Lec 2 dr Ameen (Tafree3')

8/14/2019 Lec 2 dr Ameen (Tafree3')

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2007-2008

Lecturer : Dr. Ameen Al-Ali

:: Done By ::

Husam Al-jofi & 3ahad

GGeennee MMuuttaattiioonn


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Insertions & deletions :

A number of inherited diseases are caused by the insertion of manycopies of the same triplet of nucleotides .(fragileX, Hungtington'sdisease).

e.g. 100 copies of (AGU) (AGU) (AGU) (AGU) (AGU)..(triplets)100called triplet expansion

Amino acid sequence of the protein subsequent to mutation bears noresemblance to the normal sequence.

e.g. Once you enter the glycine code and copy it 200 time the structure ofprotein will totally change Most frame shift mutations leads to premature stop codon down stream

to the mutation.90% frame shift truncation (shortness) of the protein, not complete

Frameshift : is that when u add or delete 1,2,4,7not 3 amino acidstherefore it will lead to early stop codonMy note # just remember that stop codon is important in DNA translationto prevent elongation of the gene synthesis.

Duplication :

Duplication are doubling of a section of the genome during meiosis.not one base but millions of bases a hole piece of DNA has beenduplicated which cause a serious effect

This can cause the gene to carry inappropriate promoters at its 5' end.Promoter : determines how fast the gene is transcribed1 gene1 specialized promoter (majority)1 gene with other promoter cause problemsLinked genes : 3genes 1 promoter

e.g. 3 enzyms are required to metabolize lactose under the same

promoter Normal cell protein coupled to C-terminal of another protein

(leukaemia).take N-protien of a gene and insert it another gene therefore duplicationof this new mutated gene will cause leukaemia.


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Translocation : Transfer of a piece of one chromosome to a non-homologous

chromosome. This can alter the phenotype in several ways

o Break may occur within a gene destroying its function.if u break the gene in half it will lose its functiono Gene may come under influence of different promoters &

enhancers.o Breakpoint may occur within a gene creating a hybrid gene. i.e.

protein with an N-terminal of one normal cell protein coupled toC-terminal of another (leukaemia).

gene + gene of another gene with totally new function

differ then the originals (e.g. leukaemia)

- The gene D will from the long arm to the

short arm, and the C gene will move fromthe short arm to the long 1. and when

- A gene in a given chromosome may becontrolled by another gene or factor from

another chromosome so there is acomplicated interaction between genes

present on different chromosomes and thiswhat complicates the whole case of

genetics


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Single Nucleotide Mutation : Single base substitution (also called point mutation)- Missense mutation (sickle cell disease)- Non mutation (cystic fibrosis)-

Silent mutation (one in G6PD deficiency)Due to synonymous(degeneracy of the codon) mutation (is like 1 codon ofArg is replaced by another codon of Arg )& other cause is there is amutation but it does not led to any phenotypic character or disease e.g.no change in haemoglobin between Hb g (glutamic) & Hb a (alanine), theimportance of this is to differentiate between other disease like markerse.g. Hb g are cretin to develop some kind of disease more than Hb a(diagnostic).

- Splice-site mutation (thalassemia)the intron & axon are linked by a splice site that contain (AG) in thebeginning & (GA) in the end, and if we have a mutation of the endGAGT so when the enzyme will continue synthesis till the next intron

without stopping cause all he knows is GA.

- Promoter site (altered expression)No change in protein the changeis in the rate of expression(syntheses/transcription) of thatprotein effect translation

reduction or increase of theactivity of the protein Insertion & deletion Duplication(during meiosis)

AG

AG GA

GT170 amino acid

STOP

Continue with no exon connected till next intron

EXON146 amino acid

- thalasemia


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Point mutation and small deletions :Wildtype sequences

N-phe Arg Trp Ile Ala Asp-C Small

protienAmino

acid

5-UUU CGA UGG AUA GCC AAU-3

mRNA 3-AAA GCT ACC TAT CGG TTA-5DNA 5-TTT CGA TGG ATA GCC AAT-3

Sense strand is the same as DNA but the difference is TU

MiessensemRNA 3-AAT GCT ACC TAT CGG TTA-5DNA 5-TTA CGA TGG ATA GCC AAT-3

N-Leu Arg Trp Ile Ala Asp-C

One of the T is mutated to A the (only change its in the amino acid and noframeshift happened ) which cause sicle cell anaemia

Non-sensemRNA 3-AAA GCT ATC TAT CGG TTA-5DNA 5-TTT CGA TAG ATA GCC AAT-3

N- Phe Arg StopTGG TAG, more over the transfer RNA does not know this codon (TAG) so

the protein stops here instead of 6 protiens we have 2 which is known as

premature termination (truncation of a protein) e.g. some of - thalasemia &breast cancer

Frameshift by additionmRNA 3-AAA GCT ACC ATA TCG GTT A-

5DNA 5-TTT CGA TGG TAT AGC CAA T-

3N-phe Arg Trp Tyr Ser Gln

Frameshift by deletionGCTACGAT

mRNA 3-AAA CCT ATC GGT TA-5DNA 5-TTT GGA TAG CCA AT-3

N-Phe Gly Stop


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Stop codons (termination):Non of these codons is recognized by tRNA and recognized by proteinfactors. RF1 ( releasing factor) recognizes UAA & UAG while RF2 recognizes

UGA & UAA.they are universal from human to bacteria UAG : (amber) more error reading this codon .

most common in human ,TAG in DNA, amber = yellow UAA : (ochre) most commonly used termination codon in bacteria

ochre = green UGA : (opal) is the least efficient

opal = red

e.g. of missence mutationthe only change is that Glu GAG

Val GTG but the rest didnt change,and by this it will result in forming -

Sicle cell hemoglobin

e.g of framshift mutationur read it Glu, Pro, Gln, Leuetc.

but when u delete G in u will read itlike this

Ser, Arg, Asn, Phe.etcThe same thing goes when u add

amino acidsOr when u add or delete 2

Not 3 6 9


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5'- TAC -3' sense strand3'- ATG -5' non-sense strand3'- AUG -5' tRNA5'- UAC -3' mRNA

Suppressor mutations: wrong + wrong = right

A mutation in the gene for a tRNA molecule that changes its anti-codon loopcan "suppress" nonsense mutations that occur elsewhere in protein-coding

genes.

http://www.mun.ca/biology/scarr/Suppressor_mutation.html

Generation of a 'Nonsense' Mutation : The wild type DNA sequence :

5'- CTA CAG ATT -3'3'- GAT GTC TAA -5'

Produces the mRNA 5'- CUA CAG AUU -3'which codes for the poly peptide leu gln iso ..etc. A 1st position mutation ( C T ) in the second triplet gives a 'mutant' DNA

sequence sense strand5'- CTA T AG ATT-3'3'- GAT ATC TAA -5'

Produces the mRNA which 5'-CUA U AG AUU -3' codes for thepolypeptide Leu-*because UAG is a 'stop' codon (the so-called amber stop). Chain growth inthe polypeptide terminates prematurely. So all u have is only one codoninstead of three which is GAT

Generation of an 'amber suppressor' mutation in a tRNA gene : tDNA sense strand 5'- TAC -3'

3- ATG5which produces the tRNA 3'- AUG -5' anticodon loopwhich reads the mRNA 5'- UAC -3' as

tyr-

tRNA anticodon AUC combines with mRNA UAC codon and by this we could

recognise the UACmRNAtRNA

mRNA tRNAantibodies

antigens
http://www.mun.ca/biology/scarr/Suppressor_mutation.htmlhttp://www.mun.ca/biology/scarr/Suppressor_mutation.html


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5'- TAG -3' sense strand3'- ATC -5' non-sense strand3'- AUC -5' tRNA5'- UAG -3' mRNA

A3rd-position mutation(CG) in this region produces a 'mutant' tDNA gene sense strand 5'-TAG-3'

which produces the tRNA3'-AUC-5' anticodonloopwhich reads the mRNA a 5'-UAG-3' as

-tyr-

Then, the mutated tDNA gene produces a tRNAmolecule that will read the the mutated gln codonUAG as a tyr codon, instead of as a 'stop' codon, thiswill continue synthesis of the protein withoutstopping till the next codon.

Mutation in Non-Coding Region :o Most likely to have a phenotypic effects, expect when it falls in

o Promoter region that effect the level of expressiono Spilce region

o Splice acceptor "AG" , or splice donor "GT"In the picture; IVS 1: intervening sequence 1; i.e. intron 1If a mutation takes place in an intron ( a non coding region ); it will mostlikely lead to a phenotypic effect, except if it falls in the promoter site whichaffects level of expression not structure, while in splice, the number of

amino acids will change, which is a structure mutation, i.e. case ofthalassemiaThe beat chain of haemoglobinconsists of 146 amino acids,which means that the beta Hbgene consists of 146x3= 450base approximatelyYet, the real size of the beta Hbgene is from 2800 to 3000 bases

nucleotidesWhy? Because; 3 exons=450approximately + an intron whichcontains further more bases

The problem in tahalssemia is a mutation in the splice site leading to anintron between exon 1 and exon 2, which is not supposed to be there, thiswill lead to coding of more amino acids, around 170 instead of 146

Intron always much more larger than exon, despite exon is always

more in number, not size.


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DNA slippage :o Occurs in area where there are tandemly repeated sequences (e.g. STR-

short tendom repeat CACACACA)The complimentary code will be GTGTGTGTo There are thousands of STR in human genomeo

STR have a high rate mutationHence knows as hot spots of mutationo Increase in repeat frequently are more common than decrease

DNA is a very dynamic molecule, strands open and closes due to basepairing all the time, there will be an expansion in the DNA area, leading toelongation of the hot spot, which is known as DNA slippage, an alterationin the structure will take place, resulting in mutation.

Somatic Mosaicism :

Not all cells work 100%o Cells have genetic variation because : Error in cell division Error in replication Random segregation of mitochondria

- 2 places where u find DNA nucleous & mitochondria (mitochondrial DNA)- p.s. your mitochondria comes from your mother cell only and theother half is in the tail of the sperm which does not fuse with the ova- its mutation due to( high metabolic rate in energy production therefore

it requires different mitochondrial DNA so every cell has more than 100mitochondria and when the cel is divided and it will form 2cells withdifferent population of mitochondria and therefore different population ofmitochondrial DNA)- when one mitochondria is not functioning and the other 99 isfunctioning you cannot see the disease cause it is masked

:: :: mitochondria DNA

!!

.

Change in gene expression controlo Somatic variation is normal but is frequently masked.

..

#END#

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Lec 2 dr Ameen (Tafree3')