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Powerpoint Presentation: Gene Mutations

 

Molecular Genetics Index

The Chemical Nature if the Gene
The Nucleic Acids
The Gene
Ribonucleic Acid (RNA)
The Characteristics of the Genetic Code
The Genetic Code
Gene Expression
Translation
The lac Operon
Genetic Engineering
Electrophoresis

Topic Chapters Index

 

Mutations of haemoglobin

Haemoglobin is a tetromer (it has 4 polypeptide subunits - 2 alpha and 2 beta chains). The genes for these polypeptides are found on different chromosomes. The beta chain gene is found on the short arm of chromosome 11. The alpha chain gene is found on chromosome 16. The nucleotide sequences have been worked out. Several inherited diseases occur on the beta chain, which contains 146 amino acids:

Mutation

Codon

Change to DNA sense strand

Change in Amino Acid

S (sickle cell anaemia)

6

GAG to GTG

Glu to Val

C (Cooley's syndrome)

6

GAG to AAG

Glu to Lys

GSan Jose

7

GAG to GGG

Glu to Gly

E

26

GAG to AAG

Glu to Lys

MSaskatoon

63

CAT to TAT

His to Tyr

MMilwauki

67

GTG to GAG

Val to Glu

OArabia

121

GAA to GTA

Glu to Val

The sense strand (coding strand) cDNA sequence of for beta haemoglobin

cDNA (complementary DNA) is obtained by back-transcribing the mRNA used to translate the polypeptide. So cDNA has no introns. This is done using reverse transcriptase enzyme. Here the sequence of only one strand of the cDNA is shown, this is the side which is equivalent to the mRNA strand called the sense or coding strand. The sequence reads from left to right and line by line. It has been divided up into triplets of bases corresponding to the codons. Normally it would be a continuous series of bases.

 

 

MOLECULAR GENETICS

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Gene Mutations (aka Point Mutations)

Gene mutations are changes in the structure of the DNA which affect only one gene.

 

gene mutations

 

Normal gene

normal gene

The antisense strand is the DNA strand which acts as the template for mRNA transcription.

 

Mutations

Substitutions

Substitutions will only affect a single codon. Their effects may not be serious unless they affect an amino acid that is essential for the structure and function of the finished protein molecule (e.g. sickle cell anaemia).

sickle cell

Because the genetic code is degenerate it is possible for a mutation to have no effect on the phenotype. Changes in the third base of a codon often have no effect:

codon third base change

Some substitution mutations can, however, be disastrous:

disasterous mutation

 

Inversion

Inversion mutations, like substitutions, only affect a small part of the gene:

inversion mutation

 

Addition

addition mutation

Because the genetic code has no "punctuation" the reading frame is set at the beginning of the gene. One additional base will change the whole reading frame from that point on.

 

Deletion

deletion mutation

Removing a base from the gene has a similar effect to addition. It changes the reading frame downstream of the mutation.

 

Mutations of haemoglobin continued

The sense strand (coding strand) cDNA sequence of for beta haemoglobin

Mutations of haemoglobin

 

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