MUTASI VALENTINA YURINA, M.Si

Kompetensi mahasiswa

• Mahasiswa memahami:

–Konsep mutasi

–Efek mutasi

–Klasifikasi mutasi

–Implikasi mutasi di bidang kesehatan

–Uji mutasi

MUTATIONS

– Most mutations are neutral

– More likely to be harmful than beneficial to the individual

• More likely to disrupt function than improve function

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NEUTRAL MUTATION • A large percentage of DNA is not part of any gene, and has no

known function Changes in these portions of the DNA do not alter any gene and are thus silent.

• Because of the wobble concept, the exact identity of the third base of a codon often makes no difference, and if it does, the two amino acids coded for by the old one and the new codon are often quite similar in nature, so a change in the DNA of a gene which affects one of these bases will often have no effect on the function of the gene product. This genearlly includes 1/4-1/5 of the bases in the gene

• Not all portions of a protein are equally important for the function of that protein. Even a mutation which changes an amino acid in the final gene product may not make a difference in the function of the protein, or may alter the precise nature of the function of the gene product without alter the usefulness of that product.

NEGATIVE EFFECT

• If an amino acid is altered in the active site region of a protein, it may decrease or destroy the ability of the protein to perform its function.

• Alterations in key portions of a protein may lead to changes in the specificities of the protein (conditional mutations).

Positive effect

• A small percentage of mutations may actually improve the function of the gene product or may convey a new or expanded function to that product evolution process

POSITIVE MUTATIONS

• Mutations occasionally produce

a polypeptide with an enhanced

ability to function

– Relatively rare

– May result in an organism with a greater likelihood to survive and reproduce

– Natural selection may increase the frequency of this mutation in the population

Mutation

• A mutation is a permanent change in the nucleotide sequence of the DNA molecule.

• A mutation occurs at the level of the gene

– change the DNA

– changes the mRNA

– may change protein

– may change character

• Mutations can be classified according to

– Cause

– Mechanism

Types of Mutations according to Cause:

• brought about by a mutagen

• A mutagen is a physical agent or a chemical agent that causes an alteration of the base sequence of the DNA molecule

Induced mutation

• The cause is unclear

• The rate is approximately 1in106 replicated genes.

Spontaneous mutation

• Causes of spontaneous mutations – Abnormalities in crossing over

– Abnormal segregation of chromosomes during meiosis

– Mistakes by DNA polymerase during replication

– Alteration of DNA by chemical products of normal metabolic processes

– Integration of transposable elements

– Spontaneous changes in nucleotide structure

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• heat

• ultraviolet radiation

Physical mutagens

• nitrous acid

• benzpyrenes

• aflatoxin

• base analogues

Chemical mutagens

CAUSES OF INDUCED MUTATION

HEAT

• The base C spontaneously converts to the base U readily at high temperatures.

U A

• C U A T

• G G

C

G

Ultraviolet Radiation

• Ultraviolet radiation causes the formation of thymine dimers (this can also happen to side-by-side Cytosines, or to a Thymine and a Cytosine which are beside each other, but these are less common)

• Thymine dimers block DNA replication.

• Cell excises (cuts out) the thymine dimers using enzymes and repairs the damage.

• A mutation occurs when the repair is faulty.

Nitrous Acid •

Sodium Nitrate (NaNO3) is a common food additive that also forms naturally when meats are preserved by smoking.

• In the stomach, sodium nitrate is converted to nitrous acid (HNO2), which acts as a mutagen by deamination of the NH2 group of adenine and/or cytosine to an ether group, thus altering their base pairing.

• Sodium Nitrate is one of many chemicals which, although not directly mutagenic themselves, become mutagenic when metabolized.

Benzpyrene and Aflatoxin

• Benzpyrene and Aflatoxin both produce frameshift mutations so that one or more base pairs are inserted or deleted from the DNA when it replicates.

Base Analogs

• Molecules that are structurally similar to the normal bases found in DNA.

• The base analogs can be inserted into DNA in place of the normal base.

• When DNA replicates, the analog causes mistakes in base pairing, so a mutation results.

• Examples include:

– 5-bromouracil acts as an analog of thymine.

– 2-aminopurine acts as an analog of adenine.

TYPES OF MUTATIONS

– Chromosome mutations

• Changes in chromosome structure

– Genome mutations

• Changes in chromosome number

– Single-gene mutations

• Relatively small changes in DNA structure

• Occur within a particular gene

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Chromosome Mutations

• May Involve:

–Changing the structure of a chromosome

– The loss or gain of part of a chromosome

Chromosome Mutations

• Five types exist:

Deletion

Inversion

Translocation

Nondisjunction

Duplication

Deletion

• Due to breakage

• A piece of a chromosome is lost

Inversion

• Chromosome segment breaks off

• Segment flips around backwards

• Segment reattaches

Duplication

• Occurs when a gene sequence is repeated

Translocation

• Involves two chromosomes that aren’t homologous

• Part of one chromosome is transferred to another chromosomes

Nondisjunction

• Failure of chromosomes to separate during meiosis

• Causes gamete to have too many or too few chromosomes

• Disorders: – Down Syndrome – three 21st chromosomes

– Turner Syndrome – single X chromosome

– Klinefelter’s Syndrome – XXY chromosomes

Non dis-junction of sex chromosomes

Turner’s syndrome

• Gamete with no sex chromosomes fuses with normal X gamete

• Zygote has chromosome complement 2n = 45

• Individuals are female and short in stature

• Infertile because ovaries haven’t developed normally

Klinefelter’s syndrome

• XX egg fertilised by normal Y sperm

or

• Normal X egg is fertilised by an XY sperm

• Zygote has chromosome complement 2n = 47 (44 + XXY)

• Always male

• Normally infertile

• Cannot produce sperm

Gene Mutations

• Change in the nucleotide sequence of a gene

• May only involve a single nucleotide

• May be due to copying errors, chemicals, viruses, etc.

Types of Gene Mutations

• Include: Point Mutations

Substitutions

Insertions

Deletions

Frameshift

Point Mutation • Change of a single

nucleotide

• Includes the deletion, insertion, or substitution of ONE nucleotide in a gene

Regents Biology

Sickle cell anemia

Hemoglobin protein in red blood cells

strikes 1 out of 400 African Americans

limits activity, painful & may die young

Normal round cells

Misshapen sickle cells

Only 1 out of 146 amino acids

Point Mutation

• A mistake that changes one base on a DNA molecule is called a point mutation.

• Two forms:

– Transition: one pyrimidine (T or C) substituted for the other pyrimidine or one purine substituted for the other purine (A or G).

– Transversion: purine substituted for pyrimidine or vice versa

– Transitions more common than transversions. Perhaps because transitions cause less disruption to the DNA molecule and so are less likely to be noticed by DNA repair molecules.

Fig 4.4

Frameshift Mutation

• Inserting or deleting one or more nucleotides the length of the gene is changed.

• The outcome is either an altered protein or no protein.

• Changes the “reading frame” like changing a sentence

Frameshift Mutation

• Original:

–The fat cat ate the wee rat. • Frame Shift (“a” added):

– The fat caa tet hew eer at.

Amino Acid Sequence Changed

EFFECTS OF MUTATIONS

• Mutations within the coding sequence of a gene can have various effects on the encoded polypeptide’s amino acid sequence

– Silent mutations

– Missense mutations

• Included neutral mutations

– Nonsense mutations

– Frameshift mutations

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EFFECTS OF MUTATIONS

• Silent mutations

– Amino acid sequence is not altered

• e.g., CCC CCG (pro pro)

– Genetic code is degenerate

– Alterations of the third base of a codon often do not alter the encoded amino acid

– Phenotype is not affected

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EFFECTS OF MUTATIONS

• Missense mutations

– Amino acid sequence is altered

• e.g., GAA GTA (glu val)

– Phenotype may be affected

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EFFECTS OF MUTATIONS

• Neutral mutations

– Type of missense mutation

– Amino acid sequence is altered

• e.g., CTT ATT (leu ile)

• e.g., GAA GAC (glu asp)

– No detectable effect on protein function

• Missense mutations substituting an amino acid with a similar chemistry to the original is likely to be neutral

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EFFECTS OF MUTATIONS

• Nonsense mutations

– Normal codon is changed into a stop codon

• e.g., AAA AAG (lys stop)

– Translation is prematurely terminated

• Truncated polypeptide is formed

– Protein function is generally affected

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EFFECTS OF MUTATIONS

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EFFECTS OF MUTATIONS

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Cystic fibrosis

• Broken salt channel in cells

– strikes 1 in 2500 white births

– gene codes for a protein channel that allows salt to flow across cell membrane

• broken protein doesn’t work as channel

• doesn’t allow salt out of cell, so water doesn’t flow out either

• thicker & stickier mucus coating around cells

• mucus build-ups in lungs & causes bacterial infections

• destroys lung function

– without treatment children die before 5; with treatment can live past their late 20s

Effect on Lungs Salt channel transports salt through protein channel out of cell Osmosis problems! airway

salt

H2O

H2O

salt

normal lungs

cystic fibrosis

cells lining lungs

salt channel

normal mucus

thick mucus

mucus & bacteria build up = lung infections & damage

Deletion leads to Cystic fibrosis

deletion

Loss of one amino acid!

• Many different kinds of testes can determine if an agent is mutagenic

– Ames test is commonly used

• Developed by Bruce Ames

– Uses his- strains of Salmonella typhimurium

• Mutation is due to a point mutation rendering an enzyme inactive

– Reversions can restore his+ phenotype

• Ames test monitors rate of reversion mutations

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MUTATIONS DETECTION

MUTATIONS DETECTION • Ames test

– Suspected mutagen is mixed with rat liver extract and his- Salmonella typhimurium

• Rat liver extract provides cellular enzymes that may be required to activate a mutagen

– Bacteria are plated on minimal media

• his+ revertants can be detected

• Mutation frequency calculated

– Compared to control

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DNA REPAIR

• Most mutations are harmful

– DNA repair systems are vital to the survival

– Bacteria possess several different DNA repair systems

• Absence of a single system greatly increases mutation rate

– “Mutator strains”

– Humans defective in a single DNA repair system may manifest various disease symptoms

• e.g., Higher risk of skin cancer

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