MUTATIONS:
are changes in the genetic material of the cell.
can occur at the level of an individual DNA strand (a point mutation) or to an entire chromosome (a chromosomal mutation)
MUTATIONS:
usually lead to a decrease, rather than an increase, of information.
A loss of information in the cell leads to a loss of some function.
.
However, the idea that a single mutation would lead
to a huge, dramatic change in a single generation
(like the X-Men characters) is just wrong.
TYPES OF MUTATIONS: Point mutations are caused by random
errors in the copying process (during transcription)
TYPES OF MUTATIONS: Chromosomal mutations occur when
chromosomes fail to properly separate during meiosis
• changes in the number or structure of chromosomes
• typically harmful in humans
ChromosomalMutations:
ChromosomalMutations:• can occur in four different ways: deletion, duplication, inversion
and translocation
Non-Disjunction:• is the failure of the chromosomes to properly separate during meiosis
homologous chromosomesfail to separate
Meiosis I: non-disjunction
Meiosis II:
• leads to organisms with the wrong number of chromosomes
• causes birth defects like Down’s Syndrome (trisomy-21)
Non-Disjunction:
POINTMutations:
• are changes in single base pairs of DNA
• can lead to production of faulty (misshapen) proteins, or no protein at all
POINTMutations:
• often have no effect: UCC codes for serine, for example, but so does UCU, UCA or UCG
• that change the last letter in a DNA triplet may lead to the same amino acid
POINTMutations:
• which lead to a different amino acid, however, are called missense mutations
• Depending on the amino acid, these can be good, bad or neutral
POINTMutations:
• which lead to a stop codon (UAA, UAG or UGA) are called nonsense mutations
• These lead to loss of function and are usually bad
POINTMutations:
• Sense, missense and nonsense mutations are typically caused by substitution of one nucleotide for another, changing the meaning of only one codon. • Deletion or insertion of an entire nucleotide leads to frameshift !
Frameshift Mutationsalter everything that follows:
THE DOG ATE THE RAT MAN
THE HOG ATE THE RAT MAN
THE DOG ATE HER ATM AN…
THE OGA TEH ERA TMA N…
MUTATIONS: can occur in any cell
that occur in gametes (eggs or sperm) are called germ-line mutations since they occur during the germination of new offspring (reproduction).
that occur in cells other than gametes are called somatic mutations
Somatic Mutations:
Many are harmless: freckles, for example,are often the result of such mutations.
(Three flies are shown above. Two are mutants. Would these be harmless mutations, or not?)
Some are dangerous, because theloss of information leads to the loss of some critical function.
This fruit fly was born without functional eyes, but it has a white ‘eye spot’ clearly visible on its abdomen. Flying blindly is not a favored trait !
Somatic Mutations:
Some are dangerous, because theloss of information leads to the loss of some critical function.
This fruit fly was born without functional eyes, but it has a white ‘eye spot’ clearly visible on its abdomen. Flying blindly is not a favored trait !
Somatic Mutations:
Some are dangerous, because theloss of information leads to the loss of some critical function.
Somatic Mutations:
Some are very dangerous, because their loss triggers runaway cell division, also known as . . .
Somatic Mutations:
This is a picture of skin cancer on a person’s cheek, a somatic mutation, probably triggered by UV radiation
How does this happen?
Substances which tend to cause mutations arecalled mutagens. . .
High frequencies of ultraviolet light, for example,can cause point mutations which lead to skin cancer. Let’s see how this can happen . . .
Ultraviolet Light andMutation
Sunlight contains many frequencies of radiation, butmost are filtered out by theatmosphere.
Ultraviolet Light andMutation
Sunlight contains many frequencies of radiation, butmost are filtered out by theatmosphere.
UV-B radiation does getthrough, however, triggeringa chemical change in DNA,a point mutation
Ultraviolet Light andMutation
Sunlight contains many frequencies of radiation, butmost are filtered out by theatmosphere.
UV-B radiation does getthrough, however, triggeringa chemical change in DNA,a point mutation
(This can affect the healthof organisms like the plantsshown in the graphic)
Ultraviolet Light andMutation
UV-B radiation provides energy that drives two thymine nucleotides together.
Ultraviolet Light andMutation
UV-B radiation provides energy that drives two thymine nucleotides together.
These thymine dimers pull in, effectively eliminating a single nucleotide.
Ultraviolet Light andMutation
UV-B radiation provides energy that drives two thymine nucleotides together.
These thymine dimers pull in, effectively eliminating a single nucleotide.
This single point mutation also destroys information, leading to loss of function.
Ultraviolet Light andMutation
Loss of function maydisrupt the regular cell cycle (G1, S1, G2, mitosis and cytokinesis)
Ultraviolet Light andMutation
Loss of function maydisrupt the regular cell cycle (G1, S1, G2, mitosis and cytokinesis)
If this happens, the cell may divide more often than it should !
Ultraviolet Light andMutation
Loss of function maydisrupt the regular cell cycle (G1, S1, G2, mitosis and cytokinesis)
If this happens, the cell may divide more often than it should !
Uncontrolled cell growthgoes by many names, but the generic term is cancer.
When mutations lead to cancer, they are classified as carcinogens.
Tobacco products containmany ingredients knownto cause cancer and otherforms of lung disease.
It’s also healthy for all of usto avoid mutagenswhenever possible!
It’s normal for humans
to have a certain number
of somatic mutations
during their life.
We should avoid mutagens because most mutations are highly unlikely to offer benefits, but (interestingly enough) scientists have found the study of mutations very helpful!
ALLELES:
• can be either dominant (always expressed) or recessive (hidden, and only
expressed when there are no dominant alleles present)
ALLELES:
• whatever you have, whether dominant or recessive, make up the organism’s genotype
• the traits that are actually shown are the phenotype
ALLELES:
• different combinations lead to different genotypes which may still have the same phenotype
ALLELES:
• As an example, a fly with two dominant alleles and a fly with one dominant allele and one recessive allele will both have the same phenotype !
ALLELES:
• another complication is that not all alleles are expressed all the time, in every part of the organism’s body. Their expression may be hidden.
In figuring these things out, scientists have found mutations helpful, because the errors in development reveal the locus (actual physical location) of the normal allele. So research on organisms like fruit flies often involves deliberately using mutagens to cause mutations that reveal the normal allele and, eventually, that allele’s function.