Biology by Karl Irvin Baguio (the ebook reader TXT) 📕

its anticodon.

 

One by one, amino acids are added to the growing chain until the ribosome has moved down to the end of the mRNA molecule. Because of the specificity of tRNA molecules for their individual amino acids, and because of the base pairing between codons and anticodons, the sequence of codons on the mRNA molecule determines the sequence of amino acids in the protein being constructed. And because the codon sequence of the mRNA complements the codon sequence of the DNA, the DNA molecule ultimately directs the amino acid sequencing in proteins. The primary “start” codon on an mRNA molecule is AUG, which codes for the amino acid methionine. Therefore, each mRNA transcript begins with the AUG codon, and the resulting polypeptide begins with methionine.

 

Figure 10-3 shows that the process of protein synthesis begins with the production of mRNA (upper right). The mRNA molecule proceeds to the ribosome, where it meets tRNA molecules carrying amino acids (upper left). The tRNA molecule has a base code (anticodon) that complements the mRNA code (codon) and thereby brings a specific amino acid into position. The amino acids join together in peptide bonds (bottom), and the tRNA molecules are released to pick up additional amino acid molecules.

 

After the protein has been synthesized completely, it is removed from the ribosome for further processing and to perform its function. For example, the protein may be stored in the Golgi apparatus before being released by the cell, or it may be stored in the lysosome as a digestive enzyme. Also, a protein may be used in the cell as a structural component, or it may be released as a hormone, such as insulin. After synthesis, the mRNA molecule breaks up and the nucleotides return to the nucleus. The tRNA molecules return to the cytoplasm to unite with other molecules of amino acids, and the ribosome awaits the arrival of a new mRNA molecule.

 

Figure 10-3   The process of protein synthesis.

 

DNA Structure

 

As proposed by Watson and Crick, deoxyribonucleic acid (DNA) consists of two long nucleotide chains. The two nucleotide chains twist around one another to form a double helix, a shape resembling a spiral staircase. Weak chemical bonds between the chains hold the two chains of nucleotides to one another.

 

A nucleotide in the DNA chain consists of three parts: a nitrogenous base, a phosphate group, and a molecule of deoxyribose. The nitrogenous bases of each nucleotide chain are of two major types: purines and pyrimidines. Purine bases have two fused rings of carbon and nitrogen atoms, while pyrimidines have only one ring. The two purine bases in DNA are adenine (A) and guanine (G). The pyrimidine bases in DNA are cytosine (C) and thymine (T).

 

The phosphate group of DNA is derived from a molecule of phosphoric acid. The phosphate group connects the deoxyribose molecules to one another in the nucleotide chain. Deoxyribose is a five-carbon carbohydrate. The purine and pyrimidine bases are attached to the deoxyribose molecules, and the purine and pyrimidine bases are opposite one another on the two nucleotide chains. Adenine is always opposite thymine and binds to thymine. Guanine is always opposite cytosine and binds to cytosine. Adenine and thymine are said to be complementary, as are guanine and cytosine. This is known as the principle of complementary base pairing.

 

Gene Control

 

The process of protein synthesis does not occur constantly in the cell. Rather, it occurs at intervals followed by periods of genetic “silence.” Thus, the cell regulates and controls the gene expression process.

 

The control of gene expression may occur at several levels in the cell. For example, genes rarely operate during mitosis, when the DNA fibers shorten and thicken to form chromatin. The inactive chromatin is compacted and tightly coiled, and this coiling regulates access to the genes.

 

Other levels of gene control can occur during and after transcription. In transcription, certain segments of DNA can increase and accelerate the activity of nearby genes. After transcription has taken place, the mRNA molecule can be altered to regulate gene activity. For example, researchers have found that an mRNA molecule contains many useless bits of RNA that are removed in the production of the final mRNA molecule. These useless bits of nucleic acid are called introns. The remaining pieces of mRNA, called exons, are then spliced to form the final mRNA molecule. Thus, through removal of introns and the retention of exons, the cell can alter the message received from the DNA and control gene expression.

 

The concept of gene control has been researched thoroughly in bacteria. In these microorganisms, genes have been identified as structural genes, regulator genes, and control genes (or control regions). The three units form a functional unit called the operon.

 

The operon has been examined in close detail in certain bacteria. Scientists have found, for example, that certain carbohydrates can induce the presence of the enzymes needed to digest those carbohydrates. When lactose is present, bacteria synthesize the enzyme needed to break down the lactose. Lactose acts as the inducer molecule in the following way: In the absence of lactose, a regulator gene produces a repressor, and the repressor binds to a control region called the operator. This binding prevents the structural genes from encoding the enzyme for lactose digestion. When lactose is present, however, it binds to the repressor and thereby removes the repressor at the operator site. With the operator site free, the structural genes are free to produce their lactose-digesting enzyme.

The operon system in bacteria shows how gene expression can occur in relatively simple cells. The gene is inactive until it is needed and is active when it becomes necessary to produce an enzyme. Other methods of gene control are more complex and are currently being researched.

 

Non-coding RNAs (ncRNAs)

Protein-encoding mRNA is clearly an important molecule, but the other non-coding RNAs (ncRNAs) that control mRNA transcription are increasingly becoming the focus of attention. Two such ncRNA molecules are microRNA and small interfering RNA.

 

MicroRNAs (miRNAs) are small, single-stranded RNA molecules that, along with associated proteins, bind to complementary sequences in certain mRNA molecules. Once bound, these miRNAs block translation of the mRNA by either physically preventing the ribosome from binding, or by causing the mRNA to degrade. The result—either blocked translation or mRNA degradation— depends on the extent of the base pairing between the miRNA and the target mRNA. Possibly half of human genes are regulated by miRNA.

Small interfering RNAs (siRNAs) are similar in structure and function to miRNAs. RNA interference (RNAi) is a means of disabling a gene by introducing siRNAs into a cell. Researchers employ RNAi to knock out specific genes in order to study their function.

 

Modification of chromatin structure

 

Recall that DNA is condensed and packaged with histone proteins into a complex known as chromatin (see Chapter 7). This compact structure helps DNA to fit into the nucleus and also provides opportunity for gene regulation. In order for a gene to be accessed by the transcriptional machinery, it must be “unwound” from the histone proteins. This is facilitated by certain enzymes adding acetyl groups (–COCH3) to the histones (histone acetylation). Alternatively, if a segment of DNA needs to remain unexpressed (such as the inactivated mammalian X chromosomes), a different set of enzymes will add methyl groups (–CH3) to certain bases, thus maintaining DNA’s tightly wound and inaccessible form.

 

How a cell controls the expression of its genes is almost as important as the genes themselves. Modification of DNA and its associated histone proteins has a profound effect on that gene’s expression. Furthermore, these modifications can be passed on to future generations and thus effect gene expression in progeny. This is called epigenetic inheritance. Alterations in normal modification have also been linked to some cancers, due to inappropriate gene expression.

Chapter 11: Recombinant DNA and Biotechnology

  Recombinant DNA

 

Biotechnology is a process that uses the scientific research on DNA for practical means. Biotechnology is synonymous with genetic engineering because the genes of an organism are changed during the process. Because the genes are changed, the DNA of the organism is said to be recombined. The result of the process is recombinant DNA. Recombinant DNA and biotechnology can be used to form proteins not normally produced in a cell to make drugs or vaccines or to promote human health. In addition, bacteria that carry recombinant DNA can be released into the environment under carefully controlled conditions to increase the fertility of the soil, serve as an insecticide, or relieve pollution. An organism that contains additional genes from another organism is said to be transgenic. Along with bacteria, transgenic plants and animals are also being created. Humans can also have the genes in their cells modified to produce proteins that relieve health-related deficiencies.

 

A researcher’s ability to modify an organism’s genome is possible because of genomics. Genomics is the study of an organism’s entire set of chromosomes, including their function and species evolution. The sequencing of the human genome was completed in 2003, and since then, numerous prokaryotes (such as E. coli) and eukaryotes (both vertebrates and invertebrates) have been sequenced. This allows the comparison of genomes across a wide range of organisms, providing a better understanding of their genetic similarities and evolutionary history.

 

Pharmaceutical Products

 

Gene defects in humans can lead to deficiencies in proteins such as insulin, human growth hormone, and Factor VIII that may result in problems (diabetes, dwarfism, and impaired blood clotting, respectively). Proteins for these chemicals can now be replaced by proteins manufactured through biotechnology. For insulin production, two protein chains are encoded by separate genes in plasmids inserted into bacteria. The protein chains are then chemically joined to form the final insulin product. Human growth hormone is also produced within bacteria, but special techniques are used because the bacteria do not usually produce human proteins.

 

Therapeutic proteins, such as the following, can also be produced by biotechnology:

 

     ■   Tissue plasminogen activator (TPA), a clot-dissolving protein, can now be produced in recombined mammalian cells.

     ■   Interferon, an antiviral protein produced in E. coli cells, is currently used to fight certain types of cancers and for certain skin diseases.

     ■   An antisense molecule is a molecule of RNA that reacts with and neutralizes the mRNA molecule used in protein synthesis. In doing so, the antisense molecule prevents the synthesis of a protein involved in a specific disease. For example, an antisense molecule can prohibit human host cells from producing key portions of the human immunodeficiency virus (HIV) when infection has occurred.

 

Vaccines represent another application of recombinant DNA technology. The hepatitis B vaccine now in use is composed of viral proteins manufactured by yeast cells and recombined with viral genes. The vaccine is safe because it contains no viral particles. Experimental vaccines against AIDS are being produced in the same way. One vaccine uses vaccinia (cowpox) virus as a vector. The virus has been combined with genes from several viruses, and it is hoped that injections of the vaccine will stimulate resistance to multiple diseases. Vaccines can also be produced by eliminating certain disease-inducing genes from a pathogen, leading to a harmless organism that will stimulate an immune response.

 

Diagnostic Testing

 

Recombinant DNA and biotechnology have opened a new era of diagnostic testing and have made detecting many genetic diseases possible. The basic tool of DNA analysis is a fragment of DNA called the DNA probe. A DNA probe is a relatively small, single-stranded fragment of DNA