal RNA (rRNA) and proteins and consists of two subunits; the small subunit where mRNA binds and the big subunit which contains three specific sites. Before we get into the details of these sites we must discuss transfer RNA (tRNA) which are complexes made from RNA that have a codon on one end which attaches to its complementary mRNA codon and it has an amino acid attachment site on the other end. The amino acid that a specific tRNA carries with it, is specific to its codon and this specificity is made possible by aminoacyl-tRNA synthase which is an enzyme that creates a peptide bond between a tRNA and amino acid. There are total 20 aminoacyl tRNA synthase, one for each type of amino acid. The amino acids are either derived from diet or created by the body which then by the enzyme are attached to a specific tRNA. The three sites on ribosome are the E, P, and A sites, the E site is where the tRNA exits from after providing it with an amino acid, the P site is in the middle of both sites and carries the tRNA with the growing polypeptide chain and A site is where the tRNA enters first. When an mRNA is being read, the translation begins at the codon AUG which is a start codon for methionine. To put into picture when the mRNA has an AUG codon a tRNA with the complementary codon UAC binds to it while caring the amino acid Methionine. As the mRNA continues to be read the polypeptide chain extends out through an exit tunnel region in the ribosome. Eventually, a stop codon is encountered in the mRNA which allows for binding of a release factor, a protein which breaks the bond between the polypeptide chain and the tRNA in the P site that carries it by releasing water which hydrolyzes the covalent bond. Once the polypeptide is released, it forms a secondary structure and then a quaternary structure which is all that is needed to form a protein in certain cases, however some proteins which are more complex require a quaternary structure of more than one polypeptide thus some polypeptides bind with other polypeptides producing a protein.
After the Protein is Produced: Once the protein is produced it acts as a linkage between genotype and phenotype. The proteins than function to convey the phenotype for example an eye color protein producing pigment for the eye color.
Works Cited
Brown, Donald D. “Gene Expression in Eukaryotes.” Science, vol. 211, no. 4483, 1981, pp. 667–74. JSTOR, http://www.jstor.org/stable/1685602. Accessed 3 Apr. 2024.
"Gene expression." World of Biology, Gale, 2006. Gale In Context: Science, link.gale.com/apps/doc/CV2431500269/SCIC?u=braddock_e&sid=bookmark-SCIC&xid=69166f24. Accessed 3 Apr. 2024.
"Gene expression." World of Genetics, Gale, 2007. Gale In Context: Science, link.gale.com/apps/doc/CV2433500202/SCIC?u=braddock_e&sid=bookmark-SCIC&xid=694f295a. Accessed 3 Apr. 2024.
Aamodt, Eric. "Gene Expression: Overview of Control." Genetics, 2nd ed., vol. 2, Gale, 2018, pp. 81-88. Gale In Context: Science, link.gale.com/apps/doc/CX2491300108/SCIC?u=braddock_e&sid=bookmark-SCIC&xid=31523038. Accessed 3 Apr. 2024.
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