whuy bacterial DNA is circular while eukaryotic DNA is linear?

Answer 1

There is a large gap between the chromosomal structures of prokaryotes (cells without nuclei) and eukaryotes (cells with nuclei) that I maintain cannot be bridged by the theory of evolution. There are many huge differences in transcription, storage, etc., but for the sake of a narrow topic (and because I know little about those other things) I will restrict this topic to one of the simplest differences, the shape. A prokaryotic chromosome consists of a circular strand of DNA, while a eukaryotic chromosome is a linear strand of DNA. The actual shape isn�t very important, but what it entails is. (note: If you know what a telomere is, what telomerase does, and the function of a telomere cap, you can skip the next 7 paragraphs)

Every time a linear chromosome is replicated, some of the DNA on each end is destroyed. Without some form of protection, a linear chromosome deteriorates rapidly and quickly becomes useless. When that happens, the cell is unable to repair itself and soon dies.

Linear chromosomes do exist, though, and have protection in the form of telomeres. A telomere is a section of DNA at the end of a linear chromosome made up of a large number of repeats of a small sequence of base pairs (for example, in humans the sequence is TTAGGG). This DNA acts as a sort of buffer zone for the rest of the chromosome; it is destroyed in place of vital segments.

If the protection system were that simple, the line of cells would last a little longer, but not by much. If the telomeres are being destroyed from both ends, it is only a matter of time before they are completely used up and the rest of the chromosome starts being destroyed.

This problem is solved in the form of an enzyme, telomerase, which lengthens telomeres once they have deteriorated. Telomerase is a complex protein (�complex protein� is redundant, because all proteins are complex, but it sounds better than just saying �protein�) made up of several subunits with different purposes. There is a RNA-transcriptase subunit which copies the telomere sequence, and there is at least on other subunit that is attracted to/identifies the protein caps of telomeres.

Telomerase needs regulation, though. If it was left on its own, it would continually add useless DNA as fast as it could, using up a vast amount of nucleic acids, burdening the cell�s production. Also, all of the huge amount of DNA would be copied to all daughter cells, which would then add to it and have to copy it to their daughters, etc. Fairly rapidly the cells would be unable to support themselves.

Protein caps are the last part of the protective system. At the very end of the chromosome, at the end of the telomere segment, is a protein (Pot1) that serves two purposes. First, to provide a substance telomerase can identify; second, to provide protection to the telomere tip. Experiments have shown that cells cannot survive without this additional protection, the chromosome rapidly deteriorates. This cap is removed during replication of the chromosome, and then put back once replication is complete.

The attraction function of the cap is also vital. Without something for telomerase to identify, it would be unable to find the end of the chromosome to repair it. The chromosome would be protected from normal wear and tear, but would still be eventually destroyed by replication.

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To go from circular DNA to linear DNA is not something that could take more than a single generation to happen � if it didn�t happen all at once, the cell would die, and there would be no further generations. Here is a list of the minimum required to go from circular to linear chromosomal structure:

1. The creation of telomerase, a complex protein
2. The creation of a protein that binds to the ends of telomeres
3. Regulatory mechanisms for telomerase
4. Modification of the DNA replication system to enable it to remove/replace telomere endcaps

The creation of at least two proteins, the creation of a regulatory system, and the suitable modification of the DNA replication system by a single set of mutations is highly incredible. The chances of mutations occurring such that those would all work is nearly impossible. That is not all, though.

Say that all those mutations did take place, and the DNA was such that the next daughter cell would have functional, stable linear chromosomes. That would take thousands of mutations concentrated in only a few genes. That is far, far beyond what is even remotely normal for rates of mutation.

That extremely high rate of mutations can only be explained one of two ways: either the DNA copying mechanism broke and went berserk (to use scientific terminology), or there was an extremely powerful mutagen present. In both cases, the mutations would not be restricted to the genes in question, but would affect the replication of the entire chromosome. There is no way the daughter cell could survive with DNA corrupted that badly. But even, miraculously, if the daughter cell did survive, it would not long be able to survive the mutagen or berserk DNA synthases.

Well, say all of that did occur. Circular chromosomes are much more stable than linear chromosomes. What would have caused natural selection to favor the new mutant, and cause it to evolve into all higher forms of life (all multicellular organisms have linear chromosomes), while keeping the safer, circular DNA confined to single celled organisms? And if that much change can happen in a single generation, why do yeast and humans (both eukaryotes, but far apart on the �evolutionary ladder�) use the exact same proteins to maintain their chromosomes? (Did evolution just stop?) Why do the even have similar chromosomal structures? [edited in: after posting this I learned that telomerase is the name for a family of proteins, not a specific protein. I am thus retracting the three previous questions from debate]

Answer 2

The DNA of a prokaryotic cell is literally circular; the strands of DNA form a closed circle. Eukaryotic DNA is free at the ends, making it linear instead. Also, eukaryotic DNA forms into chromosomes, which is not pure DNA. A chromosome is formed from chromatin, a construct of DNA, RNA, and protein. The protein gives the chromosome its physical structure.

Answer 3

circular dna is primitive where as linear dna is an advanced type.

Answer 4

These are the basic characteristic.