Biology Past Paper PDF - Chapter 2

2.2 The Molecule of Life 53 TABLE 2.2 (CONTINUED) Cell Part Structure Functions Rough endoplasmic Membrane system enclosing a cavity (the Sugar groups are attached to proteins reticulum c...

2.2 The Molecule of Life 53 TABLE 2.2 (CONTINUED) Cell Part Structure Functions Rough endoplasmic Membrane system enclosing a cavity (the Sugar groups are attached to proteins reticulum cisterna) and coiling through the cyto- within the cisternae; proteins are bound in plasm; externally studded with ribosomes vesicles for transport to the Golgi appara- tus and other sites; external face synthe- sizes phospholipids and cholesterol Smooth endoplasmic Membranous system of sacs and tubules; Site of lipid and steroid synthesis, lipid reticulum free of ribosomes metabolism, and drug detoxification Vesicles Small membrane-bound organelles includ- Functions depend on the type of vesicle ing lysosomes, peroxisomes, transport ves- but include transport of molecules and icles, and others various roles in metabolism Nucleus Largest organelle, surrounded by the Control center of the cell; responsible for nuclear envelope; contains fluid nucleo- transmitting genetic information and pro- plasm, nucleoli, and chromatin viding the instructions for protein synthesis Chromatin Granular, thread-like material composed of DNA contains genes DNA and histone proteins Nuclear envelope Double-membrane structure pierced by the Separates the nucleoplasm from the cyto- pores; outer membrane continuous with plasm and regulates the passage of large the cytoplasmic endoplasmic reticulum molecules into and out of the nucleus; inner membrane helps to anchor chromatin Nucleoli Dense spherical (non-membrane-bound) Site of ribosome subunit manufacture bodies composed of ribosomal RNA and proteins Central vacuole (plant Large membrane-enclosed compartment Used to store ions, waste products, pig- cells) ments, protective compounds Chloroplasts (plant cells) Membrane-enclosed organelle containing Site of photosynthesis stacked structures (grana) of chlorophyll- containing membrane sacs called thyla- koids surrounded by an inner fluid (stroma) The nucleus contains DNA. This organelle is a many high school classes. With the wealth of informa- spherical structure enclosed by a double-layered mem- tion available about many detailed aspects of DNA and brane, the nuclear envelope, and is typically the genes, the study of biology in the twenty-first century largest structure in a eukaryotic cell. Nearly 6 feet of might give you the impression that the structural DNA is coiled into the nucleus of every human cell; if details of DNA were always understood. However, the DNA in all human cells were connected end to the structure of DNA—and its function as genetic end, there would be enough to stretch to the sun and material—was not always well known. Many extraor- back about 500 times. Although the majority of DNA dinary researchers and incredible discoveries have in a eukaryotic cell is contained within the nucleus, contributed to our modern understanding of DNA mitochondria and chloroplasts also contain small cir- structure and function. In this section we provide a cular DNA molecules. brief overview of DNA structure. DNA Structure 2.2 The Molecule of Life In 1869, Swiss biologist Friedrich Miescher identified a cellular substance from the nucleus that he called Every high school or college biology course involves “nuclein.” Miescher purified nuclein from white some discussion of DNA, and DNA is routinely manip- blood cells and found that it could not be broken ulated by students in college biology laboratories and down (degraded) by protein-digesting enzymes called 54 Chapter 2 An Introduction to Genes and Genomes proteases. This discovery suggested that nuclein was components of DNA structure—an important princi- not made only of proteins. Subsequent studies deter- ple to remember because, as we explore next, these mined that this material had acidic properties, which bases are essential components of DNA. led nuclein to be renamed “nucleic acids.” DNA and The building block of DNA is the nucleotide ribonucleic acid (RNA) are the two major types of (Figure 2.3). Each nucleotide is composed of a (five- nucleic acids. While biochemists worked to identify carbon) pentose sugar called deoxyribose, a phos- the different components of nucleic acids, other scien- phate molecule, and a nitrogenous base. The bases tists carried out experiments to demonstrate that DNA are interchangeable components of a nucleotide. Each is the inherited genetic material of cells. nucleotide contains one base, either adenine (A), While evidence supporting DNA as hereditary thymine (T), guanine (G), or cytosine (C)—the so- material was building, a significant question still called A’s, T’s, G’s, and C’s of DNA. remained: what is the structure of DNA? Erwin Char- Nucleotides are the building blocks of DNA, but how gaff provided some insight into this question by iso- are these structures arranged to form a DNA molecule? lating DNA from a variety of different species and Many scientists have contributed to the answer to this revealing that the percentage of DNA bases called question, but the definitive structure of DNA was finally adenine was proportional to the percentage of bases revealed by James Watson and Francis Crick, working called thymine, and that the percentage of cytosine at the Cavendish Laboratories in Cambridge, England. bases in an organism’s DNA was roughly proportional Chemists Rosalind Franklin and Maurice Wilkins, of to the percentage of guanine. This valuable observa- University College, London, used x-ray crystallography tion suggested that the bases adenine, thymine, cyto- to provide Watson and Crick with invaluable data on sine, and guanine were somehow intricately related the structure of DNA. By firing an x-ray beam onto Nucleotide structure 5 O Nitrogenous base Phosphate group O P O CH2 59 59 O– O HOCH2 O OH 49 C H H C19 49 19 H H H H H C C H 39 29 Deoxyribose sugar 39 29 Ribose sugar OH OH (in DNA only) OH H (in RNA only) Nitrogenous bases Purines NH2 O N C N C H C N C N H C H C C C C C N H N N NH2 N Adenine (A) H Guanine (G) H Pyrimidines NH2 O O C H H C H H C CH3 N C N C N C C C C C C C O N H O N H O N H H H H Cytosine (C) Uracil (U) Thymine (T) (in RNA only) (in DNA only) FIGURE 2.3 Nucleotide Structure All DNA nucleotides consist of a nitrogenous base, A, C, G, or T; a pentose sugar; and a phosphate group. The pentose sugar in DNA is called deoxyribose because it lacks an oxygen at carbon number 2 (2’) compared with the pentose sugar, called ribose, in RNA. A base is attached to carbon number 1 (1’) of the sugar; the phosphate group is attached to carbon number 5 (5’) of the sugar. Because of their structure, adenine and guanine belong to a group of bases called purines, whereas cytosine, thymine, and uracil belong to a group called pyrimidines.  2.2 The Molecule of Life 55 crystals of DNA, Franklin and Wilkins revealed a model learned about DNA structure over the past 65 years, of DNA indicating that its structure could be helical. this description might be one of the greatest under- From these data, Chargaff’s findings, and other studies, statements ever made in a published scientific paper. Watson and Crick assembled a wire model of DNA. The significance of this discovery was appropriately Watson and Crick published “The Molecular recognized in 1962, when Watson, Crick, and Wilkins Structure of Nucleic Acids: A Structure for Deoxyri- received the Nobel Prize in Medicine. bose Nucleic Acid” in the prestigious journal Nature on Watson and Crick determined that nucleotides form April 25, 1953. The first paragraph of this paper reads, long strands of DNA and that each DNA molecule con- “We wish to suggest a structure for the salt of deoxyri- sists of two strands that join together and wrap around bose nucleic acid (D.N.A.). This structure has novel each other to form a double helix (Figure 2.4). A strand of features which are of considerable biological interest.” DNA is a string of nucleotides held together by phos- Given the importance of DNA and what we have phodiester bonds that connect the sugar of one (a) Sugar-phosphate Complementary (b) backbone base pairs 59 end 39 end O Hydrogen bond OH P OH T O O 39 H2C A O O CH2 T A O O G C 59 to 39 direction P Major A T O O O O groove P C G C O O A T H2C G C G O 59 to 39 direction O CH2 C G O O Minor P groove O O A T O O P C G A T O O T Sugars and A A T H2C phosphates O O CH2 form the DNA O “backbone” G C O P A T O O O O P O O C Phosphate G H C O O 2 CH2 39 59 O O P OH OH O Sugar (deoxyribose) 39 end 59 end FIGURE 2.4 DNA Is a Double-Stranded Helix (a) Two strands of nucleotides are joined together by hydrogen bonds between complementary base pairs. Adenine bases (A) always base pair with thymine bases (T), and cytosine (C) base pairs with guanine (G). (b) The two strands wrap around each other so that the overall structure of DNA is a double- stranded helix with a sugar-phosphate “backbone,” in which the bases are aligned in the center of the helix. 56 Chapter 2 An Introduction to Genes and Genomes nucleotide to the phosphate group of an adjacent nucleo- proteins produced by a cell, genes influence how cells, tide (Figure 2.4). The sequence of bases in a strand can tissues, and organs appear, both through the micro- vary. For instance, a nucleotide containing a C can be scope and with the naked eye. These inherited appear- connected by a phosphodiester bond to a nucleotide con- ances are called traits. Through the DNA contained in taining an A, T, G, or another nucleotide containing a C. your cells, you have inherited traits from your parents, Each strand of nucleotides has a polarity to it; such as eye color and skin color. Genes influence not there is a 5’ end and a 3’ end to the strand (Figure 2.4). only cell metabolism and behavioral and cognitive Polarity refers to the carbons of the deoxyribose sugar. abilities such as intelligence but also the inheritance of At the 5’ end of a strand, the phosphate at carbon 5 is or susceptibility to certain types of genetic diseases. not bonded to another nucleotide, but carbon 3 is Some traits are controlled by a single gene, but the involved in a phosphodiester bond. At the 3’ end, the majority of traits are determined by multiple genes, phosphate at carbon 5 is bonded to another nucleo- which produce many proteins that interact in complex tide, but carbon 3 is not. Although this aspect of nucle- ways. In Section 2.4, we explore how genes direct pro- otide structure may seem trivial, the polarity of DNA is tein synthesis in cells. Throughout this book, we con- important for replication and for the routine manipu- sider examples of genes, their functions, and their lation of DNA in the laboratory. many applications in different areas of biotechnology. Watson and Crick determined that DNA molecules consist of two interconnecting strands that wrap around each other to form a right-handed double 2.3 Chromosome Structure, DNA helix, perhaps the most famous molecular model in all of biology (Figure 2.4). The two strands are joined Replication, and Genomes together by hydrogen bonds between complemen- tary base pairs in opposite strands (Figure 2.4). Ade- Before we consider how genes function, it is important nine base pairs with thymine, and guanine base pairs that you understand how and why DNA is organized with cytosine. From this model, Chargaff’s observa- into chromosomes and how DNA is replicated in cells. tions are easily understood. The proportions of A’s and T’s are equivalent in an organism’s DNA, as are the Chromosome Structure proportions of G’s and C’s, because they pair with each other in a DNA molecule. Suppose you were presented with a challenge. If you The two strands of nucleotides in a double helix solved it, you would earn free tuition for the rest of your are antiparallel—the polarity of each strand is reversed undergraduate courses. You are given a basket contain- relative to the other (Figure 2.4). This orientation is ing 46 packages of different-colored yarn all unraveled necessary for the hydrogen bonds holding the comple- and intertwined. Your challenge is to sort the yarn into mentary base pairs to align with one another. The 46 even balls. How would you solve this challenge? If double helix resembles a twisted ladder. The rungs of you started to cut the tangled pile of yarn randomly, you this ladder consist of the complementary base pairs, probably would not succeed. Of course, if you painstak- and the sides of the ladder consist of sugar and phos- ingly unraveled the yarn and wound each color of yarn phate molecules, creating the “backbone” of DNA. into a ball, you would eventually sort it into 46 even balls. This analogy provides a highly simplified view of the challenge presented to a human cell when it has to What Is a Gene? divide and sort its DNA into even packages. Genes are units of inheritance, but what exactly is a The 3 billion base pairs (bp) of DNA in every gene? Until recently, a gene was often described as a human cell must be separated evenly when a cell sequence of nucleotides that provides cells with the divides; otherwise, the loss of DNA can have devastat- instructions to synthesize a specific protein. But ing consequences. Fortunately, such mistakes in DNA because we have learned that not all genes are used to separation are rare, in part because cells effectively make a protein, a more modern definition of a gene separate and package DNA into chromosomes. encompasses any DNA sequence that is used to pro- Inside the nucleus, DNA exists in a relatively unrav- duce RNA. For example, genes for transfer RNA eled state. This does not mean that the DNA is uncoiled (tRNA) are used to make tRNA molecules, and from its double-helical structure; rather, the DNA is although tRNAs are required for protein synthesis, somewhat loosely arranged and not fully compacted they are not translated to produce a protein. Most into tightly coiled chromosomes, although all of the genes are approximately 1,000 to 4,000 nucleotides DNA in a chromosome remains together within the (nt) long, although many smaller and larger genes nucleus. When a cell is not dividing, chromosomes in have been identified. Largely by controlling the the nucleus exist as an intricate combination of DNA  2.3 Chromosome Structure, DNA Replication, and Genomes 57 Chromosome Double-stranded DNA Chromatid Chromatid Gene Telomere Cell p arm Centromere Nucleus DNA q arm Nucleosomes Telomere Histones FIGURE 2.5 Chromosome Organization In a nondividing cell, chromosomal DNA exists in an unraveled state called chromatin. Histone proteins serve as particles around which DNA becomes tightly wound to give a “beads on a string” appearance when viewed with an elec- tron microscope. When cells divide, chromatin is further compacted into tight fibers and supercoiled looped structures. Ultimately these supercoiled loops are tightly packed together with the assistance of other proteins to create an entire chromosome, a highly compact assembly of DNA. Each chromosome consists of two sister chromatids attached by a centro- mere. Chromosome arms are the portions of the chromatid on one side of the centromere, labeled as the p and q arms. The ends of a chromosome are called telomeres. and DNA-binding proteins called histones to form 1 through 22 are known as the autosomes; the 23rd strings called chromatin. During cell division, chroma- pair are called the sex chromosomes—consisting of X tin is coiled into tight fibers that eventually wrap around and Y chromosomes. each other, so that chromosomes become highly coiled Human egg and sperm cells, known as the sex and tightly condensed packages of DNA and histones cells or gametes, contain a single set of 23 chromo- and other proteins (Figure 2.5). Compacting DNA into a somes, called the haploid number (n) of chromo- chromosome is an amazing feat when you consider that somes. All other cells of the body—such as skin cells, DNA in most human cells is about two-meters long and muscle cells, and liver cells—are known as somatic must be compacted into a nucleus that is about one- cells. Somatic cells from many organisms have two thousandth of a millimeter in diameter! sets of chromosomes, called the diploid number (2n) The size and number of chromosomes vary from of chromosomes. Human somatic cells contain 46 species to species. Most bacteria have a single circular chromosomes. Somatic cells of a normal human male chromosome, in the size range of several hundred have 22 pairs of autosomes and an X and Y chromo- thousand base pairs, which contains a few thousand some; cells of a normal female have 22 pairs of auto- genes. Eukaryotes typically contain one or more sets of somes and two X chromosomes. chromosomes, which have a linear shape, and often Sex chromosomes were so named because they these chromosomes are several million base pairs in contain genes that influence sex traits and the devel- size. Most human cells have two sets (pairs) of 23 chro- opment of reproductive organs, whereas the auto- mosomes each, for a total of 46 chromosomes. Through somes were originally thought primarily to contain the process of fertilization, you inherited 23 chromo- genes that affect body features unrelated to sex, such somes from your mother (maternal chromosomes) as skin color and eye color. Although genes involved and 23 chromosomes from your father (paternal in sex organ determination are present on the Y chro- chromosomes). These chromosome pairs are called mosome, other genes involved in sex determination homologous pairs, or homologues. Chromosomes are present on autosomes, and the majority of genes 58 Chapter 2 An Introduction to Genes and Genomes on the X chromosome are not required for develop- attaching chromosomes to the nuclear envelope. Telo- ment of the reproductive organs. meres are a subject of intense research. Changes in Several characteristics are common to most telomere length are associated with the cellular aging eukaryotic chromosomes. Each chromosome consists process (called senescence) and with the develop- of two thin, rod-like structures of DNA called sister ment of certain types of cancers. chromatids (Figure 2.5). The sister chromatids are exact replicas of each other, copied during DNA syn- Karyotype analysis for studying chromosomes thesis. During cell division, sister chromatids are sepa- Analyzing chromosome structure and abnormalities rated so that newly forming cells receive the same is called cytogenetics or cytogenetic analysis. One amount of DNA as the original cell from which they common cytogenetic method for studying chromo- arose. Each eukaryotic chromosome has a single cen- some number and basic aspects of chromosome tromere, a constricted region of the chromosome structure is to prepare a karyotype. In karyotype consisting of intertwined DNA and proteins that join analysis, cells are spread on a microscope slide and the two sister chromatids to each other. This region of then treated with chemicals to release and stain the a chromosome also contains proteins that attach chro- chromosomes. For example, G-banding, in which mosomes to organelles called microtubules, which chromosomes are treated with a DNA-binding dye play essential roles in moving chromosomes and sepa- called Giemsa stain, creates a series of alternating rating sister chromatids during cell division. light and dark bands in stained chromosomes. Each The centromere delineates each sister chromatid stained chromosome shows a unique and reproduc- into two arms—the short arm, called the p arm, and ible banding pattern that can be used to identify dif- the long arm, or q arm. Each arm of a chromosome ferent chromosomes. Chromosomes can be aligned ends with a segment known as a telomere (Figure and paired based on their staining pattern and size 2.5). Telomeres are highly conserved repetitive (Figure 2.6). In humans, chromosome 1 is the largest sequences of nucleotides that are important for chromosome and chromosome 21 is the smallest. Human male G-bands 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 X Y FIGURE 2.6 Karyotype Analysis In a karyotype, dividing cells are spread out onto a glass microscope slide to release their chromosomes. Chromosomes are stained and aligned based on their overall size, the position of the centromere, and their staining pattern to create a karyotype.  2.3 Chromosome Structure, DNA Replication, and Genomes 59 Modern methods for karyotype analysis, called spec- (parent) cell. For instance, a human skin cell divides tral karyotyping, incorporate specific probes and to produce two daughter cells, each containing 23 techniques to colorize chromosomes. This provides a pairs of chromosomes. Gametes are formed by mei- more detailed analysis of chromosome structure than osis, wherein a parent cell divides to create up to traditional karyotypes, such as the one shown in Fig- four daughter cells, which can be either sperm or egg ure 2.6, which makes it easier to identify human cells. During meiosis, the chromosome number in genetic disease conditions associated with abnormal- daughter cells is cut in half to the haploid number. ities in chromosomal structure and number. Simi- Sperm and egg cells each contain a single set of 23 larly, a number of different approaches can be used chromosomes. Through sexual reproduction, a fertil- to identify specific alterations in chromosome struc- ized egg, called the zygote, is formed. The zygote, ture, including the presence or absence of a specific which divides by mitosis to form an embryo and gene, by binding different DNA probes to chromo- eventually a complete human, contains 46 chromo- somes. One approach for doing this is called fluores- somes: 23 paternal chromosomes and 23 maternal cence in situ hybridization, or FISH. See Figure chromosomes. 3.13 for an example of FISH. Prior to cell division by either mitosis or meiosis, In Chapter 11, as well as other chapters, you will DNA must be replicated in the cell. Replication occurs learn more about how karyotype analysis and other by a process called semiconservative replication. approaches for genetic testing enable scientists to Figure 2.7 shows an overview of this process. Before diagnose genetic diseases. replication begins, the two complementary strands of the double helix must be pulled apart into single DNA replication strands. Once separated, these strands serve as tem- When a cell divides, it is essential that the newly cre- plates for copying two new strands of DNA. At the end ated cells contain equal copies of replicated DNA. of this process, two new double helices are formed. Somatic cells divide by mitosis, wherein one cell Each helix contains one original DNA (parental) strand divides to produce daughter cells, each of which con- and one newly synthesized strand—thus the term tains an identical copy of the DNA from the original semiconservative. (a) (b) A T A T A T A T A T C G C G C G C G C G C G A T G C G C C G C G C G G C A T A T A T A T C G T A T A T A T A A T Nucleotides A T G C Parental After separation, Two identical daughter molecules A T DNA both parental strands serve of DNA C G G as templates G C C T A T A A T A T T A T A C G C G T A T A G C G C G C G C T A T A Original (parental) Newly synthesized Original (parental) strand strands strand FIGURE 2.7 An Overview of DNA Replication Nucleotide strands in a DNA molecule must first be separated (a). Each strand serves as a template for the synthesis of new strands, producing two DNA molecules, each containing one original strand and one newly synthe- sized strand (b). 60 Chapter 2 An Introduction to Genes and Genomes 3) The leading strand is synthesized continuously 2) Single-strand binding proteins stabilize in the 59 39 direction by DNA polymerase. the unwound parental DNA. 39 59 1) Helicase unwinds the DNA polymerase parental double helix. Replication fork RNA primer Helicase Okazaki fragment Primase being made 59 DNA polymerase 4) The lagging strand is 39 39 synthesized discontinuously. Parental DNA 59 Primase synthesizes a short RNA primer, which is extended 5) After the RNA primer is replaced with by DNA polymerase to form DNA nucleotides by DNA polymerase, an Okazaki fragment. DNA ligase joins the Okazaki fragment DNA ligase to the growing strand. Overall direction of replication FIGURE 2.8 Semiconservative Replication of DNA DNA replication occurs in a series of stages involv- polymerase uses nucleotides present in the cell to syn- ing a number of different proteins. Because prokaryotes thesize complementary strands of DNA. DNA polymerase contain circular chromosomes, DNA replication in pro- always works in one direction, synthesizing new strands karyotes is slightly different from that in eukaryotes. in a 59-to-39 orientation and adding nucleotides to the 39 Here we consider the main components and key con- end of a newly synthesized strand (Figure 2.8) by form- cepts in DNA replication overall. But keep in mind that ing phosphodiester bonds between the phosphate of one subtle differences distinguish the replication process in nucleotide and the sugar in the previous nucleotide. prokaryotes from that in eukaryotes. Replication is initi- Because DNA polymerase proceeds only in a 59-to- ated by DNA helicase, an enzyme that separates the 39 direction, replication along one strand, the leading two strands of nucleotides, literally “unzipping” the strand, occurs in a continuous fashion (Figure 2.8). DNA by breaking hydrogen bonds between comple- Synthesis on the opposite strand, the lagging strand, mentary base pairs (Figure 2.8). The separated strands occurs in a discontinuous fashion because DNA poly- form a replication fork. As helicase unwinds the DNA, merase must wait for the replication fork to open. On single-strand binding proteins attach to each strand to hold the lagging strand, short pieces of DNA, called Okazaki them apart and prevent them from base pairing and re- fragments (named after Reiji and Tuneko Okazaki, the forming a double helix during DNA replication. Strand scientists who discovered these fragments), are synthe- separation occurs at sites called origins of replication. sized as the DNA polymerase works its way out of the Bacterial chromosomes have a single origin. Because of replication fork. Covalent bonds between Okazaki frag- their large size, eukaryotic chromosomes have multiple ments in the lagging strand are formed by DNA ligase origins. Starting DNA replication at multiple origins to ensure that there are no gaps in the phosphodiester allows eukaryotic chromosomes to be copied rapidly. backbone. Finally, the RNA primers are removed and The next step in DNA replication involves the addi- these gaps are filled by DNA polymerase. tion of short segments of RNA approximately 10 to 15 Remember the functions of enzymes involved in nucleotides long. These sequences, called RNA primers, DNA synthesis. As we will discuss, DNA polymerase are synthesized by an enzyme known as primase (in and DNA ligase are routinely used in the lab in work- eukaryotes, a form of DNA polymerase called a [alpha] ing with cloned DNA (Chapter 3). acts as the primase). Primers start the process of DNA replication because they serve as binding sites for DNA What is a genome? polymerases, the key enzymes that synthesize new DNA contains the instructions for life—genes. All of strands of DNA. Several different forms of DNA poly- the DNA in an organism’s cells is called the genome. merase are involved in copying DNA. In bacteria, the Contained in the human genome are approximately enzyme DNA polymerase III (called DNA polymerase 20,000 genes scattered among 3 billion base pairs of d [delta] in eukaryotes) binds to each single strand, DNA. The study of genomes, a discipline called moving along the strand and using it as a template to genomics, is currently one of the most active and copy a new strand of DNA. During this process, DNA rapidly advancing areas of biological science.  2.4 RNA and Protein Synthesis 61 We probably discuss genomes more than any Cell nucleus other topic in this book. In many chapters we discuss aspects of the Human Genome Project, a worldwide effort to identify all human genes on each chromo- Replication some. The Human Genome Project was an enormous DNA undertaking in genomics that has provided exciting insight into human genes and gene functions, created new areas of research, and led to the development of novel ways to diagnose, treat, and—in some cases— Transcription cure human genetic diseases. Also, as you will learn in Chapters 3 and 11, DNA sequencing (a method for determining the order of mRNA nucleotides in a DNA molecule) technology has pro- gressed so rapidly that it is now possible to sequence Cell cytoplasm individual human genomes fairly quickly, accurately, and Translation at low cost! As a result, DNA sequencing is having major implications for diagnosing and treating human genetic disease conditions. In Chapter 11, we will consider Protein many examples of genome sequencing as related to medical biotechnology. 2.4 RNA and Protein Synthesis Traits FIGURE 2.9 The Flow of Genetic Information in Cells DNA is copied into RNA during the process of transcrip- Genes govern the activities and functions within a cell tion. RNA directs the synthesis of proteins during translation. often by directing the synthesis of proteins. Some of Through proteins, genes control the metabolic and physical the myriad functions of proteins are as follows: properties or traits of an organism. Proteins are necessary for cell structure, for exam- ple, as important components of membranes, are also very similar to those of DNA. One key differ- organelles, and the cytoplasm. ence is that RNA contains a base called uracil (U) Proteins carry out essential reactions in the cell as instead of thymine (T; see Figure 2.3). The other pri- enzymes. mary difference is that RNA contains a pentose sugar Proteins perform critical roles as hormones and called ribose, which has a slightly different structure other “signaling” molecules that cells use to com- than the deoxyribose sugar contained in DNA. municate with one another. An easy way to remember the difference between Receptor proteins bind to other molecules, such as transcription and translation is to know that transla- hormones, and transport proteins, enabling mole- tion involves a change in code from RNA to protein, cules to enter and leave cells. much like translating one language to another. Through the production of mRNA, other RNAs, and Proteins act as antibodies that recognize and the synthesis of proteins, DNA controls the properties destroy foreign materials in the body, creating of a cell and its traits (Figure 2.9). This process of tran- immunity to foreign substance. scription and translation directs the flow of genetic Quite simply, cells cannot function without proteins. information in cells, controlling a cell’s activities and How does DNA make proteins? Actually, DNA does not properties. Here we study basic principles of transcrip- make proteins directly. To synthesize proteins, genes are tion and translation and aspects of how gene expres- first copied into molecules called messenger RNA sion can be controlled by cells. (mRNA) (Figure 2.9). RNA synthesis is called transcrip- tion, because genes are literally transcribed (copied) Copying the Code: Transcription from a DNA code into an RNA code. In turn, mRNA mol- ecules, which are exact copies of genes, contain informa- How is DNA used as a template to make RNA? RNA tion that is deciphered into instructions for making a polymerase is a key enzyme for transcription. Inside protein through a process known as translation. the nucleus, RNA polymerase unwinds the DNA helix RNA molecules are single-stranded, not double- and then copies one strand of DNA into RNA (Figure 2.10). stranded like DNA, but the chemical composition of Unlike DNA replication, in which the entire DNA mol- RNA is very similar to that of DNA. The bases of RNA ecule is copied, transcription occurs only in segments