IB Biology Notes for Molecular Genetics

The informaton in this document is meant to cover Topic 3.3-3.5, Topic 4.4 and Topic 7.1-7.4 of the IB Syllabus.

"This structure has novel features which are of considerable biological interest" (Watson & Crick, 1953)

DNA Structure

Deoxyribonucleic acid (DNA) is a double helix composed of two polynucleotde chains wound around each other.

htp://ghr.nlm.nih.gov/handbook/illustratons/dnastructure.jpg

● the "backbone" of the molecule consists of chains of alternatng pentose sugars (deoxyribose) and phosphate groups

○ each phosphate is atached to the 5' carbon of the pentose, and in a chain, ataches to the 3' carbon of the pentose in the next nucleotde

● atached to the 1' carbon of the pentose is one of four nitrogen-containing bases, which project in towards the axis of the double helix

○ adenine (A) and guanine (G) are purines○ thymine (T) and cytosine (C) are pyrimidines

htp://users.rcn.com/jkimball.ma.ultranet/BiologyPages/P/Pentose.gif

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IB Biology Notes for Molecular Genetics

● the base is hydrogen-bonded to its complementary base on the opposing polynucleotde strand

○ adenine always bonds with thymine (2 hydrogen bonds)○ guanine always bonds with cytosine (3 hydrogen bonds)

htp://users.rcn.com/jkimball.ma.ultranet/BiologyPages/B/BasePairing.gif

● DNA is assembled in the cell (and read by us) in the 5'→3' directon

● the two polynucleotde strands of a double helix run ant-parallel to each other, meaning one runs 5'→3' while the other runs 3'→5'

● the double helix makes a complete turn for every 10 base pairs – there is an average of 25 hydrogen bonds in just this short secton of a DNA molecule

Eukaryotc Chromosomes

In eukaryotes, chromosomes are not composed of naked DNA (as in prokaryotes). Instead they are composed of chromatn, which is about half DNA and half protein.

Nucleosomes are subunits of chromatn composed of a short length of DNA wrapped around a core of eight histone proteins. The nucleosome is held together by an additonal histone protein. The nucleosome is the fundamental packing unit of DNA, and aids in the super-coiling of DNA into chromosomes.

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The hydrogen bonds between bases are represented by the dotted lines (A=T & G≡C)

IB Biology Notes for Molecular Genetics

DNA Replicaton

DNA replicaton occurs in a 5'→3' directon. Replicaton is semi-conservatve – each new strand of DNA formed is half new DNA and half original (parent) DNA.

htp://www.estrellamountain.edu/faculty/farabee/biobk/semiconserv.gif

The process of replicaton begins at specifc nucleotde sequences called origins of replicaton on the DNA strand. In preparaton for DNA replicaton, the cell produces free nucleotdes, called deoxyribonucleoside triphosphates, because they have three phosphate groups atached instead of just one. Many enzymes are also involved.

1. Because the two strands of DNA are intertwined, the double helix must rotate in the process of replicaton. Two enzymes called topoisomerases accomplish this:

DNA gyrase can induce twistng and super-coiling of the DNA molecule. This may help facilitate unwinding of the double helix – by creatng negatve super-coils, or relaxing positve super-coils it eases tension generated by the acton of helicase.

Helicase moves along the sugar-phosphate backbones of the double helix, separatng the strands using energy from nucleotde hydrolysis. Separaton of the strands creates a replicaton fork.

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2. Synthesis of DNA occurs in the 5'→3' directon. Due to the ant-parallel nature of the DNA polynucleotde strands, synthesis of new DNA occurs diferently on each strand:

On the 3'→5' strand, the leading strand, synthesis occurs contnuously, since the new strand will be ant-parallel to the template strand. DNA polymerase III matches up free nucleotdes (dATP, dTTP, dGTP and dCTP) according to complementary base pairing rules. DNA polymerase III moves in the same directon as helicase (and hence, the replicaton fork) on the leading strand.

On the 5'→3' strand, the lagging strand, synthesis is discontnuous. Synthesis of new DNA is initated at several points along the lagging strand (by DNA polymerase III, which moves away from the replicaton fork) forming short segments called Okazaki fragments.

Primase adds a short length of RNA nucleotdes (at one point for the leading strand, and several points for the lagging strand) which acts as a primer for DNA synthesis – DNA polymerase cannot begin a new chain on a single-stranded template without at least a short region of duplex.

DNA polymerase I removes the RNA primer, replacing it with DNA. Gaps are lef between Okazaki fragments, which are then joined together by another enzyme, DNA ligase, which helps create the phosphodiester bond between adjacent nucleotdes.

The free single-stranded region is protected from degradaton by single-stranded binding protein (SSB).

Both DNA polymerase I & III possess a 3'→5' exonuclease functon, which allows for proof-reading and removing mismatched nucleotdes that were inserted erroneously during replicaton.

All nucleotdes used in the process are added according to the complementary base-pairing rules. In the case of the RNA primer, uracil (U) pairs with adenine on the template strand, as RNA does not contain thymine.

RNA

Nucleic acids required to turn the informaton stored in DNA into a polypeptde (protein).

● Single-stranded polynucleotde chains

○ pentose is ribose

○ bases are adenine, uracil, guanine & cytosine

Comparing DNA & RNA

DNA RNA

Strands Two One

Sugar Deoxyribose Ribose

Bases Adenine, Thymine, Guanine, Cytosine

Adenine, Uracil, Guanine, Cytosine

htp://www.mun.ca/biology/scarr/Fg10_09b_revised.gif

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There are three main types of RNA in cells:

● messenger RNA (mRNA) is a carrier of genetc informaton, since it is a copy of a gene sequence which is then used as a template for protein constructon

● transfer RNA (tRNA) brings amino acids to a ribosome, according to the nucleotde sequence in mRNA

● ribosomal RNA (rRNA) is a structural RNA found in ribosomes

Transcripton

Transcripton is the process by which a copy of a partcular gene is made using RNA nucleotdes. Transcripton must occur because DNA remains in the nucleus, while ribosomes, while manufactured by the nucleolus, work in the cytoplasm. mRNA is a copy of DNA, and is read by the ribosome during translaton. RNA polymerase unravels and unzips the DNA helix, and brings in RNA nucleotdes according to complementary base-pairing rules. Transcripton occurs in three stages:

1. Initaton

promoter regions are nucleotde sequences on DNA that signal initaton of transcripton

RNA polymerase binds at the promoter, which has specifc nucleotde sequences (e.g. TATA and CAAT)

a dissociatve subunit of RNA polymerase, factor sigma (σ), allows RNA polymerase to recognize and bind to the promoter regions

once RNA polymerase binds to the DNA molecule it begins to unwind the DNA , forming a bubble allowing the frst nucleoside triphosphate (ATP, UTP, GTP or CTP) to pair with the complementary DNA nucleotde

2. Elongaton

afer transcripton is initated, the sigma factor dissociates from the RNA polymerase

like DNA, the RNA is synthesized by additon of nucleotdes in the 5'→3' directon

3. Terminaton

RNA polymerase is also able to recognize a signal for chain terminaton

this signal is usually a palindromic sequence which causes formaton of a hairpin loop of mRNA, rich in G≡C pairs, and with a few A=U pairs at the end, which allows the mRNA to easily detach from its DNA template

terminaton may or may not depend on the presence and acton of another protein, the rho factor

htp://www.genetcengineering.org/chemis/Chemis-NucleicAcid/

Graphics/Transcripton.gif

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IB Biology Notes for Molecular Genetics

In eukaryotes, mRNA is usually modifed before it exits the nucleus. The DNA sequence copied by RNA polymerase contains both coding (exons) and non-coding (introns) regions. mRNA is a copy of both the exons and introns, and must be modifed to become biologically actve. In additon, there are modifcatons which increase mRNA's stability:

1. At the 5' end of mRNA, a cap of 7-methylguanosine is added shortly afer initaton. This increases the molecule's stability.

2. At the 3' end, the mRNA is polyadenylated, which means a tail of 20-250 adenines is added (poly-A tail).

3. Introns are excised (and subsequently recycled by the cell) and exons are spliced together.

htp://www.genetcengineering.org/chemis/Chemis-NucleicAcid/Graphics/Splicing.gif

Translaton

Translaton is the second stage in protein synthesis. It occurs in the cytoplasm, with the help of ribosomes and tRNA.

tRNA Structure

● 3' end always has the sequence CCA, and is the point of atachment for the amino acid

● there are regions of double helix, where bases form complementary pairs

● there are open loops where bases do not pair

● opposite the amino acid is the ant-codon loop, which corresponds to a codon in mRNA by complementary base pairing

htp://www.uic.edu/classes/bios/bios100/lecturesf04am/tRNA.gif

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Actvaton of tRNA

Aminoacyl-tRNA (a.k.a. charged tRNA) is produced in two steps; amino acid actvaton and transfer. The frst step is the adenylaton of the amino acid, which forms aminoacyl-AMP:

amino acid + ATP → aminoacyl-AMP + PPi

Then, the amino acid residue is transferred to the tRNA:

aminoacyl-AMP + tRNA → aminoacyl-tRNA + AMP

The net reacton is:

amino acid + ATP + tRNA → aminoacyl-tRNA + AMP +PPi

All of these reactons take place inside the aminoacyl-tRNA synthetase (enzyme) specifc for that tRNA. This allows tRNA to be reused afer exitng the ribosome afer dropping of its atached amino acid.

The Genetc Code

● used to turn the nucleotde sequence in mRNA into an amino acid sequence

● mRNA bases are read as triplets (codons), which are non-overlapping

In the mRNA (5' - AUG CCA UAA - 3') there are 3 codons.

● the code is universal – virtually all organisms use it

● the code is degenerate (redundant) – since there are 64 codons and only 20 amino acids, some amino acids are coded by multple codons

● AUG is the start codon – it corresponds to the amino acid methionine (met), which is the frst amino acid in any polypeptde

● UAA, UAG and UGA are stop codons (chain terminatng), which do not correspond to a tRNA/amino acid, thus stopping the translaton of mRNA (indicated by 'end' in the genetc code chart to the right)

htp://homepage.smc.edu/hgp/images/CharicaturesSmall/code.jpg

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htp://fg.cox.miami.edu/~cmallery/150/gene/c7.17.16ribosome.jpg

1. Initaton is the beginning of translaton.

A ribosome, mRNA and tRNA come together at the AUG start codon

• frst the small ribosomal subunit binds to both the mRNA and a special initator tRNA• then the large ribosomal subunit binds to the small subunit and mRNA and tRNA• proteins called initaton factors are required to bring all these components together

2. Elongaton is the successive atachment of amino acids, brought in by tRNAs

a tRNA with the correct ant-codon comes into the A site (initator tRNA is now in the P site)

the ribosome catalyses formaton of the peptde bond between the amino acids

translocaton of the ribosome occurs, and it moves to the next mRNA codon

• the tRNA formerly in the P site moves to the E site (now detached from amino acid)• the tRNA formerly in the A site moves to the P site (polypeptde is atached to it)• the new tRNA is now in the A site

the cycle repeats untl a stop codon is reached

3. Terminaton occurs when a stop codon is encountered.

Release factors bind to the codon (there are no tRNA/aa's that correspond to stop codons)

a water molecule is added instead of an amino acid, and the whole polypeptde falls of

the ribosomal subunits separate, and release the mRNA

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Ribos om e s

IB Biology Notes for Molecular Genetics

Several ribosomes may translate one molecule of mRNA simultaneously. As one moves away from the start codon, another may atach. This allows cells to synthesize vast quanttes of protein. A complex of several ribosomes on the same mRNA is called a polysome (polyribosome).

Ribosomes which are atached to the endoplasmic retculum (RER) are synthesizing proteins for export out of the cell. Afer synthesis, a vesicle containing the protein moves to the Golgi apparatus, where it may be modifed (e.g. by additon of sugars or lipids) before it moves to the cell membrane for exocytosis. For example, the protein hormone insulin would be made by ribosomes on the RER.

Ribosomes which are free-foatng in the cytoplasm are synthesizing proteins for use within the cell. For example, the enzymes used for glycolysis would be made by free ribosomes.

**Chloroplasts & mitochondria have their own ribosomes, so that they make their own proteins, including enzymes for Calvin cycle/Krebs cycle and the Electron Transport Chain proteins.

Mutatons

In living cells, DNA undergoes frequent chemical change, partcularly when being replicated. Most of these changes are repaired (by the proofreading actvity of DNA polymerases). Those which are not repaired result in a gene mutaton, defned as a change in the base sequence of a gene. There are many kinds of mutatons:

1. Single-Base Substtutons occur when one base is exchanged for another. These are also called point mutatons. The consequences of point mutatons depend on how the codon changes.

a) Missense mutatons alter the codon, so as to produce an altered amino acid in the gene's protein product. If the amino acid has similar propertes (e.g. both hydrophobic), there may be litle to no efect on protein functon. Otherwise, a change in protein functon is likely.

htp://academic.brooklyn.cuny.edu/biology/bio4fv/page/molecular%20biology/mutaton-missense.jpeg

For example, sickle cell disease results from the replacement of A by T at the 17th nucleotde of the beta chain of hemoglobin. This changes the codon from GAG (glutamic acid) to GTG (valine). This alters the tertary structure of hemoglobin in individuals with sickle cell disease, resultng in crescent-shaped erythrocytes instead of the usual biconcave disks.

htp://bsw-uiuc.net/tutorials_current/Sickle_Cell_Anemia/SC2001/intro_fles/image002.jpg

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b) Nonsense mutatons change a codon that specifes an amino acid to a stop codon. This means that the translaton of the mRNA transcribed from this mutant gene will end prematurely, truncatng the protein.

htp://academic.brooklyn.cuny.edu/biology/bio4fv/page/molecular%20biology/mutaton-nonsense.jpeg

c) Silent mutatons occur when a point mutaton changes a codon to another codon which encodes the same amino acid (due to the degeneracy of the genetc code). These mutatons cause no change in the fnal protein, and cannot be detected without sequencing the gene.

htp://academic.brooklyn.cuny.edu/biology/bio4fv/page/molecular%20biology/mutaton-silent.jpeg

d) Splice-site mutatons alter the sequence of a splice-site between exons & introns. When this occurs, introns (normally removed from mRNA) are not removed from the fnal mRNA molecule. This will alter the amino acid sequence of the protein.

2. Insertons and deletons occur when base pairs are added (insertons) or removed (deletons) from the DNA of a gene. The number of bases can range from a single base pair to thousands. Insertons and deletons which involve a number of base pairs not divisible by three (the number of bases in a codon) can have devastatng consequences on the protein product. These are called frame shif mutatons because translaton of the mRNA is shifed, altering all codons which come afer the point of mutaton:

htp://academic.brooklyn.cuny.edu/biology/bio4fv/page/molecular%20biology/mutaton-frameshif.jpeg

Frame shif mutatons ofen create new stop codons, generatng nonsense mutatons.

Insertons and deletons which involve three nucleotdes (or multples of three) may be less serious because they preserve the reading frame. A number of inherited human disorders are caused by the inserton of many copies of the same triplet of nucleotdes. Examples include Huntngton's disease (CAG) and Fragile X syndrome (CGG).

3. Duplicatons are a doubling of a secton of the genome. This can occur during meiosis, if sister chromatds that are out of alignment cross over.

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4. Translocatons are the transfer of a piece of one chromosome to a non-homologous chromosome. They are ofen reciprocal, meaning the two leukaemiaogous chromosomes exchange segments.

If the break occurs within a gene, it may destroy its functon, or create a hybrid gene. For example, patents with chronic myelogenous leukaemia (CML) ofen have a translocated chromosome (the Philadelphia chromosome, formed from chromosome 9 & 22), which contains a hybrid gene.

Biotechnology = manipulaton of organisms or their components to make useful products

PCR – Polymerase Chain Reacton

What is PCR?

● Is a technique for copying a piece of DNA a billion-fold

○ is an enzyme-mediated reacton○ the enzymes that are used for the PCR are DNA-dependent DNA polymerases (DDDP)○ enzymes are derived from thermophilic (heat-loving) bacteria○ DDDPs operate at 60-75°C, and can even survive at temperatures above 90°C○ PCR also needs a DNA template to copy, and a pair of short DNA sequences called

oligonucleotdes or "primers" to get the DNA polymerase started.PCR CYCLE

● PCR is a 3 step cycle that produces exponental number of identcal DNA molecules

1. Denaturaton

○ Samples are heated to 94 – 96°C for one to several minutes○ High temp. breaks hydrogen bonds break & separates target DNA in single strands

2. Primer Annealing

○ Temperature is lowered to 50 - 65°C ○ Primers are added○ Cooling allows lef & right primers to anneal to their complementary sequences○ Primers are designed to “bracket” the DNA region to be amplifed

3. Primer Extension

○ Temperature is raised to 72°C ○ The allows Taq polymerase to atach at each priming site○ Taq polymerase synthesizes a new DNA strand in 5’ – 3’ directon○ Extension occurs in the directon that the primer faces.○ The new double-stranded PCR product is called an "amplicon".

● The process is then repeated by cycling through the temperatures over and over again (35 to 55 tmes).

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● Each cycle results in a new DNA duplex

● each strand acts as a potental template for one or other primer

● The original template strands contnue to act as templates because the PCR process is not destructve

● Each cycle produces a greater number of the shorter amplicon molecules.

○ Eventually the majority of the amplicon in the reacton vessel is the expected length

htp://fg.cox.miami.edu/~cmallery/150/gene/c7.20.7.pcr.jpg

Gel Electrophoresis

● Is a technique used to separate nucleic acids or proteins that difer in size, electrical charge, or other physical propertes

● Electrophoresis uses an electric current to separate diferent sized molecules in a porous, sponge-like matrix

● Smaller molecules move more easily through the gel pores than larger molecules

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How does it work?

1. Restricton enzymes are added to create DNA fragments

2. DNA mixture is made by combining fragments, bufer soluton and a blue dye

3. Mixture is added to the wells of the electrophoresis chamber

4. An electric current is applied

5. DNA molecules move through the gel

○ the phosphate groups in the DNA backbone carry negatvely charged oxygen atoms – giving the DNA molecule an overall negatve charge

○ in an electric current, the negatvely charged DNA moves towards the positve pole of the electrophoresis chamber

○ smaller DNA fragments migrate faster & further down the gel

htp://img.sparknotes.com/fgures/7/749a4182b7527e44d289a612e420f40c/nucleotde.gif

htp://www.stanford.edu/group/hopes/diagnsis/gentest/f_s02gelelect.gif

Gel electrophoresis is a widely-used technique, especially in DNA profling.

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DNA Profling● Is a typical banding patern of genetcs profles (aka. DNA fngerprintng).

● The genetcs paterns are produced by electrophoresis of treated samples of DNA.

● In criminal investgatons, usually 13 sites on the DNA are tested (CODIS).

● If the banding paterns produced by testng these sites in the DNA of a crime suspect and in a sample containing DNA found at the crime scene are the same, it is considered that the two people of DNA are from the same person.

● Matching banding paterns in at least 7 sites in the DNA of children and their supposed parents is usually enough to establish likely parentage.

htp://statc.howstufworks.com/gif/dna-profling-2.jpg

Applicatons of DNA Profling

● Paternity suits ● Criminal Investgatons

● Identfcaton of people who died a long tme ago (e.g. Romanofs)

● The NFL used DNA technology to tag all of the Super Bowl XXXIV balls, ensuring their authentcity for years to come and helping to combat the growing epidemic of sports memorabilia fraud. The footballs were marked with an invisible, yet permanent, strand of synthetc DNA. The DNA strand is unique and is verifable any tme in the future using a specially calibrated laser.

Human Genome Project

Completed in 2003, the Human Genome Project (HGP) was a 13-year project coordinated by the U.S. Department of Energy and the Natonal Insttutes of Health.

htp://genomics.energy.gov/gallery/chromosomes/originals/530.jpg

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Project goals were to

● identfy all the approximately 20,000-25,000 genes in human DNA,● determine the sequences of the 3 billion chemical base pairs that make up human DNA,● store this informaton in databases,● improve tools for data analysis,● transfer related technologies to the private sector, and● address the ethical, legal, and social issues (ELSI) that may arise from the project.

Some of the outcomes of the HGP include pharmacogenomics, genetc counselling and gene therapy.

Pharmacogenomics

Pharmacogenomics is the study of how an individual's genetc inheritance afects the body's response to drugs, and holds the promise that drugs might one day be tailor-made for individuals and adapted to each person's own genetc makeup. It is in limited use today.

Antcipated benefts of pharmacogenomics:

● more powerful medicines● beter, safer drugs the frst tme● more accurate methods of determining appropriate drug dosages● advanced screening for disease● beter vaccines● improvements in the drug industry and the approval process● decrease in overall cost of healthcare

Genetc Counselling

According to the Natonal Society of Genetc Counsellors (htp://www.nsgc.org/), genetc counselling is the process of helping people understand and adapt to the medical, psychological and familial implicatons of genetc contributons to disease. This process integrates:

● Interpretaton of family and medical histories to assess the chance of disease occurrence or recurrence.

● Educaton about inheritance, testng, management, preventon, resources and research.

● Counselling to promote informed choices and adaptaton to the risk or conditon.

Gene Therapy

What is gene therapy?

○ Gene therapy = a technique for replacing an absent or faulty gene

○ has the potental for treatng disorders traceable to a single defectve gene

○ Normal allele of the defectve gene is inserted into the somatc cells of the tssue afected by the disorder

● For gene therapy of somatc cells to be permanent, the cells that receive the normal alleles must be ones that multply throughout the patents life

○ Gene therapy is used to treat cystc fbrosis and SCID (severe combined immunodefciency; replaced gene allows for the producton of the enzyme ADA – adenosine deaminase)

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How does gene therapy work?

● a clone of a "normal" gene is obtained and inserted into a carrier molecule called a vector

● the vector is used to deliver the therapeutc gene to the patent's target cells

○ Currently, the most common vector is a virus that has been genetcally altered to carry normal human DNA

● Target cells such as the patent's liver or lungs are infected with the viral vector.○ The vector then unloads its genetc material containing the therapeutc human gene into the target

cell.○ The generaton of the functonal protein product from the therapeutc gene restores the target cell

to its normal state.

● What factors have limited gene therapy as an efectve treatment for genetc disease?○ short-lived nature of gene therapy○ immune response○ problems with viral vectors – toxicity, immune and infammatory response

Gene Transfer● the process of taking a gene from one organism and insertng it into the DNA of another● cells that have had a gene from another organism inserted into them are called recombinant cells

○ ofen used to clone a gene ■ somatropin (growth hormone) and insulin are synthesized by recombant E. coli bacteria

How Gene Transfer Works1. A gene of interest (DNA fragment) is isolated from human tssues or cells that have been grown in a

laboratory culture.2. A plasmid is isolated from a bacterium (e.g. E. coli)

○ plasmid = extrachromosomal DNA molecule separate from chromosomal DNA○ capable of autonomous replicaton○ typically circular & double-stranded○ occurs naturally in bacteria○ carries genes that provide resistance to antbiotcs (e.g. ampicillin & tetracycline)

3. Both the human DNA and the bacterial plasmid are treated with restricton enzymes to produce identcal stcky ends.

4. The restricton enzymes cut the plasmid DNA at its single recogniton sequence, disruptng the tetracycline gene.

5. The DNAs are mixed together; the complementary stcky ends are atached to each other by complementary base-pairing. DNA ligase is added to bond the stcky ends.

6. The recombinant plasmid is introduced into the bacterial cell by simply adding the DNA to a bacterial culture. Under the right conditons, some bacteria will take up the plasmid from soluton by the process of transformaton.

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7. The gene cloning process occurs when bacteria with the recombinant plasmid is allowed to reproduce.8. Colonies of bacteria that carry the recombinant plasmid can be identfed by the fact that they are

resistant to ampicillin but sensitve to tetracycline (if the ampicillin gene is inserted into the tetracycline gene).

Restricton Enzymes (a.k.a. Restricton Endonucleases)● enzymes that cleave double-stranded DNA at specifc base-pair sequences

● each type of restricton enzyme recognizes a partcular sequence, known as its recogniton site

○ the restricton enzyme EcoRI binds to the following base-pair sequence: 5’ – GAATTC-3’ or 3’CTTAAG-5’

■ EcoR1scans the DNA molecule & stops when it can bind to its recogniton site.■ Once bound, it disrupts, via hydrolysis reacton, the phosphodiester bond between the guanine

and adenine nucleotdes on each strand.■ The hydrogen bonds of complementary base pairs between A and G are disrupted. The DNA

strand is cut, and two fragments are produced.

Examples of Restricton Enzymes

htp://users.rcn.com/jkimball.ma.ultranet/BiologyPages/R/RestrictonEnzymes.gif

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DNA Ligase

● enzyme that joins together cut strands of DNA● forms the phosphodiester bond between nucleotdes via a condensaton reacton● DNA ligase works best with stcky ends – for blunt ends, T4 DNA ligase is used

Genetc Screening

● Genetc screening is testng individuals in a populaton for the presence or absence of a certain gene (or a certain allele of a gene).

● It has three elements: pre-concepton, prenatal, neonatal

○ pre-concepton screening:

■ couples are screened before embarking on a pregnancy to determine if they could produce a child with a specifc genetc disorder (e.g. Tay Sachs)

○ prenatal screening:

■ screens fetus for genetc disorders and guides decisions about possible medical treatment or aborton

■ amniocentesis and chorionic villus sampling are commonly used to sample fetal DNA

○ neonatal screening:

■ newborns are screened to identfy treatable and untreatable genetc disorders

■ all provinces & territories screen newborns for phenylketonuria and hypothyroidism

● PKU is a metabolic disorder, caused by inability to metabolize the amino acid phenylalanine; treated by restrictng certain foods from the diet. Untreated, PKU can lead to severe, progressive mental retardaton, organ damage, & unusual posture.

● Hypothyroidism results from too litle thyroid hormone in the bloodstream, and slows down many body processes.

● Tests are conducted using blood samples and other tssues

● Screening is done so that further testng can be undertaken. For example, women may be tested for BRCA1& 2 - genes associated with breast cancer, so that preventatve measures and early interventon

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can be considered.

● Genetc diagnosis is currently available for the following:

○ abnormal chromosome number (e.g. Down's syndrome, trisomy 21)○ early onset conditons (sickle cell, cystc fbrosis)○ late onset conditons (Huntngton's disease, polycystc kidney disease)○ susceptbility to (e.g. hypercholesterol, alcoholism)○ carriers of recessive genes

Potental Benefts & Concerns About Genetc Screening

Advantages Disadvantages

● prenatal diagnosis of genetc diseases● predetermine carrier status (e.g. sickle cell)

● invasion of privacy – possible use of informaton by employers & insurance

● storage & use of genetc informaton for legal purposes

CloningClones are genetcally identcal organisms or a group of cells derived from the same parent cell.

There are various methods of cloning, involving both undiferentated and diferentated cells.● Many plants can be cloned by taking a cutng (leaf, stem, etc.) and rootng it in water, soil, or other

media.

● Dolly the sheep was cloned from a mammary cell:○ the nucleus of a (diploid) mammary cell was removed and

injected into an enucleated ovum○ diferent breeds of sheep were used for DNA and egg donors

(Why??)○ the ovum was zapped with electricity to fuse the nucleus with its

new cytoplasm, which actvated the cell-division cycle, as if the ovum had been fertlized

○ the "fertlized" ovum was placed in the reproductve tract of a third sheep, and Dolly resulted

htp://www.synapses.co.uk/science/cells4.gif

● Clones can also be made using undiferentated cells – afer fertlizaton, the frst few cell divisions produce undiferentated cells. Cells separated at about the 8-cell stage can be separated, and will contnue to divide as if the were the original fertlized egg. This can result in producton of eight identcal ofspring which could gestate in eight diferent surrogate mothers.

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IB Biology Notes for Molecular Genetics

Ethical Concerns Regarding Cloning

As far as we are aware, humans have not yet been cloned artfcially. Natural clones are also called monozygotc, or identcal, twins. Some concerns regarding cloning humans:

● some may regard producton of two embryos from one fertlized egg to be acceptable○ others may see this as leading to selecton of those "ft to be cloned" and producton of a genetc

super class (eugenics)● unresolved questons:

○ Is a clone its own unique human being?○ Is cloning strictly for the purpose of stem cell producton or organ harvestng legal or right?○ What about reproductve cloning?

Genetcally Modifed Organisms (GMOs)

A genetcally modifed organism is an organism whose genetc material has been altered using genetc engineering techniques (recombinant DNA technology). Transgenic organisms are GMOs formed by transferring DNA from one species to another.Examples:Transgenic sheep have been produced that produce human proteins in their milk:

● A blood protein called AAT enables us to maintain lung elastcity – it can be used to treat patents with emphysema, but cannot be produced in the laboratory in practcal quanttes. Insertng the gene into a sheep’s mammary gland tssue results in producton of AAT during lactaton.

● A human clotng factor called factor IX can be obtained from GM sheep for use in patents who are unable to synthesize it on their own.

Many commercially valuable plant species have been genetcally modifed:● Herbicide resistant crop plants are unafected by weed killer.● Rice varietes that contain β-carotene, a vitamin A precursor● Tomatoes that can ripen on the plant to full favour but remain “fresh” for a longer period of tme.

As with many aspects of biotechnology, there are benefts and ethical consideratons to discuss regarding genetcally modifed organisms:

B.t. genes (from Bacillus thuringiensis, a naturally occuring bacterium) have been inserted into corn and other plants. B.t. produces crystal proteins that are lethal to insect larvae. The beneft to farmers, is that corn plants with the B.t. genes are resistant to insect pests such as the European corn borer. However, pollen from B.t. corn plants (which would contain the B.t. genes) has proven to be harmful to Monarch caterpillars. The caterpillars consume milkweed, but if pollen from B.t. corn is blown onto milkweed plants from neighbouring cornfelds, then the caterpillars could consume it and die.

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