Microbiology - DNA and Gene Expression | Biology

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Study GuideMicrobiology–DNA and Gene Expression1.DNA StructureDuring the 1950s, biology experienced rapid scientific growth. Researchers began to understandhowgenes workand how genetic information controls cell activities. These discoveries led to thedevelopment ofmolecular genetics, a field that studiesDNAand explains how it directs theproduction of proteins in microorganisms and other living cells.1.1The Discovery of DNA StructureIn 1953,James Watson and Francis Crickproposed the correct structure of DNA. They showed thatdeoxyribonucleic acid (DNA)is made oftwo long chains of nucleotides.•The two chains twist around each other to form adouble helix.•The structure looks like aspiral staircase.•The two strands are held together byweak hydrogen bondsbetween specific nitrogenousbases.1.2The Building Blocks of DNA: NucleotidesEach DNA strand is made of repeating units callednucleotides.Every nucleotide hasthree parts:1.A nitrogenous base2.A phosphate group3.A sugar molecule called deoxyriboseNitrogenous BasesThere arefour nitrogenous basesin DNA. These bases fall into two categories:

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Study GuidePurines (two-ring structures)•Adenine (A)•Guanine (G)Pyrimidines (one-ring structures)•Cytosine (C)•Thymine (T)Both types of bases appear on each strand of the DNA double helix.The DNA Backbone and Base Pairing•Thephosphate groupsanddeoxyribose sugarsform the backbone of each DNA strand.•The nitrogenous bases attach to the deoxyribose sugars and face inward toward the oppositestrand.•Bases pair in aspecific and predictable way:oAdenine (A)always pairs withThymine (T)oGuanine (G)always pairs withCytosine (C)These base pairs are calledcomplementary base pairs.This rule is known as theprinciple of complementary base pairing.DNA ReplicationBefore a cell divides—either bybinary fissionin bacteria ormitosisin eukaryotic cells—its DNAmust be copied. This process is calledDNA replication.Steps of DNA Replication1.Unzipping the DNASpecialized enzymes separate the two DNA strands by breaking the hydrogen bonds.2.Exposing the basesAs the strands separate, the bases on each strand become exposed.3.Matching complementary bases

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Study GuideFree nucleotides in the cell pair with the exposed bases using complementary base pairing.4.Building new strandsThe enzymeDNA polymeraselinks the nucleotides together, forming a new DNA strand.Semiconservative ReplicationAfter replication is complete:•Each new DNA molecule contains:oOne original (old) strandoOne newly synthesized strandBecause half of the original DNA is conserved in each new molecule, this process is calledsemiconservative replication.Key TakeawayDNA is a double-stranded, double-helical molecule made of nucleotides. Its structure allows it to becopied accurately and efficiently. Through complementary base pairing and semiconservativereplication, DNA ensures that genetic information is faithfullypassed on to new cells.2.Protein SynthesisProtein synthesis is the process by whichgenetic information in DNA is used to build proteins,which function as enzymes, structural components, and cellular machinery.2.1. The Genetic Code•Thegenetic codeis the sequence of nitrogenous bases in DNA.•Bases are read ingroups of threecalledcodons.•Each codon specifiesone amino acid.•The sequence of codons determines thesequence of amino acids, and thus the protein.

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Study Guide2.2. RNA: The Key IntermediaryRNA differs from DNA in three ways:1.Sugar:ribose(not deoxyribose)2.Base:uracil (U)replaces thymine (T)3.Structure:single-strandedTypes of RNA•mRNA (messenger RNA):carries genetic code from DNA to ribosome•tRNA (transfer RNA):carries specific amino acids to ribosome•rRNA (ribosomal RNA):forms the structural and functional core of ribosomes2.3. Transcription (DNA → mRNA)Where:•Prokaryotes: cytoplasm•Eukaryotes: nucleusSteps:1.RNA polymerasebinds to DNA2.DNA strands separate3.Complementary RNA bases pair with DNA bases4.mRNA strand elongates5.At a stop signal, mRNA detachesKey point:•Usescomplementary base pairing•Produces an mRNA strand made ofcodons

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Study GuideFigure 1The synthesis of mRNA using a strand of DNA as a template.2.4. Translation (mRNA → Protein)Where:ribosomesStepsInitiation•mRNA binds to ribosome•First tRNA (with methionine) binds to start codon (AUG)Elongation•tRNA anticodons pair with mRNA codons•Amino acids are linked bypeptide bonds•Ribosome moves along mRNATermination•Ribosome reaches astop codon•Polypeptide chain is releasedResult:A completedprotein (polypeptide chain)

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