Unit 1: DNA Structure Flashcards
(41 cards)
1952 Alfred Hershey and Martha Chase
Labelled T2 bacteriophages’ DA with 32P and protein with 35S. Showed that DNA was passed on to virus progeny.
Allele
One of several alternative forms of a gene occupying a given locus on a chromosome
Chromosome
A discrete unit of the genome carrying many genes. Each consists of a very long molecule of duplex DNA and an approximately equal mass of proteins (in eukaryotes). It is visible as a morphological entity only during cell division.
Gene
a sequence of DNA that encodes an RNA, and in protein-coding, or structural, genes, the RNA in turn encodes a polypeptide
1928 Griffith
Gave rise to idea that genes have roots in DNA.
Did experiments with mice, “S” bacteria and “R” bacteria. Heat inactivated “S” bacteria and live “R” bacteria still killed mice because of the “transforming principle” (e.g., genes that made “S” bacteria virulent transformed into “R” bacteria making it virulent).
linkage
The tendency of genes to be inherited together as a result of their location on the same chromosome; measured by percent recombination between loci
Nucleoside
Purine or pyrimidine base linked to the 1’ carbon of a pentose sugar.
Nucleotide
A nucleoside linked to a phosphate group on either the 5’ or the 3’ carbon of the (deoxy)ribose.
Purine
A double-ringed nitrogenous base, such as adenine or guanine
Pyrimidine
A single-ringed nitrogenous base, such as cytosine, thymine or uracil.
The ultimate definition of a genome
The sequence of DNA of each chromosome
Transfection
In eukaryotic cells, it is the acquisition of new genetic markers by incorporation of added DNA.
Analogous to bacterial transformation.
Transforming principle
DNA that is taken up by a bacterium and whose expression then changes the properties of the recipient cell.
polynucleotide
a long chain of nucleotides
DNA supercoiling
Double helix winds around itself changing overall conformation, or topology, of the DNA molecule in space.
Occurs only in “closed” DNA with no free ends (circular DNA or linear DNA with anchored ends).
Causes tension in DNA. Non supercoiled DNA is said to be in the “relaxed” state.
The reactions to control supercoiling in the cell are performed by topoisomerase enzymes.
Positive Supercoiling
The right-handed, double helical form of DNA. Both strands of the double helix coil together in the same direction as the coiling of the strands.
Overwinds the DNA, fewer base pairs per turn.
Results in an increase in the linking number (+ΔL)
Negative supercoiling
The left-handed, double-helical form of DNA. Creates tension in the DNA that is relieved by the unwinding of the double helix. The result is the generation of a region in which the two strands of DNA have separated.
Underwinds the DNA, more base pairs per turn.
Results in a decrease in the linking number (−ΔL)
Topological manipulation
Topological manipulation of DNA is a central aspect of all of its functional activities (e.g., recombination, replication, and transcription) as well as of the organization of its higher order structure. All synthetic activities involving double-stranded DNA require the strands to separate.
Linking number (L)
In a closed molecule of DNA, the number of times one strand crosses over another in space.
Made up of a writhing number (W) and twisting number (T)
The critical feature about the use of the linking number is that this parameter is an invariant property of any individual closed DNA molecule. The linking number cannot be changed by any deformation short of one that involves the breaking and rejoining of strands. A circular molecule with a particular linking number can express the number in terms of different combinations of T and W, but it cannot change their sum so long as the strands are unbroken.
Topological isomers
Molecules of DNA that are the same except for their linking numbers (same sequence, different degrees of supercoiling).
Molecules with the same chemical formula but different bond connectivities, thus resulting in different topological structures.
Writhing number (W)
In DNA, the turning of the axis of the duplex in space.
Corresponds to the intuitive concept of supercoiling but does not have exactly the same quantitative definition of measurement.
For a relaxed molecule, W = 0, and the linking number equals the twist.
One of two components that make up the linking number.
Twisting number (T)
Rotation of one strand about the other.
Represents the total number of turns of the duplex and is determined by the number of base pairs per turn.
For a relaxed closed circular DNA lying flat in a plane, T is the total number of base pairs divided by the number of base pairs per turn.
Change in the Linking number equation
ΔL = ΔW + ΔT
The equation states that any change in the total number of revolutions of one DNA strand about the other can be expressed as the sum of the changes of the coiling of the duplex axis in space (ΔW) and changes in the helical repeat of the double helix itself (ΔT). In the absence of protein binding or other constraints, the twist of DNA does not tend to vary—in other words, the 10.5 base pairs per turn (bp/turn) helical repeat is a very stable conformation for DNA in solution. Thus, any ΔL is mostly likely to be expressed by a change in W; that is, by a change in supercoiling.
Specific linking difference
σ = ΔL/L0
L0 is the linking number when the DNA is relaxed.
If all of the change in the linking number is due to change in W (that is, ΔT = 0), the specific linking difference equals the supercoiling density.
In effect, σ, as defined in terms of ΔL/L0, can be assumed to correspond to supercoiling density so long as the structure of the double helix itself remains constant.
