Eukaryotic Chromosome Structure The length of DNA in the nucleus is far greater than the size of the compartment in which it is contained. To fit into this compartment the DNA has to be condensed in some manner. The degree to which DNA is condensed is expressed as its packing ratio. Packing ratio - the length of DNA divided by the length into which it is packaged For example, the shortest human chromosome contains 4.6 x 107 bp of DNA (about 10 times the genome size of E. coli). This is equivalent to 14,000 µm of extended DNA. In its most condensed state during mitosis, the chromosome is about 2 µm long. This gives a packing ratio of 7000 (14,000/2). To achieve the overall packing ratio, DNA is not packaged directly into final structure of chromatin. Instead, it contains several hierarchies of organization. The first level of packing is achieved by the winding of DNA around a protein core to produce a \"bead-like\" structure called a nucleosome. This gives a packing ratio of about 6. This structure is invariant in both the euchromatin and heterochromatin of all chromosomes. The second level of packing is the coiling of beads in a helical structure called the 30 nm fiber that is found in both interphase chromatin and mitotic chromosomes. This structure increases the packing ratio to about 40. The final packaging occurs when the fiber is organized in loops, scaffolds and domains that give a final packing ratio of about 1000 in interphase chromosomes and about 10,000 in mitotic chromosomes. Eukaryotic chromosomes consist of a DNA-protein complex that is organized in a compact manner which permits the large amount of DNA to be stored in the nucleus of the cell. The subunit designation of the chromosome is chromatin. The fundamental unit of chromatin is the nucleosome. Chromatin - the unit of analysis of the chromosome; chromatin reflects the general structure of the chromosome but is not unique to any particular chromosome Nucleosome - simplest packaging structure of DNA that is found in all eukaryotic chromosomes; DNA is wrapped around an octamer of small basic proteins called histones; 146 bp is wrapped around the core and the remaining bases link to the next nucleosome; this structure causes negative supercoiling The nucleosome consists of about 200 bp wrapped around a histone octamer that contains two copies of histone proteins H2A, H2B, H3 and H4. These are known as the core histones. Histones are basic proteins that have an affinity for DNA and are the most abundant proteins associated with DNA. The amino acid sequence of these four histones is conserved suggesting a similar function for all. The length of DNA that is associated with the nucleosome unit varies between species. But regardless of the size, two DNA components are involved. Core DNA is the DNA that is actually associated with the histone octamer. This value is invariant and is 146 base pairs. The core DNA forms two loops around the octamer, and this permits two regions that are .

1. Eukaryotic Chromosome Structure The length of DNA in the nucleus is far greater
than the size of the compartment in which it is contained. To fit into this compartment the DNA
has to be condensed in some manner. The degree to which DNA is condensed is expressed as its
packing ratio. Packing ratio - the length of DNA divided by the length into which it is packaged
For example, the shortest human chromosome contains 4.6 x 107 bp of DNA (about 10 times the
genome size of E. coli). This is equivalent to 14,000 µm of extended DNA. In its most condensed
state during mitosis, the chromosome is about 2 µm long. This gives a packing ratio of 7000
(14,000/2). To achieve the overall packing ratio, DNA is not packaged directly into final
structure of chromatin. Instead, it contains several hierarchies of organization. The first level of
packing is achieved by the winding of DNA around a protein core to produce a "bead-like"
structure called a nucleosome. This gives a packing ratio of about 6. This structure is invariant in
both the euchromatin and heterochromatin of all chromosomes. The second level of packing is
the coiling of beads in a helical structure called the 30 nm fiber that is found in both interphase
chromatin and mitotic chromosomes. This structure increases the packing ratio to about 40. The
final packaging occurs when the fiber is organized in loops, scaffolds and domains that give a
final packing ratio of about 1000 in interphase chromosomes and about 10,000 in mitotic
chromosomes. Eukaryotic chromosomes consist of a DNA-protein complex that is organized in
a compact manner which permits the large amount of DNA to be stored in the nucleus of the cell.
The subunit designation of the chromosome is chromatin. The fundamental unit of chromatin is
the nucleosome. Chromatin - the unit of analysis of the chromosome; chromatin reflects the
general structure of the chromosome but is not unique to any particular chromosome
Nucleosome - simplest packaging structure of DNA that is found in all eukaryotic chromosomes;
DNA is wrapped around an octamer of small basic proteins called histones; 146 bp is wrapped
around the core and the remaining bases link to the next nucleosome; this structure causes
negative supercoiling The nucleosome consists of about 200 bp wrapped around a histone
octamer that contains two copies of histone proteins H2A, H2B, H3 and H4. These are known as
the core histones. Histones are basic proteins that have an affinity for DNA and are the most
abundant proteins associated with DNA. The amino acid sequence of these four histones is
conserved suggesting a similar function for all. The length of DNA that is associated with the
nucleosome unit varies between species. But regardless of the size, two DNA components are
involved. Core DNA is the DNA that is actually associated with the histone octamer. This value
is invariant and is 146 base pairs. The core DNA forms two loops around the octamer, and this
permits two regions that are 80 bp apart to be brought into close proximity. Thus, two sequences
that are far apart can interact with the same regulatory protein to control gene expression. The
DNA that is between each histone octamer is called the linker DNA and can vary in length from
8 to 114 base pairs. This variation is species specific, but variation in linker DNA length has also

2. been associated with the developmental stage of the organism or specific regions of the genome.
Solution
Eukaryotic Chromosome Structure The length of DNA in the nucleus is far greater
than the size of the compartment in which it is contained. To fit into this compartment the DNA
has to be condensed in some manner. The degree to which DNA is condensed is expressed as its
packing ratio. Packing ratio - the length of DNA divided by the length into which it is packaged
For example, the shortest human chromosome contains 4.6 x 107 bp of DNA (about 10 times the
genome size of E. coli). This is equivalent to 14,000 µm of extended DNA. In its most condensed
state during mitosis, the chromosome is about 2 µm long. This gives a packing ratio of 7000
(14,000/2). To achieve the overall packing ratio, DNA is not packaged directly into final
structure of chromatin. Instead, it contains several hierarchies of organization. The first level of
packing is achieved by the winding of DNA around a protein core to produce a "bead-like"
structure called a nucleosome. This gives a packing ratio of about 6. This structure is invariant in
both the euchromatin and heterochromatin of all chromosomes. The second level of packing is
the coiling of beads in a helical structure called the 30 nm fiber that is found in both interphase
chromatin and mitotic chromosomes. This structure increases the packing ratio to about 40. The
final packaging occurs when the fiber is organized in loops, scaffolds and domains that give a
final packing ratio of about 1000 in interphase chromosomes and about 10,000 in mitotic
chromosomes. Eukaryotic chromosomes consist of a DNA-protein complex that is organized in
a compact manner which permits the large amount of DNA to be stored in the nucleus of the cell.
The subunit designation of the chromosome is chromatin. The fundamental unit of chromatin is
the nucleosome. Chromatin - the unit of analysis of the chromosome; chromatin reflects the
general structure of the chromosome but is not unique to any particular chromosome
Nucleosome - simplest packaging structure of DNA that is found in all eukaryotic chromosomes;
DNA is wrapped around an octamer of small basic proteins called histones; 146 bp is wrapped
around the core and the remaining bases link to the next nucleosome; this structure causes
negative supercoiling The nucleosome consists of about 200 bp wrapped around a histone
octamer that contains two copies of histone proteins H2A, H2B, H3 and H4. These are known as
the core histones. Histones are basic proteins that have an affinity for DNA and are the most
abundant proteins associated with DNA. The amino acid sequence of these four histones is
conserved suggesting a similar function for all. The length of DNA that is associated with the
nucleosome unit varies between species. But regardless of the size, two DNA components are
involved. Core DNA is the DNA that is actually associated with the histone octamer. This value
is invariant and is 146 base pairs. The core DNA forms two loops around the octamer, and this
permits two regions that are 80 bp apart to be brought into close proximity. Thus, two sequences

3. that are far apart can interact with the same regulatory protein to control gene expression. The
DNA that is between each histone octamer is called the linker DNA and can vary in length from
8 to 114 base pairs. This variation is species specific, but variation in linker DNA length has also
been associated with the developmental stage of the organism or specific regions of the genome.