IEI Genes and Ghromosomes

Genes1 A gene is a basic unit of inheritance which can be

inherited by the next generation.

2 A gene contains genetic information which determinesa particular characteristic in an organism.

3 lt can exist in different forms called alleles, that willdetermine the traits of an organism.

4 A gene can have many alleles. But, usually only twoforms of alleles are present in each diploid cell.

5 A gene occupies a specific position (locus) in a

chromosome.

Ghromosomes1 Chromosomes are thread-like structures in the nuclei

of plant and animal cells.

2 Each chromosome is made up of a long DNA moleculecoiled around protein molecules called histones.

3 Genes are located in the DNA molecule.

4 During interphase, chromosomes are not clearly visibleand are called chromatin.

5 However, during prophase of cell division, thechromosomes condense and can be seen clearly. Eachchromosome consists of two chromatids held togetherat the centromere (Figure 15.1).

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What You Will Learn

15.1 Genes and Chromosomes

15.2 Structure of Deoxyribonucleic Acid

15.3 Genes, DNA and Protein Synthesis

15.4 DNA Replication

1 5.5 Genetic Engineering

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1 DNA is a type of nucleic acid.

2 A DNA molecule is made up of basic unitscalled nucleotides.

3 A nucleotide is made up of a deoxyribosesugar, a nitrogenous base and a phosphategroup.

4 ln a nucleotide, the deoxyribose sugar is linkedto the phosphate group and the nitrogenousbase (Figure 15.2).

nitrogenousbase

deoxyribosesugar

Figure 15.2 Structure of a DNA nucleotide

5 There are four different types of bases: adenine(A), thymine (T), cytosine (C) and guanine (G).

Hence, there are four different types ofnucleotides.

6 Each nucleotide is joined to the next nucleotidethrough the phosphate group to form a longpolynucleotide strand (Figure 1 5.3).

(d) Structure of DNA moleculein a chromosome

Figure 15.3The nucleotides are joined togetherto form a polynucleotide

7 A DNA molecule consists of two polynucleotidestrands which are linked together at thenitrogenous bases by hydrogen bonds(Figure 15.4).

thread-like strand coiledaround prolein molecules

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Figure 15.1 Structure of chromosome which consists of DNA molecule coiled around protein molecules

Structure ofDeoxyribonucleic Acid(DNA)

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8 The thymine base of one polynucleotide strandis linked to the adenine base of anotherpolynucleotide strand by hydrogen bonds.Thecytosine base is linked to the guanine base(Figure 15.4).

9 The phosphate group and the deoxyribosesugar are the backbone in the DNA molecule as

they form a fixed structure.

CONCEPT FOCUSNitrogenous bases in a DNA molecule always comein fixed pairs - adenine pairs with thymine, andguanine with cytosine.

10 A gene is made up of a specific sequence ofDNA nucleotides. Sequences of DNAnucleotides make up the genetic code.

11 The gene contains information to synthesiseproteins such as enzymes and structuralproteins in an organism.

12 Proteins are the basic substances used to buildan organism. Enzymes are the basic substancesfor the chemical reactions in a cell, whilestructural proteins determine the characteristicsof an organism.

13 Based on the model of the DNA moleculeproposed byWatson and Crick, a DNA moleculeconsists of two polynucleotide strands coiledtogether, forming a double helix (Figure 15.5).

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from the combination of any three nitrogenousbase. .,rr:',

Genes, DNA andProtein Synthesis

1 The specific sequence of nucleotides of a genecodes for a specific amino acid sequence anddetermines the protein synthesised,

2 Every group of three bases which codes for a

specific amino acid is known as a codon.

3 The information carried by the sequence ofnucleotides is copied onto a specific sequenceof nucleotides in mRNA (ribonucleic acid)during a process called transcription.

DNA Replication1 Since A always pairs withT, and C always pairs

with G, each strand of DNA can act as atemplate for the assembly of a complementarystrand.

2 An enzyme breaks the hydrogen bondsbetween complementary bases. Free

4 Transcription occurs in the nucleus.The DNAdouble helix separates. One of the strandsserves as a template for mRNA following thesame base-pairing rules found in DNAreplication, except that uracil (U) pairs withadenine.

5 The mRNA molecule contains a specificsequence, exactly complementary to the DNAsequence of the template strand.

6 The mRNA formed moves to the cytoplasmwhere it is translated or converted byribosomes into a sequence of amino acids.

7 During translation, the ribosome reads threenucleotides at a time and assigns an aminoacid to each successive triplet. Eventually, a

whole polypeptide chain is produced.

8 Polypeptides form proteins which the cell needsfor various functions. The DNA thus controlscell activities by its ability to code for proteins.

9 A schematic diagram in Figure 15.6 shows howa trait is manifested from the basic unit ofin heritance.

nucleotides from the surroundings move in andbond with the complementary bases in theDNA. Other enzymes seal the newly attachednucleotides to each other.

3 Complementary base pairings between A andland between C and G in the DNA double helixprovide an explanation for the biochemicalfindings that quantitatively, G = C and A =I

bases in a DNA

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15.3

15.4

A sequence ofnucleotide bases inDNA is called a gene.

The gene codes thesynthesis of a specificpolypeptide chain, proteinor enzyme in an organism.

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IE Genetic Engineering1 Genetic engineering is also known as

recombinant DNA technology. The techniqueinvolves the manipulation of the genes to alterthe genetic content in an organism.

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CONCEPT FOCUSDuring replication, the parent DNA strand unravelsand free nucleotides bond with the correspondingbases on the separated DNA strands.Dan

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2 ln genetic engineering, the original traits of anorganism are altered by inserting a target genefrom another organism into the DNA.Thismodified DNA is known as recombinant DNA.

3 Hence, genetic engineering enables the genesthat control the beneficial traits to betransferred from the DNA of one species intothe DNA of another species.The organism thatresults contains genes from at least two speciesand is called a transgenic organism.

4 Genetic engineering is also used to speed upthe process of selective breeding in bothanimals and plants.The genes with thebeneficial traits are isolated and inserteddirectly into the DNA of another organism. Theorganism with the recombinant DNA is knownas a genetically modified organism (GMO).

5 The application of genetic engineeringtechniques to alter or replace defective genes iscalled gene therapy. lt involves the transfer ofthe normal genes into the genetic materials ofa cell to replace the defective genes that causegenetic disorder.Then, the body would be ableto produce the right protein or enzyme thebody needs and thus eliminates the cause ofthe disorder. Gene therapy is used to cure orprevent genetic diseases and cancer.

6 Genetic engineering has many otherapplications. For example, to produce insulin inmedicine and to produce genetically modifiedfood in agriculture.

7 Genetic engineering enables us to(a) enhance our understanding on inheritance

and the manipulation of DNA to produceceftain desirable traits;

(b) detect and treat certain diseases such assickle cell anaemia, haemophilia, cancer,hepatitis and diabetes mellitus;

(c) produce medicinal products such as growthhormone and vaccines for hepatitis B;

(d)increase the quality of agricultural productsthat are produced from animals and plants.

Human !nsulin ProductionThe procedure to obtain human insulin using theprinciples of genetic engineering involvesrecombinant DNA technology.The various stages ofproduction of human insulin using this method aregiven below.

1 Plasmids, circular DNA molecules found inbacterial cells, are modified by the addition of a

segment of human DNA which codes forproinsulin (precursor of insulin).The product isrecombinant DNA.

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Figure 15.8 Modification of plasmids

2 The plasmids are then inserted into bacteriacalled Escherichia colilE. colil.This process ofintroducing DNA into the cells is known astransformation. E. coli serves as the biologicalhost for the modified plasmid as well as thevehicle for the proinsulin gene segment toreplicate exponential ly.

3 ln order to produce large amounts of humanproinsulin molecules, the E. coli bacteria areallowed to multiply exponentially in a

controlled media in a fermentor at a suitabletem peratu re.

4 Heat is applied to kill the E. colibacteria,leaving behind the proinsulin.

Figure 15.9 Human proinsulin (darksection is insulin)

5 Specific protein-digesting enzymes cut off partof the proinsulin chain leaving behind insulin.

Figure 15.10 Human insulin

6 The final products are centrifuged to remove allcontaminating cell materials. Furtherpurification is carried out by liquidchromatography and crystallisation.

Agriculture1 Genetic engineering is used in agriculture to

produce organisms that have

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(a) resistance to infections;(b)tolerance to pesticides and herbicides;(c) tolerance to unfavourable climates.

2 Genetic engineering is used in tissue culturewhere all the beneficial genes from variousplants are isolated and then transferred to aplant. ln this way, the hybrids will be of highquality.

3 The food derived from organisms that havebeen modified genetically is called geneticallymodified food (GMF). Examples of successfullyproduced GMF are given below.(a) Tomatoes have been modified genetically so

that the fruit will last longer and the skin willnot wrinkle easily.

(b)Genes from flounder fish that live in theArctic were transferred into thechromosomes of certain plants to enablethem to grow in very cold environments.

(c) Maize plants have also been geneticallymodified to improve the nutrient content inthe grains.

CONCEPT FOCUSThe transfer of DNA from one species to another isknown as recombinant DNA rechnology.

Abuse of Knowledge in Genetics1 Genetic engineering has been successfully used

to treat various genetic diseases and hasresulted in advancement in the fields ofmedicine and agriculture.

2 Ihe advantages of genetic engineering are asfollows.(a) lt produces new strains of crops and

livestock that are highly resistant topathogens or pests.

(b) lt produces crops that are resistant toherbicides.

(c) lt produces food sources such as crops andlivestock that are rich in nutrients.

(d)lt allows the mass production of insulin,vaccines and other medicines to treatvarious diseases.

3 The disadvantages of genetic engineering areas follows.(a) Rising concern over the safety and the

effects of the genetically modified food onhealth after a long period of consumption

(b)The new strains of crops and livestock thatare produced genetically may cause theoriginal species to be defeated in thecompetition for survival and become extinct.

4

(c) The introduction of the new genes in thegenetically modified organisms (food) maycause the original gene pools to bedepleted.

(d)The introduction of the new genes mayaffect the functions of the normal genes,eventually causing cell mutation and cancer.

The abuse of genetic engineering involvessome risks and may cause problems thatthreaten the health and the survival of theorganisms.

Examples of risks that may arise are givenbelow.(a) Genetically modified food (GMF) which is

rich in protein may cause allergies orbecome toxic to the body.

(b) A transplanted gene may activate certainhuman genes, which could lead to cancer orother genetic disorders.

(c) New harmful strains of micro-organismsmay be produced as a result of geneticengineering. These micro-organisms couldbe used as biological weapons.

(d)Human cells may be manipulated to alterthe human traits, thus changing thecharacteristics of the human body.

(e) Biologists may abuse the knowledge ofgenetics to clone human beings by makingidentical copies of the cells and creatinghuman clones.

5

Ethical and Moral lssues in theApplication of Genetics1 The application and abuse of knowledge in

genetics has raised many ethical issuesconcerning human rights, religion, environmentand moral values.

2 Genetic engineering allows the selection andcreation of 'super species'. But in the selectionfor characteristics, who is to be responsible forwrong decision?

3 lf human cells can be manipulated to curediseases, should biologists try to engineer tallerpeople or change their eye colour, hair texture,sex, blood group, or appearance? What willhappen to the human species if we have theopportunity to design our own bodies? What arethe consequences if biologists develop humanclones by making identical copies of the cells?

4 New strains of crops and livestock areproduced through genetic engineering.Thishelps to produce good quality food and solvethe problem of food shortage.The concern for

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the safety of these genetically modified foodshas raised many issues. For example:(a) What are the side effects from eating these

genetically modified food on a long termbasis?

(b)Are these foods harmful to the body?(c) What effects will the new strains of crops

and livestock have on the ecosystem?

5 ln gene therapy, genetic diseases are treatedthrough the manipulation of genes.There areseveral ethical issues are involved here. Forexample, who will be given the chance to betreated for genetic diseases? What sort of genetherapy or gene replacement is encouraged?Do we allow genetic enhancement to producegenetically enhanced individuals? What is thepossibility of eugenics, the science ofimproving the heredity or qualities of thehuman race, becoming acceptable?

6 New harmful species of micro-organisms maybe produced. These harmful micro-organismsmay be dangerous pathogens that could beused as biological weapons to destroy thehuman race. Who will be responsible for theseecological disasters and biological attacks?

7 Stem cell research that uses stem cells derivedfrom human embryos or fetal tissues has raisedsome ethical issues. ln stem cell research, theembryonic stem cells that are taken from a

human embryo are capable of differentiatinginto various tissue cells when transplanted intothe cells of a fully developed individual. ls itmorally or ethically correct to destroy liveembryos for stem cell research to findtreatment for genetic diseases?

15.1A Understanding key ideas

Tick (/l if the statement is true.

1 A gene is the basic unit of inheritancein an organism.

2 Genes are located in the DNA of a

chromosome.

3 The DNA molecule is made up oftwo polynucleotide strands that arecoiled together to form a doublehelix structure.

4 Through genetic engineering, insulinis extracted from the skin.

5 A DNA molecule consists of twopolynucleotide strands which arelinked together by hydrogen bonds,

B Main concepts and facts

Draw the structure of a DNA molecule.

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MOLECULAR GENETICS

Molecular Genetics

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