Lesson 3 of 6 / DNA and gene expression
Copy each strand before division
Why does each daughter DNA contain an old strand?
In this lesson: Describe semi-conservative DNA replication, including complementary synthesis and discontinuous joining.
About 6 min
The key ideaThe two parental strands separate and each templates a complementary new strand. Each resulting DNA molecule contains one old strand and one new strand.
Explore the idea
Track the old strands
The two parental strands separate. Neither complete old double helix remains together in just one daughter molecule.
This model tracks strand origin, not fork movement. The connecting lines represent paired bases, not additional DNA backbones.
Explanation
Before cell division, DNA must be copied so that daughter cells can receive genetic information. Helicase separates the strands by disrupting hydrogen bonds between paired bases. Each exposed parental strand then serves as a template.
Free DNA nucleotides align through complementary base pairing. DNA polymerase adds nucleotides to the 3' end of a growing strand, forming phosphodiester bonds; synthesis therefore proceeds 5' to 3'. An RNA primer provides an initial end from which DNA polymerase can extend.
Because the templates are antiparallel, synthesis near a moving replication fork differs on the two sides. The leading strand can be synthesised continuously towards the fork; the lagging strand is made as short fragments in the opposite overall progression. Both new strands are nevertheless synthesised 5' to 3'.
RNA primers are removed and replaced with DNA, and DNA ligase seals remaining breaks between adjacent fragments by completing backbone links. The resulting two DNA molecules each contain one parental strand and one newly synthesised complementary strand: replication is semi-conservative.
Complementary pairing supports accurate copying, though errors can occur. DNA replication copies DNA into DNA; it is different from transcription of a particular gene into RNA.
Step by step
- 1
Separate templates
Expose bases by separating the parental strands.
- 2
Synthesize complementary DNA
Polymerase extends new strands 5' to 3', using a primer.
- 3
Complete continuous backbones
Replace primers and ligate fragments so each daughter molecule has two complete strands.
Worked example
Track parental DNA
After one replication of a DNA molecule whose two strands are labelled, where are the labelled strands?
One way to explain it
Each of the two daughter molecules contains one labelled parental strand paired with one unlabelled new strand. The original double helix is not retained intact as only one daughter molecule.
Why this answer works
- Separate the two parental templates.
- Pair each with a newly made complement.
Is this true? "The lagging strand is made by DNA polymerase in the 3'-to-5' direction."
DNA polymerase synthesises both new strands 5' to 3'. Discontinuous fragment synthesis accommodates the antiparallel template arrangement.