7. DNA and Chromosome Structure

Describe the structure of giant polytene chromosomes and how they arise.

What genetic process is occurring in a puff of a polytene chromosome? How do we know this experimentally?

During what genetic process are lampbrush chromosomes present in vertebrates?

Why might we predict that the organization of eukaryotic genetic material will be more complex than that of viruses or bacteria?

Describe the sequence of research findings that led to the development of the model of chromatin structure.

Describe the molecular composition and arrangement of the components in the nucleosome.

Describe the transitions that occur as nucleosomes are coiled and folded, ultimately forming a chromatid.

Provide a comprehensive definition of heterochromatin and list as many examples as you can.

Mammals contain a diploid genome consisting of at least 10⁹ bp. If this amount of DNA is present as chromatin fibers, where each group of 200 bp of DNA is combined with 9 histones into a nucleosome and each group of 6 nucleosomes is combined into a solenoid, achieving a final packing ratio of 50, determine (a) the total number of nucleosomes in all fibers, (b) the total number of histone molecules combined with DNA in the diploid genome, and (c) the combined length of all fibers.

Examples of histone modifications are acetylation (by histone acetyltransferase, or HAT), which is often linked to gene activation, and deacetylation (by histone deacetylases, or HDACs), which often leads to gene silencing typical of heterochromatin. Such heterochromatinization is initiated from a nucleation site and spreads bidirectionally until encountering boundaries that delimit the silenced areas. Recall from earlier in the text (see Chapter 4) the brief discussion of position effect, where repositioning of the w⁺ allele in Drosophila by translocation or inversion near heterochromatin produces intermittent w⁺ activity. In the heterozygous state (w⁺/w) a variegated eye is produced, with white and red patches. How might one explain position-effect variegation in terms of histone acetylation and/or deacetylation?