Nucleic acids are of two main types, deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Human beings have 46 chromosomes arranged in 23 pairs of autologous chromosomes and one pair of sex chromosomes. Genes are sequences of DNA carried on chromosomes that encode information for the translation of nucleic acid sequences into amino acid sequences that result in the production of proteins. Although the human genome has more than 3 billion DNA bases, the number of encoded genes is approximately 30,000. In com parison, RNA acts as an intermediate nucleic acid structure that helps convert the DNA-encoded genetic information into proteins. DNA is the template for the synthesis of RNA.

DNA and RNA are polymers made up of repeating nucleotides or bases that are linked together (Fig. 1). DNA and RNA have the same two purine bases, adenine (A) and guanine (G), but the pyrimidine bases differ. DNA has cytosine (C) and thymine (T); RNA substitutes uracil (U) for T. DNA is predominantly a double-stranded molecule with specific base pairs linked together (Fig. 2). Nucleotides are bonded together and two strands are twisted into an alpha helix (Fig. 3).

Fig1. A, Purine and pyrimidine bases and the formation of complementary base pairs. Dashed lines indicate the formation of hydrogen  bonds. B, A single-stranded DNA chain. Repeating nucleotide units are linked by phosphodiester bonds that join the 5′ carbon of one sugar to the  3 ′carbon of the next. Each nucleotide monomer consists of a sugar moiety, a phosphate residue, and a base. *In RNA, thymine is replaced by  uracil, which differs from thymine only in its lack of the methyl group. **In RNA, the sugar is ribose, which adds a 2′-hydroxyl to deoxyribose.  (Adapted from Piper MA, Unger ER: Nucleic acid probes: a primer for pathologists, Chicago, 1989, ASCP Press.)

Fig2. Structure of DNA. The DNA molecule is a double helix that consists of two sugar-phosphate backbones with four bases—cytosine  (C), guanine (G), adenine (A), and thymine (T)—attached. C and G residues and A and T residues on opposite strands pair through hydrogen bond ing. (From LeGrys V, Leinbach SS, Silverman L: Clinical applications of DNA probes in the diagnosis of genetic diseases, Crit Rev Clin Lab Sci 25:255, 1987.)

Fig3. The DNA double helix, with sugar-phosphate back bone and pairing of the bases in the core forming planar structures.  (From Jorde CB, Carey JC, Bamshead MJ et al, editors: Medical genetics, ed 3, St Louis, 2006, Mosby.)

How Does DNA Replicate?

 DNA is a very stable molecule and replication is straightfor ward. The process of replication (Fig. 4) involves one strand of the molecule acting as a template for the creation of a complementary strand. As a result of this process, two identical daughter molecules are produced. In the laboratory, the hydro gen bonds that hold the strands of the double helix can be broken apart or denatured. If complementary strands of DNA are denatured in the laboratory, they can spontaneously rejoin, or anneal. The process of denaturation and annealing can be used effectively in molecular testing.

Fig4. DNA replication. Double-stranded DNA is separated  at the replication fork. The leading strand is synthesized continuously,  whereas the lagging strand is synthesized discontinuously but joined  later by DNA ligase. (From Burtis CA, Ashwood ER, Bruns DB: Tietz funda mentals of clinical chemistry, ed 6, St Louis, 2008, Saunders.)

Production of functional protein from genetically encoded DNA is achieved by two processes, transcription and translation. Transcription is a process of generating a strand of messenger RNA (mRNA) that encodes for the gene and is expressed as a protein. Translation occurs when the mRNA moves from the nucleus of a cell into the cellular cytoplasm to the ribosomes. mRNA is translated into an amino acid sequence on the ribosome. This process manufactures a protein that was originally encoded in DNA in the cellular nucleus.

Forms of RNA

 RNA can be easily replicated and is used in molecular laboratory testing. RNA exists in three forms, mRNA, tRNA, and rRNA. All the forms of RNA exist as single-stranded polymers and are longer than DNA. The function of each form of RNA differs, as follows :

• mRNA—translates DNA coding into functional proteins (Fig. 5)

• tRNA—transports various amino acids to manufacture proteins

• rRNA—site of protein synthesis directed by mRNA

Fig5. DNA transcription and mRNA processing. A gene that  encodes for a protein contains a promoter region with a variable number of introns and exons. Transcription commences at the transcription start site. Pre-mRNA is processed by capping, polyadenylation,  and intron splicing and becomes a mature mRNA. (From Burtis CA, Ashwood ER, Bruns DB: Tietz fundamentals of clinical chemistry, ed 6, St Louis, 2008, Saunders.)

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مواضيع ذات صلة

Amplification Techniques in Molecular Biology

solation of Integration-Defective Mutants

Gene Order Permutation and the Campbell Model

Simultaneous Deletion of Chromosomal and Lambda DNA

Mapping Integrated Lambda

Expression Patterns of Developmental Genes

Enhancer Traps for Detecting and Cloning Developmental Genes

Cloning Developmental Genes

General Considerations on Signaling

DNA Strand Inheritance and Switching in Fission Yeast

DNA Cleavage at the MAT Locus

Glucose Metabolism