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Nucleic Acid Synthesis 859

orH5.Thepolymericchainnucleosomesarethenfoldedin
a 30-nm fiber whose structure is stabilized by the interac-
tion among histones and a number of other proteins collec-
tively called nonhistone chromosomal proteins (NHC),
including high mobility group (HMG), which are not par-
ticularly basic. Eventually, the fibers are condensed into
FIGURE 3 An RNA polymerase unit (filled circle), which consists
highly compacted metaphase chromosomes. The nature
of multiple factors, opens DNA helix (shown as a bubble) and
of the interactions present in interphase and metaphase
synthesizes RNA in the 5 → 3 direction.

chromosomes is not clear.
However, the implications of this compaction are pro-
The intermediate carrier in the transfer of information found. It is absolutely essential to condense the mam-
from DNA to protein is the messenger RNA (mRNA), malian genome, which in an extended linear form more
which is copied (transcribed) from only one of the two than 1 m long, to a volume which can be accommodated
strands (Fig. 3), based on base pair complementarity (ex- in the nuclear volume of 10–30 femtoliters. At the same
cept for the presence of U in RNA in the place of T; time, the genes will be buried in condensed chromatin, and
Fig. 1C). In the synthesis of both DNA (replication) and yet their specific sequences need to be exposed for various
RNA (transcription), the polynucleotide chains are syn- processes of information transfer. Thus, for both transcrip-
thesized by sequential addition of monomeric units (de- tion and replication, the chromatin has to be decondensed.
oxyribonucleotide for DNA and ribonucleotides for RNA) This was evident in early in vitro studies which showed

to the 3 end of the growing chain (Fig. 3). that both these processes are severely inhibited when DNA
The mRNA is read out by ribosomes, the ribonucleo- is complexed with histones.
protein complex which functions as the factory for pro-
tein synthesis. The codons are recognized as blocks be- F. DNA Sequence and Chromosome
cause they code for specific amino acids. Thus, the linear
Organization
polypeptide sequence is determined by the linear mRNA
sequence. The massive human genome project should achieve its
goal of determining the complete sequence of human
and mouse genomes in the near future; a “rough draft”
E. Chromosomal DNA Compaction and Its
has already been obtained. Furthermore, this genome ini-
Implications in Replication and Transcription
tiative, pursued by both government and private enter-
Metaphase chromosomes in cells undergoing mitosis are prises in the United States and other countries, has al-
visible under the light microscope. Their formation re- ready culminated in elucidating the complete sequence
4
5
quires some 10 -to10 -fold condensation of uninter- of E. coli and other bacteria, as well as yeast, a nema-
rupted linear duplex DNA which has a 2-nm diameter. tode, and the fruitfly Drosophila melanogaste. Significant
Such compaction is accomplished in a highly complex progress has been made in elucidating the nucleotide se-
and stepwise fashion. Because DNA is a polyelectrolyte quences of both human and mouse genomes by using a
with two negative charges per nucleotide, charge neutral- two-pronged approach. On one hand, the sequences of
ization and shielding is required before the polymer can transcribed regions of the genomes are being deduced
be folded in an ordered, condensed structure. In addition from sequences of randomly isolated mRNA segments
to metal ions and polyamines, the major source of the reverse transcribed into DNAs. At the same time, com-
positive charge in chromatin is the family of highly basic plete DNA sequences of fragments of whole chromo-
small proteins, called histones, which are rich in the basic somes are being directly determined. This has opened up
amino acid residues lysine and arginine needed to neu- a huge scientific challenge of deciphering the genetic in-
tralize the charge of the phosphate backbone of DNA. The formation, identifying unknown genes and their encoded
prokaryotesalsohavebasicproteins(suchasHUproteinin proteins, and the variability of gene sequences with cor-
E. coli) which induce DNA condensation. However, chro- responding changes in the protein sequences in individ-
matin compaction in eukaryotes is carried out in stages. uals. Functional genomics is a newly created discipline
The simplest folded unit of DNA is the 10-nm nucleo- which deals with the deterministic prediction of protein
some, consisting of a core histone octamer containing two functions from the primary sequences. One extension
molecules each of histone H2A, H2B, H3, and H4 around of such analysis is to ascertain the consequences of al-
which nearly two turns of the DNA is wrapped. The nu- lelic polymorphisms in the human genome, i.e., minor
cleosome cores are connected by a stretch of linear DNA changes in the sequences of cellular proteins which do
(linker) of variable length which is covered by histone H1 not cause an explicit pathological phenotype and yet