Semiconservative replication describes the mechanism by which DNA

DNA

Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...

 is replicated in all known cells.
This mechanism of replication was one of three models originally proposed

for DNA replication

DNA replication

DNA replication is a biological process that occurs in all living organisms and copies their DNA; it is the basis for biological inheritance. The process starts with one double-stranded DNA molecule and produces two identical copies of the molecule...

:

• Semiconservative replication would produce two copies that each contained one of the original strands and one new strand.
• Conservative replication would leave the two original template DNA
  DNA
  Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
  
   strands together in a double helix and would produce a copy composed of two new strands containing all of the new DNA base pairs.
• Dispersive replication would produce two copies of the DNA
  DNA
  Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
  
  , both containing distinct regions of DNA composed of either both original strands or both new strands.

The decipher

DECIPHER

DECIPHER is a web-based resource and database of array comparative genomic hybridization data from analysis of patient DNA. It documents submicroscopic chromosome abnormalities, including microdeletions and duplications, from over 6000 patients and maps them to the human genome using the Ensembl...

ing of the structure of DNA by Watson

James D. Watson

James Dewey Watson is an American molecular biologist, geneticist, and zoologist, best known as one of the co-discoverers of the structure of DNA in 1953 with Francis Crick...

 and Crick

Francis Crick

Francis Harry Compton Crick OM FRS was an English molecular biologist, biophysicist, and neuroscientist, and most noted for being one of two co-discoverers of the structure of the DNA molecule in 1953, together with James D. Watson...

 in 1953 suggested that each strand of the double helix would serve as a template for synthesis of a new strand. However, there was no way of knowing how the newly synthesized strands might combine with the template strands to form two double helical DNA

DNA

Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...

 molecules. The semiconservative model seemed most reasonable since it would allow each daughter strand to remain associated with its template strand. The semiconservative model was confirmed by the Meselson-Stahl experiment

Meselson-Stahl experiment

The Meselson–Stahl experiment was an experiment by Matthew Meselson and Franklin Stahl in 1958 which supported the hypothesis that DNA replication was semiconservative. Semiconservative replication means that when the double stranded DNA helix was replicated, each of the two double stranded DNA...

and other even more revealing experiments that allowed for autoradiographic visualization of the distribution of old and new strands within replicated chromosome

Chromosome

A chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...

s.

Biophysical evidence

The semi-conservative theory can be confirmed by making use of the fact that DNA is made up of nitrogen bases. Nitrogen

Nitrogen

Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...

 has an isotope N15 (N14 is the most common isotope) called heavy nitrogen. The experiment that confirms the predictions of the semi-conservative theory makes use of this isotope and runs as follows:

1. Bacterial (E coli) DNA is placed in a media containing heavy nitrogen(N15), which binds to the DNA, making it identifiable.
2. This DNA is then placed in a media with the presence of N14 and left to replicate only once. The new bases will contain nitrogen 14 while the originals will contain N15
3. The DNA is placed in test tubes containing caesium chloride (heavy compound) and centrifuged at 40,000 revolutions per minute.
4. The caesium chloride molecules sink to the bottom of the test tubes creating a density gradient. The DNA molecules will position at their corresponding level of density (taking into account that N15 is more dense than N14)
5. These test tubes are observed under ultraviolet rays. DNA appears as a fine layer in the test tubes at different heights according to their density.

According to the semi-conservative theory, after one replication of DNA, we should obtain 2 hybrid (part N14 part N15) molecules from each original strand of DNA. This would appear as a single line in the test tube. This result would be the same for the dispersive theory. On the other hand, according to the conservative theory, we should obtain one original DNA strand and a completely new one i.e. two fine lines in the test tube placed separately one from the other. Up to this point, either the semi-conservative or the dispersive theories could be truthful, as experimental evidence confirmed that only one line appeared after one replication. In order to conclude between those two, DNA had to be left to replicate again, still in a media containing N14.

In the dispersive theory, after 2 divisions we should obtain a single line, but further up in the test tube, as the DNA molecules become less dense as N14 becomes more abundant in the molecule According to the semi-conservative theory, 2 hybrid molecules and 2 fully N14 molecules should be produced, so two fine lines at different heights in the test tubes should be observed.
Experimental evidence confirmed that two lines were observed therefore offering compelling evidence for the semi-conservative theory.

Genetic evidence

An independent 'genetic' evidence for the semi-conservative theory was provided more recently by high throughput genomic sequencing of individual mutagenized bacteria. E. coli were treated with Ethyl methanesulfonate (EMS), known to induce G:C → A:T transitions due to generation of abnormal base O-6-ethylguanine, which is further misrecognized during DNA replication and paired with T instead of C. The sequenced DNA from individual colonies of EMS-mutagenized bacteria exhibited long stretches of solely G → A or C → T transitions, which in some cases were spanning entire bacterial genome. The elementary explanation of this observation is based on semi-conservative mechanism: one should expect the segregation between daughter strands into different cells after replication, which leads to each descendant cell having exclusively G → A or C → T conversions.