Discussion: DNA Double-Helical Structure

Discussion: DNA Double-Helical Structure

Discussion: DNA Double-Helical Structure

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Nucleic Acids

Nucleic acids are key macromolecules in the continuity of life. They carry the genetic blueprint of a cell and carry instructions for the functioning of the cell.

The two main types of nucleic acids are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). DNA is the genetic material found in all living organisms, ranging from single-celled bacteria to multicellular mammals.

The other type of nucleic acid, RNA, is mostly involved in protein synthesis. The DNA molecules never leave the nucleus but instead use an RNA intermediary to communicate with the rest of the cell. Other types of RNA are also involved in protein synthesis and its regulation.

DNA and RNA are made up of monomers known as nucleotides. The nucleotides combine with each other to form a polynucleotide, DNA or RNA. Each nucleotide is made up of three components: a nitrogenous base, a pentose (five-carbon) sugar, and a phosphate group (Figure 22). Each nitrogenous base in a nucleotide is attached to a sugar molecule, which is attached to a phosphate group.

DNA Double-Helical Structure

DNA has a double-helical structure (Figure 23). It is composed of two strands, or polymers, of nucleotides. The strands are formed with bonds between phosphate and sugar groups of adjacent nucleotides. The strands are bonded to each other at their bases with hydrogen bonds, and the strands coil about each other along their length, hence the “double helix” description, which means a double spiral.

The alternating sugar and phosphate groups lie on the outside of each strand, forming the backbone of the DNA. The nitrogenous bases are stacked in the interior, like the steps of a staircase, and these bases pair; the pairs are bound to each other by hydrogen bonds. The bases pair in such a way that the distance between the backbones of the two strands is the same all along the molecule.

Key Terms

acid a substance that donates hydrogen ions and therefore lowers pH

adhesion the attraction between water molecules and molecules of a different substance

amino acid a monomer of a protein

anion a negative ion formed by gaining electrons

atomic number the number of protons in an atom

base a substance that absorbs hydrogen ions and therefore raises pH

buffer a solution that resists a change in pH by absorbing or releasing hydrogen or hydroxide ions

carbohydrate a biological macromolecule in which the ratio of carbon to hydrogen to oxygen is 1:2:1; carbohydrates serve as energy sources and structural support in cells

cation a positive ion formed by losing electrons

cellulose a polysaccharide that makes up the cell walls of plants and provides structural support to the cell

chemical bond an interaction between two or more of the same or different elements that results in the formation of molecules

chitin a type of carbohydrate that forms the outer skeleton of arthropods, such as insects and crustaceans, and the cell walls of fungi

cohesion the intermolecular forces between water molecules caused by the polar nature of water; creates surface tension

covalent bond a type of strong bond between two or more of the same or different elements; forms when electrons are shared between elements

denaturation the loss of shape in a protein as a result of changes in temperature, pH, or exposure to chemicals

deoxyribonucleic acid (DNA) a double-stranded polymer of nucleotides that carries the hereditary information of the cell

disaccharide two sugar monomers that are linked together by a peptide bond

electron a negatively charged particle that resides outside of the nucleus in the electron orbital; lacks functional mass and has a charge of –1

electron transfer the movement of electrons from one element to another

element one of 118 unique substances that cannot be broken down into smaller substances and retain the characteristic of that substance; each element has a specified number of protons and unique properties

enzyme a catalyst in a biochemical reaction that is usually a complex or conjugated protein

evaporation the release of water molecules from liquid water to form water vapor

fat a lipid molecule composed of three fatty acids and a glycerol (triglyceride) that typically exists in a solid form at room temperature

glycogen a storage carbohydrate in animals

hormone a chemical signaling molecule, usually a protein or steroid, secreted by an endocrine gland or group of endocrine cells; acts to control or regulate specific physiological processes

hydrogen bond a weak bond between partially positively charged hydrogen atoms and partially negatively charged elements or molecules

hydrophilic describes a substance that dissolves in water; water-loving

hydrophobic describes a substance that does not dissolve in water; water-fearing

ion an atom or compound that does not contain equal numbers of protons and electrons and therefore has a net charge

ionic bond a chemical bond that forms between ions of opposite charges

isotope one or more forms of an element that have different numbers of neutrons

lipids a class of macromolecules that are nonpolar and insoluble in water

litmus paper filter paper that has been treated with a natural water-soluble dye so it can be used as a pH indicator

macromolecule a large molecule, often formed by polymerization of smaller monomers

mass number the number of protons plus neutrons in an atom

matter anything that has mass and occupies space

monosaccharide a single unit or monomer of carbohydrates

neutron a particle with no charge that resides in the nucleus of an atom; has a mass of 1

nonpolar covalent bond a type of covalent bond that forms between atoms when electrons are shared equally between atoms, resulting in no regions with partial charges as in polar covalent bonds

nucleic acid a biological macromolecule that carries the genetic information of a cell and carries instructions for the functioning of the cell

nucleotide a monomer of nucleic acids; contains a pentose sugar, a phosphate group, and a nitrogenous base

nucleus (chemistry) the dense center of an atom made up of protons and (except in the case of a hydrogen atom) neutrons

octet rule states that the outermost shell of an element with a low atomic number can hold eight electrons

oil an unsaturated fat that is a liquid at room temperature

periodic table of elements an organizational chart of elements, indicating the atomic number and mass number of each element; also provides key information about the properties of elements

pH scale a scale ranging from 0 to 14 that measures the approximate concentration of hydrogen ions of a substance

phospholipid a major constituent of the membranes of cells; composed of two fatty acids and a phosphate group attached to the glycerol backbone

polar covalent bond a type of covalent bond in which electrons are pulled toward one atom and away from another, resulting in slightly positive and slightly negative charged regions of the molecule

polypeptide a long chain of amino acids linked by peptide bonds

polysaccharide a long chain of monosaccharides; may be branched or unbranched

protein a biological macromolecule composed of one or more chains of amino acids

proton a positively charged particle that resides in the nucleus of an atom; has a mass of 1 and a charge of +1

radioactive isotope an isotope that spontaneously emits particles or energy to form a more stable element

ribonucleic acid (RNA) a single-stranded polymer of nucleotides that is involved in protein synthesis

saturated fatty acid a long-chain hydrocarbon with single covalent bonds in the carbon chain; the number of hydrogen atoms attached to the carbon skeleton is maximized

solvent a substance capable of dissolving another substance

starch a storage carbohydrate in plants

steroid a type of lipid composed of four fused hydrocarbon rings

surface tension the cohesive force at the surface of a body of liquid that prevents the molecules from separating

temperature a measure of molecular motion

trans-fat a form of unsaturated fat with the hydrogen atoms neighboring the double bond across from each other rather than on the same side of the double bond

triglyceride a fat molecule; consists of three fatty acids linked to a glycerol molecule

unsaturated fatty acid a long-chain hydrocarbon that has one or more than one double bonds in the hydrocarbon chain

van der Waals interaction a weak attraction or interaction between molecules caused by slightly positively charged or slightly negatively charged atoms