Nucleic Acids and Proteins PDF - Biology Textbook

AREA OF STUDY 1 WHAT IS THE ROLE OF NUCLEIC ACIDS AND PROTEINS IN MAINTAINING LIFE? The relationship between 1 nucleic acids and proteins KEY KNOWLEDGE In this topic, you will investigate: The relationship between nucleic acids and proteins nucleic acids as information molecules that encode instructions for the synthesis of proteins: the structure of DNA, the three main forms of RNA (mRNA, rRNA and tRNA) and comparison of their respective nucleotides the genetic code as a universal triplet code that is degenerate and the steps in gene expression, including transcription, RNA processing in eukaryotic cells and translation by ribosomes the structure of genes: exons, introns and promoter and operator regions the basic elements of gene regulation: prokaryotic trp operon as a simplified example of the regulatory process amino acids as the monomers of a polypeptide chain and the resultant hierarchical levels of structure that give rise to a functional protein proteins as a diverse group of molecules that collectively make an organism’s proteome, including enzymes as catalysts in biochemical pathways the role of rough endoplasmic reticulum, Golgi apparatus and associated vesicles in the export of proteins from a cell via the protein secretory pathway. Source: VCE Biology Study Design (2022–2026) extracts © VCAA; reproduced by permission. PRACTICAL WORK AND INVESTIGATIONS Practical work is a central component of learning and assessment. Experiments and investigations, supported by a practical investigation eLogbook and teacher-led videos, are included in this topic to provide opportunities to undertake investigations and communicate findings. 1.1 Overview Numerous videos and interactivities are available just where you need them, at the point of learning, in your digital formats, learnON and eBookPLUS, and at www.jacplus.com.au. 1.1.1 Introduction Cells are what define us as living organisms, allowing FIGURE 1.1 RNA polymerase unwinding a DNA us to reproduce, adapt, survive and grow. Understanding strand (seen in violet) and building a new RNA the structure and components of cells is fundamental to strand (seen in red) in the process of transcription understanding life itself. How do we control what enters and leaves our cells? How do we synthesise the proteins that allow us to thrive? Our cells are incredible in their ability not only to synthesise proteins, but also to regulate their production so they are only made where and when they are required. Proteins are fundamental to our survival, forming diverse structures with a variety of functions and making up everything from our enzymes to our fibrous tissues. In this topic, we will examine our cells’ amazing ability to make proteins from a DNA blueprint. DNA is the basis of the incredible diversity of life on Earth. LEARNING SEQUENCE 1.1 Overview.................................................................................................................................................................................................. 4 1.2 BACKGROUND KNOWLEDGE Reviewing cells.......................................................................................................................5 1.3 Nucleic acids as information molecules................................................................................................................................... 18 1.4 The genetic code and protein synthesis................................................................................................................................... 28 1.5 The structure of genes..................................................................................................................................................................... 40 1.6 Gene regulation.................................................................................................................................................................................. 45 1.7 Amino acids and polypeptides..................................................................................................................................................... 52 1.8 The proteome...................................................................................................................................................................................... 61 1.9 Organelles involved in the protein secretory pathway......................................................................................................... 65 1.10 Review................................................................................................................................................................................................... 72 Resources Resourceseses eWorkbook eWorkbook — Topic 1 (ewbk-1880) Practical investigation eLogbook Practical investigation eLogbook — Topic 1 (elog-0001) Digital documents Key science skills — VCE Biology Units 1–4 (doc-34326) Key terms glossary — Topic 1 (doc-34409) Key ideas summary — Topic 1 (doc-34412) Exam question booklet Exam question booklet — Topic 1 (eqb-0012) 4 Jacaranda Nature of Biology 2 VCE Units 3 & 4 Sixth Edition 1.2 BACKGROUND KNOWLEDGE Reviewing cells BACKGROUND KNOWLEDGE This subtopic will review concepts from Units 1 and 2 that will help you understand various key knowledge points of the Study Design for Units 3 and 4. This content is not examinable in Units 3 and 4. Cells as the basic structural feature of life on Earth, including the distinction between prokaryotic and eukaryotic cells The structure and specialisation of plant and animal cell organelles for distinct functions The structure and function of the plasma membrane in the passage of water, hydrophobic and hydrophilic substances via osmosis, facilitated diffusion and active transport Source: Adapted from VCE Biology Study Design (2022–2026) extracts © VCAA; reproduced by permission. 1.2.1 What are cells? Cells are the basic structural and functional units of life, and all living organisms are built of one or more cells. Cells, with only a very few exceptions, are too small to be seen with an unaided eye. Throughout Units 3 and 4, you will learn about the structure and function of various types of cells. FIGURE 1.2 Comparing the size of various cells, organelles and non-cellular pathogens. Each diagram zooms in on the previous diagram. Red Human egg blood cell 130 μm Mitochondrion Sperm cell HIV Ribosome 60 × 5 μm Skin cell E coli bacterium Yeast cell 30 nm 130 nm Yeast cell 30 μm 3 × 0.6 μm 3 × 4 μm Red blood cell Mitochondrion Inﬂuenza virus Measles virus 8 μm 4 × 0.8 μm 130 nm 220 nm How big are cells? Cells and organisms vary greatly in size. Some examples of these are compared in figure 1.2. Most animal cells fall within the size range of 10 to 40 μm. Among the smallest human cells are red blood cells, with diameters for normal cells in the range of 6 to 8 μm. Only a few single cells are large enough to see with the unaided human eye, for example human egg cells with diameters around 0.1 mm. The common amoeba (Amoeba proteus) is a unicellular organism with an average size ranging from 0.25 to 0.75 mm. (You would see an amoeba as about the size of a full stop.) Microbial cells are on average 10 times smaller than plant and animal cells, with diameters in the range of 0.4 to 2.0 µm and lengths in the range of 0.5 to 5 µm. (However, the smallest bacterium, Pelagibacter ubique, consists of a cell just 0.2 µm in diameter.) Cells constantly need to transport materials as they exchange ions with the extracellular environment, gain nutrients and remove wastes. Cells need to be small in order to maximise their surface area to volume ratio, allowing for the movement of ions, nutrients and wastes to occur quickly. Without a large surface area to volume ratio, cells will not survive. There is no fixed shape for cells. Cells vary in shape and their shapes often reflect their functions. For example, immune cells such as dendritic cells and macrophages have very different cellular shapes compared to other cells such as red blood cells. TOPIC 1 The relationship between nucleic acids and proteins 5 FIGURE 1.3 a. Various shapes of some different cells b. A 3D representation of a dendritic cell a. b. Monocyte Osteoclast Dendritic cell Macrophage 1.2.2 Comparing eukaryotic and prokaryotic cells Prokaryotes and eukaryotes Although cells vary greatly in complexity, they can be distinguished into two main types — prokaryotic cells and eukaryotic cells. FIGURE 1.4 Distinction of different cell types. Note that the term protista or protoctista is also sometimes used to describe eukaryotes that are not animals, plants or fungi. Cells Prokaryotic Eukaryotic cells cells Bacteria Archaea Animalia Plantae Fungi Chromista Protozoa The microscopically tiny creatures that we call ‘microbes’ are a diverse group of organisms. The microbes comprise two different classification groups, namely bacteria and archaea. The cells of all these microbes can be distinguished from the cells of the other major groups of living organisms: fungi, plants and animals. The key distinguishing feature of archaea and bacteria is that their cells lack a membrane-bound nucleus (see figures 1.5 and 1.6). Cells with this characteristic are described as prokaryotic cells, prokaryotic cells cells within and organisms displaying this feature are called prokaryotes. Like all other kinds of prokaryotes that lack a membrane- organism, archaea and bacteria have DNA in their cells, but the DNA in prokaryotic bound nucleus cells is dispersed, not enclosed within a separate membrane-bound compartment. eukaryotic cells cells within eukaryotes that have a membrane- In contrast, the cells of all other organisms have a definite nucleus (see figure 1.5) bound nucleus and other bordered by a double membrane. Organisms with these characteristics are called membrane-bound organelles eukaryotes. The nucleus of a eukaryotic cell contains DNA, the genetic material of prokaryotes any cells or organisms without a membrane-bound nucleus cells. Eukaryotic cells contain many membrane-bound cell organelles that are not eukaryotes any cells or organisms present in prokaryotic cells. with a membrane-bound nucleus 6 Jacaranda Nature of Biology 2 VCE Units 3 & 4 Sixth Edition “c01TheRelationshipBetweenNucleicAcidAndProteins_PrintPDF” — 2021/6/15 — 13:05 — page 7 — #5 FIGURE 1.5 Comparing the structure of a typical prokaryotic cell with that of a eukaryotic cell. Note that a prokaryotic cell has a simple architecture in contrast to a eukaryotic cell. Eukaryotic cell Prokaryotic cell Nucleus Capsule Ribosomes Ribosomes Golgi DNA apparatus Cell wall Endoplasmic reticulum Plasma Plasma membrane membrane Mitochondria Cytoplasm Cytoplasm Lysosome FIGURE 1.6 a. A prokaryotic cell in the process of dividing. Note the dispersed genetic material (stained red). b. A confocal fluorescence microscope view of eukaryotic cells (human breast cells). Note the discrete rounded nuclei (shown in blue). (Image courtesy of Leigh Ackland) a. b. The distinguishing feature between a prokaryotic cell and a eukaryotic cell is the absence of a membrane-bound nucleus in prokaryotes. This can be linked back to the naming of prokaryotes and eukaryotes: pro = before + karyon = nut, kernel, nucleus eu = well (= true) + karyon = nut, kernel, nucleus. TOPIC 1 The relationship between nucleic acids and proteins 7 TABLE 1.1 Comparison of prokaryotic and eukaryotic cells Feature Prokaryote Eukaryote Size Small: typically 12 µm diameter Larger: typically in range 10–100 µm Chromosomes Present as single circular DNA molecule Present as multiple linear DNA molecules Ribosomes Present; small size (20 nm or 70S)* Present; large size (25–30 nm or 80S)* Cell membrane Present Present Cell wall Present and chemically complex Present in plants, fungi, and some protists, but chemically simple; absent in animal cells Membrane-bound nucleus Absent Present Membrane-bound cell Absent Present, e.g. lysosomes, mitochondria organelles Cytoskeleton Absent Present Number of cells Unicellular Usually multicellular but can be unicellular (e.g. protists such as Amoeba and Euglena, algae such as Chlorella and yeasts) * S denotes Svedberg units, which measure the time it takes for a particle to settle to the bottom of a solution. For ribosomes, this time can be correlated to particle size. Although there are some differences in aspects of the structure of eukaryotic and prokaryotic cells, there are many similarities in their structures and functioning. Both prokaryotic and eukaryotic cells: have DNA as their genetic material have cell membranes that selectively control the entry and exit of dissolved materials into and out of the cell use the same chemical building blocks, including carbon, nitrogen, oxygen, hydrogen and phosphorus, to build the organic molecules that form their structure and enable their function produce proteins through the same mechanism (transcription and translation) use ATP as their source of energy to drive the energy-requiring activities of their cells. Resources Resourceseses eWorkbook Worksheet 1.1 Reviewing cell size and surface area to volume ratios (ewbk-1962) Video eLesson Living organisms are made of cells (eles-4165) 1.2.3 Organelles Eukaryotic cells are organised internally into various compartments, each enclosed by a membrane. These compartments are known as organelles and give eukaryotes a much more complex structure than prokaryotes. Compartmentalisation in eukaryotic cells is about efficiency. Separating the cell into specific components allows for the creation of specific microenvironments within a cell. That way, each organelle can have all the advantages it needs to perform to the best of its ability. Think about how a house is subdivided into rooms to support different functions: you shower in the bathroom, not in the kitchen; the stove is in the kitchen, not in the bedroom, and so on. A eukaryotic cell can be likened to a house — its various compartments are like different rooms where different tasks are carried out. The conditions (such as pH and ion concentration) within the different kinds of compartment can vary from each other and from the cellular environment in which they are found. 8 Jacaranda Nature of Biology 2 VCE Units 3 & 4 Sixth Edition Some of the main organelles in eukaryotes are explored in figure 1.7 and table 1.2. When writing organelle names, you should try to write the full name of the organelle to reduce the chance for confusion. Many of these organelles will be further explored in subtopic 1.9. FIGURE 1.7 Two examples of eukaryotic cells: a. a generalised animal cell; b. a generalised plant cell a. Animal cell b. Plant cell Cytosol Protein Nucleolus Mitochondrion Endoplasmic Nuclear reticulum ﬁlament Nucleus Cytosol Ribosome envelope Plasma (also on membrane endoplasmic Nucleus reticulum) Lysosome Mitochondrion Endoplasmic Nuclear envelope reticulum Nucleolus Golgi Plasma Ribosome apparatus Endosome membrane Endoplasmic Vesicle reticulum Cell wall Peroxisome Lysosome Peroxisome Centriole Microtubule Protein Vacuole microtubule Chloroplast Golgi apparatus Filament Vesicle TABLE 1.2 Summary of the structure and function of various organelles General role Organelle Structure Function Storage and Nucleus Membrane surrounded by Houses chromosomes, made transcription of the nuclear envelope (double of chromatin (DNA, the genetic genetic membrane), perforated by material and proteins) information nuclear pores Contains the nucleolus, where The nuclear envelope ribosomal sub-units are is continuous with the synthesised and assembled endoplasmic reticulum. Nuclear pores regulate entry and exit of materials including proteins and RNA. Ribosomes Consist of two major Protein synthesis components: the small The small ribosomal subunits ribosomal sub-units and the read the mRNA, and the large large sub-units. subunits join amino acids to Ribosomes can be located form a polypeptide chain. within the cytosol or bound to the endoplasmic reticulum. Endomembrane Endoplasmic Part of an interconnected Smooth endoplasmic reticulum system reticulum network of flattened, is involved in the synthesis regulates the membrane-enclosed sacs or of lipids, metabolism of synthesis tube-like structures carbohydrates, calcium storage, and transport The membranes of the and detoxication of drugs and of specific endoplasmic reticulum are poisons. proteins continuous with the outer Rough endoplasmic reticulum nuclear membrane. is involved in the synthesis of specific proteins from bound ribosomes. Golgi apparatus Consists of a collection of fused, Modifies, sorts, tags, packages flattened sacs, enclosed in a and distributes proteins to be single membrane secreted via vesicles. (continued) TOPIC 1 The relationship between nucleic acids and proteins 9 TABLE 1.2 Summary of the structure and function of various organelles (continued) General role Organelle Structure Function Digestion, Lysosomes Membrane-enclosed sacs of Breaks down ingested breakdown and hydrolytic enzymes (found only substances, cell macromolecules storage in animal cells) and damaged organelles for recycling. Vacuoles Large membrane-bound vesicles Digestion, storage, waste (found only in plant cells) disposal, water balance, cell growth and protection Peroxisomes Metabolic compartment bound by An enzyme that transfers a single membrane hydrogen to water, producing hydrogen peroxide (H2 O2 ) as a by-product, which is converted to water by other enzymes in the peroxisomes Conversion Mitochondria Bound by a double membrane; The site of cellular respiration, of inorganic the inner membrane has a series where ATP synthesis occurs for to organic of folds (cristae) containing the cell compounds enzymes for ATP synthesis. Chloroplast Double membrane around Conducts photosynthesis, fluid stroma, which contains a process by which inorganic membranous thylakoids stacks compounds are converted (sacs) in the grana (found only in to chemical energy, resulting in plant cells) the production of oxygen and energy-rich organic compounds (simple and complex sugars). Controlling the Plasma or cell Consists of a phospholipid bilayer The structural boundary that entry and exit of membrane with transport and receptor controls the entry of raw materials substances proteins, enclosed by a single into the cell, such as amino acids, membrane the building blocks of proteins INVESTIGATION 1.1 elog-0003 Viewing and staining cells Aim To describe the microscopic structure of a variety of cells and show the effect of staining on distinguishing organelles Resources Resourceseses eWorkbook Worksheet 1.2 Labelling organelles (ewbk-1964) Weblink Cells resources 10 Jacaranda Nature of Biology 2 VCE Units 3 & 4 Sixth Edition 1.2.4 The plasma membrane The cells of all living organisms have boundaries that separate their internal environments from their surroundings. From single-celled organisms, such as amoebae or bacteria, to multicellular organisms, such as mushrooms, palm trees and human beings, each of their living cells has an active boundary called the plasma membrane, also known as the cell membrane. The plasma membrane is the active boundary around all living cells, consisting of a phospholipid bilayer and associated proteins, that separates the cell contents from their external environment. The function of the membrane The plasma membrane boundary can be thought of as a busy gatekeeper selectively controlling the entry and exit of materials into and out of cells. As such, the plasma membrane is said to be semi-permeable or selectively permeable, meaning that it allows only some substances to cross it — in or out. It can exclude some substances from entering the cell while permitting entry of other substances and allowing for the elimination of certain substances. Without such a boundary, life could not exist, and indeed could not have evolved. The plasma membrane carries out several important functions for a cell. The plasma membrane: is an active and selective boundary denotes cell identity (which is vital in the immune response) receives external signals transports materials. The plasma membrane forms the active boundary of a cell, separating the cell from its external environment and other cells. The plasma membrane forms the boundary of a compartment in which the cytosol — the internal environment of a living cell — can be held within a narrow range of conditions that plasma membrane partially are different from those of the external environment. permeable boundary of a cell controlling entry to and exit of Within the cell, similar membranes form the active boundaries of cell organelles, substances from a cell including the nucleus, the endoplasmic reticulum, the Golgi apparatus and semi-permeable allows only lysosomes. In other cell organelles, such as mitochondria and chloroplasts, certain molecules to cross by membranes form both the external boundary and part of the internal structure. diffusion Because of the presence of their membrane boundaries, membrane-bound cell selectively permeable another term for semi-permeable, where organelles can maintain internal environments that differ from those in the only particular molecules can surrounding cytosol and so can perform different functions. pass through endoplasmic reticulum cell Transporting materials organelle consisting of a system of membrane-bound channels As mentioned, the membrane is vital to the transportation of materials, acting as a that transport substances within semi-permeable barrier. Various factors affect a substance’s ability to cross a the cell membrane, including: Golgi apparatus organelle molecular size that packages material into vesicles for export from a cell charge (positive or negative) (also known as Golgi complex or solubility in aqueous solution (hydrophobic/nonpolar, or hydrophilic/ Golgi body) polar) lysosomes membrane-bound concentration gradient. vesicles containing digestive enzymes hydrophobic substances that Hydrophilic (water-loving) molecules dissolve readily in water. tend to be insoluble in water; also termed nonpolar Hydrophobic (water-fearing) molecules are usually lipophilic (lipid-loving) and hydrophilic substances that dissolve readily in organic solvents such as benzene. dissolves easily in water; also termed polar TOPIC 1 The relationship between nucleic acids and proteins 11 FIGURE 1.8 Diagram showing the semi-permeable nature of a phospholipid bilayer membrane Gases Small Nonpolar Large Ions Charged uncharged molecules uncharged polar polar polar molecules molecules molecules CO2 Ethanol Oestrogen Glucose K+ Amino acids N2 H2O Benzene Sucrose Mg 2+ ATP O2 Ca 2+ Substances can cross a membrane by several different methods. These can be passive simple diffusion the movement (not requiring energy) or active (requiring energy). of substances from a region of higher concentration to one of lower Passive methods include the following: concentration of that substance; that is, down its concentration Simple diffusion is the means of transport of small lipophilic substances. Water gradient can also move across the plasma membrane by diffusion; this is a special case of osmosis a specialised process of diffusion known as osmosis. passive transport in which water Facilitated diffusion involves protein transporters and is the means of transport molecules move across a partially permeable membrane from an area of dissolved hydrophilic substances down their concentration gradients. of high water (low solute) to an area of low water (high solute) Active methods include the following: facilitated diffusion form of Active transport involves protein transporters known as pumps and is the means diffusion involving a specific carrier molecule for the substance of transport of dissolved hydrophilic substances against their concentration active transport net movement of gradients. dissolved substances across a cell Bulk transport of macromolecules and fluid includes: membrane by an energy-requiring endocytosis (movement into the cell) process that moves substances against a concentration gradient exocytosis (movement out of the cell). from a region of lower to higher concentration These are summarised in table 1.3. endocytosis an energy-requiring process of bulk transport, in which solids or liquids move into the cell Resources by engulfment Resourceseses exocytosis an energy-requiring Video eLesson Mechanisms of membrane transport (eles-2463) process of bulk transport, in which Interactivity Movement across membranes (int-0109) solids or liquids move out of the cell via vesicles 12 Jacaranda Nature of Biology 2 VCE Units 3 & 4 Sixth Edition TABLE 1.3 Summary of modes of transport Simple Facilitated Active diffusion Osmosis diffusion transport Bulk transport Direction of High to low High to low High to low Low to high High to low movement concentration concentration concentration concentration of concentration of solute of water of solute solute of solute (high water = low dissolved solute) Is energy No No No Yes Yes required? Extra None None Protein Protein pumps Vesicles requirements transporter Use Nonpolar Water Large polar Movement Bulk movement molecules and and charged of various of various small polar molecules molecules molecules, such molecules against the as the transport concentration of synthesised gradient proteins from a cell via vesicles The structure of the membrane FIGURE 1.9 Coloured transmission electron micrograph Small, but vitally important, the plasma membrane (TEM) showing the plasma membrane (red) of an is just 8 nanometres (nm) wide and so is only intestinal brush border. In the inset, the structure of visible using a transmission electron microscope the membrane is evident. (TEM). A TEM image of the plasma membrane (as seen in figure 1.9) has a ‘train track’ appearance with two dark lines separated by a more lightly stained region. These images were important clues in elucidating the structure of the plasma membrane. The plasma membrane has the following major components, which can be observed in figure 1.10: phospholipids. Various kinds of phospholipids are the main structural components of the plasma membrane. They are organised as two layers (leaflets). proteins. Some proteins are embedded in the plasma membrane; others are attached at the membrane surfaces. carbohydrate groups. These are attached to phospholipids major type some lipids, forming glycolipids, and to some of lipid found in plasma proteins, forming glycoproteins. Both of these membranes and the main structural component of plasma occur at the membrane surfaces. membranes proteins macromolecules built of amino acid sub-units and linked by peptide bonds to form a chain, sometimes termed a polypeptide carbohydrate groups molecules that are associated with the plasma membrane and are associated with cell to cell communication and signalling TOPIC 1 The relationship between nucleic acids and proteins 13 The fluid mosaic model The fluid mosaic model describes the structure of the plasma membrane. This model also applies to the membranes that form the outer boundary of cell organelles, such as the membranes that surround the cell nucleus and other cell organelles. The fluid mosaic model proposes that the plasma membrane and other intracellular membranes should be considered as fluid layers in which proteins are embedded. The term ‘fluid’ comes from the fact that the fatty chains of the phospholipids are like a thick oily fluid, and the term ‘mosaic’ comes from the fact that the external surface (when viewed from above) has the appearance of a mosaic because of the various embedded proteins set in a uniform background. FIGURE 1.10 Diagram showing the fluid mosaic model of membrane structure Peripheral protein Exterior Carbohydrate Glycoprotein Glycolipid Integral Leaﬂets protein Phospholipid bilayer Hydrophobic core Fatty acid tails Hydrophilic Integral polar head Cytosol protein Peripheral Cholesterol proteins Phospholipids The plasma membrane consists of a double layer (bilayer) of phospholipids. Each phospholipid molecule consists of two fatty acid chains joined to a phosphate-containing group. The phosphate-containing group forms the water-loving (hydrophilic or polar) head of the molecule. The fatty acid chains constitute the water-fearing (hydrophobic or nonpolar) tail of each phospholipid molecule. As seen in figures 1.10 fluid mosaic model a model and 1.11, the two layers of phospholipids in a plasma membrane are arranged so which proposes that the plasma that the hydrophilic polar heads are exposed to both the external environment of the membrane and other intracellular cell and the cytosol (the internal environment of the cell). In contrast, the two membranes should be considered layers of hydrophobic nonpolar tails face each other in the central region of the plasma as two-dimensional fluids in which proteins are embedded membrane. 14 Jacaranda Nature of Biology 2 VCE Units 3 & 4 Sixth Edition FIGURE 1.11 a. Chemical structure of a phospholipid b. Diagram showing part of the bilayer of phospholipid molecules in the plasma membrane a. b. External environment + CH3 H3C – N – CH3 Hydrophilic H–C–H Hydrophilic polar head H–C–H O polar head – O–P–O Phosphate group O H H H–C C C–H H O O C=O C=O Hydrophobic H–C–H H–C–H Hydrophobic H–C–H H–C–H nonpolar tail H–C–H H–C–H nonpolar tails H–C–H H–C–H H–C–H H–C–H H–C–H H–C–H H–C–H H–C–H C– H–C–H H C– H–C–H H– H C– H–C–H H H– C– H–C–H H– H H– C– H H–C–H Cytosol C– H–C–H H– H C– H– H H–C–H C– H– H H–C–H C– H– H H–C–H C– H H–C–H H FIGURE 1.12 Ribbon model of a Proteins trans-membrane protein Proteins form the second essential part of the structure of the plasma membrane. Many different kinds of protein make up the plasma membrane. They can be broadly grouped into: integral proteins peripheral proteins. Integral proteins, as their name implies, are fundamental components of the plasma membrane. These proteins are embedded in the phospholipid bilayer. Typically, they span the width of the plasma membrane, with part of the protein being exposed on both sides of the membrane; these proteins are described as being trans-membrane. Trans-membrane proteins serve many functions, including as transporters, receptors, channels and carriers. Integral proteins can be separated from the plasma membrane only by harsh treatments that disrupt the phospholipid bilayer, such as treatment with strong detergents. integral proteins proteins Figure 1.12 shows a ribbon model of a trans-membrane protein that functions that are embedded in the phospholipid bilayer as an acid-sensing (H+ ) ion channel. The red and blue lines are not part of the trans-membrane proteins protein but indicate the extracellular (red) and the cytoplasmic (blue) sides of the proteins that are embedded plasma membrane. Most of the protein extends into the extracellular space, some is within and span the plasma embedded in the plasma membrane, and a small part lies within the cell. membrane, allowing them to have parts exposed to both the Peripheral proteins are either anchored to the exterior of the plasma membrane intracellular and extracellular environment through bonding with lipids, or indirectly associated with the plasma membrane peripheral proteins proteins through interactions with integral proteins in the membrane. that are anchored to the exterior of the plasma membrane through bonding with either lipids or integral proteins TOPIC 1 The relationship between nucleic acids and proteins 15 Carbohydrates In many cases, carbohydrate groups, such as sugars, are attached to the FIGURE 1.13 A computer exposed parts of proteins on the outer side of the membrane, creating generated image of combinations called glycoproteins (see figure 1.13). Some carbohydrates the plasma membrane. instead covalently link directly to the lipids in the membrane; these are Glycoproteins and glycolipids referred to as glycolipids. are shown in red. Carbohydrates on the cell surface have many functions, including: cell-to-cell communication acting as receptors, distinguishing cells as ‘self’ (a feature that is vital in the immune system, which will be covered in Topic 5). Glycoproteins in particular are of vital importance in immune recognition and include molecules of the major histocompatibility complex, which is found on the surface of all nucleated cells. The prefix ‘glyco’ means sugar. Sugars attached to a protein = glycoprotein Sugars attached to a lipid = glycolipid FIGURE 1.14 Diagram showing two integral proteins embedded in and spanning the plasma membrane. The left image shows associated carbohydrates. N Carbohydrate Outside of cell C C N Cytosol Cholesterol In animal cells only, cholesterol is an essential component of the plasma membrane, acting in a fluid manner similar to an iceberg. It makes up about 20 percent of the membrane by mass. Cholesterol molecules are inserted alongside phospholipid molecules in both leaflets of the membrane. 16 Jacaranda Nature of Biology 2 VCE Units 3 & 4 Sixth Edition Cholesterol acts on the plasma membrane in several ways: At low temperatures, cholesterol molecules maintain the fluidity of the membrane by keeping phospholipid molecules separated and preventing the membrane from become too stiff. At high temperatures, cholesterol stabilises the membrane by raising its melting point and preventing it from becoming excessively fluid. INVESTIGATION 1.2 elog-0005 Membrane transport across a semi-permeable membrane Aim To observe the semi-permeability of an artificial membrane and relate this to plasma membranes Resources Resourceseses eWorkbook Worksheet 1.3 Structure of the membrane and membrane transport (ewbk-1966) KEY IDEAS Cells are the major structural unit of life. They can vary in size and shape. Prokaryotic cells do not contain membrane-bound organelles (such as nuclei). Eukaryotic cells contain membrane-bound organelles, with DNA situated within the nucleus. Organelles are compartments that carry out specific functions. The major structural component of the plasma membrane is a bilayer of phospholipid molecules, each with a hydrophilic head and hydrophobic tail. A major role of the plasma membrane of a cell is to act as a gatekeeper that controls the entry and exit of materials into and out of the cell. Passive methods of membrane transport include facilitated diffusion, simple diffusion and osmosis. Active methods include active transport, exocytosis and endocytosis. 1.2 Activities To answer questions online and to receive immediate feedback and sample responses for every question, go to your learnON title at www.jacplus.com.au. A downloadable solutions file is also available in the resources tab. 1.2 Quick quiz 1.2 Exercise 1.2 Exercise 1. Compare and contrast eukaryotic and prokaryotic cells. 2. Outline the role of the two main organelles in the endomembrane system that regulate and transport specific proteins. 3. Describe the function(s) of the plasma membrane in a eukaryotic cell. 4. The plasma membrane is often described as a ‘fluid mosaic’. Why? 5. Describe the importance of the polar head and nonpolar tails in the plasma membrane. 6. Explain the importance of cholesterol as a component of animal cell membranes. 7. Specifically identify the substances that can pass easily through a plasma membrane (including charge) and the substances that cannot pass through a plasma membrane (including charge). TOPIC 1 The relationship between nucleic acids and proteins 17 1.3 Nucleic acids as information molecules KEY KNOWLEDGE Nucleic acids as information molecules that encode instructions for the synthesis of proteins: the structure of DNA, the three main forms of RNA (mRNA, rRNA and tRNA) and a comparison of their respective nucleotides Source: VCE Biology Study Design (2022–2026) extracts © VCAA; reproduced by permission. 1.3.1 What are nucleic acids? Nucleic acids are biomolecules that are vital for the continuity of life. They are found nucleotides basic building blocks in all organisms and provide a genetic blueprint that provides the information for or sub-units of DNA and RNA consisting of a phosphate group, protein synthesis. Nucleic acids are made up of sub-units known as nucleotides. a base and a five-carbon sugar There are two kinds of nucleic acid: deoxyribonucleic acid (DNA) nucleic acid consisting of nucleotide deoxyribonucleic acid (DNA), which is located in chromosomes in the nucleus sub-units that contain the sugar deoxyribose and the bases A, of eukaryotic cells. It is the genetic material that contains hereditary information C, G and T; DNA forms the major and is transmitted from generation to generation. component of chromosomes ribonucleic acid (RNA), which is formed against a template strand of DNA. ribonucleic acid (RNA) nucleic acid consisting of a single chain of nucleotide sub-units that contain the All nucleic acids are polymers made up of sub-units (or monomers) known as sugar ribose and the bases A, U, C nucleotides. Each nucleotide has: and G; RNA a 5-carbon (pentose) sugar a phosphate a nitrogenous base. FIGURE 1.15 A nucleotide with a sugar, a phosphate and a nitrogen-containing base Phosphate Nitrogenous base Sugar (ﬁve-carbon sugar) Although DNA and RNA are both made of nucleotides, there are some distinctions between the two, outlined below in table 1.4. TABLE 1.4 Comparison of DNA and RNA DNA RNA Type of sugar Deoxyribose Ribose Nitrogen-containing bases Adenine, cytosine, guanine and Adenine, cytosine, guanine and thymine uracil Number of strands Two One 18 Jacaranda Nature of Biology 2 VCE Units 3 & 4 Sixth Edition TIP: Be careful when spelling biological terminology in your responses. If the spelling means that your term will be confused with something else, it will not be accepted. For example, when spelling cytosine, cytosin or cytocine would be accepted, but cystine would not be accepted (as it is too close to the amino acid cysteine). In some cases, the symbol for the nucleotide may be appropriate (A, C, G, T, U). 1.3.2 The structure of DNA Our DNA is what makes every single person unique. DNA is vital FIGURE 1.16 Part of a DNA double to code for all the proteins in our bodies — from the melanin helix revealed through scanning that determines our skin colour, to enzymes such as lactase, tunnelling microscopy which allows us to break down lactose. Differences in our genetic code lead to the production of different proteins that allow us to have different traits. Each DNA molecule is made of two complementary chains of nucleotides that run anti-parallel (in opposite directions). Each of the chains of nucleotides is made up of a sugar-phosphate backbone (bonded through phosphodiester bonds). The terms 3′ and 5′ are very important in understanding the direction of the each chain of the nucleic acid. The 5′ end is the phosphate end, which is attached to the 5′ carbon of the sugar. The 3′ end is the hydroxyl end of the sugar, which is associated with the 3′ carbon. One strand runs 3′ to 5′ and the opposite strand runs 5′ to 3′. Resources Resourceseses Video eLesson DNA structure (eles-4211) FIGURE 1.17 DNA made up of monomers of nucleotides Nucleotide Nucleotide A T 5′ 3′ Deoxyribose G C T A Phosphate C G 3′ 5′ complementary a molecule having a specific chemical structure that allows it to bond Hydrogen in a ‘lock-and-key’ fashion to Nitrogenous bond another structure base TOPIC 1 The relationship between nucleic acids and proteins 19 It can be observed that the sugar (deoxyribose) and phosphate parts are FIGURE 1.18 The base pairing the same in each nucleotide. However, there are four different kinds of rules in DNA nucleotides because four different kinds of nitrogen-containing bases are involved: adenine (A), thymine (T), cytosine (C) and guanine (G). H Hydrogen bonds form between complementary nitrogenous bases on CH3 O H–N opposite strands. The two chains form a double-helical N N–H N molecule of DNA. N N The base pairs between the two strands, namely A with T and C with N O G, are said to be complementary pairs. A and T bond with 2 hydrogen Thymine Adenine bonds, and C and G bond with 3 hydrogen bonds (as seen in figure 1.18). H O H–N N Base pairing rules in DNA N–H N N N A pairs with T (arrow in the target). N N H O C pairs with G (car in the garage). H Cytosine Guanine FIGURE 1.19 The double helix structure of DNA. The two chains are held together by hydrogen bonds between int-0133 complementary bases. DNA double helix 3′ 5′ G C T A A T 1 spiral coil - 3.4 nm T T A C G A T G C C G A T T T A 3′ 5′ Adenine (A) Thymine (T) Guanine (G) Cytosine (C) The DNA double helix combines with certain proteins, in particular histones, as it condenses to form a chromosome (see figure 1.20a). As the DNA winds around clusters of histone proteins, it forms structures called nucleosomes (see figure 1.20b). 20 Jacaranda Nature of Biology 2 VCE Units 3 & 4 Sixth Edition FIGURE 1.20 a. Diagram showing the coiling and supercoiling of one molecule of a DNA double helix to form a eukaryotic chromosome b. A model of a nucleosome showing the DNA double helix (grey) coiling around a cluster of histone proteins (shown in colours) (Image b. courtesy of Dr Song Tan, Pennsylvania State University) a. Nucleus Chromosome b. Chromatid Nucleosome core particle Centromere with kinetochore Cell Chromatid Nucleosomes Histone DNA elix uble h Base pairs Do GA TCA T C AG T Histone proteins DNA The total length of the DNA double helix molecule in an ‘average’ human chromosome is about five centimetres. By coiling and supercoiling, this long DNA molecule becomes compressed into a microscopic chromosome. In one cell alone, the length of all the DNA is around two metres. In the entire human body, the length of all DNA is trillions of metres long. INVESTIGATION 1.3 elog-0007 Extraction of DNA Aim To extract and observe DNA from the nucleus of a cell Representing DNA There are many ways we can represent a DNA strand. Part of a single chain of DNA could be shown as follows: … -nucleotide-nucleotide-nucleotide-nucleotide-nucleotide- … OR it could be shown as:... − P − sugar − P − sugar − P − sugar − P − sugar − P − sugar −... | | | | | base base base base base OR the specific bases in the nucleotides in one chain could be shown as: 5′ … A T T A G C T T G A G G C G … 3′ TOPIC 1 The relationship between nucleic acids and proteins 21 DNA is not always represented in diagrams as a double helix. Figure 1.21 shows some of the many ways of representing DNA. The representation used will depend on the purpose of the diagram. Each provides different information about DNA. FIGURE 1.21 Different ways of representing DNA a. d. Intron 1 Intron 2 A T C G G A T C Exon 1 Exon 2 Exon 3 T b. A T C A A G T 879 bp 286 bp c. T AGA G C C G C G C TC A G C TG G A C A TC TG A G C G C A G G C TG TC C A GCT EXTENSION: Mitochondrial DNA When we consider DNA, our minds often jump straight to linear nuclear FIGURE 1.22 Mitochondrial DNA within the mitochondrion DNA. However, DNA is also found ATP synthase within our mitochondria. This type of DNA is circular and is referred to as mitochondrial DNA (mtDNA). Like nuclear Mitochondrial DNA DNA, it is made up of nucleotides joined with phosphodiester bonds. mtDNA contains around 40 genes that code for proteins involved in cellular respiration. Mitochondrial DNA is an extension concept for this subtopic but will be explored further in Unit 4 in Topic 10. Outer membrane To access more information on this extension concept, please download the Intermembrane space digital document. Inner membrane Ribosome Matrix Resources Resourceseses Digital document Extension: Mitochondrial DNA (doc-35832) 22 Jacaranda Nature of Biology 2 VCE Units 3 & 4 Sixth Edition “c01TheRelationshipBetweenNucleicAcidAndProteins_PrintPDF” — 2021/10/20 — 11:40 — page 23 — #21 1.3.3 Forms of RNA Ribonucleic acid (RNA) is also a polymer of nucleotides (see figure 1.23). It differs from DNA in that it: is an unpaired chain of nucleotides contains the sugar ribose contains uracil rather than thymine. FIGURE 1.23 The four nucleotide sub-units, uracil, adenine, guanine and cytosine, from which the three forms of RNA are constructed Uracil C Guanine Adenine Cytosine G A U Base Nucleotide Ribose Phosphate RNA has three main forms with different functions and structures. messenger RNA (mRNA) form The three different forms of RNA are all produced in the nucleus using DNA of RNA synthesised by the as a template: transcription of a DNA template strand in the nucleus; mRNA messenger RNA (mRNA), which carries the genetic message from the carries a copy of the genetic DNA within the nucleus to the ribosomes, where the message is translated information into the cytoplasm ribosomes organelles that are into a particular protein. Each group of three nucleotides in mRNA (known major sites of protein production as a codon) provides the information for the addition of one amino acid. in cells in both eukaryotes and A special form of mRNA known as pre-mRNA is made through transcription prokaryotes in the nucleus. ribosomal RNA (rRNA) stable ribosomal RNA (rRNA), which, together with particular proteins, makes form of RNA found in ribosomes transfer RNA (tRNA) form of the ribosomes found in cytosol RNA that can attach to specific transfer RNA (tRNA), molecules that carry amino acids to ribosomes that amino acids and carry them to a are free in the cytoplasm, where they are used to construct proteins. An ribosome during translation anticodon (a set of three nucleotides) binds to the complementary anticodon sequence of three bases in a transfer RNA codon on mRNA. molecule that can pair with the complementary codon of In each of the forms of RNA, the strand of nucleotides is folded in a different way a messenger RNA molecule (see figure 1.24). TOPIC 1 The relationship between nucleic acids and proteins 23 FIGURE 1.24 The different forms of RNA Amino acid Codon Messenger RNA (mRNA) Ribosomal RNA (rRNA) Anticodon Transfer RNA (tRNA) In figure 1.25, ribosomes (blue) attach to the mRNA strand (pink). A tRNA molecule carrying a corresponding amino acid binds to the ribosome. As the ribosome moves onto the next base along the mRNA, a protein (green) grows from the ribosome. FIGURE 1.25 A transmission electron micrograph (TEM) of a fragment of an mRNA translation unit from the salivary gland cell of a midge (Chironomus sp.) 24 Jacaranda Nature of Biology 2 VCE Units 3 & 4 Sixth Edition SAMPLE PROBLEM 1 Comparing DNA and RNA tlvd-1090 Compare and contrast the monomers of DNA and RNA. (2 marks) THINK WRITE 1. In compare and contrast questions, both similarities and differences must be addressed. 2. Note down similarities. DNA and RNA are both made up of nucleotide monomers, which contain phosphate, a sugar and a nitrogenous base (1 mark). 3. Note down differences. DNA contains thymine as a base instead of uracil, which TIP As this question is asking about is in RNA. DNA contains the deoxyribose sugar, whereas monomers, stating one is double-stranded and RNA contains the ribose sugar (1 mark). one is single-stranded is incorrect. INVESTIGATION 1.4 elog-0009 Building a model of DNA and RNA Aim To construct a model to examine the structure of DNA and RNA and understand differences between these two nucleic acids Resources Resourceseses eWorkbook Worksheet 1.4 Comparing DNA and RNA (ewbk-1968) Interactivity RNA structure (int-0111) Weblink DNA and genes resources KEY IDEAS Proteins, polysaccharides and nucleic acids are polymeric organic molecules built out of a very large number of repeating sub-units. The nucleic acids, double-helical DNA and single-stranded RNA, are built out of a very large number of repeating sub-units called nucleotides. Each nucleotide consists of a sugar, a phosphate and a nitrogen-containing base, with the sugar in DNA being deoxyribose and that in RNA being ribose. Each DNA molecule consists of two chains of nucleotides that are complementary to each other and held together by hydrogen bonds. In DNA the nitrogenous bases are adenine, cytosine, guanine and thymine. In RNA, thymine is replaced with uracil. Each RNA molecule consists of a single strand of nucleotides. There are three main types of RNA. Messenger RNA (mRNA) carries the genetic material in DNA from the nucleus to ribosomes. Ribosomal RNA (rRNA) makes up ribosomes. Transfer RNA (tRNA) carries amino acids to ribosomes. TOPIC 1 The relationship between nucleic acids and proteins 25 1.3 Activities To answer questions online and to receive immediate feedback and sample responses for every question, go to your learnON title at www.jacplus.com.au. A downloadable solutions file is also available in the resources tab. 1.3 Quick quiz 1.3 Exercise 1.3 Exam questions 1.3 Exercise 1. MC Nucleic acids come in two main forms — deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Which of the following is a similarity between polymers of DNA and RNA? A. They have the same number of oxygen atoms. B. They contain phosphodiester bonds. C. They contain an identical sugar. D. They contain the nitrogenous base thymine. 2. a. Draw and label a double stranded of DNA with four nucleotides. b. DNA is a double helix. How does a double helix form? Identify the bonds that hold the nucleotides in position. 3. Within a DNA strand, which would be harder to separate into two strands: DNA composed predominantly of AT base pairs, or DNA composed predominantly of GC base pairs? Why? 4. How is pre-mRNA made? What is the name of the process? Where does this happen? 5. Describe the roles messenger RNA (mRNA) and transfer RNA (tRNA) perform in protein synthesis. 6. If a double-stranded DNA molecule contains 13.5% cytosine, what would be the percentages of the other three nitrogenous bases? 7. a. A strand of DNA has the sequence 3′ ATGCCGGATA 5′. What would be the sequence of the other strand? b. What is meant by 3′ and 5′ in regards to nucleic acids? 1.3 Exam questions Use the following information to answer Questions 1 and 2. The diagram below represents part of a DNA molecule. X Y 5′ 3′ Z 3′ 5′ Question 1 (1 mark) Source: VCAA 2015 Biology Exam, Section A, Q3 MC A single DNA nucleotide is shown by sub-unit(s) A. X alone. B. X and Y together. C. Y and Z together. D. X, Y and Z together. 26 Jacaranda Nature of Biology 2 VCE Units 3 & 4 Sixth Edition Question 2 (1 mark) Source: Adapted from VCAA 2015 Biology Exam, Section A, Q4 MC A feature of DNA that can be seen in the diagram is A. the anti-parallel arrangement of the two strands of nucleotides. B. the process of semi-conservative replication. C. its ribose sugar–phosphate backbone. D. its double-helix structure. Question 3 (1 mark) Source: Adapted from VCAA 2016 Biology Exam, Section A, Q4 MC A portion of one strand of a macromolecule has the sequence -CGATTCGGTTAA-. The complementary strand would be A. -CGATTCGGTTAA- B. -AATTGGCTTAGC- C. -GCTAAGCCAATT- D. -GCUAAGCCAAUU- Question 4 (4 marks) Source: Adapted from VCAA 2011 Biology Exam 1, Section B, Q1b The following figure represents a portion of a plant cell. N Q Examine the figure above and answer the following. a. i. Identify the type of nucleic acid found in Structure N. 1 mark ii. Outline the specific function of this nucleic acid. 1 mark b. i. Identify the type of nucleic acid found in Structure Q. 1 mark ii. Outline the specific function of this nucleic acid. 1 mark Question 5 (3 marks) Source: Adapted from VCAA 2020 Biology Exam, Section B, Q2a and b RNA molecules consist of long strands of nucleotides. Each nucleotide consists of three sub-units. a. Label the three sub-units on the diagram of the RNA nucleotide below. 1 mark b. Describe the role in a cell of the two types of RNA listed: i. tRNA 1 mark ii. mRNA 1 mark More exam questions are available in your learnON title. TOPIC 1 The relationship between nucleic acids and proteins 27 1.4 The genetic code and protein synthesis KEY KNOWLEDGE The genetic code as a universal triplet code that is degenerate and the steps in gene expression including transcription, RNA processing in eukaryotic cells and translation by ribosomes Source: VCE Biology Study Design (2022–2026) extracts © VCAA; reproduced by permission. DNA is an information molecule that provides the code to produce proteins. How does DNA control functions within cells? All metabolic reactions in cells are controlled by enzymes that, almost without exception, are proteins built of one or more polypeptide chains — that is, chains of amino acids. Hence, the DNA in the nucleus of a eukaryotic cell controls all the metabolic processes in a cell through the polypeptide chains for which the DNA dictates production. 1.4.1 Features of the genetic code The genetic instructions for all organisms are written in a code that uses an ‘alphabet’ of four letters only, namely A, T, C and G. The DNA of genes is an information-carrying molecule. Before DNA was identified as the genetic material, many biologists thought that DNA was too simple a molecule to contain complex genetic instructions. How can a large amount of information be encoded by a code that has a small number of elements? The genetic code in the DNA of protein-encoding genes typically contains information for joining amino acids to form polypeptides. We can consider this as coded or decoded information, as shown in table 1.5. TABLE 1.5 Coded and decoded information Coded information Decoded information Nucleotide sequences in DNA template strand Order of amino acids in polypeptides The genetic code as a triplet code Consider two observations: 1. Genes typically contain coded information for assembling amino acids to form polypeptides. 2. Polypeptides are made of combinations of 20 different amino acid sub-units. From these observations, it may be inferred that the genetic code must have at least 20 different instructions or pieces of information. genetic code representation of There are only 4 possible nucleotides that code for proteins. How many different genetic information through a non- nucleotides would be required to account for these 20 amino acids? overlapping series of groups of three bases (triplets) in a DNA template If a sequence of only one or two nucleotides coded for one amino acid, there would chain not be enough combinations to code for all 20 amino acids. Thus, one genetic triplet code the idea that the instruction consists of a group of three bases, such as AAT, GCT and so on. Because genetic code consists of triplets of this, the genetic code is referred to as a triplet code. or three-base sequences TABLE 1.6 Comparing the number of instructions coded for by different numbers of nucleotides Number of nucleotides in one instruction Total number of different instructions possible 1 (e.g. T) 4 2 (e.g. AA, AT, GA) 16 3 (e.g. TTA, GCC, AAA) 64 4 (e.g. GGGA, TGCA, AATG) 256 28 Jacaranda Nature of Biology 2 VCE Units 3 & 4 Sixth Edition This code is non-overlapping. So, a fragment of DNA consisting of 12 bases contains four pieces of information or instructions, with each triplet leading to the addition of a single amino acid. EXTENSION: Breaking the code The genetic code was originally unknown and had to be broken. In 1961, the first piece of the genetic code was broken when the three-base triplet ‘AAA’ in DNA was decoded as ‘Add the amino acid phe into a protein being constructed’. We will see later in this topic that the translation of each DNA triplet occurs through an intermediate molecule, messenger RNA (mRNA). By 1966, all 64 pieces of the genetic code had been deciphered. The different amino acids that each triplet codes for are shown in table 1.7. Note that some combinations lead to instructions ‘START adding amino acids’ and ‘STOP adding amino acids’. Refer to table 1.12 in section 1.7.1 to learn more about the different amino acids. Refer to Appendix: Amino acid data to see the structures of the amino acids and their three-letter and single-letter abbreviations. The genetic code as a universal code The code is essentially the same in bacteria, in plants and in animals — it is said to be universal. The same sequence of nucleotides codes for the same amino acid universal the property of the (for example, CCA codes for proline in plants, animals and bacteria). genetic code in which the code is essentially the same across all The information in the DNA template strand also includes a START instruction organisms (TAC) and three STOP instructions (ATT, ATC or ACT). TABLE 1.7 Triplets of DNA and the corresponding amino acids (and in three cases, the STOP signal). Each amino acid is represented with a different colour. Amino acids of similar colours (blue, pink, purple, orange or green) have similar properties. SECOND BASE T C A G TTT Phe TCT Ser TAT Tyr TGT Cys TTC Phe TCC Ser TAC Tyr TGC Cys T TTA Leu TCA Ser TAA STOP TGA STOP TTG Leu TCG Ser TAG STOP TGG Trp CTT Leu CCT Pro CAT His CGT Arg CTC Leu CCC Pro CAC His CGC Arg C CTA Leu CCA Pro CAA

Nucleic Acids and Proteins PDF - Biology Textbook

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