DNA is tightly packed into the metaphase chromosome. Packing ratio can be expressed as [total length of DNA]/ [length of metaphase chromosome]. Values of 8,000 X are realistic. The first level of packing, the nucleosome, involves DNA wound around histone proteins. The histones are highly conserved, indicating that they are involved in a very fundamental process. A second level of packing is into a 30 nm chromatin fiber. Additional packing involves folding of loops of chromatin fibers against a scaffold consisting of non-histone proteins. The packing allows the long DNA molecule to be transmitted to the daughter cells via the metaphase chromosome.
The centromere and telomere are key parts of chromosomes. The regions around these are rich in heterochromatin (dark staining, genetically inactive material, which often contains short DNA repeat sequences). The centromere is the site where sister chromatids attach to each other, and where spindle fibers attach (also termed kinetochore). Specific DNA sequences are associated with the centromeres in yeast. Alpha-satellite DNA repeats are found in the vicinity of centromeres in humans.
The telomere corresponds to the tip of chromosomes. Classical genetic studies suggested that the chromosome tips were special, and only a few specfic regions could serve in that role. The reason may relate to the otherwise steady loss of sequences near the tips associated with replication. Similar short repeats (e.g., 5’ TTAGGG 3’ in vertebrates) are associated with the telomeres. These sequences are added to the chromosome tips by the enzyme telomerase. This may allow cells which might otherwise die to continue dividing.
Sex chromosomes were first recognized cytologically shortly after 1900. Systems include XO male / XX female; XY male/ XX female and ZW female/ ZZ male. Sex chromosomes are normally inherited such that males pass their X to their daughters and their Y to their sons, and females pass one X to offspring of each sex. The non-sex chromosomes are termed autosomes.
X-linked inheritance was first recognized in Drosophila by Thomas Hunt Morgan.
Crosses involving white-eyed and wildtype (red-eyed) flies gave different results in reciprocal crosses. Results were consistent with the pattern of transmission of the X and Y chromosomes. Characteristics of X-linked inheritance include: different results in reciprocal crosses, heterozygous females pass specific X-linked alleles to half their sons and half their daughters, males with an X-linked allele pass it to all of their daughters and none of their sons. Human pedigrees for traits like hemophilia and colorblindness show this pattern of inheritance.
Experiments on nondisjunction in Drosophila by Bridges demonstrated that genes were on chromosomes. A cross of white-eyed females to red-eyed males gave red-eyed females and white-eyed males. Rare white-eyed females and red-eyed males were found at an incidence of about 1/2000. These were found to be XXY and XO, respectively, in their chromosome constitution. Thus the genetic exceptions were also chromosomal exceptions. The chance of both things happening together due to chance are vanishingly small. These experiments and subsequent ones have shown that sex determination in Drosophila is based on the ratio of X chromosomes and autosomes. A ratio of 0.5 or less gives male development, 1.0 or greater gives female development and between 0.5 and 1.0 gives intersex development.