Question

What does it mean when it is said that DNA replication is ‘semiconservative’?

Answer 1

During the process of cell division, the genetic material is divided by means of a process called “DNA replication.” Replication of DNA (deoxyribonucleic acid) results in the formation of two new double stranded DNA molecules. The new DNA helix now contains one old strand (parental) and one new strand hence, this process is named as “semiconservative replication.” DNA polymerase II is an enzyme that essentially enables DNA replication in semiconservative manner.

During DNA semiconservative replication, the usual antiparallel nature of the deoxyribonucleic acid in combination with DNA polymerase enzyme and the novel synthesis may finally need a replication fork with asymmetrical nature.

During the process of cell division, the genetic material is divided by means of a process called “DNA replication.” Replication of DNA (deoxyribonucleic acid) results in the formation of two new double stranded DNA molecules. The new DNA helix now contains one old strand (parental) and one new strand hence, this process is named as “semiconservative replication.”

DNA replication in the lagging strand: The lagging strand receives several RNA primers. Replication process in the laggingstrand is more complicated due to its orientation (its direction of replication is opposite to that of growing replication fork).

The DNA template of lagging strand is read by a primase, which synthesise complementary RNA primer. The primed segments are extended by DNA polymerase forming the Okazaki fragments, now the RNA primers are removed. The synthesis of lagging stand occurs using short DNA segments called Okazaki fragments each consisting of 100 to 200 nucleotides in humans (1000 to 2000 nucleotides in bacteria). The replication occurs in 5 to 3 direction, but away from the replication fork.  DNA ligase connects the Okazaki fragments by catalysing the formation of phosphodiester bond between the Okazaki fragments of complementary strand.

Experiment:

The experiments done by Meselson and Stahl’s supported the model of semiconservative replication of DNA. In their experiment, they obtained heavy DNA and light DNA by growing Escherichia coli (E. coli) bacteria with a radioactive nitrogen isotope ¹⁵N, and the more common isotope of nitrogen ¹⁴N respectively.

The results showed that, the first generation bacteria grow in ¹⁴N medium yielded a single band of DNA and the density of this band is in between that of ¹⁵N DNA and ¹⁴N DNA, indicating that one strand of each duplex is contained ¹⁴N and another is containing ¹⁵N. After two generations in the same ¹⁴N medium, the results showed the occurrence of two bands. One of the bands is with intermediate density, (in which one strand contains ¹⁴N and another is containing ¹⁵N) and another with lower density, (in which both the strands contained ¹⁴N).