2. Chromosomal Aberrations : Change in structure of chromosome.
(i) Deletion : Loss of a part or segment of chromosome which leads to loss of some gene is called as deletion.
It is of 2 types :-
(i) Terminal deletion - Loss of chromosomal segment from one or both ends.
eg. The cry -du-chat syndrome is an example of terminal deletion in short arm of 5th chromosome.
(ii) Intercalary deletion - Loss of chromosomal part between the ends.
(ii) Inversion : Breakage of chromosomal segment but reunion on same chromosome in reverse orders. It leads to change in distance between genes on chromosome or sequence of genes on chromosome so crossing over is affected.
It is of 2 types :-
(i) Paracentric - If inversion occur only in one arm and inverted segment does not include centromere.
(ii) Pericentric - In this type of inversion inverted segment include centromere.
(iii) Duplication :
Occurence of a chromosomal segment twice on a chromosome. If in this segment any recessive gene is present, then it gives it's expression due to homozygous condition. If in this segment any recessive but lethal gene is present, it leads to death of organism.
Example : In drosophila "Bar eye character" is observed due to duplication in X-chromosome. Bar eye is a character where eyes are narrower as compared to normal eye shape.
IV Translocation : In this, a part of the chromosome is broken and may be joined with non homologous chromosome. This is also known asIllegitimate crossing over (illegeal crossing over) Three types of translocation –
(A) Simple Translocation→ When a chromosomal segment breaks and attached to the terminal end of a non- homologous chromosome.
(B) Interstitial or shift translocation→ If a segment of chromosome breaks and gets inserted in interstitial position of a non homologous chromosome.
c) Reciprocal Translocation→ Exchange of segments between two non-homologous chromosome.
eg. Chronic myloid leukemia [C M L] is a type of blood cancer. This disease is a result of reciprocal transiocation between 22 and 9 chromosome.
Note : If exchange of segments takes place in between homologous chromosomes then it is called crossing over.
Gene Mutation or point mutation
Two types :-
2. Frame shift mutation.
A. Substitution : Replacement of one nitrogenous base by another nitrogenous base is called as substitution.
- It causes change in one codon in genetic code which leads to change in one amino acid in structure of protein. eg. Sickle cell anaemia
Main Point :
- Change may not occur because for one animo acid more than one type of codons are present.
Substitution is of two types :-1. Transition 2. Transversion.
1. Transition : Replacement of one purine by another purine or replacement of pyrimidine by another pyrimidine.
Methods of Transition :-
1. By Tautomerisation :- By this method transition is induced by HNO2. HNO2 changes normal structure of nitrogenous base and changed nitrogenous base is called as Tautomer.
- In structure of adenine and guanine, amino group is present, HNO2 changes it into imino group.
- In the structure of cytosine and thymine, keto group is present. Which is changed into enol group by HNO2.
- In first DNA replication, Tautomer of adenine pairs with a normal cytosine and Tautomer of thymine pairs with normal guanine.
- It is unusual pairing which is called as forbidden pairing so a wrong type of DNA is formed in cell.
- In second DNA replication normal cytosine pairs with normal guanine and normal guanine pairs with normal cytosine.
- It is usual pairing so transition completes in two DNA replication (Tautomers always perform forbidden pairing)
2. By Ionisation :
- By this method transition is induced by ionising radiation like X-ray. These radiations convert nitrogenous bases in their ions and ions perform forbidden pairing. So by this method transition is completed in two DNA replications.
3. By Base Analogues :
- Transition is induced by chemicals which are same as nitrogenous base in function. They are called base analogues or duplicates of nitrogenous base. eg. Aminopurine is base analogue to Adenine (purine) 5–Bromo uracil is base analogue to thymine (pyrimidine),5-Iodo uracil is base analogue to guanine,5-Chloro uracil is base analogue to cytosine.
- In I DNA replication base analogues get establish in normal structure of DNA.
- In II DNA replication they perform forbidden pairing.
- In III DNA replication transition is completed.
- Replacement of purine by pyrimidine or pyrimidine by purine is called transversion.
- EMS r Ethyl methane sulphonate
- M MS r Methyl methane sulphonate
- These chemicals causes depurination means they remove one purine from structure of DNA. So a gap is formed.
- If this gap is filled by another purine then it is called as transition.
- *But if this gap is filled by pyrimidine then it is called as transversion.
- So EMS and MMS may cause both transition and transversion.
Frame shift mutation/Gibberish mutation :
- (1) Acredine (2) proflavin These chemicals causes loss or addition of one or two nitrogenous bases in structure of DNA so complete reading of genetic code is changed. It leads to change in all animo acids in structure of protein so a new protein is formed which is completely different from previous protein.
A TG ACG GAC AGA AAC .................................
A TG CGG A CA GAA AC .................................
- So frame shift mutations are more harmful as compared to substitution.
- Thallesemia (lethal genetic disorder) Mutagens :
- Mutagens are those substances which cause mutations :-
1. Radiation :- are two types
(i) Ionising :- a, b, g, X-ray
(ii) Non ionising :- U. V. rays.
- U. V. rays has less penetration power and skin of higher organisms absorb radiations. So they don't cause any effect in higher animals, but U. V. rays and radiations are effective mutagens in microbes and due to more effect leads to death of microbes. So U. V. rays are used to sterilize operation theatre.
- Radiations mainly cause chromosomal aberrations which cause major change in organisms. So chromosomal mutations are more important in evolution.
- U. V. rays and HNO2 cause deamination of nitrogenous base means they remove amino group from nitrogenous base by deamination of,
Adenine ¾ → Hypoxanthine Guanine ¾ → Xanthine Cytosine ¾ → Uracil U.V. rays do not cause deamination in thymine. By U.V. rays two adjacent thymine bind together and form thymine-dimer.
2. Chemical mutagens:-eg. Mustard gas (first identified Chemical Mutagens) Carbon tetra sulphide, Nitrous acid (HNO2) Organic peroxide, Ethyl urethane, Pesticides etc.
DDT (Dichloro Diphenyl Trichloro Ethane) LSD (Lysergic acid diethylamide)
- Chemical mutagens are more harmful than radiations because body is not protected against chemicals.
- Source of chemical mutagens are food, air and water.
- Effect of radiation is localised, while chemical mutagens spread in complete body through blood circulation and when they reach in gonads they cause germinal mutation.
- Chemicals also cause chromosomal mutations.
Antibiotics : 1. Neomycin 2. Kenamycin 3. Streptomycin
These antibiotics combine with small subunit of prokaryotic ribosome and Cause misreading of genetic code or induce error in translation.
Main Point :
- Same effect of puromycin antiboitic in eukaryotes.
Mis-sense mutation :- When a nucleotide change in genetic code cause the change of one amino acid of a polypeptide chain it is called mis-sense mutation.
Non-sense mutation :- When a nucleotide change in one codon causes termination of polypepetide synthesis by producing non-sense codon.
Same sense codon :- A change in one nucleotide in a codon does not change amino acid in polypeptide chain, because both codons code same amino acid.