Large segment deletions and splice site mutations | Biochemistry | First aid lectures | Lec9#Drjamil
Channel: Dr. Jamil Lectures
Date: 2022-05-23

In this lecture we will discuss about Large segment m
deletions ,Meiosis crossing over or recombination,unequal cross over,Examples of lagre segment deletions,Splice site mutations,Splicing,examples of splice site mutations.


https://youtu.be/mMxl2X-gDMo
https://youtu.be/lJg9vLZjKtk
https://youtu.be/aP06bUvyqCA


Instagram account    Dr_jamil56


Large segment deletions
Large segments of DNA can be deleted from a chromosome during unequal
crossover in meiosis.
Cross over or recombination
It is a normal part of meiosis1 in which the non-sister chromatids of homologous
chromosomes exchange there segments between them and resultant chromosomes are
mixture of maternal and paternal alleles causing recombination\genetic diversity and no
genetic information is deleted
Unequal crossover
Sometimes during crossing over one of the two non-sister chromatids losing
information(deletion) and one gains extra genes (addition)
Examples
1. α-Thalassemia is the well example of genetic disease in which α-genes is
responsible for synthesis of α-globin chain of haemoglobin,is deleted at chromosome
16 because of unequal crossover so decrease in production of haemoglobin.
2. Cri-du-chat happens because of deletion of the short arm of chromosome 5 during
crossing over.
Clinical Features
Mental retardation,microcephaly,white-set eyes and kitten like cry(crying like cat)


Mutation in splice sites
Mutation in splice sites affect the accuracy of introns removal from hnRna (pre-mRNA)
during post transcriptional processing (splicing) .
In eukaryotic transcription the mRNA form is not a mature mRNA ruther it is pre-mRNA
because axons are interpreted by introns and these introns have to be removed and exons
joined together to form mature mRNA(splicing).
Introns have 5`donor splice site and 3` acceptor splice site.5` donor splice site contains G-U
sequence and it is the site where splicing starts by spliceosomes enzyme and nick happens
physically.3`acceptor splice contain AG sequence and it is the site where splicing ends up
causing removal of introns and exons will join together
Mutations
Mutations in 5`donor splice site (GC) and mutations in 3`acceptor site (AG) may lead to
removal of exons along with introns from mRNA or may cause the remaining of introns along
with exons in mature mRNA so exons informations are interrupted by introns
Examples of splice site
1. β-Thalassemia: Deletion of B-genes on chromosome 11 leads to β-Thalassemia
because β genes are responsible for formation of β-chains of haemoglobin.
2. Tay-sach disease: Deletion of hexa-gene on chromosome 15 causes Tay-sach
disease.Hexa gene is responsible for synthesis of β-hexosaminidase enzyme.This
enzyme is responsible for breakdown of fatty acids in brain.So β-hexosaminidase
deficiency leading to accumulation of fatty acids in brain that affect nerve cells
3. Gaucher disease: Deletion of GBA (glucocerebrosidase) gene on chromosome 1
leads to gaucher disease.GBA gene encodes glucocerebrosidase that degrades
glucocerebroside lipids through out the body.So deficiency of this enzyme leads to
accumulation of glucocerebroside lipids throughout the body especially in bone
marrow spleen and liver causing gaucher disease



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