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Definition of Semi-Conservative DNA Model

The DNA inside a cell contains the entire blueprint for the organization of that cell or the organism it is a part of. All of an organism's characteristics, from its size and shape to aspects of its metabolism, are encoded by the helical strands of DNA molecules. When these molecules replicate, it is essential that none of this information is lost.

Cells and DNA
- Eukaryotic cells are characterized by the presence of a membrane-enclosed nucleus. It is in the nucleus that a cell's genetic information, or DNA, is found. In eukaryotes, DNA is generally linear and organized into chromosomes. Eukaryotic cells also contain small amounts of genetic material in their mitochondria or chloroplasts. Prokaryotic cells, which includes bacteria and archaea, do not possess a membrane-enclosed nucleus and their genetic material is found in the cytosol. The DNA of prokaryotes generally exists in circular form, but some bacterial species possess linear DNA, too.
DNA Structure
- DNA molecules have a characteristic double helical structure. They are actually composed of two intertwining strands of nucleotides held together through hydrogen bonding. There are four nucleotides -- adenine, thymine, cytosine, and guanine -- and they can only bond in very specific ways. Adenine and thymine on two opposite strands of DNA can only form hydrogen bonds with each other, and the same is the case for cytosine and guanine. Each such bond between nucleotides on the two strands of DNA is called a base pair.
Chromosomes
- DNA molecules can be very long and are arranged into a more compact form known as chromosomes in eukaryotic cells. In addition to genetic material, chromosomes also contain specialized proteins that help with the compaction as well as with controlling and regulating gene function. The number and size of chromosomes varies between different species. The arrangement of chromosomes also changes as a cell progresses through the cell cycle; dividing cells contain duplicated and arranged chromosomes ready to be partitioned into the two incipient cells.
Semi-Conservative DNA Replication
- DNA molecules contain crucial information for the functioning of an organism. Since each nucleotide in a DNA strand can only bond with one other specific nucleotide in the other strand, both strands contain, in essence, the same information. During replication, the two strands unwind and each one is replicated by specialized proteins. The result is two, new molecules of DNA, each of which contains one "old" DNA strand, and one "new" one. This is known as semi-conservative DNA replication.

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