UNIT 3: Molecular Genetics
Chapter 6: DNA: Hereditary Molecules of Life pg. 268 6.5: DNA Organization in Eukaryotes and Prokaryotes
pg. 291 – 294
DNA found in a Eukaryotic Cell, if unwound would be too large to fit within
the nucleus. The total length of DNA would cover 1.8 m. The entire DNA
must fit within a nucleus with a diameter of 10 μm.
The Packing of Eukaryotic DNA
Nucleosome - is a unit of DNA storage, consisting of eight histones with
DNA with DNA strands wrapped around them; the DNA around each
nucleosome is about 147 nucleotides in length.
Solenoid – is a group of six nucleosomes.
Supercoiling – is the continuous twisting of prokaryotic DNA that reduces
the volume of the DNA.
DNA is wound around special proteins called histones. Histones are
positively charged proteins and the DNA molecule is negatively charged,
creating an attraction between the two. This attraction causes the DNA to
coil tightly around a cluster of eight histones. This cluster is called a
nucleosome. Nucleosomes are linked together by small stretches of DNA.
DNA length has been reduced by 7 times.
The DNA is further packaged by coiling strings nucleosomes into cylinders
called solenoids, having a length of 30 nm. Each coil of solenoids consists of
six nucleosomes.
Figure 1: DNA Packaging, to fit inside the nucleus, DNA strands wrap around clusters of eight histones
proteins to form a nucleosome. Further packing reduces the volume of six nucleosomes to just 30nm. A
length of about 147 nucleotides is wrapped around each group of histones. pg. 291
Prokaryotic DNA Organization
The DNA organization in both eubacteria and archaea is simpler then
eukaryote’s DNA. Eubacteria consists of only one chromosome that is
circular. The genetic information is free floating in the cytosol and not
contained in a nucleus. The genetic packaging is less constricting and easily
access by enzymes involved in replication.
Also within the cell are smaller circular portions of genetic information,
called plasmids. This information can be exchanged between eubacteria
through a process called conjugation, where a bridge is form between two
eubacteria cells. These plasmids can be incorporated into the larger circular
DNA of the recipient cell. The DNA is still twisted into a tight ball,
Supercoiling similar to the eukaryotic cell.
Figure 2: Prokaryotic DNA is one long chromosome that may be circular.
Figure 3: Neighbouring bacteria can replicate plasmids, donating the replicated plasmid to the other
bacterium.
Telomeres: A Key Difference
Telomere – is a repeating sequence of DNA at the end of a chromosome
that protects coding regions from being lost during replication.
There is a problem with replication of DNA eukaryotes is the template
strand of DNA where the lagging strand is being build. After the last RNA
primer is removed, nucleotides are attached to fill this gap. This leaves an
exposed section of DNA that may get lost or damaged. Over time the DNA
molecule will continually shorten.
To ensure the coding regions of DNA at the ends are not damaged, telomeric
sequences are attached. Telomeres are repetitive non-coding nucleotides
attached to the ends of eukaryotic DNA molecules. Each time DNA is
replicated it will lose a small portion of the non-coding region, but the
coding regions of DNA remain completed.