Genetic Code

Molecules of DNA contain information, coded in the sequence of nucleotides, for the synthesis of proteins. A sequence of DNA nucleotides containing the information that specifies the amino acid sequence of a single polypeptide chain is known as a gene. A gene is thus a unit of hereditary information. A single molecule of DNA contains many genes.

The total genetic information coded in the DNA of a typical cell in an organism is known as its genome. The human genome contains between 50,000 and 100,000 genes, the information required for producing 50,000 to 100,000 different proteins. Currently, scientists from around the world are collaborating in the Human Genome Project to determine the nucleotide sequence of the human genome that will involve locating the position of the approximately 3 billion nu-cleotides that make up the human genome.

It is easy to misunderstand the relationship between genes, DNA molecules, and chromosomes. In all human cells (other than the eggs or sperm), there are 46 separate DNA molecules in the cell nucleus, each molecule containing many genes. Each DNA molecule is packaged into a single chromosome composed of DNA and proteins, so there are 46 chromosomes in each cell. A chromosome contains not only its DNA molecule, but a special class of proteins called histone proteins, or simply histones. The cell's nucleus is a marvel of packaging; the very long DNA molecules, having lengths a thousand times greater than the diameter of the nucleus, fit into the nucleus by coiling around clusters of histones at frequent intervals to form complexes known as nucleosomes. There are about 25 million of these complexes on the chromosomes, resembling beads on a string.

Although DNA contains the information specifying the amino acid sequences in proteins, it does not itself participate directly in the assembly of protein molecules. Most of a cell's DNA is in the nucleus (a small amount is in the mitochondria), whereas most protein synthesis occurs in the cytoplasm. The transfer of information from DNA to the site of protein synthesis is the function of RNA molecules, whose synthesis is governed by the information coded in DNA. Genetic information flows from DNA to RNA and then to protein (Figure 5-1). The process of transferring genetic information from DNA to RNA in the nucleus is known as transcription; the process that uses the coded information in RNA to assemble a protein in the cytoplasm is known as translation.

transcription translation

As described in Chapter 2, a molecule of DNAcon-sists of two chains of nucleotides coiled around each other to form a double helix (see Figure 2-24). Each DNA nucleotide contains one of four bases—adenine (A), guanine (G), cytosine (C), or thymine (T)—and each of these bases is specifically paired by hydrogen bonds with a base on the opposite chain of the double helix. In this base pairing, A and T bond together and G and C bond together. Thus, both nucleotide chains contain a specifically ordered sequence of bases, one chain being complementary to the other. This specificity of base pairing, as we shall see, forms the basis of the transfer of information from DNA to RNA and of the duplication of DNA during cell division.

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

I. Basic Cell Functions

5. Genetic Information and Protein Synthesis

© The McGraw-Hill Companies, 2001

Genetic Information and Protein Synthesis CHAPTER FIVE

Genetic Information and Protein Synthesis CHAPTER FIVE

Cytoplasm

Nucleus

Transcription

Cytoplasm

Nucleus

Transcription

Translation

Translation

Amino acids Proteins

Proteins having other functions

Amino acids Proteins

Enzymes

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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