Gene expression refers to the mechanism by which information from a gene directs the synthesis of proteins. This process is utilized by eukaryotes and prokaryotes. Gene expression involves various steps. These includes; transcription and translation. The central dogma indicates the flow of genetic information from the Deoxyribonucleic Acid to Ribonucleic Acid and synthesis of the required protein.
Transcription is the production Ribonucleic Acid copies of Deoxyribonucleic acids in the nucleus. The DNA consists of two anti parallel complementary strands, each having 5’ and 3’ ends. The strands serve as a template strand for the production of an RNA transcript. The process is performed in the nucleus of the cell by RNA polymerase enzyme. A new Ribonucleic acid nucleotide is added to the growing RNA strand. The RNA molecule produced is complementary to the template 3’ to 5’ DNA strand. It is also complementary to the coding 5’ to 3’ parent DNA strand. The resulting RNA strand during transcription is usually identical to the coding of the parent DNA strand. However, the thymine (T) bases are replaced with uracils (U) bases in the formed RNA. Transcription is performed by three types RNA polymerases. Each RNA polymerase requires a promoter and transcription factors to initiate the process. First, RNA polymerase I is responsible for transcription of ribosomal RNA. Secondly, RNA polymerase II transcribes all protein coding genes and some non- coding RNAs. Thirdly, RNA polymerase III transcribes transfer RNA genes, 5S ribosomal RNA, and some small non-coding RNAs such as 7SK. The process of transcription is terminated when the enzyme reaches a sequence called the terminator. During transcription, eukaryotic genes leave a primary transcript of RNA (pre-mRNA). This has to be modified to become a mature messenger RNA. The mature messenger RNA is then transported to the cytoplasm from the nucleus. The modifications include 5’ capping, 3’ cleavage and polyadenylation and RNA- splicing.
The 5’ capping modification consist of a set of enzymatic reactions that add 7-methylguanosine to the 5’ end of the primary transcript of mRNA. This protects the RNA from degradation by exonucleases. The 7-methylguanosine cap is bound by cap binding complex heterodimer, which aids mRNA transportation to cytoplasm. The 3’ cleavage and polyadenylation occur if polyadenylation signal sequence, 5’-AAUAA-3’, is present in the primary transcript of RNA. The signal sequence is usually contained between protein-coding sequence and the terminator. The primary transcript mRNA is first cleaved and about 200 adenines are added form pol (A) tail. The poly (A) tail is then bound by poly (A) - binding proteins. The poly (A) binding proteins are necessary for mRNA export and translation initiation. In addition, RNA splicing is a very important modification of the primary transcript of RNA. The eukaryotic pre-mRNA consist exons and introns segments. During the process of splicing, RNA- protein catalytic complex, spliceosome, is catalyzed to two transesterification reactions. This helps removes an intron and release it the form of lariat structure. They also splice the exons together. In alternative splicing, the introns or exons are either removed or retained in the mature mRNA. Alternative splicing creates different transcripts from a single gene. The transcripts can be translated into different proteins. Therefore, RNA splicing extends the complexity of gene expression. Reverse transcription is also the assembling of the genetic information from RNA to form a new DNA. It is known to occur in case of HIV.
The messenger RNA (mRNA) carries genetic information for the synthesis of one or more proteins. Translation mostly occurs in the cytoplasm of the cell. The ribosomes bind the mRNA in the cytoplasm. The messenger RNA consists of the 5’ untranslated region, protein coding region and 3’ untranslated region. Three joined nucleotides in the coding region forms a codon. The beginning codon is AUG comprising adenine, uracil and guanine). Each codon corresponds to the complementary anticodon triplet in transfer RNA (rRNA). The transfer RNAs of the anticodon carry a similar type amino acid. The amino acids are joined together by the ribosome. It helps tRNA to bind to messenger RNA and takes the amino acid from tRNA. During translation, every molecule of messenger RNA is usually translated to many protein molecules. Translation usually ends with a stop codon which can be UAA, UGA, or UAG.
