DNA Replication Flashcards

Discovering DNA Structure

• Erwin Chargaff:

  • Base composition varies between species.

  • Number of Adenine (A) bases equals Thymine (T) bases, and Cytosine (C) equals Guanine (G).

• Maurice Wilkins & Rosalind Franklin: X-ray diffraction.

• James Watson & Francis Crick: Determined DNA structure to be a double helix in 1953.

DNA Replication

• DNA replication copies DNA molecules to produce two identical copies.

• Occurs before cell division (during interphase).

• Daughter cells each receive a copy of the replicated DNA.

Semiconservative Model

• Each parent DNA strand serves as a template for a new strand.

• Resulting DNA molecules are hybrids (original + new).

Steps of DNA Replication

1. Parent DNA molecule has two template strands.

2. Template strands separate.

3. New strands are synthesized using parent DNA strands as templates.

Ingredients for DNA Replication

• Parent DNA molecule: provides template strands.

• Proteins/enzymes: help in the synthesis of new DNA strands.

• Deoxyribonucleoside triphosphates (dNTPs): nucleotide building blocks.

  • Examples: Deoxythymidine triphosphate (dTTP), Deoxyadenosine triphosphate (dATP).

Origin of Replication (ori)

• DNA replication begins at the origin of replication (ori) site.

• DNA strands separate at the ori site.

• Prokaryotes:

  • Circular genomes with one origin of replication.

  • Typically have one chromosome.

• Eukaryotes:

  • Multiple origins of replication.

  • Linear chromosomes that are much longer.

  • Typically have multiple chromosomes.

Nucleotide Joining

• Condensation reactions catalyzed by DNA Polymerase III.

• New strand synthesis proceeds in the 5’ to 3’ direction.

Initiation of Replication

• Enzymes and proteins assemble at replication forks.

• Helicase: Separates the two strands (unwinds the DNA helix).

• Single-strand DNA binding proteins: hold the two parent strands apart.

• DNA Primase: synthesizes RNA primer sequence.

• Topoisomerase: cuts and swivels DNA to relieve coiling tension.

Replication Fork

• Y-shaped region where DNA is unwound.

• DNA replication proceeds in two directions.

Leading vs. Lagging Strand

• DNA chains run in opposite directions (antiparallel).

  • Strand 1: 5’ to 3’

  • Strand 2: 3’ to 5’

• DNA synthesis only occurs in the 5’ to 3’ direction.

• Leading strand: elongates new DNA strand towards the replication fork.

  • DNA polymerase III synthesizes the new DNA strand.

• Lagging strand: elongates away from the replication fork.

  • Formation of Okazaki fragments (100 – 200 nucleotides long in eukaryotes).

Lagging Strand Synthesis (continued)

• DNA polymerase I: removes RNA primers and replaces them with new DNA.

• DNA ligase: forms covalent bonds between DNA fragments to make the new DNA strand continuous.

Key Enzymes

• Helicase

• Single-strand DNA binding proteins

• Topoisomerase

• Primase

• DNA polymerase III

• DNA polymerase I

• DNA ligase