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  Section: General Biochemistry » Nucleic Acid Synthesis
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Size, Structure, Organization, and Complexity of Genomes

Except for certain viruses, DNA is the genetic material for all organisms and self-replicating units, including viruses and such intracellular organelles as chloroplasts (in plants), kinetoplasts (in protozoa), and mitochondria (in most eukaryotes). GenomicDNAis double helical (except for the genomes of certain bacterial viruses), and its size is related to the complexity of the organism (Table I). In subcellular organelles, viruses, and plasmids, the genome often exists as a circular molecule consisting of up to several thousand base pairs. The genome of bacteria, such as that of the widely studied enteric strain E. coli, is present as a single, circular, double-stranded molecule containing about 4.7 million base pairs. By and large, the genome of many small self-replicating entities is circular DNA, without any terminus in the unbranched polymeric chain.

In contrast, the large nuclear genomes of more complex organisms (from lower eukaryotes such as unicellular yeast with a genome size only an order of magnitude larger than that of E. coli, to mammals with genomes larger by three orders of magnitude) consist of multiple, distinct, linear subunits organized in chromosomes. Depending on the stage of the cell cycle, the structure of chromosomes (collectively called chromatin) varies from the highly extended and amorphous state occurring in much of the (interphase) nucleus to highly compacted, linear, organized chromosomes (metaphase) after completion of DNA duplication followed by cell division (mitosis). This complex organization of eukaryotic genomes is a distinctive feature which separates them from the prokaryotes.

TABLE I Genomic DNA Characterized in Biologya
Organism Structure Total size (bp) Number of genes Sequence
Bacteriophage Linear, circular 5∼200×103 10∼100 Completed for many species
Virus   Up to 2×105 10∼100 Completed for many species
Bacteria E. coli circular 4.6×106 ∼4300 Completed
yeast (S. cerevisiae)
Linear 1.4×107 ∼6000 Completed
Drosophila Linear 1.4×108 1.4×104 Partially completed
Human Linear 3×109 4×104 to 1×105 Partially completed
aAs of Feb 2001 the data are to be renewed continuously and are available at the website http://ncbi.nlm.
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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