DNA Structure, Function, and Replication Mechanisms

Comprehensive vocabulary flashcards covering the discovery of genetic material, structural features of DNA, packaging into chromatin, semiconservative replication experiments, and the enzymes of prokaryotic and eukaryotic replication.

Friedrich Miescher

The Swiss physician and biologist who first identified and isolated nucleic acids from the nuclei of human white blood cells in the 1860s.

Transforming Principle

The hereditary material transferred from heat-killed pathogenic $S$ strain cells to living nonpathogenic $R$ strain cells that permanently transforms their phenotype, first demonstrated by Frederick Griffith in 1928.

Smooth ($S$) Strain of Streptococcus pneumoniae

A virulent, pathogenic bacterial strain characterized by smooth colony morphology due to an outer polysaccharide capsule that protects the bacteria from host immune destruction, causing fatal pneumonia in mice.

Rough ($R$) Strain of Streptococcus pneumoniae

A nonpathogenic bacterial strain that lacks a protective polysaccharide capsule due to an inactive mutated gene, forming colonies with a rough appearance that are readily destroyed by host immune cells.

Avery, MacLeod, and McCarty Experiment

An experimental investigation that systematically tested the macromolecular components of heat-killed Streptococcus pneumoniae to conclusively demonstrate that DNA, rather than protein or RNA, is the transforming genetic material in bacteria.

Bacteriophage

A type of virus that specifically infects bacterial cells, composed solely of an outer protective protein coat and inner genetic material consisting of nucleic acid.

Hershey-Chase Blender Experiment

A 1952 experiment using radioactive $^{35}\text{S}$ to label viral protein coats and $^{32}\text{P}$ to label viral DNA, demonstrating that only $^{32}\text{P}$ enters infected Escherichia coli cells, proving that DNA is the genetic material in viruses.

Chargaff's Rules

The biochemical principle discovered by Erwin Chargaff stating that in double-stranded DNA across all species, the amount of adenine equals thymine ($A = T$) and the amount of cytosine equals guanine ($C = G$).

Purines

Nitrogenous bases characterized by a double-ring structure consisting of a six-membered ring fused to a five-membered ring; specifically adenine ($A$) and guanine ($G$).

Pyrimidines

Nitrogenous bases characterized by a single six-membered ring structure; specifically cytosine ($C$), thymine ($T$) in DNA, and uracil ($U$) in RNA.

Phosphodiester Bond

A strong covalent bond consisting of two ester linkages that connects adjacent nucleotides by linking the $5'$ phosphate group of one nucleotide to the $3'$ hydroxyl group on the pentose sugar of another.

Antiparallel

The orientation of the two complementary strands in a double-stranded DNA helix, running side-by-side in opposite directions ($5' \rightarrow 3'$ on one strand and $3' \rightarrow 5'$ on the other).

Major and Minor Grooves

The asymmetrical wider and narrower spiral depressions along the exterior surface of the DNA double helix that serve as essential binding sites for DNA-binding proteins during replication and transcription.

Nucleoid

The central, non-membrane-bound cytoplasmic region within a prokaryotic cell where the single circular double-stranded DNA chromosome resides.

Histone

A basic protein octamer composed of eight subunits (arranged in two layers of four) around which eukaryotic DNA winds nearly two complete times during chromosome packaging.

Nucleosome

The fundamental structural repeating unit of eukaryotic chromatin, composed of a segment of double-stranded DNA wrapped approximately twice around a core octamer of histone proteins.

$30\text{-nm}$ Fiber

A higher-order chromatin structure formed when a chain of nucleosomes coils into a dense, telephone-cord-like cylinder approximately $30\text{ nm}$ in diameter via linker DNA.

Scaffold Proteins

Non-histone framework proteins in the eukaryotic cell nucleus that anchor looping $30\text{-nm}$ fibers into $300\text{-nm}$ chromatin loops to achieve full chromosome compaction.

Semiconservative Replication

The confirmed model of DNA replication in which parental double-stranded DNA unwinds and each separated strand acts as a template, producing two daughter duplexes that each contain one original parental strand and one newly synthesized strand.

Meselson-Stahl Experiment

The 1958 density-gradient centrifugation experiment using Escherichia coli grown sequentially in heavy nitrogen ($^{15}\text{N}$) and light nitrogen ($^{14}\text{N}$) that proved DNA replicates in a semiconservative manner.

Conservative Replication Model

A disproven replication hypothesis proposing that the original double-stranded parental DNA remains entirely intact together as a single molecule while directing the synthesis of a completely new double-stranded daughter DNA molecule.

Dispersive Replication Model

A disproven replication hypothesis suggesting that the parental DNA molecule fragments into segments during synthesis, resulting in daughter duplexes composed of interspersed patches of old and newly synthesized DNA.

Origin of Replication

A specific, $A\text{-}T$-rich nucleotide sequence along the DNA molecule where replication proteins bind and initiate strand separation; prokaryotes possess a single origin, whereas eukaryotes possess thousands per chromosome.

Helicase

An enzyme that utilizes ATP to break the hydrogen bonds between complementary base pairs, unwinding parental double-stranded DNA into single strands and establishing bidirectional replication forks.

Single-Strand Binding Proteins (SSBs)

Proteins that coat and stabilize exposed single-stranded template DNA behind the helicase, preventing the hydrophobic bases from prematurely re-annealing into a double helix.

Topoisomerase

An enzyme operating ahead of the advancing replication fork that relieves supercoiling tension by making transient cuts in the DNA backbone, allowing overwound strands to rotate and relax before resealing them.

RNA Primase

A specialized RNA polymerase that synthesizes a short RNA primer ($5\text{ to }10$ nucleotides in length) in the $5' \rightarrow 3'$ direction, supplying the necessary free $3'\text{-OH}$ group required for DNA polymerase activity.

DNA Polymerase III

The primary bacterial replication enzyme that continuously or discontinuously synthesizes new DNA strands in the $5' \rightarrow 3'$ direction by adding deoxynucleotides complementary to the template strand starting from an RNA primer.

Leading Strand

The continuously synthesized daughter strand of DNA that elongates smoothly in the $5' \rightarrow 3'$ direction toward the opening replication fork.

Lagging Strand

The discontinuously synthesized daughter strand of DNA that elongates in the $5' \rightarrow 3'$ direction away from the replication fork as a series of short segments.

Okazaki Fragments

Short, discontinuous segments of newly synthesized DNA along the lagging strand, each initiated by a separate RNA primer.

DNA Polymerase I

A bacterial enzyme with exonuclease activity that removes RNA primers from both the leading and lagging strands and replaces them with complementary DNA nucleotides, leaving a single-bond nick.

DNA Ligase

The enzyme that acts as the molecular 'glue gun' of the cell by catalyzing the formation of a phosphodiester bond to seal single-stranded nicks between adjacent Okazaki fragments.

DNA Polymerase II

A prokaryotic DNA polymerase that functions primarily as a repair enzyme rather than carrying out the bulk synthesis of chromosomal DNA.

Telomeres

Non-coding repetitive nucleotide sequences ($\text{TTAGGG}$ repeated up to $1{,}000$ times) situated at the ends of linear eukaryotic chromosomes that safeguard genomic integrity against progressive shortening during replication.

Telomerase

A ribonucleoprotein complex containing an internal RNA template and a catalytic protein component that elongates the ends of linear chromosomes in germ cells and adult stem cells, preventing replicative senescence.

Dolly the Sheep

The first large mammal successfully cloned from an adult somatic cell, confirming that differentiated animal nuclei retain the full genomic information necessary to regenerate a complete organism.

Rosalind Franklin

The English biophysicist whose X-ray diffraction photograph (Photo 51) provided critical measurements revealing the helical structure, dimensions, and regular repeating turns of the DNA double helix.