1 Structure and Function of the Human Genome
11
Organization of Chromatin
Structure of chromatin
DNA in chromosomes is associated with proteins called histones. Two molecules of histones H2A, H2B, H3, and H4 (Figure SB) form a multimerk
protein complex called a the nucleosome (Figure SA). Double-stranded
DNA is wound around the nucleosome in 2 turns over a length of 146
base pairs (Figure SA). By electron microscopy consecutive nucleosomes
appear as so-called "beads-on-a-string" or "10 nm-Filament" (Figure
SB,C). Under high salt concentrations 10nm ftlaments condense to form
the "30 nm fiber" (Figure SD,E). Histone H1 organizes the chromatin to
form a solenoid, which is an assembly of 6 nucleosomes arranged in a bouquet-like structure (Figure SE,F).
Chromosomes
During mitosis, higher degrees of chromatin condensation occur under the
influence of non-histone proteins and lead to the formation of the metaphase chromosome (Figure 6A). Cytogenetic techniques permit the staining
of specific chromosome bands which represent either AT-rich ( dark) or CGrich (light) regions of the chromosome. In Figure 6B the left half of each
human chromosome shows staining during metaphase, whereas the right
half shows a higher resolution as it is seen in prometaphase. This banding
pattern, although species-specific, is highly conserved throughout evolution.
Figure 1 (see above) shows an electron micrograph of a chromosome that
has been treated with detergents. A remnant of the protein scaffold can be
appreciated, while long loops of DNA spillout from the scaffold.
Dimensions of the human genome
• The diploid set of human chromosomes is 46, XY (i.e. 22 homologous
autosomes: 22 from the father, 22 from the mother, plus 2 sex chromosomes: XY or XX)
• The length of the haploid human genome is 3 x 10 9 base pairs (bp)
11
Organization of Chromatin
Structure of chromatin
DNA in chromosomes is associated with proteins called histones. Two molecules of histones H2A, H2B, H3, and H4 (Figure SB) form a multimerk
protein complex called a the nucleosome (Figure SA). Double-stranded
DNA is wound around the nucleosome in 2 turns over a length of 146
base pairs (Figure SA). By electron microscopy consecutive nucleosomes
appear as so-called "beads-on-a-string" or "10 nm-Filament" (Figure
SB,C). Under high salt concentrations 10nm ftlaments condense to form
the "30 nm fiber" (Figure SD,E). Histone H1 organizes the chromatin to
form a solenoid, which is an assembly of 6 nucleosomes arranged in a bouquet-like structure (Figure SE,F).
Chromosomes
During mitosis, higher degrees of chromatin condensation occur under the
influence of non-histone proteins and lead to the formation of the metaphase chromosome (Figure 6A). Cytogenetic techniques permit the staining
of specific chromosome bands which represent either AT-rich ( dark) or CGrich (light) regions of the chromosome. In Figure 6B the left half of each
human chromosome shows staining during metaphase, whereas the right
half shows a higher resolution as it is seen in prometaphase. This banding
pattern, although species-specific, is highly conserved throughout evolution.
Figure 1 (see above) shows an electron micrograph of a chromosome that
has been treated with detergents. A remnant of the protein scaffold can be
appreciated, while long loops of DNA spillout from the scaffold.
Dimensions of the human genome
• The diploid set of human chromosomes is 46, XY (i.e. 22 homologous
autosomes: 22 from the father, 22 from the mother, plus 2 sex chromosomes: XY or XX)
• The length of the haploid human genome is 3 x 10 9 base pairs (bp)