Anyone who has taken high school biology has learned about the process of DNA replication. However, DNA comprises an incredibly small part of the cell. The reproduction of all the organelles (mitochondria, lysomes, peroxisomes, etc) has not been mentioned in a single biology textbook that I’ve seen, yet it has to be a vital part of cell reproduction, both in terms of the necessity of the organelles in the daughter cells and in terms of the amount of energy it must require. How does this process work? At what point in the cell cycle does it occur? What does the cell do with the rough ER prior to the creation of a new nuclear membrane, since the ER is attached to the nucleus? How is this process different in meiosis than in mitosis? I know that sperm only contributes DNA, not organelles (which is why mitochondrial DNA is solely maternal) – how does that work? Most importantly, why is it that none of the bio classes I’ve taken have seen fit to mention this process?
Very good question! Coincidentally, I was just reading in a biology book how some organelles reproduce themselves, and such info is NOT contained in the DNA. Mitochondria was one example, IIRC. Also, chloroplasts (in plant cells) was mentioned. You may wish to read up on a process known as “endosymbiosis” or “the endosymbiotic theory”. Yes, this sounds seems strange to me, too. However, it seems as though some organelles are present and duplicate independent of the DNR, if I am understanding this correctly. - Jinx
Vastard, what biology classes have you taken? Maybe you’re still in introductory courses? I just finished taking both Molecular Biology and Cell Biology this semester, so let’s see if what I learned can help you out any.
Mitochondria and chloroplasts: As Jinx mentioned, the common theory concerning these 2 organelles is “endosymbiosis.” Mitochondria and chloroplasts are believed to have originally been indipendent prokaryotic cells that were engulfed by single-celled eukaryotic cells long long ago. The two proto-organelles gave their hosts the ability to function aerobically (with oxygen), which provided the cells with a much higher energy per unit food ratio. Seeing this as rather advantageous, it is believed that the host cells managed to take some of the vital DNA present in the proto-organelles so that they would become permenant residents of the host. Mitochondria and chloroplasts have retained their own replication machinery and reproduce entirely on their own. Since the eukaryotic cell’s replication machinery has no control over this, the number of mitochondria or chloroplasts within a cell is not conserved in its daughter cells. These two organelles are distributed randomly and take care of themselves.
Sperm, ovum, and mitochondrial DNA: As you said, the sperm contains just half of the father’s DNA. The egg however, has half the mother’s DNA plus a huge cytoplasm filled with organelles to support the growth of a new organism. If you’ve ever seen to scale drawings or microscope captures, you’d notice the huge difference in size between an egg and a sperm.
Lysosomes: The enzymes present in lysosomes are all synthesized by the ER and prepped in the Golgi apparatus. When these enzymes (hydrolases, esterases, etc) are mature, they are sent away from the Golgi complex in vesicles. These vesicles fuse together to form a lysosome. These, I believe, divide randomly also during cell replication.
ER: I’m not sure about how rough or smooth ER are divided, but my guess is that it’s pretty random also. There’s more than one ER extending from the nucleus, so it might all just separate during telophase as the cell splits into two parts.
Since they float around in the cytoplasm and can become a part of the daughter cells without any effort, I don’t think any energy is expended by the cell. I’ve never heard or read anything that says the number of the organelles is conserved throughout generations.
A lot of organelles are simply conglomerations of membrane and/or cytoskeleton components and are constantly being created and destroyed. ER is contiguous with both the cell membrane and the nuclear membrane. I was going to say that when the cell pinches in two, the internal membranes simply get pinched off too, but now I’m recalling that some internal cellular structures actually dissolve during mitosis and reform afterward. I don’t think ER does that, but I can’t remember for sure.
Ribosomes haven’t been mentioned either. They’re made up of RNA and proteins, both of which are coded for in the nuclear DNA.