DNA is the carrier of genetic information which transmits traits from one generation to the next generation. Genes are the units of heredity that are formed by DNA. These genes transfer the information to proteins which express in the cells to perform specific functions or maintain specific structures. The whole process can be termed central dogma inheritance. DNA transfers the information to RNA by transcription and RNA undergoes translation to produce polypeptides. The process in which DNA is converted to functional protein can be called gene expression.
The theory of central dogma indicates that DNA that carries the information undergoes transcription to copy its information to m-RNA and m-RNA translates it to proteins. So the information moves in a single direction. According to central dogma, genetic information may be transferred from nucleic acids to proteins as well as nucleic acids, but proteins cannot transfer information to other proteins. Ribosomes help in the process of translation.
Biological information flows in the following general directions: DNA replication, DNA information is copied to mRNA termed transcription, and proteins are synthesized by using mRNA as a template termed translation.
An enzyme called RNA polymerase uses one strand of DNA, to synthesize a complementary RNA strand by using a template derived from the noncoding strand of DNA. The resulting primary transcript carries the same data as that of the coding DNA strand. However, due to biochemical variations present between DNA and RNA, primary transcripts are not identical to coding strands of DNA.
The nucleus, which is the location of DNA, is the site of transcription in eukaryotes and prokaryotic transcription occurs in the cytosol. As a result, an mRNA from a eukaryotic cell must first leave the nucleus before it undergoes the process of translation to make proteins. Transcription and translation are carried out in the cytosol by prokaryotic cells, which are without nuclei.
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Ribosomes produce polypeptide chains from mRNA template molecules during translation. Eukaryotes translate in the cytoplasm of the cells, where ribosomes are free or attached to the endoplasmic reticulum (ER). The process of transcription and translation is conducted in the cytoplasm in prokaryotes, which lack a nucleus.
As a result of the interaction between the initiator tRNA and the translation AUG start codon, tRNA brings an fMet (formylated methionine). Bacteria begin synthesizing polypeptide chains at its N terminal end due to its role in initiation.
Shine-Dalgarno sequence (AGGAGG) of mRNA in E. coli and rRNA form a complementary base pairing. The 30S subunit finds its place on the mRNA template. This results in the binding of the 50S subunit to the initiation complex creating a complete ribosome.
The tRNA carries three nucleotide codes for a particular amino acid at one end and a single amino acid at the other end. The elongation process begins with the entry of charged tRNA to the aminoacyl site which then enters the peptidyl site and at last to the exit site to get out of the ribosome. The resulting polypeptide chain has the same nucleotides as that of mRNA. The translation process undergoes termination when they find termination codons for which there are no complementary nucleotides. Both the subunits separate from one another and mRNA. The resulting polypeptide may undergo post-translational modifications to enhance its efficiency.
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Genetic code is a combination of three nucleotides together which make codons and codes for a specific amino acid. Proteins are made up of twenty different amino acids which are made up of nucleotides. There are 64 codons from four nucleotides U, C, A, and G. They have termination codons (UGA, UAG, and UAA) which terminate translation. Stop codons do not code for amino acids, but they function as termination signals that trigger the release of the polypeptide from the ribosome after the polypeptide is complete. Each mRNA contains the AUG codon, which is not only responsible for coding for methionine, but also signifies the start of the protein chain. Generally, polypeptides begin with methionine, but the initial amino acid may be removed later on.
These are universal, comma less, and non-ambiguous. They show degeneracy which means a single amino acid may have different codons. They always move from 5l to 3l direction. Starting codons determine the reading frame, which is the way mRNA sequences are divided into three nucleotide groups within the ribosome. Certain organisms and some eukaryotes' mitochondria have been found to differ slightly in their genetic code, once believed to be identical in all forms of life.
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Central dogma carries the information from DNA to RNA and RNA to proteins.
Transcription helps in copying the information from DNA to mRNA.
The translation process synthesizes polypeptide chains from amino acids.
Replication helps in producing two copies of DNA which can be equally distributed to two daughter cells after cell division.
Diversity of life on Earth can be attributed to a genetic code shared by all organisms.
The theory of central dogma indicates that DNA that carries the information undergoes transcription to copy its information to m-RNA and m-RNA translates it to proteins. Genetic code is a combination of three nucleotides together which make codons and codes for a specific amino acid. This tutorial helps to understand the basic concepts of the central dogma inheritance mechanism. Steps involved in central dogma such as transcription and translation have been described in this tutorial. In conclusion, this tutorial is useful for understanding the importance of central dogma.
Q1. Write a note on exceptions to central dogma.
Ans. Retroviruses can be the best example of an exception to central dogma. They have an enzyme called reverse transcriptase which transcribes RNA to DNA which is the reverse transcription process.
Q2. What are the enzymes used in replication? Write down the functions of two enzymes.
Ans. DNA polymerase, ligase, helicase, primase, SSB proteins, and topoisomerase are the enzymes used in replication. Ligase is used for the sewing of DNA strands by forming phosphodiester bonds. Helicase breaks the hydrogen bonds to separate both the DNA strands.
Q3. How does prokaryotic transcription differ from eukaryotic transcription?
Ans. Cytoplasm is the place for transcription in prokaryotes and nucleus is the place for eukaryotic transcription. RNA undergoes post-transcriptional modification in eukaryotes, but they are directly used for translation in prokaryotes.
Q4. What are the properties of genetic code?
Ans. Genetic codes are degenerative and made of three nucleotide codons. They always move in a single direction, they have start codons and stop codons. They are universal.
Q5. How are the post-transcriptional modifications done?
Ans. Capping of 7-methylguanosine at 5l end, the addition of polyadenylated tail at 3l end, and splicing of introns and joining of exons are done during post-transcriptional modifications.
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