DNA Structure and Function

Griffith's transforming principle

Material from heat-killed bacteria could transform live bacteria.

Avery, MacLeod, and McCarty

Demonstrated that nucleic acids were the transforming informational component.

Hershey-Chase experiment

Radioactive phosphorus showed that phages inject DNA, not protein, to direct reproduction.

Chargaff's rules

The amount of adenine equals thymine; cytosine equals guanine.

Franklin's X-ray diffraction data

Helped Watson and Crick determine DNA's double-helical structure.

Complementary base pairing

Adenine pairs with thymine; cytosine pairs with guanine through hydrogen bonds.

Meselson and Stahl's experiment

Heavy and light nitrogen experiments in E. coli demonstrated semiconservative replication.

Semiconservative replication

Each parental strand serves as a template for a new complementary strand.

Antiparallel DNA strands

One strand runs 5′ to 3′; the other runs 3′ to 5′.

DNA synthesis direction

DNA polymerase adds nucleotides to the free 3′ end, synthesizing 5′ to 3′.

Helicase

Separates DNA strands at the origin of replication.

Topoisomerase

Relieves additional coiling and overwinding ahead of the replication fork.

Single-strand binding proteins

Stabilize separated DNA strands and keep them open.

Primase

Synthesizes a short RNA primer that provides DNA polymerase with a starting point.

RNA primer length

A primer is typically 5-10 nucleotides long.

DNA polymerase III

Synthesizes the new daughter DNA strand.

DNA polymerase I

Replaces RNA primers with DNA.

DNA ligase

Seals gaps between DNA fragments by forming phosphodiester bonds.

Leading strand synthesis

Proceeds continuously toward the replication fork.

Lagging strand synthesis

Proceeds discontinuously away from the fork in Okazaki fragments.

DNA polymerase proofreading

Detects and corrects mismatched bases during replication.

Mismatch repair

Proteins detect incorrect bases after replication, remove them, and resynthesize the sequence.

Thymine dimers

UV damage can cause adjacent thymines to bond covalently.

Nucleotide excision repair

A nuclease removes damaged DNA; polymerase fills the gap and ligase seals it.

Mutation

A permanent change in nucleotide sequence that can result from uncorrected errors.

Point mutation types

Silent, missense, and nonsense mutations.

Frameshift mutations

Insertions or deletions that alter the reading frame.

Chromosome-level alterations

Larger mutations affecting chromosomes.

Eukaryotic DNA packaging

Genetic information is organized into chromatin and packaged with histone proteins into chromosomes.

DNA replication and inheritance

Accurate copying transmits genetic information across cell divisions and generations.