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  Section: General Biochemistry » Nucleic Acid Synthesis
 
 
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Information Storage, Processing, and Transfer

 
     
 

The central dogma of molecular biology is that information is transferred from DNA to RNA to proteins. The proteins (which include the enzymes and structural components of cells) are directly responsible for most cellular activities and functions. The information needed for all functions of all organisms is stored in the genomic DNA sequence, which contains discrete units defined as genes. Each gene encodes a protein whose function and activity are determined by its primary sequence. The discovery of colinearity of theDNAnucleotide sequence and the amino acid sequence of the encoded polypeptide in prokaryotes and their viruses led to the discovery of the genetic code which postulates that a three-nucleotide sequence in DNA, called a codon, is responsible for insertion of a specific amino acid in the polypeptide chain during its synthesis.

Thus, the information content in the genomic DNA of a cell needs not only to be preserved and passed on to the progeny cells during replication, an essential characteristic and requirement of all living organisms, but also has to be processed and transferred via proteins to the ultimate cellular activities, including the metabolism.

Elucidation of the double-helical structure of DNA lends itself to an elegant but simple mechanism of perpetuation of the DNA information during duplication, called semi-conservative replication. In this model (Fig. 2), the two strands of DNA separate, and each then acts as the template for synthesis of a new daughter strand based on base pair complementarity and strand polarity. Thus, the two strands of the DNA double helix, though not identical in sequence, are equivalent in information content.

DNA polymerization reaction. (A) According to the base pairing rules, a deoxythymidinetriphosphate (dTTP) is added at the 3´-OH end of the top strand through a transesterification reaction catalyzed by a DNA polymerase. (B) Two units of DNA polymerase form a heterodimer complex to carry out replication in a semi-conservative way. Because the reaction goes only in the 5´→3´ direction, one side (the leading strand) is synthesized continuously, while the other (the lagging strand) consists of short DNA fragments (Okazaki fragment). DNA replication is initiated by an RNA primer (waved line) which is synthesized by a primase. There are a number of accessory but essential proteins besides the polymerase unit.
FIGURE 2 DNA polymerization reaction. (A) According to the base pairing rules, a deoxythymidinetriphosphate (dTTP) is added at the 3´-OH end of the top strand through a transesterification reaction catalyzed by a DNA polymerase. (B) Two units of DNA polymerase form a heterodimer complex to carry out replication in a semi-conservative way. Because the reaction goes only in the 5´→3´ direction, one side (the leading strand) is synthesized continuously, while the other (the lagging strand) consists of short DNA fragments (Okazaki fragment). DNA replication is initiated by an RNA primer (waved line) which is synthesized by a primase. There are a number of accessory but essential proteins besides the polymerase unit.

An RNA polymerase unit (filled circle), which consists of multiple factors, opens DNA helix (shown as a bubble) and synthesizes RNA in the 5´→3´ direction.
FIGURE 3 An RNA polymerase unit
(filled circle), which consists of multiple
factors, opens DNA helix (shown as a
bubble) and synthesizes RNA in the
5´→3´ direction.
The intermediate carrier in the transfer of information from DNA to protein is the messenger RNA (mRNA), which is copied (transcribed) from only one of the two strands (Fig. 3), based on base pair complementarity (except for the presence of U in RNA in the place of T; Fig. 1C). In the synthesis of both DNA (replication) and RNA (transcription), the polynucleotide chains are synthesized by sequential addition of monomeric units (deoxyribonucleotide for DNA and
ribonucleotides for RNA) to the 3 ´ end of the growing chain (Fig. 3).

The mRNA is read out by ribosomes, the ribonucleoprotein complex which functions as the factory for protein synthesis. The codons are recognized as blocks because they code for specific amino acids. Thus, the linear polypeptide sequence is determined by the linear mRNA sequence.
Content of Nucleic Acid Synthesis
» Nucleic Acids
» Structure and Function of Nucleic Acids
    » Basic Chemical Structure
    » Base Pairing in Nucleic Acids: Double Helical Structure of Dna
    » Size, Structure, Organization, and Complexity of Genomes
    » Information Storage, Processing, and Transfer
    » Chromosomal Dna Compaction and Its Implications in Replication and Transcription
    » DNA Sequence and Chromosome Organization
    » Repetitive Sequences: Selfish DNA
    » Chromatin Remodeling and Histone Acetylation
» Nucleic Acid Syntheses
    » Similarity of DNA and RNA Synthesis
    » DNA Replication Vs Transcription: Enzymatic Processes
    » Multiplicity of DNA and RNA Polymerases
» DNA Replication and Its Regulation
    » DNA Replication
    » Regulation of DNA Replication
    » Regulation of Bacterial DNA Replication at the Level of Initiation
    » DNA Chain Elongation and Termination in Prokaryotes
    » General Features of Eukaryotic DNA Replication
    » Licensing of Eukaryotic Genome Replication
    » Fidelity of DNA Replication
    » Replication of Telomeres—The End Game
    » Telomere Shortening: Linkage Between Telomere Length and Limited Life Span
» Maintenance of Genome Integrity
» DNA Manipulations and their Applications
» Transcriptional Processes
    » Recognition of Prokaryotic Promoters and Role of S-Factors
    » Regulation of Transcription in Bacteria
    » Eukaryotic Transcription
    » RNA Splicing in Metazoans
    » Regulation of Transcription in Eukaryotes
    » Fidelity of Transcription (RNA Editing)
» Chemical Synthesis of Nucleic Acids (Oligonucleotides)
» Bibliography of Nucleic Acid Synthesis
 
     
 
 
     



     
 
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