Active euchromatin is more loosely bound the to nucleosome; this is
partly due to acetylation of histone tails, and partly to alterations
in DNA topology caused by remodelling machines.
Telomeric heterochromatin lacks nucleosomes, and is instead bound
to Rap and Sir. Its distal end is ssDNA.
DNA base is 0.33 nm 'tall'. Human haploid genome is 3 giga base
pairs long. 0.33 × 10−9 m bp−1 × 3 ×
109 bp × 2 (diploid cell) = 1.98 m
Prokaryotic and eukaryotic genes are both composed of DNA; however,
in prokaryotes, this is lightly packaged into a single circular
chromosome, whereas in eukaryotes, it is present as several long linear
chromosomes with characteristic telomeres and centromeres. The
prokaryotic genophore is found free in the cytoplasm, whereas the
eukaryotic chromosomes are found within a membrane-bound nucleus, and
are packaged with a variety of proteins, particularly histones, into
'chromatin'. Prokaryotic genomes are generally small, and almost all
the DNA codes for useful polypeptides. However, in most eukaryotes, the
majority of the genome neither codes for protein, nor performs
essential housekeeping (like centromeres), but is simply 'junk', often
found within genes themselves, splitting a contiguous DNA message into
several sections that have to be spliced back together after
transcription. This is not the case in bacteria, although archaeal
prokaryotes possess both histones, and some self-splicing genes. In
both prokaryotes and eukaryotes, the genes are delimited by start and
stop regions (promoters and terminators), but in eukaryotes, this
further tuned by the presence of many enhancer regions to which
regulatory proteins may bind. However, both groups use a TATA box with
a conserved A/T rich motif to recruit RNA polymerase to the gene to
begin transcription.
All 'junk' DNA is useful to itself, in that the only purpose of DNA
is to get replicated. Some repetitive DNA is useful to other genes, in
that it forms the telomeres and centromeres of chromosomes.