Regulation of Gene Expression

Although selective expression of some combinations of genes during cellular differentiation accounts for the wide range of cellular phenotypes in complex organisms, regulation of gene expression does not end when cells become terminally differentiated. Except for red blood cells, which have no nuclei, all cells increase or decrease expression of some of their genes to bring functional capacity into alignment with changing physiological demands. We therefore consider briefly some pertinent, but general, aspects of gene regulation distilled from the wealth of information that has come to light in recent years. For a detailed understanding of this important topic, the student is urged to consult one of the many textbooks dedicated to molecular biology.

DNA Structure

We may consider the gene to be a stretch of deoxyribonucleic acid (DNA) that contains both the coded information that governs its regulation and the amino acid sequence of one, or sometimes more than one, protein. The sum total of all of the genes is called the genome, which in humans is estimated to contain between 30 and 35 thousand pairs of genes. DNA consists of a chain of many millions of deoxyribose (a five-carbon sugar) molecules linked together by phosphate groups that form ester bonds with the 30 hydroxyl group of one sugar and the 50 hydroxyl group of its neighbor (Fig. 1). Carbon 1 of each deoxyribose is attached by N-glycosidic linkage to an organic base, which may be adenine (A), guanine (G), thymine (T), or cytosine (C). A nucleotide is the fundamental unit of the nucleic acid polymer and consists of a base, a sugar, and a phosphate group. The information stored in DNA is encoded in the sequence of its constituent nucleotides. The ability of purine bases A and G to form complementary pairs with pyrimidine bases T and C on an adjacent strand of DNA is the fundamental property that permits accurate replication of DNA and transmission of stored information (Fig. 2). In the cell, DNA forms a double helix of two strands oriented in opposite directions, with each A on one strand pairing with a T on the complementary strand and each G pairing with a C. The long double strands of DNA are organized into nucleosomes, each of which consists of a stretch of about 180 nucleotides tightly wound around a complex of eight histone molecules. The nucleosomes are linked by stretches of about 30 nucleotides, and the whole strand of nucleoproteins is tightly coiled in a higher order of organization to form chromosomes. Each chromosome contains one double helix of DNA in which 50 to 250 million base pairs code for thousands of genes lined up like a string of beads. Humans have 23 pairs of chromosomes, with one member of each pair inherited from each parent.

Gene Transcription

For the information encoded in the DNA to guide protein synthesis, a complementary strand of ribonucleic acid (RNA) must be synthesized to serve as the template that guides assembly of amino acids into proteins. The synthesis of RNA transfers encoded information from DNA to RNA and is therefore called transcription. RNA differs chemically from DNA in two ways: Its sugar is ribose instead of deoxyribose, and it contains uracil (U) instead of thymine. Only part of each gene is transcribed. Untranscribed portions include sequences of bases involved in regulation of transcription and

FIGURE 1 Composition of DNA. DNA is a polymer of the five-carbon sugar, deoxyribose, in diester linkage with phosphate (P) forming ester bonds with hydroxyl groups on carbons 3 and 5 on adjacent sugar molecules. The purine (adenine or guanine) and pyrimidine (thymine or cytosine) bases are linked to carbon 1 of each sugar. The numbering system for the five carbons of deoxyribose are shown in blue at the top of the figure. The chemical bonds forming the backbone of the DNA chain are also shown in blue. The 50 or 30 ends refer to the carbons in deoxyribose.

FIGURE 1 Composition of DNA. DNA is a polymer of the five-carbon sugar, deoxyribose, in diester linkage with phosphate (P) forming ester bonds with hydroxyl groups on carbons 3 and 5 on adjacent sugar molecules. The purine (adenine or guanine) and pyrimidine (thymine or cytosine) bases are linked to carbon 1 of each sugar. The numbering system for the five carbons of deoxyribose are shown in blue at the top of the figure. The chemical bonds forming the backbone of the DNA chain are also shown in blue. The 50 or 30 ends refer to the carbons in deoxyribose.

Get Rid of Gallstones Naturally

Get Rid of Gallstones Naturally

One of the main home remedies that you need to follow to prevent gallstones is a healthy lifestyle. You need to maintain a healthy body weight to prevent gallstones. The following are the best home remedies that will help you to treat and prevent gallstones.

Get My Free Ebook


Post a comment