Central dogma is the backbone of molecular biology all the basic concept revolves around it. It's the step by step transfer of information within the cell at molecular level. This term was first coined by Francis Crick in 1957 and later on was publically published in 1958 in a local newspaper. The transfer of genetic verses from DNA to RNA and RNA to polypeptides takes place through a special mechanism or framework known as central dogma. DNA is the hereditary matter in the cell contains all the pieces of information that are ought to be transferred from one generation to the other. Central dogma involves DNA, RNA and biological polymer known as protein composed of polypeptide chains. The biological relationship between DNA, RNA and proteins is known as Central dogma. One way flow of information between DNA and protein is another definition of central dogma. This is the concept that hereditary information is irreversibly transferred from DNA to mRNA by a process known as transcription and then from mRNA to proteins by translation. The concept has been modified to take into account the transfer of information in the other direction from RNA to DNA by reverse transcription, carried out by some viruses.
DNA replication, transcription and translation and are the three core processes involved in central dogma. The first step is the replication of DNA strand which is followed by transcription of DNA in to RNA and the final process in central dogma is the formation of proteins from RNA this process is known as translation. There are three cases suggested by Central Dogma (1) General (2) Special (3) unknown. It says that 3*3=9 so there are nine situations that are observed in the genetic process.
(1) DNA replicated from DNA
(2) RNA made from DNA by transcription
(3) Proteins made from RNA by translation
(4) DNA made from RNA by reverse transcriptase
(5) RNA made from RNA from RNA replication
(6) Protein made from DNA by direct translation
(7) Protein codes for DNA
(8) Protein codes for RNA
(9) Protein codes for protein
• DNA is the hereditary material of the cell, it's inert and stable. All necessary pieces of information are stored on it. DNA replication follows the semi conservative model which was supported by Meselsohn and Stahl's experiment conducted in 1958. According to this model new molecule of DNA (daughter DNA) consists of two strands one intact strand from the parent known as template strand and other one is the newly synthesized strand. DNA replication is a very complex process. The enzymes that can take part in replication process are helicases, DNA binding proteins, DNA polymerases and DNA ligases. Helicases enzyme separates the two DNA strands. Energy is supplied by ATP for this process. DNA binding proteins keep the strands separated during replication. DNA polymerases catalyse the polymerisation of nucleotides to form a polynuctleotide chain in the 5'3'direction; it allows the continuous replication of the strand. The other strand is not replicated continuously but is replicated in short parts in intervals. The pieces of nucleotide chains are joined together by DNA ligase. DNA replication is started at number of sites known as replication forks. It's not possible to replicate the whole DNA at the single instant.
• Transcription is the process of synthesis of mRNA from DNA template in the nucleus. RNA polymerases and transcription factors take part in this process. DNA codes for the replication of mRNA. mRNA is a large polymer, chemically similar to DNA but different in having one chain of nucleotides, consists of ribose sugar instead of deoxyribose. mRNA contains base uracil instead of thymine. DNA acts as a template for making mRNA by complementary base pairing. Thus a sequence of this sort is obtained
DNA BASE SEQUENCE: T A G G C T T G A T C G
RNA BASE SEQUENCE: A U C C G A A C U A G C
The triplet sequence of nucleotide in mRNA is known as codon. Codon is always read in 5'3'direction. The H.bonds in DNA is broken by using helicase enzyme. Polymerase attaches itself at specific sequence known as promoter site, initiating transcription. On reaching the end of the process a stop sequence known as terminator, enzyme detaches and mRNA molecule moves away from DNA.
• mRNA then moves to cytoplasm, where it combines with ribosomes to direct protein synthesis by a process called translation. rRNA has two units smaller unit has binding site for mRNA, bigger unit binds tRNA. Peptidyl transferase catalyzes the formation of peptide bond between proline and histidine. Process is initiated by codon AUG and is stopped by (UAA, UGA, UAG) codon.
Central dogma is diagrammatically expressed like this
DNA RNA Protein.
Fransic Crick proposed that protein can't encode for either DNA or RNA. It's irreversible.
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