This course is organized around four core themes. Almost
everything in the course touches on one or more of these themes:
Mendelian genetics originated with the discovery of single genes - single units of inheritance. Cytology originated in the study of chromosomes, which carry genes. For many decades, cytogenetics employed the methods of microscopy to learn about inheritance on a macroscopic scale. One therefore had the choice of working with one gene at a time using phenotypes, or to observe the whole genome at a time under the microscope. Until recent decades, the best one could do was to map a given gene to a particular chromosomal band, a very broad level of mapping.
Modern cytogenetics attempts to bring together:
At the same time, the methods of cytogenetics really only apply to organisms with large cells and chromosomes. It is not feasible to study bacterial genetics under the light microscope, because of their extremely small size.
Today, we distinguish between the bacteria
and archaea, which are prokaryotes, and the higher
organisms, which are eukaryotes. Prokaryotes are
comparatively simple organisms, with all cellular processes
taking place in a single compartment. Eukaryotes are highly
organized, with specialized organelles which
compartmentalize key cellular functions. The key distinction
between the two groups is the presence of the nucleus - in
Greek, karyon. Thus, eu (true) and karyon - true
nucleus. Cytogenetics, then, is a science devoted specifically to
to eukaryotes. It is therefore critical for us to have a
clear picture of the nature of the eukaryotic cell, the
differences between eukaryotic cells and prokaryotic
cells, and in particular, the differences between
eukaryotic chromosomes and prokaryotic chromosomes.
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Prokaryotes are generally less complex than eukaryotes. They are
also much older. The phylogenetic tree illustrates the major steps
leading to the evolution of the fundamental groups of higher
organisms: Plants, Animals and Fungi. The two short branches at
the bottom of the tree represent the Archaea and Bacteria. The Tree of Life
illustrates two evolutionary events that had a profound impact
on the evolution of the major groups. Two slender strands
connect the earliest group, Bacteria, with the early eukaryotes.
One connects to a common eukaryotic ancestor, and one connects
to the plant lineage.
Once eukaryotes evolved, somewhere about 2 billion years ago, it took a while for multicellular eukaryotes to develop. Single-celled eukaryotes were around for at least half a billion years before there is evidence for the development of plants, fungi, etc. This accounts for some common features we see across eukaryotes, and underlies the statement that eukaryotes are more fundamentally alike than they are different. One precondition for the evolution of eukaryotes, and especially multicellular eukaryotes, was the presence of high levels of oxygen produced by cyanobacteria, starting around 2.33 billion years ago. This is referred to as the Great Oxygenation Event. Hedges SB, Marin J, Suleski M, Paymer M, Kumar S (2015) Tree of Life Reveals Clock-Like Speciation and Diversification Mol. Biol. Evol. 32:835-845 doi:10.1093/molbev/msv037
Understanding the eukaryotic context of the cell leads to a deeper understanding of the concepts underlying this course. Many of the processes we will discuss are tied to the cell's characteristics, which in turn are a reflection of the needs and possibilities of a cell with a true nucleus - a eukaryote.