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Chapter 6

Nucleic Acids

 
Genetic information is encoded in the nucleotide polymer of DNA. The DNA molecule is a double-stranded helix of two complementary strands of nucleotides that form hydrogen bonds between A–T and G–C basepairs. The genetic information is duplicated during DNA replication when DNA polymerases synthesize a new complementary nucleotide polymer, using each of the two parental strands in the double helix as templates. Viruses, which are simple complexes of short DNA (or RNA) molecules encapsulated in protein coats, exploit the host cell DNA replication machinery for their own amplification and propagation. Although DNA is a stable molecule, it sometimes suffers damage from UV light or chemicals, or errors introduced by DNA polymerase during replication. If not repaired, this damage can cause mutations. Several repair mechanisms function to remove damaged nucleotides or incorrectly paired bases. Exchange of genetic information between DNA molecules in chromosomes is an important mechanism that drives evolution and establishes diversity among organisms. Genetic information exchange occurs during recombination when double-stranded DNA molecules are broken and rejoined with different DNA molecules. Before the genetic information stored in DNA can be expressed into a co-linear sequence of amino acids in proteins, it must be transcribed into RNAs by multiprotein enzyme complexes called RNA polymerases. Plants, like other eukaryotes, contain three types of nuclear RNA polymerase. Chloroplast (and probably mitochondrial) genes are transcribed by prokaryote-type RNA polymerases, which reflects their evolutionary origin. Most RNAs transcribed by RNA polymerases must undergo extensive processing before they can function in the translation of proteins or in the formation of ribo-nucleoprotein complexes such as ribosomes. The protein-coding regions of many genes are also interrupted by non-coding sequences, called introns, which must be spliced out to produce functional RNAs. In some cases the genetic information contained in genes differs from the information found in their corresponding RNAs, which have nucleotides added, deleted, or substituted. This RNA editing usually restores cryptic DNA information to produce proteins with evolutionary conserved amino acid sequences.

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