Biomedical Engineering Reference
In-Depth Information
Example:
5→ AACGCTAGATC →3
5→ AAC
AGTC
GCTAGATC →3
3← T TGCGATCTAG ←5
3← T TG
TCAG
CGATCTAG ←5
where AGTC bases were inserted between the third and fourth letters in the upper strand,
and TCAG bases were inserted into the lower strand between the third and fourth letters.
Insertion of a base pair into an mRNA gene can cause changes to the resulting protein that
it expresses. For example:
mRNA from a gene before addition of single base pair:
5
/
-A-U-G
Met
-
-
A-C-C
Thr
-
-
G-A-C
Asp
-
-
C-C-G
Pro
-
-
A-A-A
Lys
-
-
G-G-G
Gly
-
-
A-C-C
Thr
-
-
/
3
mRNA after addition of G:
(frameshift)
G-A-C
Asp
-
G
-C-C
A-G-G
Arg
G-A-C-C
Asp
5
/
-A-U-G
Met
-
-
A-C-C
Thr
-
-
-
-
-
G-A-A
Glu
-
-
-
-
-
-
/
3
Ala
14.1.2.3.3. DELETIONS
Removal of a single base pair or a section of DNA is called deletion. Deletions have
similar effects as insertions that have profound effects on cellular metabolism. By deleting
or adding one or more bases, we can alter the whole composition of a protein, not just
a single amino acid. A deletion can shift the
reading frame
when translating the resulting
mRNA.
Back mutations or
reversions
are possible. Sometimes a deletion may get reversed by an
addition or vice versa. Revertants are cells for which the original wild-type phenotype has
been restored. Restoration of a function can occur due to a direct change at the original muta-
tion (e.g. if the original mutation was CAA to UAA, then a second mutation for UAA to CAA
restores the original genotype and phenotype). Second-site revertants can occur that restore
phenotype (
suppressor mutations
) but not genotype (e.g. a second deletion mutation that
restores the gene to the normal reading frame or a mutation in another gene that restores
the wild-type phenotype).
14.1.3. Large-Scale Mutations
14.1.3.1. Chromosomal Structural Mutations
Large-scalemutations involve changes of large sections of DNA (Deletions, insertions, inver-
sions, etc.) within the same chromosome or inter-chromosomal exchange of DNA material.
Deletion of large segments of chromosomes can result in a loss of a gene/genes from the
genome of an organism. If deletion involves the regions of DNA that control the expression of
a gene/group of genes, the entire group of genes may stop expression or their expression
may go out of control. Insertion and inversion can occur during replication and cause major
effects.
Amplifications
occur due to the duplication of segments of DNA during replication. Some
segments of DNA are duplicated several times, leading to multiple copies of the chromo-
somal regions. This process increases the dosage of certain genes, called gene amplification.
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