Altered gene functions cum abnormalties

1. KRVS CHAITANYA
ALTERED
GENE
FUNCTIONS

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STRUCTURE OF RNA
DNA alone cannot account for the expression of genes. RNA is needed to
help carry out the instructions in DNA.
Like DNA, RNA is made up of nucleotide consisting of a 5-carbon sugar
ribose, a phosphate group, and a nitrogenous base. However, there are three
main differences between DNA and RNA:
1. RNA uses the sugar ribose instead of deoxyribose.
2. RNA is generally single-stranded instead of double-stranded.
3. RNA contains uracil in place of thymine.

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CENTRAL DOGMA OF BIOLOGY
A gene that encodes a polypeptide is expressed in two
steps.
In this process, information flows from DNA right arrow
→ right arrow RNA right arrow → right arrow protein, a
directional relationship known as the central dogma of
molecular biology.

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THE GENETIC CODE
• The first step in decoding genetic messages is transcription, during which a nucleotide
sequence is copied from DNA to RNA. The next step is to join amino acids together to
form a protein.
• The order in which amino acids are joined together determine the shape, properties, and
function of a protein.
• The four bases of RNA form a language with just four nucleotide bases: adenine (A),
cytosine (C), guanine (G), and uracil (U).
• The genetic code is read in three-base words called codons.
• Each codon corresponds to a single amino acid (or signals the starting and stopping
points of a sequence)

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GENETIC ALTERATIONS
• Genetic alternations include chromosomal abnormalities and gene mutations.
Chromosomal abnormalities generally arise during cell division.
• They can be numeric, involving the number of chromosomes, or structural,
involving the atypical configuration of one or more chromosomes.
• Many different chromosome abnormalities have been identified, some of
which are associated with genetic disorders and diseases like cancer.

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Gene mutations are permanent changes in DNA gene sequence. They
can arise during normal DNA replication or in response to
environmental factors. There are many classes of gene mutations.
Certain mutations cause disease.

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Chromosomal abnormalities
 Each human has 46 chromosomes (23 pairs) .
 If a human does not have 46 chromosomes, a chromosomal
abnormality has occurred.
 An umbrella term for a gain or loss of chromosome is aneuploidy.
 Chromosome abnormalities often occur during cell division
(meiosis and mitosis).
 There are two main groups of chromosome abnormalities —
numeric and structural.

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Numeric abnormalities, as the name suggests, involve the number of
chromosomes.
Monosomy occurs when one of the two chromosomes is missing from a pair.
• An example of a Monosomy disorder is Turner syndrome, in which part or
all of a female’s second X chromosome is missing.
Trisomy occurs in individuals with an extra chromosome.
• For example, those with Down syndrome have three copies of chromosome
21 instead of two copies.

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MUTATIONS
• A gene mutation is a permanent change in the DNA sequence of a
gene.
• Mutations can occur in a single base pair or in a large segment of a
chromosome and even span multiple genes.
• Mutations can result from endogenous (occurring during DNA
replication) or exogenous (environmental) factors.
• There are two main categories of mutations: germ line and somatic

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Germline (hereditary) mutations
• Germline mutations are inherited from a parent (ie, mutation was
present in the parent’s egg or sperm cells).
• A person with a germline mutation will have the mutation in every
cell in the body.
• Germline mutations are the cause of some diseases, such as cystic
fibrosis and cancer (eg, breast and ovarian cancer, melanoma).

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Somatic (acquired) mutations
• Somatic mutations can occur at any point in a person’s life.
• These mutations are often caused by environmental or lifestyle factors and
can also result from mistakes during cell division.
• This type of mutation is not passed down from parents to children and thus,
is not present in every cell in the body.
• Although several types of hereditary cancers can be linked to germline
mutations in genes that alter the gene’s original function (eg, tumor
suppression), most cancers arise from somatic mutations.

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• Somatic mutations arise after conception and can affect any of the
body’s cells, except for germ cells.
• Approximately 10% of cancers demonstrate both germline and
somatic mutations.
• Alterations in genes, whether they occur in a germline or somatic
fashion, change the function of the gene, which may contribute to
the development or spread of cancer.

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Single nucleotide polymorphisms
• A single nucleotide polymorphism (SNP, pronounced snip) is one
difference in a single base pair, or nucleotide, in a section of DNA.
• SNPs result in genetic variation in humans. SNPs can occur with a
gene or near a gene, but they are most commonly found in the
DNA between genes.
• To be designated as a SNP, the change in the base pair must be
found in at least 1% of the population.

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• SNPs are common and normal variations in the DNA and are responsible for
many of the normal differences between people such as eye color, hair color
and blood type.
• Many SNPs have no negative effects on a person’s health, but some
variations may influence the risk of developing certain health problems such
as diabetes, heart disease or cancer.
• On average, SNPs occur once in every 300 nucleotide base pairs, which
means that the human genome has roughly 10 million SNPs.

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Genetic code table. Each three-letter sequence of mRNA nucleotides corresponds
to a specific amino acid, or to a stop codon. UGA, UAA, and UAG are stop codons.
AUG is the codon for methionine, and is also the start codon.

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There are three kinds of substitution mutations:
Silent mutations do not affect the sequence of amino acids during
translation.
Nonsense mutations result in a stop codon where an amino acid
should be, causing translation to stop prematurely.
Missense mutations change the amino acid specified by a codon.

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Insertions and deletions
• An insertion occurs when one or more bases are added to a DNA
sequence.
• A deletion occurs when one or more bases are removed from a DNA
sequence.
• Because the genetic code is read in codons (three bases at a time),
inserting or deleting bases may change the "reading frame" of the
sequence.
• These types of mutations are called frameshift mutations.

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Common mistakes and misconceptions
• Amino acids are not made during protein synthesis. Some students
think that the purpose of protein synthesis is to create amino acids.
However, amino acids are not being made during translation, they
are being used as building blocks to make proteins.

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Mutations do not always have drastic or negative effects. Often people
hear the term "mutation" in the media and understand it to mean that a
person will have a disease or disfigurement. Mutations are the source of
genetic variety, so although some mutations are harmful, most are
unnoticeable, and many are even good!

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• Insertions and deletions that are multiples of three nucleotides will
not cause frameshift mutations. Rather, one or more amino acids will
just be added to or deleted from the protein.
• Insertions and deletions that are not multiples of three nucleotides,
however, can dramatically alter the amino acid sequence of the
protein.

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