DNA Replication – MCDB 1A Lecture

Vocabulary flashcards covering the classic experiments and molecular machinery involved in DNA replication.

Transformation (Griffith)

Process where genetic material from heat-killed pathogenic (S) cells turned harmless (R) cells into pathogenic forms.

Griffith Experiment

1928 study showing that a ‘transforming principle’ (later shown to be DNA) can transfer pathogenicity between bacterial strains.

Hershey–Chase Experiment

1952 phage study using 35S-labeled protein and 32P-labeled DNA that proved DNA, not protein, enters bacteria to program viral reproduction.

Radioactive 35S

Isotope used to label proteins (sulfur in amino acids); remained in the supernatant during Hershey–Chase, indicating proteins stayed outside bacteria.

Radioactive 32P

Isotope used to label DNA (phosphate backbone); found in bacterial pellet in Hershey–Chase, showing DNA entered the cells.

Meselson–Stahl Experiment

Density-gradient centrifugation of 15N- and 14N-labeled DNA demonstrating that DNA replication is semiconservative.

Semiconservative Replication

Model in which each daughter DNA molecule consists of one parental strand and one newly synthesized strand.

Conservative Replication

Rejected model proposing that the parental double helix remains intact and an entirely new double helix is made.

Dispersive Replication

Rejected model suggesting parental DNA is interspersed in both strands of daughter molecules.

Origin of Replication

Specific DNA sequence where replication begins; initiator proteins bind and open a replication bubble.

Replication Bubble

Locally unwound region of DNA where synthesis proceeds in both directions from an origin.

Replication Fork

Y-shaped end of a replication bubble where parental strands are unwound and new DNA is synthesized.

DNA Helicase

Enzyme that unwinds the DNA double helix at replication forks.

Topoisomerase

Enzyme that relieves overwinding ahead of replication forks by cutting, swiveling, and rejoining DNA strands.

Single-Strand Binding Proteins (SSB)

Proteins that bind and stabilize separated DNA strands, preventing reannealing.

Primase

RNA polymerase that synthesizes short RNA primers to provide a 3′ OH for DNA polymerases.

Primer

Short RNA (or DNA) sequence with a free 3′ OH to which DNA polymerase can add nucleotides.

DNA Polymerase III

Main bacterial replicative polymerase; reads template 3′→5′ and builds new strand 5′→3′ at ~500 nt/s.

DNA Polymerase I

Enzyme that removes RNA primers and fills the gaps with DNA nucleotides.

Sliding Clamp

Ring-shaped protein that holds DNA polymerase III onto the DNA for high processivity.

Leading Strand

DNA strand synthesized continuously toward the replication fork in the 5′→3′ direction.

Lagging Strand

DNA strand synthesized discontinuously away from the fork as Okazaki fragments.

Okazaki Fragment

Short DNA fragment (~1000–2000 nt in bacteria) produced during lagging strand synthesis.

DNA Ligase

Enzyme that seals nicks by forming phosphodiester bonds, joining Okazaki fragments into a continuous strand.

Telomere

Repeating TTAGGG DNA sequence at eukaryotic chromosome ends; shortens slightly each replication cycle.

5′→3′ Directionality

Intrinsic polarity of DNA; polymerases add nucleotides only to the 3′ OH end, synthesizing 5′→3′.

Phosphodiester Bond

Covalent linkage between the 5′ phosphate of one nucleotide and the 3′ hydroxyl of the next.