What are mutations?
Mutations are heritable changes in genetic information.
If a mutation occurs at the translational level, will it be dangerous?
Not necessarily. If one codon is read incorrectly and only one abnormal protein is produced, then it might not have any detrimental effects on the cell.
If a mutation occurs at the transcriptional level, will it be dangerous?
No. The transcribed mRNA will be translated only a couple of times being degraded so only a small number of the abnormal protein will be produced.
If a mutation occurs at the DNA level, will it be dangerous?
Yes. Since the cell's DNA stores all genetic information, the mutation will cause all translated proteins to be abnormal. Mutations originate at the DNA level and not at the RNA or protein level. The EFFECTS of a mutation are usually found at the protein level.
Where do genetic mutations come from?
They can be inherited or they can come on spontaneously. Spontaneous mutations can come from DNA replication errors, environmental errors, or randomly.
How are mutations classified?
They can be classified by either their effects on DNA or on protein DNA -> point, frame-shiftProtein -> missense, nonsense
What is a point mutation?
a point mutation is when one nucleotide in DNA is mutated into a different nucleotide. This may result in one amino acid being different in the mutant protein, or may not change the amino acid at all; thus, this mutation may not affect the function of the protein
What is a frameshift mutation?
mutation that shifts the "reading" frame of the genetic message by inserting or deleting a nucleotide. All the codons downstream of the mutation will be changed and will usually result in a nonfunctional protein
What is a non-sense mutation?
A nonsense mutation is a genetic mutation in a DNA sequence that results in a shorter, unfinished protein product. (stop codon)
What is a missense mutation?
Point mutation in which a single nucleotide is changed, resulting in a codon that codes for a different amino acid.
What are the different types of missense mutations?
silent mutations, conservative mutations, and nonconservative mutations
What are silent mutations?
have no effect on the amino acid produced by a codon because of redundancy in the genetic code
What is a conservative mutation?
A conservative mutation is a specific type of missense mutation where one amino acid is replaced with another amino acid that has similar biochemical characteristics. They do not alter protein length and may or may not alter secondary structure, protein stability and protein function.
What is a non-conservative mutation?
A specific type of missense mutation where one amino acid is replaced with another amino acid that has completely different biochemical characteristics
Point Mutations
-Any sort of change or alteration to the composition of DNA other than recombination is called a genetic mutation. Mutations can take place due to the incorrect pairing of base nucleotides during replication-when the DNA polymerase fails to find the mistake. Such mutations that occur naturally are known as spontaneous mutations. Mutations in nucleotide sequence can also occur due to outside physical or chemical agents called mutagens. Such mutations are called induced mutations. *Point Mutations-When a mutation occurs on a single nucleotide in the DNA molecule, it is called a point mutation. A point mutation can occur in a coding or non-coding region of the DNA. In either case, if that mutation does not cause significant changes, it is called a silent mutation. -A mutation in the non-coding region will not cause negative effects because the non-coding region is not used to synthesize proteins. But what about a mutation in the coding region? Remember that codons are a sequence of three nucleotides used to link amino acids. There are 64 codons and only 29 amino acids, which means that different codons can code fro the same amino acid (genetic code is degenerate). Therefore, if a point mutation causes a change in the codon but the new codon still codes for the same amino acid, no change in protein structure will take place. -A point mutation in which one nucleotide is substituted for another and this causes a change from one amino acid to another is called a missense mutation. A missense mutation may or may not lead to problems. Sickle cell anemia is an example of a missense mutation that alters the structure of hemoglobin because a glutamic acid is replaced with valine. -A nonsense mutation is a mutation that changes a codon that codes for an amino acid to a stop codon. This causes the polypeptide to terminate prematurely, which usually produces non-functional proteins.
What is a spontaneous mutation?
Mutations that occur as a result of natural biological and/or chemical processes are considered spontaneous.
What is a mutagen?
an agent, such as radiation or a chemical substance, that causes genetic mutation.
What is an induced mutation?
A mutation that results from the influence of extraneous factors; the result of either natural or artificial agents
Notes on Insertion, Deletion, and Frameshift Mutations
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Summary of Gene Mutations
-A mutation can be either an insertion/deletion mutation or a point mutation. If it is an insertion/deletion mutation then it can be a frameshift mutation or a non-frameshift mutation. If it is a point mutation, it can be a silent mutation or a missense mutation. -Both insertions/deletions and point mutation can cause nonsense mutations. Such mutations arise when the codon that codes for an amino acid is changed to a stop codon. This terminates the polypeptide early and forms a non-functional protein. -Gene mutations on DNA sequences arise from either many natural processes (spontaneous) or they can be caused by outside factors (induced) called mutagens. -An insertion/deletion of a number of nucleotides that is not a multiple of three will cause the reading frame to shift, which will change the amino acid sequence of the polypeptide chain. This is a frameshift mutation. -An insertion/deletion of nuceltoides that is a mutple of three will simply insert or remove a number of amino acids equaling to the number of new codons added/removed. This is a non-frameshift mutation because the majority of the amino acid sequence is unchanged. -If a point mutation does not actually change the amino acids produced, it is called a silent mutation. If a point mutation changes the codon and amino acid, it is called a missense mutation. Point mutations are also non-frameshift mutations because they one change a single codon and do not alter the other amino acids (do not shift the reading frame).
What are the different types of base substitutions that can lead to a point mutation?
transition, transversion, and mispairing
transition mutation
A point mutation in which a pyrimidine is susbstituted for a pyrimidine, or a purine is substituted for a purine.
transversion mutation
A point mutation in which a pyrimidine is substitued for a purine, or vice versa.
mispairing mutation
A not pairing with T, or G not pairing with C.
Large-scale mutations
-chromosomal; always cause disorders or death-nondisjunction, translocation, inversions, duplications, large deletions
translocation mutation
The process in which a segment of a chromosome breaks off and attaches to another chromosome.that is not a homologous chromosome
inversion mutation
A mutation involving a piece of a chromosome that breaks off and reattaches in reverse orientation.
nondisjunction mutation
A chromosome fails to separate from its homologous chromosome during Meiosis.
duplication mutation
a mutation that involves duplication of a region of DNA on the same strand
large deletion
many genes and all the DNA between them deleted
What are mutagens?
chemical or physical agents in the environment that interact with DNA and may cause a mutation
endogenous mutagens
These are mutagens that originate or grow within an organism. These include reactive oxygen species (ROS) created during the body during oxidative phosphorylation. These reactive oxygens are highly reactive with different cell components including DNA. By reacting with DNA, it can significant damage to a cell's DNA and genetic code. An example of this type of damage is the double-strand break. Reactive oxygen species can cause a double-strand break in which the DNA strand is broken completely. It can also cause base modification, which can lead to point mutations. ROS levels are generally maintained low in the cell due to anti-oxidants; however, these levels can get extremely high and lead to oxidative stress. This is why antioxidants are important. They neutralize ROS and thus prevent them from damaging cellular components.
exogenous mutagens
These are mutagens that originate outside of the body. -Intercalators are an example of an exogenous mutagen. One of them is ethidium bromide. What ethdium bromide will do is jump into a DNA double helix and stick itself between the two strands. When these intercalators intercalate into the DNA, they can distort the structure of DNA and cause damage.-Another example are base analogues, such as 5-bromouracil. These analogues pretend to be certain bases; however, they act much differently than normal nucleotides. Once it is incorporated into DNA, it shifts between two forms. Its keto form pairs best with adenine while its enol form pairs best guanine.
intercalators
Class of chemical mutagens composed of flat, planar molecules that can sandwich themselves between successive base pairs and disrupt the machinery of replication, recombination, or repair.
base analogues
similar enough to the "real" DNA bases to be incorporated into DNA, but they don't have the same faithfulness in pairing; produce substitution mutations
What are carcinogens?
Cancer causing agents; some carcinogens can also be mutagens. Some examples of carcinogens are tobacco, asbestos, and UV radiation
Are mutations good or bad?
They can be good, bad or neutral. It depends on a huge number of factors, including the environment the organism lives in. -For example, streptococcus penumoniae is the bacteria typically associated with pneumonia. One of the popular treatments is an antibiotic such as penicillin. Sometimes you can find some mutated streptococcus bacteria that will have a special trait that prevents the antibiotic from killing the bacteria. We call this a "good mutation" for the bacteria because it provides the bacteria with an advantage.-Cystic fibrosis is usually caused by a mutation in the CFTR gene. This defect makes the mucus in the lungs extremely thick, causing breathing difficulties. This can be considered a "bad mutation
What is a gene?
segment of DNA that codes for a protein
What is an allele?
A version of a gene
Homozygous
An organism that has two identical alleles for a trait
heterozygous
An organism that has two different alleles for a trait
dominant allele
An allele whose trait always shows up in the organism when the allele is present.
recessive allele
An allele that is masked when a dominant allele is present
genotype
An organism's genetic makeup, or allele combinations; it is possible for two genotypes to have the same phenotypes
phenotype
An organism's physical appearance, or visible traits.
Mendel's Law of Segregation Notes
-Mendel proposed that each pea plant contains two hereditary factors (aka genes) for any given trait. He argued that when gametes are formed, the two genes behave like "particles:" and separate from one another. This means that the games (sex cells)that are formed only have one of the pairs of genes for that particular trait. In addition, the two genes do not mix together. When he bred a tall pea plant (TT) with a short pea plant (tt), he noticed all the pea plants were tall. Since the tall trait is dominant over the short trait, all four possibilities will resemble the tall parent (Tt). This idea that the two hereditary genes for any given trait segregate from one another during gamete formation became known as Mendel's Law of Segregation. At the time of Mendel however, no knowledge about gamete formation and meiosis existed. Nowadays we know that segregation is a direct result of the separation of homologous chromosomes during meiosis. In both parents, meiosis forms haploid gametes that contain only copy of the gene (law of segregation). When the two gametes combine, they reform the diploid number of chromosomes.
Mendel's Law of Segregation
the two copies of a gene segregate from each other during transmission from parent to offspring
Mendel's Law of Dominance
-states that one of the factors for a pair of inherited traits will be dominant and the other recessive, unless both factors are recessive-Gregor Mendel conducted experiments with the highly variable pea plants. He worked with true-breeding (genetically pure) lines of pea plants. A true-breeding for a given trait will produce only that trait generation after generation. Traits-height (tall vs short)-color of seeds (yellow vs green)-color of pods (green vs yellow)-shape of seeds (round vs wrinkled)-shape of pods (inflated vs constricted)-color of seed coat (white vs gray)Breeding Experiments 1. Mendel crossed twp different true-breeding plants with contrasting traits (TT with tt). He found that the offspring plant (F1 generation) resembled one of the two parents (Tt). He then took two F1 generation plants and crossed them (Tt and Tt). 75% were tall and 25% were short. He concluded that because the short trait reappeared in the second generation, the trait for the short plant was not "lost" during the crossing process. Principle of Dominance -From these experiments, Mendel argued that each trait in a given plant is controlled by two "hereditary factors". TT x tt -> Tt-In the F1 generation offspring, the "hereditary factor" of the tall parent (T) seemed to mask the expression of the "hereditary factor" from the short parent (t). -In such a case, we say that the tall hereditary factor is dominant over the short one, which is said to be recessive. This principle came to be known as the principle of dominance. -The "hereditary factor" that Mendel spoke of was eventually discovered to be genes (sequences of DNA).
Genes, Alleles, and Loci on Chromosomes
-In biology, a locus refers to a specific location on a chromosome that contains a gene for some trait. A gene is a segment of DNA that codes for a protein that is used to express that trait.-In diploid organisms, chromosomes always come in pairs. In any homologous pair, one chromosome comes from the male parent and the other comes from the female parent. -Homologous Chromosomes1) have similar size and structure2) carry genetic information (genes) about the same types of physical traits. For instance, if a chromosome contains a gene for seed color, then its homologous chromosome will also contain a corresponding gene for seed color. Such pair of genes that code for polypeptides that express the same physical trait are called alleles. -Because homologous chromosomes have similar size and shape, each allele pair are found at a similar locus (location) on the chromosomes -Even though each allele pair contains similar genes, the genes do not have to code for identical traits. For example, the male allele can code for green seeds while the female allele can code for yellow seeds
Complete dominance
a relationship in which one allele is completely dominant over another; only 1 allele in the genotype is seen in the phenotype
Codominance
situation in which both alleles of a gene contribute to the phenotype of the organism; traits of both alleles show up in the genotype; both alleles in genotype are seen in the phenotype
incomplete dominance
A pattern of inheritance in which two alleles, inherited from the parents, are neither dominant nor recessive. The resulting offspring have a phenotype that is a blending of the parental traits; a mixture of alleles in the genotype is seen in the phenotype
What is independent assortment?
genes for different traits can segregate independently during the formation of gametes
What is a dihybrid cross?
Cross in which the inheritance of two different traits is followed
What must we assume if we are to use Hardy-Weinberg equation?
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