0.2 Eukaryotic genetics  (Page 2/9)

Myoglobin consists of a single globin subunit plus heme and carries oxygen within muscles. Because of their similar sequence and gene organization (both have three exons in exactly the same location along the gene), it is believed that both the globin and myoglobin are derived from a common ancestor gene.

Plants called legumes have the ability to use certain kinds of bacteria as a means of getting their needed nitrogen through a process of nitrogen fixation. An example is soybeans. The roots develop a sac where bacteria can fix nitrogen. The bacteria and the plant have a symbiotic relationship; the plant provides the bacteria with food, and the bacteria fixes nitrogen for the plant. Leghemoglobin is crucial in this process because it binds oxygen within the sac which allows the bacteria to fix nitrogen. The bacteria cannot function in the presence of oxygen. The sequence of leghemoglobin is related to the sequence of the other globins, but, interestingly, the middle exon is split in leghemoglobin, giving this particular globin gene 4 exons. Since the gene organization is close to that of the rest of the globin family and the protein sequence of leghemoglobin and globin are related, it is clear that these genes all share a common ancestor. It is not known if the ancestor had three or four exons.

The characteristics of eukaryotic genes and genomes have been very well considered in MITOPENCOURSEWARE ( PDF ), especially in model eukaryotic organisms, the yeast Saccharomyces cerevisiae and the mouse Mus musculus.

Lecture 26. gene regulation in eukaryotes

( (External Link) )

Because of essential differences in eukaryotic gene and genome structures compared with those of prokaryotes, as described in the above lecture, there are a number of ways that gene regulation in eukaryotes differs from gene regulation in prokaryotes.

Eukaryotic genes are not organized into operons. Eukaryotic regulatory genes are not usually linked to the genes they regulate. Some of the regulatory proteins must ultimately be compartmentalized to the nucleus, even when signaling begins at the cell membrane or in the cytoplasm. Eukaryotic DNA is wrapped around nucleosomes.

Now we will consider how one can use genetics to begin analysis of the mechanisms by which eukaryotic gene expression can be regulated.

The latest estimates are that a human cell, a eukaryotic cell, contains 20,000–25,000 genes.

• Some of these are expressed in all cells all the time. These so-called housekeeping genes are responsible for the routine metabolic functions (e.g. respiration) common to all cells.
• Some are expressed as a cell enters a particular pathway of differentiation.
• Some are expressed all the time in only those cells that have differentiated in a particular way. For example, a plasma cell expresses continuously the genes for the antibody it synthesizes.
• Some are expressed only as conditions around and in the cell change. For example, the arrival of a hormone may turn on (or off) certain genes in that cell.

How is gene expression regulated?