|
|
 |
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. |
|
|
|
