Genetic material must contain complex information, be replicated accurately, code for the phenotype, and have the capacity to vary.
Evidence that DNA is the source of genetic information came from the finding by Avery, MacLeod, and McCarty that transformation depended on DNA and from the demonstration by Hershey and Chase that viral DNA is passed on to progeny phages.
James Watson and Francis Crick, using data provided by Rosalind Franklin and Maurice Wilkins, proposed a model for the three-
A DNA nucleotide consists of a deoxyribose sugar, a phosphate group, and a nitrogenous base. An RNA nucleotide consists of a ribose sugar, a phosphate group, and a nitrogenous base.
The bases of a DNA nucleotide are of two types: purines (adenine and guanine) and pyrimidines (cytosine and thymine). RNA contains the pyrimidine uracil instead of thymine.
Nucleotides are joined together by phosphodiester linkages to form a polynucleotide strand. Each polynucleotide strand has a free phosphate group at its 5′ end and a free hydroxyl group at its 3′ end.
DNA consists of two nucleotide strands that wind around each other to form a double helix. The sugars and phosphates lie on the outside of the helix, and the bases are stacked in the interior. The two strands are joined together by hydrogen bonding between bases in each strand. The two strands are antiparallel and complementary.
DNA molecules can form a number of different secondary structures, depending on the conditions in which the DNA is placed and on its base sequence.
The structure of DNA has several important genetic implications. Genetic information resides in the base sequence of DNA, which ultimately specifies the amino acid sequence of proteins. Complementarity of the bases on DNA’s two strands allows genetic information to be replicated.
The central dogma of molecular biology proposes that information flows in one direction, from DNA to RNA to protein. Exceptions to the central dogma are now known.
Chromosomes contain very long DNA molecules that are tightly packed. Supercoiling results from strain produced when rotations are added to or removed from a relaxed DNA molecule.
A bacterial chromosome consists of a single, circular DNA molecule that is bound to proteins and exists as a series of large loops.
Each eukaryotic chromosome contains a single, long linear DNA molecule that is bound to histone proteins. Euchromatin undergoes the normal cycle of decondensation and condensation in the cell cycle. Heterochromatin remains highly condensed throughout the cell cycle.
The nucleosome consists of a core of eight histone proteins and the DNA that wraps around the core. Nucleosomes are folded into a 30-
Epigenetic changes are stable alterations of gene expression that do not require changes in DNA sequences. Epigenetic changes can take place through alterations of chromatin structure.
Centromeres are chromosomal regions where spindle microtubules attach. Telomeres stabilize the ends of chromosomes.
Eukaryotic DNA exhibits three classes of sequences. Unique-