Molecular Basis of Inheritance
DNA Replication
Before a cell divides, it copies all of its DNA so each new cell gets a full set. The two strands unzip, and each one acts as a template to build a matching new strand.
DNA is a double helix: two strands held together by base pairs (A with T, G with C). To copy it, the strands separate. An enzyme, DNA polymerase, reads each old strand and adds the matching bases to build a new partner strand.
The result is two double helices, each made of one old strand and one new strand. This is called semiconservative replication, because half of every new molecule is conserved from the original.
Where students slip up
Replication is semiconservative, not conservative. Each new DNA keeps one original strand and one fresh strand. Students often picture the old DNA staying whole while a brand new copy is made from scratch. That is wrong: the old strands split up, one into each daughter molecule.
Hold onto this
Two strands in, two hybrids out. Each keeps one old strand.
Why it helps in the exam: the Meselson-Stahl experiment proved semiconservative replication, and the enzymes (helicase, polymerase, ligase) are repeat questions.
Where this connects
Copying keeps A across from T and G across from C, the same base-pairing rule the cell uses to read a gene into RNA. Learn the pairing once and both copying and transcription fall out of it.
How the same rule reads a gene →Quick check
DNA replication is called semiconservative because each new molecule has:
- ATwo brand new strands
- BOne old strand and one new strand
- CTwo old strands
- DOnly fragments of the old strand