Bio Midterm 2: DNA Replication & DNA packaging

DNA replication

The process of copying DNA before cell division.

Semiconservative replication

Each daughter DNA molecule contains one original strand and one newly synthesized strand.

Meselson-Stahl experiment (1958)

Used heavy and light nitrogen isotopes to demonstrate semiconservative DNA replication.

Origin of replication

The DNA location where replication begins.

Replication bubble

An opened region of DNA with two replication forks moving in opposite directions.

Replication fork

The Y-shaped region where DNA strands separate and new DNA is synthesized.

Helicase

Unwinds DNA by separating its two strands.

Topoisomerase

Relieves twisting strain ahead of the replication fork.

Primase

Synthesizes short RNA primers to start DNA synthesis.

DNA polymerase

Adds DNA nucleotides to a growing strand in the 5′ to 3′ direction.

DNA ligase

Seals breaks in the sugar-phosphate backbone, joining DNA fragments.

Single-strand binding proteins

Keep separated DNA strands from pairing back together.

DNA primer

A short nucleic acid sequence providing a starting point for DNA polymerase.

Why does DNA synthesis occur 5′ to 3′?

DNA polymerase adds nucleotides only to the free 3′ end of the growing strand.

Leading strand

Synthesized continuously toward the advancing replication fork.

Lagging strand

Synthesized discontinuously away from the advancing fork in short DNA fragments.

Okazaki fragments

Short DNA segments synthesized on the lagging strand.

Why is the lagging strand discontinuous?

DNA strands are antiparallel, and DNA polymerase can synthesize only 5′ to 3′.

Circular vs. linear DNA replication

Circular bacterial chromosomes often have one main origin; linear eukaryotic chromosomes have many origins.

Replication fidelity

The accuracy with which DNA is copied.

DNA polymerase proofreading

Removes incorrectly added nucleotides and replaces them with correct ones.

Mismatch repair

Repair system that corrects mismatched bases remaining after DNA replication.

Why is DNA replication accurate?

Correct nucleotide selection, polymerase proofreading, and mismatch repair reduce errors.

Telomeres

Repetitive DNA sequences that protect the ends of linear chromosomes.

End-replication problem

The final RNA primer on a lagging strand leaves a gap that cannot be filled by ordinary DNA polymerase.

Telomerase

Enzyme that extends chromosome ends using its own RNA template.

Why do telomeres shorten?

After removal of the final lagging-strand primer, DNA polymerase cannot fill the terminal gap.

Where is telomerase active?

It is active in certain cells, including many germ and stem cells, and is often reactivated in cancer cells.

Histones

Proteins around which eukaryotic DNA wraps.

Nucleosome

DNA wrapped around a core of histone proteins.

Chromatin

DNA and associated proteins that package and organize eukaryotic chromosomes.

Why is DNA packaging important?

It allows DNA to fit in cells, protects it, and helps regulate access to genes.