FIGURE 4.25 Histone Tails May Be Acetylated
The N-terminal domains of some of the histone proteins are free for acetylation as indicated by "acetyl." The single letter system for naming amino acids is used.
Histones Are Highly Conserved and Originated Among the Archaebacteria
Of all known proteins, the eukaryotic core histones, especially H3 and H4, are the most highly conserved during evolution. For example, only two amino acids are different, out of 102, between the H4 of cows and peas. The linker histone, H1, is more variable in composition.
Typical bacteria (i.e., the eubacteria) do not possess histones. [Large numbers of "histone-like proteins" are found bound to bacterial chromosomes. Despite the name, these are not homologous in sequence to true histones nor do they form nucleosomes for packaging DNA.] However, some members of the genetically distinct lineage of archaebacteria (e.g., the methane bacteria), do possess histones. Archaeal histones vary significantly from each other. They are 65-70 amino acids long and are missing the tails characteristic of eukaryotic histones. Archaeal nucleosomes accommodate a little under 80 bp of DNA and contain a tetramer of the archaeal histone. They are probably homologous to the (H3 + H4)2 tetramers found in the core of the eukaryotic nucleosome.
FIGURE 4.26 Looping of 30 Nanometer Fiber on Chromosome Axis
A) A chain of nucleosomes is coiled further with six nucleosomes forming each turn. B) The coiled nucleosomes form a helix, known as a 30 nm fiber. C) The 30 nm fibers form loops that are periodically anchored to a protein scaffolding.
When chromosomes are visible under the light microscope, it is because they are present in a cell that has been caught in the act of dividing. Between cell divisions, most of the DNA is less condensed. It consists of a single, extended molecule of double helical DNA and does not look at all like typical chromosome pictures. Just before cell division, the DNA condenses and folds up, as described above. The typical metaphase chromosome, seen in most pictures, has replicated its DNA some time previously, and is about to divide into two daughter chromosomes, as shown in Figure 4.28. It therefore consists of two identical double helical DNA molecules that are still held together at the centromere. These are known as chromatids. Note that between cell
chromatid Single double-helical DNA molecule making up whole or half of a chromosome. A chromatid also contains histones and other DNA-associated proteins.
FIGURE 4.27 Summary of the Folding of DNA in Eukaryotic Chromosomes
The DNA helix (A) is wrapped around (B) eight histones (the core). The linker DNA regions unite the nucleosomes to give a "string with beads." This in turn is coiled helically (C) (not clearly indicated) to form a 30 nm fiber. The 30 nm fibers are further folded by looping and attachment to a protein scaffold. Finally, during mitosis the DNA is folded yet again to yield very thick chromosomes.
Size a DNA helix
B 'String with beads'
30 nm fiber
D Looped 30nm fibers
30 nm fibers e Mitotic chromosome
Pair of interphase chromosomes lUncondensed
Between rounds of cell division, chromosomes consist of single chromatids, and are referred to as interphase chromosomes. Before the next cell division, the DNA is replicated and each chromosome consists of two DNA molecules or chromatids linked at the centromere. Just prior to mitosis, condensation occurs, making the chromosomes (and chromatids) visible. The chromosomes are best viewed while spread out during the middle part (metaphase) of mitosis. Each daughter cell will acquire one of the chromatids and the process begins
Anaphase chromosomes (two sets -one for each daughter cell)
Dna replication (s-phase)
Separation in mitosis
(^y Condensed o
Double stranded DNA
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