Understanding Gene Mutations in Molecular Biology

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In this lecture…..
In this lecture…..
 
Mutation:
Gene (point) Mutation
Chromosomal Mutation
 
Gene Mutation
Gene Mutation
 
 
A gene mutation occurs when the nucleotide sequence of
the DNA is altered and a new sequence is passed on to the
offspring. The change may be either a substitution of one
or a few nucleotides for others or an insertion or deletion
of one or a few pairs of nucleotides.
 
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Genomes of bacteria exist on a single double-stranded
circular DNA molecule that contains approximately 4000
kb of DNA and are regulated by operons, the majority of
bacterial genes exist on one circular chromosome, there
are other genetic elements within the bacterial genome.
A mutation is a change in the nucleotide sequence and can
create new cellular functionalities or lead to the
dysfunction of others. Mutations can occur spontaneously
or be caused by exposure to mutation-inducing agents.
 
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A spontaneous mutation is one that occurs as a result of
natural processes in cells, for example DNA replication
errors. These can be distinguished from induced
mutations; those that occur as a result of interaction of
DNA with an outside agent or mutagen that causes DNA
damage.
Mutagens may be of physical, chemical, or of biological
origin. Mostly they act on the DNA directly, causing
damage which may result in errors during
replication. Although, severely damaged DNA can prevent
replication and cause cell death.
 
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Bacterial genes with similar functions often share one
promoter (RNA polymerase binding site) and are
transcribed simultaneously; this system is called an
operon. Typical operons consist of several structural genes
that code for the enzymes required for the pathway.
Regulation occurs through transcription factors binding to
a short sequence of DNA between the promoter region and
the structural genes called an operator
 
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Results of mutations can produce changes in structural or
colony characteristics or loss in sensitivity to antibiotics.
Some potential consequences of mutations are as follows:
Auxotrophs: have a mutation which leaves an essential
nutrient process dysfunctional.
Resistant mutants: can withstand the stress of exposure to
inhibitory molecules or antibiotics secondary to acquired
mutation.
Regulatory mutants: have disruptions on regulatory
sequences like promotor regions.
Constitutive mutants: continuously express genes that
usually switch on and off as in operons.
 
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Mutation rates have been measured in a great variety of
organisms, mostly for mutants that exhibit visible and
prominant effects.
Mutation rates are generally lower in bacteria and other
microorganisms than in more complex species. In humans
and other multicellular organisms, the rate typically ranges
from about 1 per 100,000 to 1 per 1,000,000 gametes. There
is, however, considerable variation from gene to gene as well
as from organism to organism.
 
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Mutations are of fundamental importance in molecular
biology for several reasons:
1- 
Mutations are important as the major source of genetic
variation that drives evolutionary change.
2- 
Mutations may have deleterious or (rarely) advantageous
consequences to an organism.
3- 
Mutant organisms are important tools for molecular
biologists in characterizing the genes involved in cellular
processes.
 
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At the molecular level, in eukaryotes and prokaryotes the
simplest type of mutation is a nucleotide substitution, in
which a nucleotide pair in a DNA duplex is replaced with a
different nucleotide pair. Mutations that alter a single
nucleotide pair are called point mutations.
Other kinds of mutations cause more drastic changes in
DNA, such as expansions of trinucleotide repeats,
extensive insertions and deletions, and major
chromosomal rearrangements.
 
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Other kinds of mutations cause more drastic changes in
DNA, such as expansions of trinucleotide repeats,
extensive insertions and deletions, and major
chromosomal rearrangements.
 
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Transitions and transversions can lead to:
1- 
silent mutation
2- missense mutation
3- nonsense mutation
 
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Nucleotide substitutions in a protein-coding gene may or
may not change the amino acid in the encoded protein.
Mutations that change the nucleotide sequence without
changing the amino acid sequence are called synonymous
mutations or silent mutations. Mutational changes in
nucleotides that are outside of coding regions can also be
silent. However, some noncoding sequences do have
essential functions in gene regulation and, in this case,
mutations in these sequences would have phenotypic
effects.
 
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Nucleotide substitutions in protein-coding regions that do
result in changed amino acids are called missense
mutations or nonsynonymous mutations.
This type of mutation is a change in one DNA
base pair that results in the substitution of one amino acid
for another in the protein made by a gene.
A change in the amino acid sequence of a protein may
alter the biological properties of the protein.
 
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A nucleotide substitution that creates a new stop codon is
called a nonsense mutation. Because nonsense mutations
cause premature chain termination during protein
synthesis, the remaining polypeptide fragment is nearly
always nonfunctional.
A nonsense mutation is also a change in one DNA base pair.
Instead of substituting one amino acid for another,
however, the altered DNA sequence prematurely signals
the cell to stop building a protein. This type of mutation
results in a shortened protein that may function
improperly or not at all.
 
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Chromosomes, which carry the hereditary material, or
DNA, are contained in the nucleus of each cell.
Chromosomes come in pairs, with one member of each
pair inherited from each parent. The two members of a
pair are called homologous chromosomes. Each cell of an
organism and all individuals of the same species have, as a
rule, the same number of chromosomes.
 
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Changes in the number, size, or organization of
chromosomes within a species are termed chromosomal
mutations, chromosomal abnormalities, or
chromosomal aberrations. Changes in number may occur
by the fusion of two chromosomes into one, by fission of
one chromosome into two, or by addition or subtraction of
one or more whole chromosomes or sets of chromosomes.
 
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Changes in the structure of chromosomes may occur
by 
inversion
, when a chromosomal segment rotates 180
degrees within the same location; by 
duplication
, when a
segment is added; by deletion, when a segment is lost; or
by 
translocation
, when a segment changes from one
location to another in the same or a different
chromosome. These are the processes by which
chromosomes evolve. Inversions, translocations, fusions,
and fissions do not change the amount of DNA. The
importance of these mutations in evolution is that they
change the linkage relationships between genes. Genes
that were closely linked to each other become separated
and vice versa; this can affect their expression because
genes are often transcribed sequentially, two or more at a
time.
 
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Gene mutations play a significant role in molecular biology, leading to alterations in DNA sequences that can impact offspring. These mutations can arise spontaneously or be induced by various factors, such as mutagens. Understanding gene mutations is crucial for comprehending the genetic basis of cellular functionalities and dysfunctions. Bacterial genes, organized in operons, can undergo mutations with diverse consequences like changes in structural characteristics or antibiotic sensitivity. Gene mutations give rise to different mutant types, such as auxotrophs, resistant mutants, and regulatory disruptions. By studying gene mutations, researchers uncover insights into the molecular mechanisms underlying genetic variability.


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  1. Molecular Biology lecture 4 Asst. Prof. Dr. Dalya Basil Hanna

  2. In this lecture.. Mutation: Gene (point) Mutation Chromosomal Mutation

  3. Gene Mutation A gene mutation occurs when the nucleotide sequence of the DNA is altered and a new sequence is passed on to the offspring. The change may be either a substitution of one or a few nucleotides for others or an insertion or deletion of one or a few pairs of nucleotides.

  4. Gene Mutation Genomes of bacteria exist on a single double-stranded circular DNA molecule that contains approximately 4000 kb of DNA and are regulated by operons, the majority of bacterial genes exist on one circular chromosome, there are other genetic elements within the bacterial genome. A mutation is a change in the nucleotide sequence and can create new cellular functionalities dysfunction of others. Mutations can occur spontaneously or be caused by exposure to mutation-inducing agents. or lead to the

  5. Gene Mutation A spontaneous mutation is one that occurs as a result of natural processes in cells, for example DNA replication errors. These can be mutations; those that occur as a result of interaction of DNA with an outside agent or mutagen that causes DNA damage. Mutagens may be of physical, chemical, or of biological origin. Mostly they act on the DNA directly, causing damage which may replication. Although, severely damaged DNA can prevent replication and cause cell death. distinguished from induced result in errors during

  6. Gene Mutation Bacterial genes with similar functions often share one promoter (RNA polymerase transcribed simultaneously; this system is called an operon. Typical operons consist of several structural genes that code for the enzymes required for the pathway. Regulation occurs through transcription factors binding to a short sequence of DNA between the promoter region and the structural genes called an operator binding site) and are

  7. Gene Mutation Results of mutations can produce changes in structural or colony characteristics or loss in sensitivity to antibiotics. Some potential consequences of mutations are as follows: Auxotrophs: have a mutation which leaves an essential nutrient process dysfunctional. Resistant mutants: can withstand the stress of exposure to inhibitory molecules or antibiotics secondary to acquired mutation. mutants: have sequences like promotor regions. Regulatory disruptions on regulatory Constitutive mutants: continuously express genes that usually switch on and off as in operons.

  8. Gene Mutation Mutation rates have been measured in a great variety of organisms, mostly for mutants that exhibit visible and prominant effects. Mutation rates are generally lower in bacteria and other microorganisms than in more complex species. In humans and other multicellular organisms, the rate typically ranges from about 1 per 100,000 to 1 per 1,000,000 gametes. There is, however, considerable variation from gene to gene as well as from organism to organism.

  9. Gene Mutation Mutations are of fundamental importance in molecular biology for several reasons: 1- Mutations are important as the major source of genetic variation that drives evolutionary change. 2- Mutations may have deleterious or (rarely) advantageous consequences to an organism. 3- Mutant organisms are important tools for molecular biologists in characterizing the genes involved in cellular processes.

  10. Gene Mutation At the molecular level, in eukaryotes and prokaryotes the simplest type of mutation is a nucleotide substitution, in which a nucleotide pair in a DNA duplex is replaced with a different nucleotide pair. Mutations that alter a single nucleotide pair are called point mutations. Other kinds of mutations cause more drastic changes in DNA, such as expansions extensive insertions and chromosomal rearrangements. of trinucleotide deletions, repeats, major and

  11. Gene Mutation

  12. Gene Mutation

  13. Gene Mutation Other kinds of mutations cause more drastic changes in DNA, such as expansions extensive insertions and chromosomal rearrangements. of trinucleotide deletions, repeats, major and

  14. Gene (Point) Mutation Transitions and transversions can lead to: 1- silent mutation 2- missense mutation 3- nonsense mutation

  15. Silent mutations Nucleotide substitutions in a protein-coding gene may or may not change the amino acid in the encoded protein. Mutations that change the nucleotide sequence without changing the amino acid sequence are called synonymous mutations or silent mutations. Mutational changes in nucleotides that are outside of coding regions can also be silent. However, some noncoding sequences do have essential functions in gene regulation and, in this case, mutations in these sequences would have phenotypic effects.

  16. Silent mutations

  17. Missense mutations Nucleotide substitutions in protein-coding regions that do result in changed amino acids are called missense mutations or nonsynonymous mutations. type of mutation base pair that results in the substitution of one amino acid for another in the protein made by a gene. This is a change in one DNA A change in the amino acid sequence of a protein may alter the biological properties of the protein.

  18. Missense mutations

  19. Nonsense mutations A nucleotide substitution that creates a new stop codon is called a nonsense mutation. Because nonsense mutations cause premature chain synthesis, the remaining polypeptide fragment is nearly always nonfunctional. termination during protein A nonsense mutation is also a change in one DNA base pair. Instead of substituting one amino acid for another, however, the altered DNA sequence prematurely signals the cell to stop building a protein. This type of mutation results in a shortened improperly or not at all. protein that may function

  20. Nonsense mutations

  21. Chromosomal mutation Chromosomes, which carry the hereditary material, or DNA, are contained in Chromosomes come in pairs, with one member of each pair inherited from each parent. The two members of a pair are called homologous chromosomes. Each cell of an organism and all individuals of the same species have, as a rule, the same number of chromosomes. the nucleus of each cell.

  22. Chromosomal mutation Changes chromosomes within a species are termed chromosomal mutations, chromosomal chromosomal aberrations. Changes in number may occur by the fusion of two chromosomes into one, by fission of one chromosome into two, or by addition or subtraction of one or more whole chromosomes or sets of chromosomes. in the number, size, or organization of abnormalities, or

  23. Chromosomal mutation Changes in the structure of chromosomes may occur by inversion, when a chromosomal segment rotates 180 degrees within the same location; by duplication, when a segment is added; by deletion, when a segment is lost; or by translocation, when a segment changes from one location to another in chromosome. These are chromosomes evolve. Inversions, translocations, fusions, and fissions do not change the amount of DNA. The importance of these mutations in evolution is that they change the linkage relationships between genes. Genes that were closely linked to each other become separated and vice versa; this can affect their expression because genes are often transcribed sequentially, two or more at a time. the the same processes or a different by which

  24. Chromosomal mutation

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