The genetic material of most organisms is DNA, which stores the information required for the development, functioning, and reproduction of the organism. DNA is a long piece of nucleotides where both coding and non-coding regions can be identified within the nucleotide sequence. The coding regions contain the information for the production of functional proteins in the organisms. The process of production of the polypeptide chain of the protein inside the cell is known as protein synthesis. Transcription of DNA into mRNA and translation of mRNA into an amino acid sequence of the protein are the sequential steps of the protein synthesis.
Key Areas Covered
1. What are Genes
– Definition, Features, Role
2. How to Transcribe DNA into mRNA
– Process of Transcription
Key Terms: DNA, Genes, mRNA, Protein Synthesis, Transcription, Translation
What are Genes
Genes are pieces of DNA that contain protein-coding regions in the genome of a particular organism. Protein-coding region and regulatory region are the two segments of a gene. The protein-coding region of eukaryotic genes contains introns and exons. The regulatory sequence of a gene contains nucleotide sequences such as promoter, enhancer, and silencer, which are responsible for the regulation of the gene expression. Genes are included in chromosomes as shown in figure 1.
Figure 1: Gene
A complete set of genes of a particular species is inherited by an organism during reproduction. The alternative forms of a gene are known as alleles. Different alleles give phenotypic variations to the organisms in a particular population.
How to Transcribe DNA into mRNA
Protein synthesis is the process of production of a polypeptide chain of a functional protein based on the information stored in a gene. The two steps of protein synthesis are transcription and translation. Transcription is the first step of protein synthesis; here, a mRNA molecule is produced based on the information encoded within the corresponding gene.
RNA polymerase is the enzyme involved in transcription. Transcription is initiated by the binding of RNA polymerase to the promoter of the gene. This binding is facilitated by transcription factors in both prokaryotes and eukaryotes. The six transcription factors associated with RNA polymerase in eukaryotes are TFIIA, TFIIB, TFIID, TFIIE, TFIIF, and TFIIH. The binding of RNA polymerase to the DNA double-helix forms a transcription bubble. RNA polymerase reads the antisense DNA strand from 3′ to 5′ direction. The transcription bubble consists of approximately 14 bases of the unwound double-stranded promoter. Then, complementary RNA nucleotides are added to the antisense strand from the transcription initiation site from 5’to 3′ direction. The addition of nucleotides is stopped at the transcription termination site. To the end of the 3′ end, a polyadenylate tail is also added by the enzyme. The process of transcription is shown in figure 2.
Figure 2: Transcription
The primary transcript synthesized by RNA polymerase is known as pre-mRNA. The eukaryotic pre-mRNA consists of introns that should be cleaved off during RNA splicing in post-translational modifications. The alternative splicing of eukaryotic pre-mRNA produces multiple proteins from a single gene. Generally, eukaryotic genes are organized into gene clusters known as operons. The genes in a particular operon are responsible for a particular function in the cell such as metabolism. These operons are transcribed at once, and they produce several proteins from a single mRNA molecule.
Conclusion
Protein synthesis is a process of producing proteins based on the information encoded in a gene. Transcription is the first step of protein synthesis. During transcription, the protein-coding region of the gene is transcribed into a mRNA molecule, which ultimately undergoes translation for the synthesis of the polypeptide chain.
Reference:
1. Venters, B. J, and B. F Pugh. “How eukaryotic genes are transcribed.” Critical reviews in biochemistry and molecular biology., U.S. National Library of Medicine, June 2009, Available here.
Image Courtesy:
1. “Chromosome-DNA-gene” By Thomas Splettstoesser (www.scistyle.com) – Own work (CC BY-SA 4.0) via Commons Wikimedia
2. “Process of transcription (13080846733)” By Genomics Education Programme – Process of transcription (CC BY 2.0) via Commons Wikimedia
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