DNA, Gene Expression, And Biotechnology

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Unit 8: DNA, Gene Expression,and Biotechnology1

Learning Objectives Describe what DNA is and what it does.Explain the process of DNA replication.Explain the process of gene expression.Explain the causes and effects of damage to thegenetic code.Describe biotechnology and its implications forhuman health.Discuss biotechnology in agriculture.Discuss biotechnology today and tomorrow.2

What is the most common reason why DNAanalyses overturn incorrect criminalconvictions? In more than three-quarters of the cases,inaccurate eyewitness testimony played animportant role in the guilty verdict. Julius Ruffin Ken Wyniemko3

Selfish dictators may owe their behaviourpartly to their genes, according to astudy that claims to have found agenetic link to ruthlessness.–Nature, April 2008Whether a man has one type of geneversus another could help decidewhether he’s good “husband material,” anew study suggests.–Washington Post, September 20084

The DNA molecule containsinstructions for the development andfunctioning of all living organisms.5

Two Important Features of DNA1. DNA contains the instructions onhow to create a body and controlits growth and development.2. The instructions encoded in theDNA molecule are passed downfrom parent to offspring.6

Cast of Characters Erwin Chargaff: established base-pairing rules James Watson and Francis Crick: brash, young,inexperienced; not taken seriously by too manypeople Sir William Lawrence Bragg: head of the CavendishLaboratory where Watson and Crick did their work;Nobel laureate, serious competitor to 7

Cast of Characters Linus Pauling: wizard of Caltech; world's leadingstructural chemist; odds-on favorite to solve thestructure of DNA Peter Pauling: office-mate of Watson and Crick;unofficial communications link between competinggroups in California and England Maurice Wilkins and Rosalind Franklin, whoselaboratories at King's College produced criticalevidence critical.8

Chargaff's Rules9

A Short Summary ofPauling's Involvement withDNA.10

Denying Pauling’s Request11

Watson's Early AttitudeToward DNA Crick’s Early Attitude TowardDNA12

Chargaff’s Rules 1950 - Erwin Chargaffreported that DNAcomposition varies from onespecies another. Such evidence of moleculardiversity made DNA a morecredible candidate for thegenetic material than protein. In DNA of each species hestudied, # adenines #thymine, # guanines #cytosine.13

Base PairingThe C G:A T ratio varies from organism to organism (particularly amongthe bacteria), but within the limits of experimental error, A T and C GRelative Proportions (%) of Bases in 220.521.5Grasshopper 29.329.320.520.7Sea .718.722.817.1E. coli24.723.626.025.714

Base Pairing The rules of base pairing are: the purine adenine (A) always pairs withthe pyrimidine thymine (T) the pyrimidine cytosine (C) always pairs with thepurine guanine (G) This is consistent with there not being enoughspace (20 Å) for two purines to fit within the helixand too much space for two pyrimidines to getclose enough to each other to form hydrogenbonds between them.15


Base Pairing Why not A with C and G withT? only with A & T and with C& G are thereopportunities toestablish hydrogenbonds between them (2between A & T; 3betweenC & G).17

Which answer will base pair withthe following TCAAGAGTACAUCAAGAGUACA18




DNA “Double Helix”Nucleic acids and nucleotides22

DNA replication is a biologicalprocess that occurs in all livingorganisms and copies their DNA; it isthe basis for biological inheritance.23

DNA Replication Double-stranded DNA molecule produces twoidentical copies of the moleculeSemiconservative replication: each strand of theoriginal double-stranded DNA molecule serves astemplate for the production of the complementarystrand.Cellular proofreading and error-checkingmechanisms ensure near perfect fidelity for DNAreplication.24

Key Enzymes DNA Helicase Unwinds the DNA double helixDNA Polymerase Builds a new duplex DNA strand by addingnucleotides. Also performs proof-reading and error correction.25

Let's look at the details: Helicase unwinds the double-stranded DNADNA polymerase "walks" down the DNA strands andadds new nucleotides to each strand.The nucleotides pair with the complementarynucleotides on the existing stand (A with T, G withC).A subunit of the DNA polymerase proofreads thenew DNADNA ligase seals up the fragments into one longcontinuous strandThe new copies automatically wind up again26


Genes are sections of DNA thatcontain instructions for makingproteins.Why is DNA considered the universalcode for all life on earth?28



Genes Sequence of bases in a DNA molecule Carries information necessary for producing afunctional product, usually a protein molecule orRNA Average gene is 3000 bases long31


Genes Instruction set for producing one particularmolecule, usually a proteinExamples fibroin, the chief component of silk triacylglyceride lipase (enzyme that breaks downdietary fat)33

Genes Within a species, individuals sometimes have slightlydifferent instruction sets for a given protein andthese instructions can result in a different version ofthe same trait.These alternate versions of a gene that codes forthe same character are called alleles.Any single feature of an organism is referred to as atrait.34

Different people can have free orattached earlobes. The DNA thatencodes for making free or attachedearlobes is called a(n) , and thereare two different versions of it, called.; genestrait; allelesgene; traitgene; alleles35

Not all DNA contains instructions formaking proteins.

Not all DNA contains instructions formaking proteins. Comparing the amount of DNA present in variousspecies reveals a paradox: There does not appear to be any relationshipbetween the size of an organism’s genome andthe organism’s complexity Complexity can be assessed in a variety of ways,such as by counting the number of different celltypes in the organism37

The Proportionof the DNAThat Codes forGenes

Introns & Exons Introns: non-coding regions of DNA may take the form of short (or long) sequencesthat are repeated thousands of times may also consist of gene fragments, duplicateversions of genes, and pseudogenesExons: protein-coding region in the DNA. nucleic acid sequence in DNA OR RNA transcript following genetic splicing

How do genes work? Every cell contains all of the information neededto manufacture every protein in the body buthaving the instructions is not the same as havingthe productsExample: skin cells on your arm contain genes forproducing liver cells, RBCS, muscle tissue—butthey don’t

Which molecule acts as a “middle man”between the nucleus, where transcriptionoccurs, and the cytoplasm, wheretranslation occurs? 1 and 3 arecorrect.

Transcription A single copy of one specific gene within the DNA ismade. Recognize & bind Transcribe Termination Capping & editing44

Step 1 – Recognize, Bind, andUnwind RNA polymerase (enzyme) recognizes a promotersite, a part of the DNA molecule that indicates thestart of a geneAt the promoter site, the molecule binds to onestrand of the DNA and, like a court reportertranscribing everything that is said in a courtroom,begins to read the gene’s messageAt the point where the RNA polymerase binds to thepromoter, the DNA molecule unwinds just a bit, sothat only one strand of the DNA can be read45

Step 2 – Transcribe As the DNA strand is processed through the RNApolymerase, the RNA polymerase builds a copy—called a “transcript”—of the gene from the DNAmolecule.This copy is called messenger RNA (mRNA) becauseonce the copy of the gene is created, it can moveelsewhere in the cell and its message can betranslated into a protein.46

mRNA constructed from four different ribonucleotides,each of which pairs up with an exposed base onthe now unwound and separated DNA: If the DNA strand has a Thymine (T), an Adenine(A) is added to the mRNA. If the DNA strand has a Adenine (A), a Uracil (U)is added to the mRNA. If the DNA strand has a Guanine (G), a Cytosine(C) is added to the mRNA. If the DNA strand has a Cytosine (C), a Guanine(G) is added to the mRNA.47

mRNA Because the mRNA transcribes a specific sequenceof DNA letters (the gene), the transcript carries theDNA’s information.And because it is separate from the DNA, themRNA transcript can move throughout the cell, tothe places where the information is needed, whileleaving the original information within the DNA.48

Step 3 – Re-wind As the RNA polymerase moves down the unwoundstrand of DNA, the DNA that has already beentranscribed twists back into its original double-helixform.49

Step 4 – Terminate When the RNA polymerase encounters a sequenceof bases on the DNA at the end of the gene (calleda termination sequence), the court reportermolecule stops creating the transcript anddetaches from the DNA molecule.At that point, the mRNA molecule is released as afree-floating single-strand copy of the gene50

Step 4 – Terminate Prokaryotes: once the mRNA transcript separatesfrom the DNA it is ready to be translated into aprotein51

Step 4 – Terminate Eukaryotes: transcript must first be editeda) cap and tail may be added at the beginningand end of the transcriptb) protects the mRNA from damage and help theprotein-making machinery recognize the mRNAc) Introns are snipped out Once the mRNA transcript has been edited, it isready to leave the nucleus for the cytoplasm whereit will be translated into a protein52


Translation Once the mRNA molecule moves out of the cell’snucleus and into the cytoplasm, the translationprocess begins.In translation, the information carried by thenucleotide sequence of the mRNA is read andingredients present in the cell’s cytoplasm are usedto produce a protein.54

Several ingredients must be present inthe cytoplasm for translation to occur. Free amino acidsRibosomal unitsTransfer RNA55



Translation Step 1 – Recognize and Initiate Protein-BuildingTranslation begins in the cell’s cytoplasm when aribosome, essentially a two-piece protein-buildingfactory, recognizes and assembles around a “startsequence” – AUG – on the mRNA transcript.58

Translation As the ribosomal subunits assemble themselves intoa ribosome, one side of a tRNA molecule(anticodon) also recognizes the start sequence(codon) and binds to the mRNA at that point.That initiator tRNA has the amino acid methioninebound to its other side. This will be the first aminoacid in the protein that is to be.59

Translation Step 2 – Elongate After the mRNA start sequence, the next (codon)three bases on the mRNA specify which aminoacid-carrying tRNA molecule should bind to themRNA next. Example – If the next three bases on the mRNAtranscript are GUU, a tRNA molecule thatrecognizes that sequence will attach to themRNA at that point. The GUU-recognizing tRNAmolecule always has the amino acid valineattached.60

Translation The process continues in the same manner.This is the beginning of protein synthesis because allproteins are chains of amino acids, like beads on astring.61

Translation The mRNA continues to be “threaded” through theribosome, with the ribosome moving down themRNA strand reading and translating its message inlittle three-base chunks.Each three-base sequence specifies the next aminoacid, lengthening the growing amino acid strand.After the amino acid carried by a tRNA molecule isattached to the growing protein, the tRNAmolecule detaches from the mRNA and floatsaway.62

Translation Step 3 – Terminate Eventually, the ribosome arrives at the three-basesequence on the mRNA that signals the end oftranslation. Once the ribosome encounters this sequence,the assembly of the protein is complete. Translation ends and the amino acid strand andmRNA molecule are released from the ribosome.63

Translation When it is complete, the protein—such as insulin ora digestive enzyme—may be used within the cell orpackaged for delivery via the bloodstream tosomewhere else in the body where it is needed.In bacteria an mRNA strand may last from a fewseconds to more than an hour; in mammals, mRNAmay last several days.Depending on how long it lasts, the same mRNAstrand may be translated hundreds of times.Eventually, it is broken down by enzymes in thecytoplasm.64


Translation Protein SynthesisProtein Synthesis66

Causes and effects of mutation Alteration of the sequence of bases in DNA can lead to changes in the structure andfunction of the proteins produced can have a range of effects67

Mutations can have a range of effects serious, even deadly, problem little or no detrimental effect. beneficial to the organism68

Mutations Bad reputation Tend to be disruptive Very, very rare69

Mutations 2 types point mutations - one base pair is changed chromosomal - entire sections of a chromosomeare altered70


Breast Cancer in Humans Two human genes, called BRCA1 and BRCA2 When functioning properly, help to preventbreast cancer by deterring cells from dividinguncontrollablyMore than 200 different changes in the DNAsequences of these genes have been detected,each of which results in an increased risk ofdeveloping breast cancer.72

Spontaneous mutationsSome mutations arise by accident as longstrands of DNA are duplicating themselves.Most errors are repaired by DNA repair enzymesbut some still slip by and there’s not much wecan do about them.73

Chemical-induced mutationsMany chemicals, such as those found incigarette smoke and in internal combustionengines, can also

Genes Sequence of bases in a DNA molecule Carries information necessary for producing a functional product, usually a protein molecule or RNA Average gene is 3000 bases long 31 . 32 . Genes Instruction set for producing one particular molecule, usually a protein Examples fibroin, the chief component of silk triacylglyceride lipase (enzyme that breaks down dietary fat) 33 .

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