Chapter 3: Molecules of Cells PDF

**Chapter 3: Molecules of Cells** **3.1 Life\'s molecular diversity is based on the properties of carbon** Carbon is the star player in molecule formation. Most cellular molecules are made of carbon atoms bonded to each other and other elements. Carbon\'s unique ability to form large, complex mole...

**Chapter 3: Molecules of Cells** **3.1 Life\'s molecular diversity is based on the properties of carbon** Carbon is the star player in molecule formation. Most cellular molecules are made of carbon atoms bonded to each other and other elements. Carbon\'s unique ability to form large, complex molecules is crucial for building structures and performing life functions. These carbon-based molecules, known as organic compounds, typically also contain hydrogen atoms. Carbon atoms are central to the chemistry of life because they have four electrons in a valence shell that can hold eight. This allows carbon to form four covalent bonds with other atoms, enabling the creation of a wide variety of complex and diverse organic molecules essential for life. Figure 3.1A illustrates a ball-and-stick model of methane (CH₄), highlighting carbon\'s four bonds that angle out toward the corners of a tetrahedron. This tetrahedral shape occurs when carbon forms four single bonds. In molecules with multiple carbon atoms, each carbon can branch in up to four directions. Different shapes arise when carbon forms double bonds, leading to the diverse and complex shapes of organic molecules. The shape of a molecule typically determines its function. Carbon chains are the backbone of most organic molecules. Figure 3.1B shows that these \"carbon skeletons\" can vary in four ways: they can differ in length, be straight, branched, or arranged in rings. Additionally, carbon skeletons can include double bonds, which can vary in number and location. Butane and isobutane both have the molecular formula C₄H₁₀ but differ in their carbon skeleton arrangement. Similarly, two other molecules with the formula C₄H₈ have different three-dimensional shapes due to the location of the double bond. Compounds with the same formula but different structural arrangements are called isomers. These different shapes contribute significantly to the diversity and properties of organic molecules. Isomers can form due to different spatial arrangements when four different groups are attached to a carbon atom. This type of isomer is crucial in the pharmaceutical industry, as the two isomers of a drug may vary in effectiveness or have different (and sometimes harmful) effects. There are 3 types of isomers: - Structural: differ in covalent bonds - Geometric: vary in arrangement of atoms around a double bond - Enantiomenrs/stereoisomers: which are mirror images of each other Methane and similar compounds are hydrocarbons, made up solely of carbon and hydrogen. They are key components of petroleum, which supplies much of the world\'s energy. While hydrocarbons are uncommon in living organisms, hydrocarbon chains are present in some molecules, such as fats, which provide fuel for the body. **Terms to know** **Hydrocarbons**-molecules consisting of only carbon and hydrogen. **organic compounds**-Carbon-based molecules are called organic compounds, and they usually contain hydrogen atoms in addition to carbon. **3.2 A few chemical groups are key to the functioning of biological molecules** The properties of an organic compound are influenced by both the size and shape of its carbon backbone and the atoms attached to that structure. Figure 3.2 illustrates the significant impact of chemical groups. Testosterone and estradiol, hormones responsible for male and female characteristics in vertebrates, differ only in their highlighted atomic groups. These small differences influence the molecules\' functions. Table 3.2 highlights six important chemical groups. The first five are functional groups that participate in chemical reactions and make compounds hydrophilic (water-loving) and soluble in water. The sixth group, the methyl group, is nonpolar and not reactive but influences molecular shape and function. - **Hydroxyl Group (OH)** - **Description:** A hydroxyl group consists of a hydrogen atom bonded to an oxygen atom. Ethanol, shown in the table, and other organic compounds containing hydroxyl groups are called alcohols. - **Memory tip:** \"Hydro\" (water) and \"ox\" (oxygen) = OH - **Example:** Ethanol (found in alcoholic beverages) - **Carbonyl Group (C=O)** - **Description:** In a carbonyl group, a carbon atom is linked by a double bond to an oxygen atom. The carbonyl group can be located within or at the end of a carbon skeleton. - **Memory tip:** \"Carbon\" (C) and \"yl\" (double bond to oxygen) = C=O - **Example:** Simple sugars (contain carbonyl and hydroxyl groups) - **Carboxyl Group (COOH)** - **Description:** A carboxyl group consists of a carbon double-bonded to an oxygen atom and bonded to a hydroxyl group. The carboxyl group can act as an acid by contributing an H+ to a solution and thus becoming ionized. - **Memory tip:** \"Carb\" (carbon) and \"oxyl\" (oxygen and hydroxyl) = COOH - **Example:** Carboxylic acids (such as acetic acid in vinegar) - **Amino Group (NH₂)** - **Description:** An amino group has a nitrogen bonded to two hydrogens. It can act as a base by picking up an H+ from a solution and becoming ionized. Organic compounds with an amino group are called amines. The building blocks of proteins---amino acids---contain an amino and a carboxyl group. - **Memory tip:** \"Amino\" has an \"n\" for **N**itrogen - **Example:** Amino acids (building blocks of proteins) - **Phosphate Group (PO₄)** - **Description:** A phosphate group consists of a phosphorus atom bonded to four oxygen atoms. It too is usually ionized, as you can see by the negatively charged oxygens in the figure. Compounds with phosphate groups are called organic phosphates and are often involved in energy transfers, as is the energy-rich compound ATP (adenosine triphosphate). - **Memory tip:** \"Phos\" (phosphorus) and \"phate\" (four oxygens) = PO₄ - **Example:** ATP (adenosine triphosphate, involved in energy transfers) - **Methyl Group (CH₃)** - **Description:** A methyl group consists of a carbon bonded to three hydrogen atoms. The methylated compound in the table---a component of DNA---affects the expression of genes. - **Memory tip:** \"Meth\" (one carbon) **3.3 Cells make large molecules from a limited set of small molecules** Despite the complexity of life on Earth, the important molecules of all living things fall into four main classes: carbohydrates, lipids, proteins, and nucleic acids. Carbohydrates, proteins, and nucleic acids can be gigantic and are called macromolecules. For example, a protein may consist of thousands of atoms. Cells create most of their macromolecules by linking smaller molecules into chains called polymers. A polymer is a long molecule made up of many identical or similar building blocks, like a train made of connected cars. These building blocks are called monomers. **Terms to know**  **Making Polymers** Cells form polymers by linking monomers through a dehydration reaction, which removes a water molecule as two monomers bond. Each monomer contributes part of the water molecule released. One monomer loses a hydroxyl group, and the other loses a hydrogen atom, forming H₂O and creating a new covalent bond. This process is consistent across different monomers and polymers. **Breaking Polymers** Cells break down macromolecules through a process called hydrolysis. This digestion process is necessary because most organic molecules in food are polymers too large to enter cells. Hydrolysis, the reverse of a dehydration reaction, involves breaking bonds between monomers by adding a water molecule. The hydroxyl group from the water attaches to one monomer, and hydrogen attaches to the adjacent monomer. Lactose-intolerant individuals cannot hydrolyze the bond in lactose due to a lack of the enzyme lactase. Both dehydration reactions and hydrolysis need enzymes to make and break bonds. Enzymes are specialized macromolecules that accelerate chemical reactions in cells. **The Diversity of Polymers** The diversity of macromolecules in the living world is vast. Surprisingly, cells make all their thousands of different macromolecules from about 40 to 50 common components and a few rare ones. Proteins are built from only 20 kinds of amino acids, and DNA is built from just four kinds of monomers called nucleotides. The key to the great diversity of polymers is the variation in the sequence in which monomers are strung together. The variety in polymers accounts for the uniqueness of each organism. However, the monomers themselves are essentially universal. Proteins in all organisms, from humans to trees to ants, are assembled from the same 20 amino acids. Life follows a simple yet elegant molecular logic: small molecules common to all organisms are ordered into large molecules, which vary from species to species and even from individual to individual within the same species. **Carbohydrates 3.4 Monosaccharides are the simplest carbohydrates** Let\'s begin our exploration of biological molecules with carbohydrates. This class includes small sugar molecules, like those found in soft drinks, and large polysaccharides, such as the starch in pasta and potatoes. Simple sugars, or monosaccharides, are the monomers of carbohydrates. Honey mainly consists of monosaccharides like glucose and fructose. These single-unit sugars can be linked together by dehydration reactions to form more complex sugars and polysaccharides. Monosaccharides typically have molecular formulas that are multiples of CH₂O. For example, glucose, a key monosaccharide in the chemistry of life, has the formula C₆H₁₂O₆. The structure of glucose includes six carbons and features two trademarks of sugars: several hydroxyl groups (-OH) and a carbonyl group (C=O). Glucose and fructose are isomers with the same molecular formula, C₆H₁₂O₆, but different arrangements of their atoms, specifically the positions of the carbonyl groups. These minor differences in molecular shape give isomers distinct properties, such as how they react with other molecules. This also makes fructose taste significantly sweeter than glucose. The carbon skeletons of glucose and fructose each have six carbon atoms. Other monosaccharides can have three to seven carbons. Common examples include five-carbon sugars (pentoses) and six-carbon sugars (hexoses). Most sugar names end in -ose, and most enzyme names end in -ase, like lactase, which digests lactose. While sugars are often drawn with linear carbon skeletons, in aqueous solutions, most five- and six-carbon sugars form rings. For glucose, carbon 1 bonds to the oxygen attached to carbon 5, with carbon 6 extending above the ring. The ring diagram of glucose can be simplified by not showing the carbon atoms at the corners, and the bonds are often drawn with varied thickness to indicate the flat structure with attached atoms extending above and below it. The simplified ring symbol is commonly used to represent glucose. Monosaccharides, especially glucose, are the primary fuel molecules for cellular work. Cells release energy from glucose when they break it down. An aqueous solution of glucose, known as dextrose, can be injected into the bloodstream of sick or injured patients to provide an immediate energy source for tissue repair. Cells also use the carbon skeletons of monosaccharides to create other organic molecules, such as amino acids and fatty acids. This dual role of sugars as energy sources and building blocks highlights the transformation of energy and matter. A diagram of a structure Description automatically generated with medium confidence **Terms to know**  **3.5 Two monosaccharides are linked to form a disaccharide** Cells create **disaccharides** from two monosaccharide monomers through a dehydration reaction. For example, maltose (malt sugar) is formed from two glucose monomers, where one monomer gives up a hydroxyl group and the other gives up a hydrogen atom, releasing water and linking the two monomers with an oxygen atom. Maltose is found in germinating seeds and is used in making beer, malt whiskey, and malted milk candy. Sucrose, the most common disaccharide, consists of a glucose monomer linked to a fructose monomer. It is transported in plant sap and provides energy and raw materials to all parts of the plant. We extract sucrose from sugarcane stems or sugar beet roots to use as table sugar. **3.7 Polysaccharides are long chains of sugar units** Polysaccharides are large macromolecules made up of hundreds to thousands of monosaccharides linked together through dehydration reactions. They can serve as storage molecules or structural compounds. Common types of polysaccharides include starch, glycogen, and cellulose. Starch is a storage polysaccharide in plants, made up of long chains of glucose monomers. These molecules coil into a helical shape and can be either unbranched or branched. Starch granules act as carbohydrate \"banks\" from which plant cells can withdraw glucose for energy or building materials. Humans and most animals have enzymes that can break down plant starch into glucose. Major sources of starch in the human diet include potatoes and grains like wheat, corn, and rice. Animals store glucose in a polysaccharide called glycogen, which is more highly branched than starch. Glycogen is stored as granules in the liver and muscle cells, where it can be hydrolyzed to release glucose when needed. Cellulose, the most abundant organic compound on Earth, is a key component of plant cell walls. It is a polymer of glucose, with its monomers linked in a different orientation compared to starch and glycogen. Cellulose molecules are arranged parallel to each other and joined by hydrogen bonds, forming cable-like microfibrils. These microfibrils combine with other polymers to provide strong support for trees and structures made from lumber. Animals lack the enzymes needed to break down the glucose linkages in cellulose, so it is not a nutrient for humans. However, cellulose contributes to digestive health as \"insoluble fiber,\" which passes unchanged through the digestive tract. Fresh fruits, vegetables, and whole grains are rich in fiber. Some microorganisms possess enzymes that can break down cellulose. Cows and termites host these microorganisms in their digestive tracts, allowing them to derive energy from cellulose. Decomposing fungi also digest cellulose, aiding in the recycling of its chemical elements within ecosystems. Chitin is a structural polysaccharide that insects and crustaceans use to build their exoskeletons. It is also found in the cell walls of fungi. Almost all carbohydrates are hydrophilic due to the many hydroxyl groups attached to their sugar monomers. This water-loving nature makes materials like cotton bath towels, which are mostly cellulose, highly absorbent. However, not all biological molecules are hydrophilic. **3.8 Fats are lipids that are mostly energy-storage molecules** Lipids are a diverse group of molecules that do not mix well with water, making them hydrophobic. Unlike carbohydrates and most other biological molecules, lipids repel water. This behavior is evident in an unshaken bottle of salad dressing, where the oil (a type of lipid) separates from the vinegar (mostly water). Lipids differ from carbohydrates, proteins, and nucleic acids in that they are neither large macromolecules nor polymers made from similar monomers. The next few modules will explore the structures and functions of three important types of lipids: fats, phospholipids, and steroids. Fat is a large lipid composed of two smaller molecules: glycerol and fatty acids. Glycerol has three carbons, each with a hydroxyl group (-OH). A fatty acid includes a carboxyl group (-COOH) and a hydrocarbon chain, typically 16 or 18 carbons long. The nonpolar C-H bonds in the hydrocarbon chains make fats hydrophobic. Figure 3.8A demonstrates how a fatty acid molecule can link to a glycerol molecule through a dehydration reaction. When three fatty acids link to glycerol, they form a fat, also known as triglyceride. This term is commonly seen on food labels and medical tests for blood fat levels. An unsaturated fatty acid has one or more double bonds in its hydrocarbon chain, causing kinks or bends. Each carbon atom connected by a double bond has one fewer hydrogen atom. In contrast, a saturated fatty acid has no double bonds, and the maximum number of hydrogen atoms attached to each carbon atom, making it \"saturated\" with hydrogen. Most animal fats are saturated, meaning their fatty acid tails lack double bonds and pack closely together, making them solid at room temperature. In contrast, plant and fish fats generally contain unsaturated fatty acids with kinks in their tails, preventing tight packing. As a result, unsaturated fats are usually liquid at room temperature and are referred to as oils. \"Partially hydrogenated oils\" on a food label indicate that unsaturated fats have been converted to saturated fats by adding hydrogen. This process also creates trans fats, which recent research links to health risks. The main function of fats is energy storage, with a gram of fat storing more than twice as much energy as a gram of polysaccharide. For immobile plants, the bulky energy storage form of starch is not an issue, while mobile animals, like humans, benefit from carrying their energy reserves in the form of fat. However, burning off excess fat requires more effort. A reasonable amount of body fat is normal and healthy, as it stores long-term fuel in adipose cells, which swell and shrink as fat is deposited and withdrawn. Additionally, fatty tissue cushions vital organs and insulates the body. **Terms to know** **Triglyceride**-A triglyceride is a type of fat found in your blood. It is made up of one glycerol molecule linked to three fatty acid molecules through dehydration reactions. Triglycerides are a form of energy storage in the body, and high levels can be an indicator of health issues. They are commonly mentioned on food labels and in medical tests for blood fat levels. **3.10 Phospholipids and steroids are important lipids with a variety of functions** Cells rely on phospholipids, the main component of cell membranes, for their existence. Phospholipids are structurally like fats but contain only two fatty acids attached to glycerol, instead of three. Additionally, a negatively charged phosphate group is attached to glycerol\'s third carbon. Phospholipids interact with water to form the structure of cell membranes. The two ends of a phospholipid have different relationships with water: the hydrophobic tails cluster together in the center, away from water, while the hydrophilic phosphate heads face the watery environment on either side. This arrangement results in a double-layered sheet. Various proteins are associated with these phospholipid membrane structures in cell membranes. Steroids are lipids with a carbon skeleton consisting of four fused rings. Cholesterol, a common component in animal cell membranes, is a precursor for other steroids, including sex hormones. Different steroids vary in the chemical groups attached to the rings. High cholesterol levels in the blood may contribute to atherosclerosis. **3.12 Proteins have a wide range of functions and structures** Proteins are essential for nearly every dynamic function in your body. They are polymers made up of amino acids, and among all of life\'s molecules, they are the most structurally and functionally complex and diverse. Your body contains tens of thousands of different proteins, with their most crucial role being enzymes. Enzymes are chemical catalysts that accelerate and regulate nearly all chemical reactions in your cells. Lactase is one example of an enzyme. Proteins have various roles in your body. Transport proteins in cell membranes move sugar molecules and nutrients into cells. Defensive proteins, like antibodies, travel through your bloodstream to support your immune system. Signal proteins, including hormones, help coordinate bodily activities. Receptor proteins in cell membranes receive and transmit signals into cells. Muscle cells contain contractile proteins, while structural proteins are present in the fibers of tendons and ligaments. Collagen, a structural protein, forms the long, strong fibers of connective tissues and makes up 40% of the protein in your body. Storage proteins provide amino acids to developing embryos. Examples of these proteins are found in eggs and seeds. The functions of different types of proteins depend on their unique shapes. Lysozyme, an enzyme found in sweat, tears, and saliva, has a globular shape. Lysozyme consists of one long polymer of amino acids, represented by the purple ribbon in a ribbon model. Its general shape is called globular. This overall shape is more apparent in a space-filling model of lysozyme, where the colors represent different atoms of carbon, oxygen, nitrogen, and hydrogen. The barely visible yellow balls represent sulfur atoms that form the stabilizing bonds shown as yellow lines in the ribbon model. Most enzymes and many other proteins are globular. Structural proteins, like those in hair, tendons, and ligaments, are typically long and thin, known as fibrous proteins. For example, a spider\'s web is made of fibrous silk proteins, which are stronger than steel strands of the same weight. Descriptions like globular and fibrous refer to a protein\'s general shape, but each protein also has a specific three-dimensional shape. The coils and twists of lysozyme\'s ribbon represent its unique structure. Nearly all proteins must recognize and bind to other molecules to function. Lysozyme destroys bacterial cells by binding to molecules on their surface, with its specific shape enabling it to recognize and attach to its target. The function of a protein depends on its shape. When a protein undergoes denaturation, it unravels and loses its specific shape, resulting in a loss of function. Excessive heat can cause denaturation, as seen when frying an egg, where the clear proteins around the yolk become solid, white, and opaque. In the right cellular environment, a newly synthesized amino acid chain folds into its functional shape. If a protein doesn\'t fold correctly, it can lead to diseases like Alzheimer\'s and Parkinson\'s, which involve the accumulation of misfolded proteins. Prions are infectious, misshapen proteins linked to serious degenerative brain diseases like mad cow disease. These diseases highlight the crucial relationship between a protein\'s unique three-dimensional shape and its proper function. **Terms to know**  **Enzyme**-A macromolecule, usually a protein, that serves as a biological catalyst, changing the rate of a chemical reaction without being consumed by the reaction. **3.13 Proteins are made from amino acids linked by peptide bonds** Amino acids, the monomers of proteins, each have an amino group and a carboxyl group, which are covalently bonded to a central carbon atom. The other two partners bonded to this carbon are a hydrogen atom and a variable chemical group, symbolized by the letter R. In the simplest amino acid, glycine, the R group is just a hydrogen atom. In other amino acids, the R group consists of one or more carbon atoms with various functional groups attached. All 20 amino acids are grouped based on whether their R groups are hydrophobic or hydrophilic. Hydrophobic amino acids have nonpolar R groups, like the C-H bonds in leucine (Leu). Hydrophilic amino acids have R groups that may be polar or charged. At cellular pH, the amino and carboxyl groups attached to the central carbon are usually in their ionized form. Cells link amino acids to form polymers through a dehydration reaction, which connects the carboxyl group of one amino acid to the amino group of the next, removing a water molecule. This covalent linkage is called a peptide bond. The product of this reaction is a dipeptide, made from two amino acids. Additional amino acids can be added to form a chain, known as polypeptides. Thousands of different proteins can be made from just 20 amino acids due to the sequence in which they are arranged. Like how varying the sequence of letters creates thousands of English words, the protein \"alphabet\" of 20 amino acids forms much longer \"words.\" Most polypeptides are at least 100 amino acids long, with some being 1,000 or more. Each polypeptide has a unique sequence of amino acids. A long polypeptide chain with a specific sequence is not the same as a functional protein, just as a strand of yarn is not the same as a knitted sweater. The R groups of amino acids influence protein structure by causing hydrophobic amino acids to cluster in the center of globular proteins and hydrophilic amino acids to face outward, aiding in solubility. Hydrogen bonds, ionic bonds, and disulfide bridges between R groups also help determine a protein\'s shape. The unique sequence of amino acids in a polypeptide dictates how the protein folds into its three-dimensional shape. **3.14 A protein\'s functional shape results from four levels of structure** The primary structure of a protein is the exact sequence of amino acids in its polypeptide chain. Segments of the chain coil or fold into local patterns, known as secondary structure. The overall three-dimensional shape of a protein is called tertiary structure. Proteins with more than one polypeptide chain have quaternary structure. **Terms to know**  **alpha helix**-a common protein structure characterized by a right-handed spiral, stabilized by hydrogen bonds between amino acids in the polypeptide chain. **pleated sheets**-also known as beta sheets, are a common protein structure where beta strands are connected laterally by hydrogen bonds, forming a sheet-like arrangement. **quaternary structure**-The fourth level of protein structure; the shape resulting from the association of two or more polypeptide subunits. **3.15 The nucleic acids DNA and RNA are information-rich polymers of nucleotides** The primary structure of a polypeptide determines the shape of a protein, and this structure is determined by the amino acid sequence, which is programmed by genes. Genes consist of DNA, a type of nucleic acid found in the nuclei of cells. The other type of nucleic acid is RNA, which helps assemble polypeptides according to DNA\'s instructions. Nucleic acids are crucial for the storage, transfer, and expression of hereditary information. **Monomers of Nucleic Acids** Nucleic acids are made up of monomers called nucleotides. Each nucleotide consists of three parts: a five-carbon sugar, a phosphate group, and a nitrogenous base. - **Five-Carbon Sugar**: In DNA, the sugar is deoxyribose, while in RNA, it is ribose. The difference between these sugars is that deoxyribose lacks one oxygen atom compared to ribose. - **Phosphate Group**: This group is negatively charged and is linked to one side of the sugar in both DNA and RNA nucleotides. - **Nitrogenous Base**: This is a molecular structure containing nitrogen and carbon. The nitrogen atoms tend to take up H+ in aqueous solutions, which is why it is called a nitrogenous base. DNA nucleotides have four different nitrogenous bases: adenine (A), thymine (T), cytosine (C), and guanine (G). RNA nucleotides also contain adenine, cytosine, and guanine, but have uracil (U) instead of thymine. **Nucleotide Polymers** A nucleic acid polymer, or polynucleotide, is formed from its monomers through dehydration reactions. In this process, the sugar of one nucleotide bonds to the phosphate group of the next, creating a repeating sugar-phosphate backbone. The nitrogenous bases are not part of this backbone. RNA usually consists of a single polynucleotide strand, while DNA contains two polynucleotides that form a double helix. The nitrogenous bases protrude from the sugar-phosphate backbones and pair in the center: A with T, and C with G. Hydrogen bonds between paired bases hold the two DNA strands together, making them complementary. If one strand has the sequence -AGCACT-, the other will be -TCGTGA-. The genetic material inherited by humans and other organisms is DNA, which resides in cells as long structures called chromosomes, each carrying many genes. DNA uniquely provides directions for its own replication. During cell division, DNA makes two identical copies of each chromosome. Complementary base pairing is crucial in this process, as the double helix unzips and new complementary strands form along the separated strands, ensuring genetic instructions are passed to each daughter cell. The instructions in DNA program all a cell\'s activities by directing the synthesis of proteins. DNA and RNA play crucial roles in the production of proteins, a process known as gene expression. The instructions in DNA program all a cell\'s activities by directing the synthesis of proteins through a process called gene expression. Here\'s a breakdown of how it works: - **Transcription**: A gene in the DNA directs the synthesis of an RNA molecule. This process is called transcription. During transcription, the base-pairing rules ensure that the information is accurately copied from DNA to RNA. The only difference is that in RNA, uracil (U) pairs with adenine (A) instead of thymine (T). - **Translation**: The RNA molecule then interacts with the cell\'s protein-building machinery. This is where the gene\'s instructions, written in \"nucleic acid language,\" are translated into \"protein language,\" which is the amino acid sequence of a polypeptide. - **Gene Expression**: The flow of genetic information from DNA to RNA to protein is summarized as DNA→RNA→protein. This process illustrates the crucial biological theme of information transfer within cells. Complementary base pairing is crucial for relaying information from DNA to RNA. During transcription, the base pairs in DNA match up with complementary nucleotides in RNA, ensuring accurate information transfer. However, base pairing can also occur within RNA molecules themselves. This internal base pairing allows RNA to fold into specific three-dimensional shapes necessary for their various functions. An organism\'s genes determine the proteins and thus the structures and functions of its body. **Terms to know**