• A mutation is the permanent alteration of the nucleotide sequence
of the genome of an organism, virus, or extra chromosomal DNA or
other genetic elements.
• Mutations result from errors during DNA replication or other types of
damage to DNA
4. Frameshift mutations
• A frame shift mutations also called a framing error or reading frame shift.
• It is a genetic mutations caused by the addition or deletion of nitrogenous
base in DNA or MRNA are known as frame shift mutations.
• Because these shifts the reading frame of codons from the site of change
onward on deletion may be neutralized by one addition or vice versa.
5. TYPES OF FRAME SHIFT MUTATIONS
There are two types of frame shift mutations
1.Deletions mutations:-These mutations are caused due to the loss or
deletion of one or more nucleotides.
2.Insertion mutations: These mutations are caused by the addition of
one or more extra nucleotides in a DNA molecule at one or more
6. MECHANISM OF ORIGIN OF FRAME SHIFT MUTATIONS
• Acridine dyes have been found to cause deletion or insertion of a
single base pair Acridines like 5-aminoacridine and proflavin become
intecalated between two adjacent purinces and increase the distance
between them from 3.4A to 6.8A.At the time of DNA
replication,either a nitrogenous base pair is introduced in the gap or a
nitrogenous base pair is lost.
• Frame shift mutations caused by the deletions are also introduced by
the removal of ethylated bases.
7. MUTATIONS DUE TO CHROMOSOME ABERRATION
• The occasional breaks in the chromosomes and fusion of the broken
ends causes many changes in the chromosomes called chromosomal
• Likewise chromosomal aberrations are also the kinds of mutations
produced as a result of segmental change in the chromosome.
• These were observed of C.B Bridges and A.H Sturtevant in Drosophila
as early as 1923. The Breaks may occur either in single
chromosome,in a pair or in non homologous chromosomes.
8. • The structural changes in the chromosomes are of various types these
changes may involve either the loss of broken fragments of chromosome or
addition of same fragment.
• In more complicated changes segments get exchanged between two
different chromosomes being termed as translocatons.
• Thus,in brief stuctural changes may be of following types:
• Translocations is the transfer of a portion of one chromosome to a
• At present,a variety of translocations are recognized which fall among
the following three categories.
• These involve a single break in the chromosomes and the transfer of
broken piece of this chromosome to the end of another since the
terms of unbroken chromosomes are nonsticky,such terminals
chromosome attachment are rarely, if ever found
• These involve three breaks and the transfer of a two break section of
one chromosome within the break produced in a nonhomologous
chromosomes.Shift are more common than simple translocations.
11. Reciprocal translocation or inter changes
• These are produced by single breaks in two homologous chromosoe
an exchange of chromosome section between them,these are the
most frequent and best studied translocation.
• Deletion is the loss of a chromosomal segment from any
• Depending upon the length of the lost chromosomal segment,the
loss of genes involved may very from single genes to block
containing several genes.
• The loss of chromosomal segment occurs when a portion of
chromosome gets detached due to certain reasons and the lost
segment does not survive because it lacks the centromere.
• The portion of the chromosome carrying the centromere fucntion
as a genetically defficient chromosome.
13. Types of deletions
• 1.Terminal deletions:-
• To the loss of segment from one or the other end of chromosomes.
The terminal acentric part of the chromosome is unable to survive
and causes terminal deletion.
• The terminal deletion is,caused by a single breks in a chromosome.
• 2.Intercalary or Intestitial deletion:
• It denotes the loss of an intercalary segment of the chromosomes
with the reunion of terminal segments. The intercalary deletion is
characterised by two point breaks and the reunion of terminal parts.
• Sometimes a segment or a part of the chromosomes becomes repeated in
the same chromosomes. These additional duplicated segments are called
• They arise as a result of unequal crossing over between chromosomal
• In Drosophila mutant bar eyes which are narrow and constricted are due to
small duplicated genes.
• It was observed that each additional duplicate segments made the eye
smaller and smaller.Sometimes duplications when act in opposite direction
serve as supressors.
• Inversion is a kind of chromosomes aberration in which a
chromosomal segment exist in reverse relationship to the rest of its
• Inversions are beleived to arise by breakages at the point of
intersection of a chromosome loop and reunion with new partners.
• Any organism may be homozygous or heterozygous for an inversion.
During meiosis,inversion heterozygotes synapse by forming a looped
• In salivary gland chromosomes and other favourable material,an
material,an inversion loop makes it possible to determine with
precision the extent of the aberration.
19. Types of Inversion
• A single crossing over or an odd number of crossovers in the inverted region
will result into the formation of a dicentric chromosome(having two
centromeres) and an acentric chromosome(with no centromere). Of
remaining two chromatids one will be normal and the other will carry the
• In a pericentric inversion(where centromere is present within the inverted
segment), although at pachytene,the configuration observed is similar to to
those describedabove for paracentric inversion,the products of crossing over
the configuraions at subsequent stages of meiosis differ,In the case,two of
four chromatids resulting after meiosis will have deficiencies and duplications.
• However,unlike paracentric inversion,no dicentric bridge or acentro fragment
will be observed.