Key Terms
Conclusion
DNA replication is semi-conservative. Each daughter DNA has one parental strand and one newly synthesized strand.
Method
Let phage infect bacteria, then blended the mixture to detach phage coats; spun it in a centrifuge.
Memory
PURe As Gold (Purines = A and G)
Result
A sugar-phosphate backbone running in one direction. One end of the strand has a free 5' phosphate.
Anti-parallel
The 3' end of one strand faces the 5' end of the other. This is not optional; the geometry of base pairing requires it.
How it works
1. Denature DNA (separate strands by heating) 2.
Conservative
Both original strands stay together; new strands form a separate double helix. One old, one new - as two complete units.
Semi-conservative
Each original strand serves as a template; each daughter molecule has one old strand + one new strand.
Dispersive
Both daughter molecules contain a mix of old and new DNA segments interspersed throughout.
After synthesis
1. RNA primers removed by 3' exonuclease activity of DNA pol I 2.
Human genome
3 billion base pairs (haploid); 6 billion replicated during S phase. Multiple origins allow the whole genome to replicat
Problem
Linear chromosomes can't fully replicate their ends. When the RNA primer at the 5' end of the lagging strand is removed,
Solution
Telomerase
Telomeres
Repetitive non-coding sequences at chromosome ends. In humans: TTAGGG repeated 100-1,000 times.
Telomerase
Enzyme with a built-in RNA template. Attaches to the 3' end of the chromosome.