Eukaryotic Genomes: The Organization and Control

Chromatin n Actually a complex of DNA and proteins n DNA Packing histone protein n + charged amino acids (lots of argenine & lysine) n ~ phosphates of DNA are charged n Nucleosome- (basic unit of DNA packing) DNA wound around a cluster of 8 histones. “beads on a string” n Heterochromatin highly condensed interphase DNA (can not be transcribed) n Euchromatin less compacted interphase DNA (can be transcribed) n Which is which depends on the type of cell….

DNA Packing n D:\bc_campbell_biology_7\0,7052, ,00.html D:\bc_campbell_biology_7\0,7052, ,00.html

Lots of DNA…but…… Lots of genes???? Not really: Most Eukaryotic DNA does NOT code for mRNA 30-50% is introns!! Another 10-15% is “Tandemly Repetitive Sequences”- -short sequences that repeat up to 1000s of times -unique nucleotide ratio gives it a lower density during centrifugation so sometimes is called “satellite DNA” -mostly located at telomeres & centromeres Still more (up to 25-40%) is “Interspersed non-coding”- -thought to be mostly transposons - some are considered “pseudogenes” -still others are thought to be inactive parts of multigene families

Multigene Families: A collection of genes which are similar or identical. Presumably of common ancestral origin… Can be clustered or dispersed. Identical- Probably arose from repeated duplication -many code for rRNA (make lots quickly!) Non- Identical- Mutations in the duplicated gene(s) Most are just PSEUDOGENES: Non-functional genes which are similar but lack expression mechanisms. Some ARE functional. Example: Hemoglobin (again) For 8 th edition, see: section 21.3 & 21.4 (pp )

REMEMBER!!! Unlike prokaryotic operons, eukaryotic genes coding for enzymatic pathways may be scattered over different chromosomes!! Yet- they can be coordinately expressed by common…. Transcription factors D:\bc_campbell_biology_7\0,7052, ,00.html

Molecular Biology of Cancer n Oncogene cancer-causing genes n Proto-oncogene normal cellular genes n How? 1-movement of DNA; chromosome fragments that have rejoined incorrectly 2-amplification; increases the number of copies of proto-oncogenes 3-proto-oncogene point mutation; protein product more active or more resistant to degradation n Tumor-suppressor genes changes in genes that prevent uncontrolled cell growth (cancer growth stimulated by the absence of suppression)