Biology 12

DNA double helix

histone

molecules

H1 molecules

2 nm

11 nm

nucleosome

30 nm

supporting

protein

structure

300 nm

protein

folding

structure 700 nm

condensed

chromosome

1400 nm

loops

230 MHR • Unit 3 Molecular Genetics

to produce bead-like structural units called

nucleosomes. Each nucleosome bead is a short

segment of DNA (about 200 base pairs) wrapped

twice around a cluster of eight histone molecules.

The attraction between the acidic DNA and the

highly alkaline histone molecules helps to keep

the arrangement in place.

A short stretch of DNA extends between each

nucleosome, as shown in Figure 7.19. This short

segment of DNA is bound to a single molecule of a

type of histone known as H1. The interaction among

H1 molecules helps to draw the arrangement into a

tight, regular array.

In turn, this array (which has a total thickness

of about 30 nm) undergoes another level of packing.

It forms loops that attach to a supporting structure

of non-histone proteins. As the cell prepares to

reproduce, the protein structure folds back on itself

to condense the chromatin even further. The result

is the short, thick chromosomes you have seen in

typical karyotypes. In a living cell, some regions of

chromatin will be found in the 30 nm arrays during

the bulk of the cell life cycle, while other regions

are more condensed. You will learn more about the

effect of this arrangement on cell functions in the

next chapter.

In the next section, you will learn about the

processes involved in DNA replication and how

they ensure the accurate transmission of hereditary

material.

Figure 7.18The successive ordering of genetic material within a eukaryotic cell.

The DNA double helix winds around histone

proteins to form a string of nucleosomes.

A

These nucleosomes form a regular, tightly

packed array to produce lengths of chromatin

fibre that are 30 nm wide.

B

Chromatin fibres form loops attached to a

protein scaffold. Some chromatin fibres only

assume this form as the cell prepares to divide,

while other fibres remain in this looped

structure throughout the life cycle of the cell.

C

The supporting protein structure folds even

further during mitosis to condense the genetic

material into a short chromosome.

D