MadSci Network: Cell Biology |
Dear Ryan,
Thank you for your question to MadSci Network! Protists are
interesting creatures. They have only one cell, like bacteria. The one
cell of the protist is much more complicated than the one cell of a
bacteria. Inside a protist cell there are more structures than inside a
bacteria cell. These structures work on keeping the protist functioning
at a higher level. There are many kinds of structures that do different
things. The structures are called organelles. One kind of
organelle that you might have heard of is the mitochondria.
The group of protists that Stentor is in is called the Ciliates.
Ciliates have a very unique kind of
organelle. This unique organelle is called a macronucleus. The
macronucleus is a copy of the Stentor's DNA. The DNA of the
macronucleus acts like the DNA of most other organisms, except for one
major difference. The macronucleus is not directly involved in
reproduction!
Instead, Stentor uses a micronucleus for reproduction. When
two
Stentor run into each other, they may latch on and decide to mate.
Before mating takes place, the macronucleus is destroyed and the
micronucleus copies itself, and becomes ready for the exchange.
The
micronucleus is the DNA that is exchanged between organisms when
Stentor species mate.
Once two Stentor species have exchanged their micronuclei,
the
micronuclei reorganize themselves. Once the micronucleus is
reorganized,
copies are made which become macronuclei. Then the protist divides
many
times by asexual reproduction. After dividing many times, the
Stentor will be ready to mate again.
Summary:
It is common for protozoa to reproduce both sexually and asexually. When
they reproduce, it is always the micronuclei that is passed on.
The
macronuclei are interesting organelles that are made by the
micronuclei.
The macronuclei control the cell like DNA of other organisms, but
they are
not used to pass on genetics to their offspring. Usually organisms only
have one kind of nucleus. The genus Stentor is in the phylum
Ciliophora, more commonly called the Ciliates. It is the Ciliates that
have developed a system of having a macronucleus for running the
cell and
a micronucleus to pass on genetic material.
Unfortunately, there are not many places on the Internet that discuss the
biology of Stentor, so I have no diagrams for you to look at. The
only source of information on these interesting animals I was able to find
was a college-level text book entitled Invertebrate Zoology by
Robert D. Barnes. If you live near a college campus, you should have
someone take you there and show you how you can find a copy of this book.
In it there are diagrams that illustrate the reproduction cycle that I
have explained.
Another MAD Scientist, Allison Gong has answered the question, How does
Vorticella reproduce? Vorticella is another genus of
Ciliate. This answer describes the asexual portion of Ciliate
reproduction.
If you are curious about why Ciliates have two kinds of nuclei, what
other organelles protists have, or anything else, please ask us another
question!
John Carlson
MAD Scientist
Try the links in the MadSci Library for more information on Cell Biology.