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# SECTION 4: Protection and Survival ### Overview Sometimes the mutation is lethal because it disrupts some essential cellular function, causing cell death, and the mutation is destroyed along with the cell. Often, the mutated cell is detected by immune cells and destroyed because it is abnormal. O...

# SECTION 4: Protection and Survival ### Overview Sometimes the mutation is lethal because it disrupts some essential cellular function, causing cell death, and the mutation is destroyed along with the cell. Often, the mutated cell is detected by immune cells and destroyed because it is abnormal. Other mutations do not kill the cell but alter its function in some way that may cause disease (e.g., in cancer). A persistent mutation in the genome that has not led to cell death can be passed from parent to child and may cause inherited disease, e.g., phenylketonuria or cystic fibrosis. ### SPOT CHECK 1. What are histones? 2. Which base always pairs with cytosine in DNA? ### Protein Synthesis #### Learning Outcomes After studying this section, you should be able to: - Describe the origin and structure of mRNA - Explain the mechanism of transcription - Outline the mechanism of translation DNA holds the cell's essential biological information, written within the base code in the center of the double helix. The products of this information are almost always proteins. Proteins are essential to all aspects of body function, forming the major structural elements of the body, as well as the enzymes essential for all biochemical processes within it. The building blocks of human proteins are about 20 different amino acids. As the cell's DNA is too big to leave the nucleus, an intermediary molecule is needed to carry the genetic instructions from the nucleus to the cytoplasm, where proteins are made. This is called messenger ribonucleic acid (mRNA). Protein synthesis is summarized in Figure 17.5. ### Messenger Ribonucleic Acid Messenger ribonucleic acid (mRNA) is a single-stranded chain of nucleotides synthesized in the nucleus from the appropriate gene, whenever the cell needs to make the protein for which that gene codes. RNA is different in structure to DNA in three main ways: - It is single-stranded instead of double-stranded. - It contains the sugar ribose instead of deoxyribose. - It uses the base uracil instead of thymine. ### Figure 17.4 The structural relationship between DNA, chromatin, and chromosomes. ### Notable Points - Chromosomes - Chromatin - Histones (structural proteins) - DNA - Adenine - Thymine - Guanine - Cytosine - Deoxyribose sugar - Phosphate group