January 12, 2010 Each chromosome has many genes (often thousands) Many eukaryotes (including humans) do not tolerate having an abnormal number of chromosomes-especially the somatic chromosomes One too many (i.e., three) is a trisomy-> down syndrome One too few (i.e., one) is a monosomy-> no known cases; none survive to birth (all eukaryotic have at least 2 chromosomes (1 pair)) (somatic chromosomes= other than sex cells) Eukaryotic chromosomes are composed of chromatin- a complex of DNA and proteins The long single DNA molecule of each chromosome is highly compacted via coiling around histone complexes (8 protein molecules) forming a unit know as a nucleosome Nucleosome compact to form a larger unit known as a solenoid (beads on string then compact further to solenoid level) Compaction to the solenoid level is characteristic of interphase chromatic called euchromatin Euchromatin DNA is generally available for transcription During mitosis (and meiosis) the chromatin compacts further via loops and coils of loop...