Biology - Gene Expression (Molecular Genetics) - Quick Guides | Biology

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Study GuideBiology–Gene Expression (Molecular Genetics)1. DNA Defined1.1. A New Era in BiologyIn the 1950s, biology entered an exciting new phase. Scientists made rapid discoveries that helpedexplain how genes work inside cells. These breakthroughs led to the development ofmoleculargenetics, a field that focuses on how genetic information is used by living organisms.1.2 What Is DNA and Why Is It Important?At the center of molecular genetics isdeoxyribonucleic acid, better known asDNA. DNA is themolecule that stores all genetic information. This information determines how cells function and whattraits an organism inherits.DNA does not work alone. Instead, it passes instructions along in a step-by-step process that leads tothe production of proteins—the molecules that do most of the work in cells.1.3 From DNA to Protein: The Information PathwayThe flow of genetic information follows a clear sequence:DNA → mRNA → proteinHere’s how it works:•DNAholds the original genetic instructions.•These instructions are copied intomessenger RNA (mRNA).•mRNA then provides the code needed to buildproteins.This one-directional flow of information—from DNA to RNA to protein—is called theCentral Dogmaof molecular biology. It explains how genetic information is expressed in cells.

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Study Guide1.4 Discovering the Structure of DNAA major breakthrough came in1953, when scientistsJames D. WatsonandFrancis H. C. Crickproposed a model for the structure of DNA. Their discovery was so important that they later receivedtheNobel Prize in 1962.Understanding DNA’s structure helped scientists explain:•How genes control cell activities•How DNA copies itself duringmitosis•How hereditary traits are passed from one generation to the next1.5 Why This Discovery MattersThe discovery of DNA’s structure opened the door to many new scientific advances. It clarified howproteins are made and laid the foundation for modern fields such asbiotechnologyandgeneticengineering. These areas of science continue to shape medicine, agriculture, and research today.In short, learning about DNA helps us understand how life works at its most basic level—and whygenetics is such a powerful part of biology.2. DNA Replication2.1 Why DNA Replication Is ImportantBefore a cell can divide bymitosis, it must first make an exact copy of its DNA. This step is crucialbecause, at the end of cell division,each new (daughter) cell must receive the same geneticinformationas the original cell.In human cells, this means that46 chromosomesare copied to make92 chromosomes. Thesecopies are then evenly divided so each daughter cell ends up with46 chromosomesagain.Step 1: Unzipping the DNADNA replication begins when special enzymes separate the DNA molecule. Think of the DNA doublehelix like azipper being pulled apart.•The two strands of DNA separate.

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Study Guide•This exposes the nitrogen bases along each strand.•These bases are eitherpurines or pyrimidines, which pair in specific ways.Step 2: Matching the BasesOnce the strands are separated, each exposed base attracts itscomplementary base:•Adenine pairs with thymine.•Cytosine pairs with guanine.Inside the nucleus, there are plenty ofnucleotidesavailable. Each nucleotide contains:•A nitrogen base•A deoxyribose sugar•A phosphate groupThe enzymeDNA polymeraselinks these nucleotides together, forming a new strand alongside theoriginal one.Step 3: Building New DNA StrandsEach original DNA strand acts as atemplatefor making a new strand. When the new strand forms, itjoins with the original strand to create a new double helix.This method is calledsemiconservative replicationbecause:•Each new DNA molecule containsone original (old) strandandone newly made strand.2.2 Leading Strand vs. Lagging StrandDNA polymerase can only build DNA in one direction:5′ to 3′. This creates two different situationsduring replication:•Leading strand:○Made smoothly and continuously in the 5′–3′ direction.•Lagging strand:○Built in short sections because itruns in the opposite direction.

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Study Guide○These short pieces are calledOkazaki fragments.○Another enzyme,DNA ligase, later joins these fragments together to form a completestrand.2.3 When Does DNA Replication Happen?DNA replication occurs during theS phase (synthesis phase)of the cell cycle.After replication:•The DNA shortens and thickens.•The copied chromosomes become visible aschromatidsduringprophaseof mitosis.2.4 The Final ResultAs mitosis continues, the cell divides intotwo daughter cells. Each daughter cell receives:•Thesame amount of DNA•Thesame type of DNAThis careful copying process ensures that genetic information is passed onaccurately and equally,allowing cells—and organisms—to grow, repair, and function properly.Inshort, DNA replication is the cell’s way of making sure nothing important is lost when it divides.3. Quiz DNA Replication1. QuestionWhat is the function of the enzyme DNA polymerase?Answer Choices•gluing together Okazaki fragments•joiningtogether nucleotides during replication•“unzipping” the two strands of DNA

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Study GuideCorrect Answerjoining together nucleotides during replicationWhy This Is CorrectDNA polymerase is the enzyme responsible for building new DNA strands during DNA replication. Itworks by adding and joining nucleotides together to form a complete strand that is complementary tothe original DNA template.For example:As DNA replicates, DNA polymerase connects each new nucleotide in the correct order, creating anexact copy of the DNA strand.The other options describe the functions of different enzymes:•Okazaki fragments are joined byDNA ligase•DNA strands are “unzipped” byhelicase2.QuestionOkazaki fragments occur with replicating:Answer Choices•both strands•the lagging strand•the leading strandCorrect Answerthe lagging strandWhy This Is CorrectDNA replication happens in opposite directions on the two strands of DNA. Theleading strandismade continuously, while thelagging strandis made in short segments calledOkazaki fragments.For example:

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Study GuideAs the lagging strand is copied, DNA polymerase can only add nucleotides in one direction, so itbuilds the strand in pieces. These pieces are later joined together to form a complete strand.3.QuestionWhen does DNA replication occur during the cell cycle?Answer Choices•G1 phase•G2 phase•S phaseCorrect AnswerS phaseWhy This Is CorrectDNA replication takes place during theS (Synthesis) phaseof the cell cycle. This is the stage whenthe cell copies its DNA so that each daughter cell will receive an identical set of chromosomes duringcell division.For example:Before a cell enters mitosis, it must duplicate its DNA. This duplication happens in the S phase,ensuring that both new cells have complete genetic information.4. Protein Synthesis4.1 Why Proteins MatterBy the 1950s and 1960s, scientists clearly understood thatDNA plays a central role in makingproteins. Proteins areessential because they perform many different jobs in the cell.For example, proteins:•Act asenzymesthat speed up chemical reactions

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Study Guide•Formstructuresinside cells•Make uphormoneslike insulin•Buildmuscle fibers,hair,skin, andnailsIn fact, the human body containshundreds of thousands of different proteins, each with a specificrole.4.2 The Protein “Code”What makes one protein different from another is theorder of amino acidsthat form it. Proteins arebuilt from20 different amino acids, and the sequence of these amino acids acts like a code.This amino acid sequence is determined by agenetic code in DNA. The code is based on theorderof nitrogenous basesin DNA. Understanding how this base sequence is converted into an aminoacid sequence is the key to understandingprotein synthesis.4.3 What Is Needed for Protein Synthesis?For protein synthesis to occur, several important components must be present:•A supply of the20 amino acids•A set ofspecific enzymes•DNA, which contains the original instructions•RNA, which helps carry out those instructions4.4 RNA: The Messenger Between DNA and ProteinRibonucleic acid (RNA)carries genetic instructions from DNA in the nucleus to the cytoplasm, whereproteins are made.RNA is similar to DNA but has two important differences:1.RNA containsribosesugar instead of deoxyribose.2.RNA uses the baseuracil (U)instead of thymine (T).

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Study Guide4.5 The Three Types of RNARibosomal RNA (rRNA)•rRNA combines with proteins to formribosomes.•Ribosomes are the cell’sprotein-building sites, often described as chemical “workbenches.”•They are found in the cytoplasm and attached to therough endoplasmic reticulum.Transfer RNA (tRNA)•tRNA carriesamino acidsfrom the cytoplasm to the ribosomes.•Each tRNA is specific—it binds only toone particular amino acid.Messenger RNA (mRNA)•mRNA is made in thenucleususing DNA as a template.•It carries the genetic instructions from DNA to the ribosomes.•The enzymeRNA polymeraseis responsible for making mRNA (as well as tRNA and rRNA).There are alsonon-coding RNAs (ncRNAs). These do not directly help build proteins but play rolesin controlling genes, which are discussed later in the chapter.4.6 Transcription: Making mRNA from DNAThe first major step in protein synthesis istranscription.Here’s how transcription works:•RNA polymerasebinds to a specific region of DNA.•Onlyone strand of DNAis used as a template.•The enzyme reads the DNA bases one by one.•A complementarymRNA strandis built in the5′–3′ direction.•When astop signalis reached, mRNA synthesis ends.The DNA strands then rewind into a double helix, while the mRNA leaves the nucleus through a poreand enters the cytoplasm.

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