PLNT 3140 Introductory Cytogenetics - 2023

Setting the Eukaryotic Context

Cytogenetics is the study of chromosomes and heredity

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:

• organization of chromosomes in the nucleus with the requirements of gene expression, in different developmental and environmental contexts
• behavior of chromosomes with transmission of phenotypes to progeny
• changes in chromosomal structure and number with speciation
• the evolution of the genome with the evolution of the species

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.

The distinction between prokaryotes and eukaryotes is a fundamental concept in biology

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. The Tree of Life from http://tolweb.org/tree/home.pages/aboutoverview.html The Tree of Life navigation picture is licensed under the Creative Commons Attribution-No Derivative Works License - Version 3.0.

Although prokaryotes are generally less complex than eukaryotes, they are 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