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Introduction

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High-Yield Terms

  • Phosphodiester bond: chemical bond between the 5'-phosphate of one base and the 3'-hydroxyl of an adjacent base in a nucleic acid

  • Watson-Crick base-pair: refers to the normal hydrogen bonding that occurs in a duplex of DNA, or an RNA-DNA duplex, where A bonds to T and G bonds to C

  • Annealing: with respect to nucleic acids this refers to DNA or RNA pairing by hydrogen bonds to a complementary sequence, forming a double-stranded polynucleotide

  • Nucleosome: a structure formed by DNA and protein interaction that consisting of an octamer protein structure composed of 2 subunits each of histones H2A, H2B, H3, and H4

  • Chromatin: the combination of DNA and proteins that make up the contents of the nucleus of a cell

  • Heterochromatin: densely packed region of a chromosome often found near the centromeres, is generally transcriptionally silent

  • Euchromatin: loosely packed regions of a chromosome where active gene transcription can occur

  • Telomere: a region of repetitive nucleotide sequences at each end of a chromatid

  • Cyclin: any of a number of proteins whose levels fluctuate during the eukaryotic cell cycle; these proteins regulate the activity of a family of kinases involved in controlling passage through the cell cycle

  • Proof-reading: in the process of DNA replication this refers to the 3′ → 5′ exonuclease activity of DNA polymerase, which allows it to remove an incorrectly incorporated nucleotide, recognized by non–Watson-Crick base-pairing

  • Okazaki fragment: short stretches of newly synthesized DNA representing the lagging strand

  • Topoisomerase: enzymes that cleave the DNA strand, thereby relieving torsional stresses incurred during the process of DNA replicaiton

  • Epigenetics: defines the mechanism by which changes in the pattern of inherited gene expression occur in the absence of alterations or changes in the nucleotide composition of a given gene

  • Imprinting: refers to the fact that the expression of some genes depends on whether or not they are inherited maternally or paternally

  • Trinucleotide repeat: sequences of 3 nucleotides repeated in tandem on the same chromosome a number of times, abnormal expansion of the repeat is associated with the genesis of disease

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DNA Structure

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DNA is a specialized form of polynucleotides. Polynucleotides are formed by the condensation of 2 or more nucleotides. The condensation most commonly occurs between the alcohol of a 5′-phosphate of one nucleotide and the 3′-hydroxyl of a second, with the elimination of H2O, forming a phosphodiester bond. The formation of phosphodiester bonds in DNA and RNA exhibits directionality. The primary structure of DNA and RNA (the linear arrangement of the nucleotides) proceeds in the 5′ → 3′ direction (Figure 35-1). The common representation of the primary structure of DNA or RNA molecules is to write the nucleotide sequences from left to right synonymous with the 5′ → 3′ direction.

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FIGURE 35-1:

A segment of one strand of a DNA molecule in which the purine and pyrimidine bases guanine (G), cytosine (C), thymine (T), and adenine (A) are ...

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