Genetic Code, Protein Creation, and Metabolic Processes
DNA, Genes, and Proteins: The Blueprint of Life
DNA is the key to inheritance. Its structure allows it to carry a code of instructions that directs and regulates the synthesis of enzymes and other specific proteins. As discussed in Chapter 1, proteins consist of polypeptides, and a polypeptide is composed of amino acids linked together by peptide bonds. Natural proteins are formed by 20 different amino acids.
The piece of DNA whose function is encoding the amino acid sequence of a polypeptide chain is called a gene. Genes are hereditary units found in the chromosomes of the cell nucleus. They are passed from parents to children, empowering common traits that identify its kind.
Protein Synthesis
Transcription
The information in the DNA molecule is transcribed into messenger RNA (mRNA) chains. In this process, which involves a series of enzymes, the double-stranded DNA is partially unwound, allowing the strands to be copied. Once formed, mRNA chains leave the nucleus and are directed to ribosomes in the cytoplasm.
Translation
Transfer RNA (tRNA) is responsible for capturing amino acids in the cytoplasm and leading them to the ribosomes. Each tRNA molecule has a trinucleotide called an anticodon that is complementary to an mRNA codon. Ribosomal RNA (rRNA) is a component of ribosomes. As we have seen, ribosomes are the organelles where proteins are synthesized.
Enzymes: Catalysts of Life
Enzymes are proteins that help chemical reactions occur faster. Without enzymes, our bodies would cease to function.
Cell Metabolism: Energy and Matter Exchange
Metabolism is the set of biochemical reactions common to all living things that occur in cells for the collection and exchange of matter and energy with the environment, and for macromolecular synthesis from simple compounds. Its aim is to maintain life processes (nutrition, growth, reproduction) and homeostasis. Metabolism is divided into anabolism and catabolism. These two processes, catabolism and anabolism, are integrated into cellular metabolism, as both are required for the synthesis of certain molecules. Ultimately, metabolism encompasses all vital activities occurring within an organism.
The Genetic Code: Language of Life
DNA molecules are formed by the combination of four different types of nucleotides, which are represented by their initial nitrogenous bases: A, T, C, and G. These four nucleotides are the "letters" through which DNA "writes" its message.
Imagine that the mRNA strand is a string of words. Each word is formed by three letters, and each letter represents a nucleotide. Each "word" formed by three nucleotides is called a codon. Codons are formed during transcription as the complement of a group of three nucleotides, or trinucleotide, from DNA. Each three-letter "word," or codon, on the mRNA specifies a particular amino acid. This set of three-letter "words" (codons), which determines the sequence of amino acids in proteins, constitutes the genetic code.