DNA Structure, Replication, and Genetic Material ch 6

Vocabulary practice flashcards covering the experimental discovery of DNA as the genetic material and the chemical components and physical structure of the DNA double helix.

Smooth (S) strain

A virulent strain of Streptococcus pneumoniae that produces smooth colonies on growth media and causes lethal infections in mice.

Rough (R) strain

A nonvirulent mutant strain of Streptococcus pneumoniae that forms rough colonies on growth media and does not cause lethal infections when injected into mice.

Bacterial Transformation

The phenomenon discovered by Frederick Griffith (1928) in which genetic material from heat-killed virulent S bacteria converts living, nonvirulent R cells into lethal S-type bacteria.

Transforming Principle

The chemical substance isolated and purified from heat-killed S bacteria by Avery, MacLeod, and McCarty (1944) that contains the genetic material necessary to permanently transform R cells into S cells.

Avery, MacLeod, and McCarty Experiment

A 1944 experiment demonstrating that treating heat-killed S cell extract with protease, RNase, or ultracentrifugation preserved transforming activity, whereas treatment with DNase abolished transformation, proving DNA is the transforming substance.

DNase (Deoxyribonuclease)

An enzyme that destroys DNA, which was the only enzyme treatment found to abolish the transforming ability of heat-killed S cell extracts.

Bacteriophage (Phage)

A type of virus that infects bacterial host cells and is composed of roughly equal parts protein and DNA.

Phage Ghosts

The empty outer viral protein coats that remain outside the host bacterial cell wall after the injection of phage DNA.

Hershey-Chase Experiment (1952)

A blender experiment by Alfred Hershey and Martha Chase that used radioactive isotopes to track T2 bacteriophage components during infection, proving that DNA, and not protein, enters the host cell to direct progeny formation.

Phosphorus-32 ($^{32}\text{P}$)

A radioactive isotope used by Hershey and Chase to selectively label bacteriophage DNA, which was recovered inside host bacterial cells and transmitted to viral progeny.

Sulfur-35 ($^{35}\text{S}$)

A radioactive isotope used by Hershey and Chase to selectively label bacteriophage proteins, which remained in the supernatant with empty phage ghosts rather than entering the host cells.

Chargaff's Rules

The biochemical observation that the nucleotide composition of DNA varies across species, but within any organism, the ratio of adenine to thymine ($\text{A}:\text{T}$) and guanine to cytosine ($\text{G}:\text{C}$) is always very close to unity.

Pauling's Triple Helix Model

An incorrect 1952 structural proposal by Linus Pauling and Robert Corey featuring three intertwined chains with the negatively charged phosphates packed in the center and the bases projecting outward.

Photograph 51

The renowned X-ray diffraction image of B-DNA solved by Rosalind Franklin and Raymond Gosling in 1952 that revealed an X-shaped pattern characteristic of a double helix.

Watson and Crick Model (1953)

The correct molecular model of DNA published in Nature, proposing a right-handed double helix with antiparallel sugar-phosphate backbones on the outside and complementary purine-pyrimidine base pairs on the inside.

Deoxyribose

A five-carbon sugar in DNA with an oxygen atom in the ring structure, hydrogens at the $2'$ carbon, and hydroxyl groups attached to the $1'$, $3'$, and $5'$ carbons.

Phosphate Group

A functional group in nucleic acids consisting of a central phosphorus atom double-bonded to one oxygen and bound to three hydroxyl or negatively charged oxygen atoms.

Purines

Double-ring nitrogenous bases found in DNA, comprising adenine (A) and guanine (G).

Pyrimidines

Single-ring nitrogenous bases found in DNA, comprising thymine (T) and cytosine (C).

Nucleoside

A molecular subunit composed solely of a nitrogenous base covalently linked to the $1'$ carbon of a deoxyribose sugar.

Nucleotide

The monomer building block of DNA consisting of a nucleoside bound to a phosphate group at the $5'$ carbon of the deoxyribose sugar.

Phosphodiester Bond

The covalent linkage connecting the $3'$ carbon of one deoxyribose sugar to the $5'$ carbon of the next via a phosphate group.

$5'$-to-$3'$ Polarity

The directional orientation of a DNA strand, defined by a terminal phosphate group at the $5'$ end and an available hydroxyl group at the $3'$ end.

Complementary Base Pairing

The specific pairing across a DNA double helix where a purine bonds with a pyrimidine, specifically adenine with thymine and guanine with cytosine.

Hydrogen Bonds in DNA

Weak electrostatic bonds that hold complementary strands together, specifically $2$ bonds between adenine and thymine and $3$ bonds between guanine and cytosine.

Antiparallel Strands

The structural arrangement of the DNA double helix where one strand runs in the $5' \rightarrow 3'$ direction and the complementary strand runs in the opposite $3' \rightarrow 5'$ direction.

DNA Helical Dimensions

The structural measurements of the B-DNA double helix: a diameter of $20\,\text{Å}$, a distance of $3.4\,\text{Å}$ between adjacent base pairs, and a repeat length of $34\,\text{Å}$ per complete helical turn ($10$ base pairs).

Major and Minor Grooves

The unequal structural indentations produced along the cylindrical surface of the double helix due to the vertical displacement between the two spiraling sugar-phosphate backbones.