to Fa*sbm @o

crg

B.va

o

o

Remember : - DNA atul RNA are poltmers oJ'rtucleotidas. clesigtruted bytlu, letters oJ'tlrcir titt'oeetk)us base s, A (acleritrc I.T ( thtnitte ). LI (w'acil t.C (c't'tosirre) and G (guattittc). Tlrc bascs are an'anged into threcsrtrukirtg trp tlrc coclorts wlic'lt are rlrc wits of tlrc getrctic code. Particularcoclorrs ceusa tlrc irtsertiott of spcciJ'ic antirto acicls ittto tlrc polryeptidesso tlrut iJ the coclotts are altcrecl. tlrc ut'ortg anino acicl nrut' ba iusertctl.Also rententber tlrut A is conrylemetrtart to T itt DliA ard to LI itt RIVA.G is contplenrcnter'\' to C irt bcttlt RNA arrcl DNA. C onryslenrcntctt'\' nteenstlrut tltey u'ill conbine togcrlter br htdt'ogert ltorrclirtg.

In sickle cel lanaemia the mutation af lects the amino acid sequence of partof the B-globin protein chain of the haemoglobin (Fig 2 ). Tliis results in thelormation of abnormal haemogiobin-S which in conditions of low oxyuentension causes the red blood cel ls to col lapsc into sickle shapes.

Fig 2. The mutation which results in Sickle Cell Anaemia

In sickle cell anaemia, the base T marked (in the normal diagram)with anarrow is substituted with a base A. This means that the resultant codon onthe mRNA is GUA instead of GAA and so the amino acid valine is insertedinto the protein in place of glutamic acid. This results in the fbrmation ofhaemoglobin-S. This type of gene mutation is called a 'missense'mutation

and only changes one amino acid in the polypeptide chain.

Sometimes a base alteration in a codon does not alter the amino acid sequenceof the polypeptide made by the gene. This is called a'samesense'mutation.It occurs because the genetic code is degenerate. Degenerate means thatsome amino acids can be coded for by several mRNA codons which vary intheir third base. For instance. serine is coded for by UCU, UCC, UCA andUCG. If the mutation changed the third base in UCA to make UCU, thenew codon will still insert serine into the polypeptide.

Gene Mutationsstud.ving this Factsheet .vou should gain a knowledge of:the nature and causes of 'gene mutation:the ef'f'ects of gene mutations;gene mutation as a source of variat ion which mav be of select ivevalue.

This Factsheet assumes that the student has a basic knorvledse of thc natureof DNA. RNA. replication. the qenetic code and protein svnthcsis. Thesewere covered in Factsheet 22. Protein sl,nthesis I - nucleic acids imd Factsheet49. Protein synthesis II - mechanisms.

The nature of gene mutationA mutation is a sudden inheritable change in the genetic material. Genemutation is sometimes cal led point mutation because the mutation may'only involve change of one nucleotide (base)in the codon sequence ol t l iegene. The repl icat ion of DNA and transcript ion of messenger RNA(mRNA) is not 100% etf icient with the result that q_gg.rqqnql errorsoccur. These errors ma)' cause mutations. The mutation ma)/ be due tothe insert ion (addit ion) or delet ion of a nucleotide. the substi tut ion ol onenucleotide lor another or inversion of two nucleotides (Fis I ).

Fig 1. Types of point mutation.

Altering the base sequences in the codclns of the gene may alter thesequence in which amino acids are assembled into the polypeptide madeby that gene. The altered polypeptide may then have an altered eftect orfunction which in turn may alter one or more characterist ics of theorganism. The result ing mutation is harmful, e.g. sickle cel l anaemia,or i t may confer evolut ionary/ or survival benefi t e.g. in industr ialmelanism which occurs in many butterllies and moths. If the mutatedallele is situated on a sex chromosome then the mutated condition will besex-linked. as in haemophilia and red-green colour blindness. In thesecondit ions the mutated al lele is on the X-chromosome.

OriginalDNA

Corresponding mRNA

Insertion of A as lburth nucleotideNew DNA

Altered mRNA codons

Deletion of the third nucleotide. TNew DNA

Altered mRNA codons

Substitution of fburth nucleotide with T.Neu,DNA

Altered mRNA codons

Inversion of second and third nucleotidesNew DNA

Altered mRNA codons

AGT CCC AAA CCA

LICA GGG UUU GGU

AGT ACC CAA ACC A

L]CA UGG GUU UGG U

AGC CCA AAC CA

UCG GGU UUG GU

AGT TCC AAA CCA

U C A A G G L ] U U G G L '

AGC TCC AAA CCA

UCG AGG UUU GGU

NormalA n r i n o a c i d s e q u e n c eo f p - g l o b i n c h a i no f h a e m o g l o b i n :

C o d o n s o n m R N A :

glutamic

+I code

I to rGAA

pro l i ne g lu tamic

+ +I code I code

l f o r I f o rCCA GAA

tII

C T T

t rl lr l

GGT CTT

pro l i ne va l i ne g lu tamic ac id a+ + t.^,-. LtI codes

I codes

| :"1.' I =I fo r

l fo r I fo r

I iCCA GUA GAA

acid aA ;

l ' "

t :l =I F

a 7& it r . =f f i ,w ' i

a

F

9.

f f i h Eg :f f !

.j

F

tII

C T T

t tt lt l

GGT CAT

ac id

B a s e s o n D N A :

A m i n o a c i d s e q u e n c e

o f p - g l o b i n c h a i no f h a e m o g l o b i n :

C o d o n s o n m R N A :

+Sick le Cel l

B a s e s o n D N A :

The substitution of A tor T sives valine instead of clutamic acid

Gene Mutations Bio Factsbeet

A 'nonsense' mutation changes an amino-acicl-speci l i ing coclon into achain-terminating codon. For instance. i f UCA. which codes tor serine. issubsti tuted, ma-king UGA. then this is a codon which halts svnthcsis of thepol l ,pept ide.

A 'fiameshift' mutation is a mutation iuising trorn the insertion or deletionof one or more nucieotides (other than in mult iples of three) that causes thegene to be misreacl during translation into the polvpeptide. In this t1,pe ofmutatictn. se veral or all of the codons after thc mutation are altered and scrscveral or all of the amino acids assernblecl together are incorrect. Theresult ing pol-vpepticle wil l probabll ,be completel\ , inactivc and biological l l 'use less.

Causes of mutationl. Spontaneous mutations result l iom errors in the repl icat ion o1'DNA.Replication of DNA is a complicated process involvins DNA pol l ,meraseand a larse number of other enzvmes. These enzvmes make rare mistakesand the se cause base changes in the comrrosition of DNA.

2. Induced mutations are chan-ee s in the DNA caused b-v the ef l-ects o1'mutagens'. These ma1, be radiat ion clr chemical in nature and causc genesto mutate at rates that are much hishcr than their spontaneous l 'ate.Examples are:

ionising radiat ions. e.g. X-rays. cosmic rays lrom space. alpha(u)rays (hel ium nuclei). beta (F) ra1's (electrons or positrons) and gamma(]) rays which are electromagnetic. Alpha. beta and samma rays aretrom radioactive sources. The1, 3c1 bl,direct lv breaking the geneticmolecules and by breaking up water molecules into reactive ionisedlragments thiit can damage tlie DNA indirectiy.

non-ionising radiat ions. The commonest example is ultra-violetI ight between wavelengths 2-50 to 170 pm. This is absorbed bi, , thenitrogenous bases in DNA and modif ies them in various ways. Onernain effect is tliat thymine dimers are produced - adjacent thvminebases in one DNA st rand couple together ra ther than wi th thecomplementary adenines in the other strand. Thus the DNA hel ixdevelops a bulge where it is not coupled. Luckily there is a li-eht repairsystem that in the l ight uses repair enzymes to make good the DNAdamage. However, some people have a gene mutation which causesthe repair enzyme to be damaged. These people are at r isk lromdeveloping skin cancer (xeroderma pigmentosum) i f they sunbathe.

chemicals. Mustard gas is an alkylating agent r','l i ich adds a methvlor similar alkyl group to guanine. altering i ts abi l i ty to base pair. Thiscauses the release of guanine lrom DNA so that the posit ion occupiedb1, guanine can then be lllled by another base. Many other chemicalsare base analogues which have sirni iar structures to the normal basesand so can become incorporated into the DNA in place of them duringrcpl icat ion. An example is 5-bromouraci l . Other mutagenic chemicalsinclude dioxin, colchicine. cafteine. some pesticides and several tobaccoproducts.

Gene mutation and natural selectionMany gene mutations are harmful, such as the human conditions of sickle-cell anaemia, haemophilia, muscular dystrophy. albinoism. thalassaemia,fibrocystic disease of the pancreas and Huntington's chorea. Withoutmedical treatment and support, the individuals suffering trom theseconditions probably would not survive. However. many gene mutationscan be of advanta_ee.

For example. the mutant sickle-cell allele for haemoglobin-S is codominantto the normal haemoglobin-A al lele. Although an individual who ishomozygous fbr allele S (genotype SS) will have sickling of all his or herred cells, causing a severe, usually f'atal. anaemia, an individual who isheterozygous ( -eenotype SA) wi l l p roduce 50c/ r s ick le ce l ls wi thhaemoglobin-S and 507c norrnal cells with haemoglobin-A. This individualcan survive and may reproduce to pass on the sickle cell trait.

Natural selection plavs a pmt when heterozygous sickle cell trait individualsIive in areas u'here malaria is prevalent. The maliuiai parasite lives in normalred cel ls ancl f-eeds on haemoglobin A. I t cannot feed successlulh onhaemoglohin-S and llnds difticultt,in entering sicklecl red cells. Thus sickle-cell carriers do not suf-fer trom malaria in areas r.l,herc n-lauv normal peclplecontract malaria. The lrequencv of the sickle-cel l al lele is raised in thesemalarial areas above i ts frequencv in malaria-lree areas. This could bebecause the survival rate and reprocluctive rate of sickle-cell trait carriers ishigher in malarial areas than in non-malarial areas. or because more normalpcople die oi 'maiaria than sickie-cel l trait carr iers do.

A clcarer example ol'natural sele ction operating on a mutated gene is shownin the example of thc pcpperecl moth, Bistott betularia. A sin-sle -eene locusis involvecl u' i th twc.r rnain al leles. The recessive al lele (c)determines the' t1 ' 'p ica l ' phenot l , 'pe in the double recess ive homozygotes (cc) . Th isphenot.""pe is light colourecl moths with a 'peppering' of black spots onthc wings and bod"v'-. Thesc were the predominant lbrms of the moth presentin Britain in the first half of thc nineteenth centur),. In 18219 a dark-colouredvarietv callecl 'carbonarill ' \\'as discovered in Manchester. This phenotvpewas clue to a dominant mutation into al lele C. The 'carbonaria' moths hadthe eenotypes CC or Cc. Between 1849 and 189,5 the 'carbonaria' varietyincreasecl in trequencl', lbrming 98% ol the population in the Manchesterarea.

This increase in trequencv \\ 'as associated with the Industr ial Revolut ionin the nineteentl') centurv and its associated pollution. The carbon particlesand sulphurdioxide lalling out fiom tactory chimneys killed the light colouredlichen on tree truntris and rocks. and tormed a black. sooty coating on thesesurlhces. The 'tvpical' torms of the moth were no longer camoullaged onthese surfaces and so werc predated on (bv birds such as robins andthrushes) morc than the 'carbonaria' fbrms which were camoufla-sed. Thusthe 'carbonaria' lbrms survived and were selected but the 'tvnical' lormswere eaten and became scarce.

This phenomenon became known as 'industrial melanism' and occurredin manl' species of butterilies and moths. However. it did not occur in non-industrial. non-polluted regions of the country - herc the 'typical' formssurvived.

In 1956 the Clean Air Act u'as introduced with the result that industrialpollution bv soot and sulphur dioxide was greatly curtailed. This resultedin a return of lichen qrowth and a lightning of the surf'aces of trees androcks. The 'carbonaria' tbrms are no longer camoullaged and now get eatenwhile the 'typical' torms are camouflaged and have an improved chance ofsurvival (Fig 3 ).

Fig 3. Changes in the frequencies of ' typical ' and 'carbonaria'

peppered moths in industrial areas of England

+II

: 100

-.=.E

-

.J

=

J O. ,ra

\ i/t

--J

;typ\ca,l

\

\t- carbonaria

1850 1900 1950

+ f time/years

t lindustrial cleanrevolut ion air act

Gene Mutations Bio Factsbeet

Othcr examples in nature of gcne mutations wliich have produced benetlcialvariations to the mutated organism ilre:. the development of antibiot ic resistauce in certain strains of bacteria:o the development ol 'w,artarin (rat poison) resistance in rats:. the development of DDT resistance in mosquitocs ancl man1, other

species of inscct:. thc development of camoullaging shell colours ancl banding on the

shells o1'the snai l . CrTa(a nentorol is. This reduces predation brrthrushes:

. thc dcvelopment o1 resistance to thc myxclmatosis virus b1, manvrabbits.

Remember :- otlter getrc mutatiotts u'l iclt lou nru\. be asketl oboutittcluclc lruemoplil iu. recl-E'r,ett colout' lt l intlrrcss. cr.s/ic.fibt'osis andalplru-l attitt ' t ' l tsirt cleJiciartct' u,lt iclt results irt arr irt lrct' ited for.m oJempltvsenta ( lwtg c l iseasc). You ntot ' be gi rct t data intcrprctat i t )nqt tesl i ot t s u lx t t t t r t t l ta r nt t t ta l iot t .s .

A t'en' conunon en'ot' is to cortlfuse base tleletiott of gerte nurtatiort u'it lrq a t rc cl a I a t i t ttt ttJ' c lt rttnl( ) s( )nr ( n t tt I tt I i r t t t.

Exam Hint: - ln addition to questions abaut the nature of genemutations and their importance as sources of variation for naturalselection to act upon, examiners often set questions on theinheritance patterns of gene mutations in family trees. ln answeringthese it is important that the genetic diagrams are laid out in thecorrect, acceptable format. Some examples are shown in thespecimen questions and mark schemes below. Questions ofteninvolve data interpretation.

Most gene mutations in nature will have only slight eftects or unnoticableeff-ects and mav also conler survival benefi ts. Polygenic systems withmany dill-erent alleles probabli'arise as a result of many such gene mutationsover long periods of time. Thus. though an individual gene mutation givesrise to discontinuous variat ion. man)' minor mutations to the same geneover thousands of years mav eive rise to the continuous variation due topolygenes which can consist of hundreds of di l ferent al leles.

Practice Questionsl. Cystic f ibrosis (CF) is the most common recessive genetic disorder in

the UK Caucasian population u,ith an overall birth fiequency of aboutI in 2-500. I t is characterised bv severe respiratory problems andinadequate pancreatic lunction. caused bv accumulation of stickl,mucus.There is no cure at present but improved treatments can increase thelif-e expectancy to about 30 years. death usually being due to respiratorylailure. Males u'ith CF are usualll,' sterile. Cystic llbrosis is due to amutation in the gene encoding the cystic f ibrosis transmembraneconductance regulator (CFTR). This protein regulates the transport ofchloride across cell membranes. The mutant allele is recessive andheterozygotes do not exhibit symptoms. The mutant allele occurs inabout 4c/r of the population ancl usually arises by deletion of threeadjacent nucleotides.

(a) ( i ) Suggest how the de le t ion cou ld occur and in f luence rheeffectiveness of the CFTR gene. 5

(ii) Name two other tvpes of sene mutation. 2

(b) (i) Bv means of a genetic diagram. show the probability of twocarrier parents producin_e a CF child. -5

(ii) What would be the expected percenrage incidence of cysticfibrosis in the population'? Explain your answer. 5

(iii) Since cystic flbrosis is a Iethal condition, why does its alielepersist in the population'l I

2. The drawings belou' shou' the appeffance of normal human red cellsand red cells fiom iur individual sufferine tiom sickle cell anaemia. Thecondition is caused bv a mutant co-dominant allele.In the homozygousconcl i t ic ln the al lele causes sickle cel l anaemia which has serious. ot ienfatal. eff'ccts on suf'ferers. rvhile in the heterozygous condition it causessicklc cell trait which is harmless and sives resistlncc against inl-ectionbv malarial parasites. The abnormal gene also changes the permeabilitl,'o1 recl cel l membranes so that the reci cel ls lose Dotassium ions.

normal red cells sickle cel ls

(at The mutatior-l al-lects the amino acid sequence ol the p-globin chain ofhaemoglobin. The nomral amino acid sequence. together with the mRNAancl DNA bases which code tbr the sequence are shou'n belou,.

amino ac id

sequence: r,al irre /hist idine / leuciuc /threonine /prol ine /glutamic acid /slutarr ic acid

m R \ Acodons:

D\A

bases:

GI. IA CAI . I CT IC ' ACL] CCA GAA G A A

-t.

C,{I CITA C;AC; TC;A C}C;T c-l-f cTI

+(i) If the base marked with an lrrow was changed to A(adenine)

what would be the eflect on the amino acicl sequence of the

-elobin'l Explain vour answer. 4(i i ) What wil l be the probable el lect is this change in amino acid

sequence on the structure of the haemo_elobin'? Explain vouranswer. 2

(i i i )Name this type of gene mutation. I

(b) The sickle cell hacmoglobin is knou'n as haemo-elobin S. This typeol haemoglobin is lbund mainly'in people who live in the malarialbelts of the world. or in their descendents. Suggest an explanationwhv this is so. 4

Alkaptonuria is a harmless. rare autosomal -eenetic def-ect, due to amutant allele. in humans. The family tree belou' shows the pedigree ofa tamil l 'af1'ected b1, 'alkaptonuria. ( lndividuals are numbered i to l4)

unaf'fected f'emale

afl-ected lemale

unattected male

allected male

(a) (i) Is the condition dominant or recessive? Explain vour answer

(ii) State the numbers of all the inclivicluals that are certain to beheterozy-eous for this gene 3

(iii) What is the probability that individual l4 is heterozy-eous fbrthis gene? I

(b) Alkaptonuria occurs due to point mutation of an allele of a -eene.State two other point mutation diseases of humans. 2

Gene Mutations Bio Factsheet

Anslr,ersl. (a) ( i) nucleotides could be omitted cluring repl icat ion of DNA (in

meiosi s/gamete tbmration ) :thus rnRNA cloes not include thc omitted nucleoticles (clurinst ranscr ip t iou r :thus an amino acid u, i l lbe omitted lrom the polypeptide chain:during translation:thus CFTR protein will not work/work properll,/be ineftective:

-5

(i i ) substi tut ion: insert ioniaddit ion: inversiorr: max 2

(b) ( i ) (use su i tab le symbols . e .s . C for normal a l le ie . c lor cvst icllbrosis allcle t

P C c C c :gametes C

(a) ( i ) recess ive:because obv ious heteroz l ,gotes/car r iers don ' t show theconditior/the alle les in 6/8/ l3 ntust have come liom the parentsn'ho do not shou' the conclition: 1

( i i ) l + l : 3 + 4 : l 0 + l l : ( k r s c

( i i i ) 2 /3 :

I mark lbr each incorrect)

(b) sicklc cell anaemia:haemophilia/colour bl indness/albinoi sn-r/an)' orher valid det'ec t:

3I

1

F C C C c C c c c :

t \ , / tnormal carriers sutferer :

probabi l i ty I in4:

( i i ) CF males are steri le so their genes can be disregarded:4% of the population iue carriers:thus chances of carr iers crossing = .04 x .04 = .0016 or 0.16% :(allow other \,r'avs of showing 1l-sures)probabilitl, of carier cross producing a CF child is I in 4:thus expected incidence wil l be 0.\6c/r =0.04c/r:

(if sa1, ,lQQ = 0.O4c/( allow I mark onl1,. unless explained)2-500 s

( i i i )because new mutat ions (o f the same type) are constant lyhappening: 1

2. (a) ( i) DNA codon would become CAT:this would tbrm GUA by transcriprior.l ro tlie MRNA:GUA codes tbr valine:thus sixth amino acid in chain/penultimate amino acid is chan-uedtrom elutamic acid to valine:

( i i ) w i l l a f fec t /a l ter the cross-bonding in the g lob in cha in /po lypept ide:which u'ill alter the 3-D shape/confbrmation/tertiarl' structureof the molecule:

( i i i ) subst i tut ion:

malarial parasite develops inside recl cells of humans:cannot survive on haemoglobin S as substrate/cannot survive inreduced potassiunr ion environments:thus sickle cell sutl-erers are resisrant to malarial inf'ection but usuallvdie from sickle cell anaemia when young:sickle cell trait heterozygotes a-re resistant to malarial inl-ection anddo not die lrom sickle cell anaemia:thus reproduce norrnally raising incidence of mutant gene in thepopulation of the malarial zone:(probably) have a greater reproductive capacity than malarial

( h )

Acknowledgements:Tlri.t FLtctslu'tt xus t'cseur(hcLl untl urittctr h.t' Murtirt GriftinCurriculum Press, u nit 3058,The Big Peg, 120 vyse Street, Birminghant B I8 6NF Bio F actsheetsnruv bc copictl .fi'cc tt.f clurg,c b.t'teuchirrg stu.fJ tn'stutlertts, prtttitlatl thut tlteir school is urcgistt'r'ed yrbst'riber. li1t purt ttl tht,st Fuctshcct.t nttt.t hc rcpnxltrcetl, sfttrctl in u rctrietul.r . \ ' . t / . ' rr i . ()r t t 'uttsnti t l ( ' ( l , i t t ut l ot l tcr. f ornt rtr hv unt tLt l tct ' nteuns. v i t l t t tul t l tc pri , ,r pt 'rnrissit t t tol t l ra puhl ishcr: /SSA' I35l-5136

suf'ferers (within the population): max 4