Lesson 3 of 6 / DNA and gene expression
Copy each strand before division
Why does each daughter DNA contain an old strand?
I can describe semi-conservative DNA replication, including complementary synthesis and discontinuous joining.
I can describe DNA replication.Syllabus
Syllabus 8876, Core 2(b). Describe DNA replication.
About 6 min
Make a guess. You are not marked.
After DNA replicates, does one daughter molecule keep both old strands while the other gets two new ones?
Show the answer
No. Each has one old and one new strand.
Replication is semi-conservative: each parental strand templates a new partner.
The key idea
The 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 unwinds the double helix and unzips the two strands by breaking the hydrogen bonds between complementary bases. Each exposed parental strand then acts 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.
DNA is a stable store of information for three reasons. The sugar-phosphate backbone of each strand is held together by strong covalent phosphodiester bonds. The two strands are held together by many hydrogen bonds between complementary base pairs: each hydrogen bond is weak, but together they hold the strands firmly. The bases sit on the inside of the double helix, where the backbones shield them from chemical attack.
A base analogue is a molecule with a similar shape to one of the normal bases. If DNA polymerase adds a base analogue instead of the normal nucleotide, it cannot pair correctly with the template, or the next nucleotide cannot be joined to it, so elongation of the new strand stops. Some anticancer drugs are base analogues: they stop replication in rapidly dividing cells.
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.
Watch out for this
A student says: "The lagging strand is made by DNA polymerase in the 3'-to-5' direction." What is wrong with this?
Show the answer
DNA polymerase synthesises both new strands 5' to 3'. Discontinuous fragment synthesis accommodates the antiparallel template arrangement.
Check your understanding
What does DNA ligase do during replication?
Show the answer
Seal breaks in the sugar-phosphate backbone between fragments.
This completes a continuous strand after fragment synthesis and primer replacement.
Exam-style questions
Check your understanding
Which enzyme is NOT needed to replicate DNA?
Show the answer
RNA polymerase
RNA polymerase transcribes genes. Replication uses helicase, DNA polymerase and DNA ligase (with primase for primers).
Quick recall
Why does each daughter DNA contain an old strand?