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Genetics summary chapter 19 VU amsterdam
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- Vrije Universiteit Amsterdam (VU)
This summary covers complete chapter 19 of genetics at the Vrije Universiteit Amsterdam.
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Chapter 19 Mutations- Mutation: heritable change in genetic material. The structure of DNA is permanently changed and this alteration
can be passed on from mother to daughter cells. If a mutation occurs in the reproductive cells, it may be passed
from parents to offspring.--> enable adaptation to the environment of species due to variety.
- Homologous recombination: the process whereby identical/similar DNA segments are exchanged between
homologous chromosomes. This occurs during meiosis. It enhances genetic diversity, it also helps to repair DNA
and ensures the proper segregation of chromosomes .
A gene mutation occurs when the sequence of the DNA within a gene is altered in a permanent way substitution,
removal, addition etc...
- Point mutation: is a change of a single base pair within the DNA.
a. Base substitution: changing one of the bases for another.
- Transition: a change of a pyrimidine (CUT) to another pyrimidine or a purine to another purine (GA). common
- Transversion: when a purine and a pyrimidine are interchanged.
Another way a gene mutation may occur is if a short sequence of base pairs is deleted/added to the chromosomal DNA
can effect the function of the encoded protein. Gene mutations can alter the coding sequence within a gene.
- Silent mutations: are those that do not alter the amino acid sequence of a polypeptide even though the base
sequence is changed. Because the genetic code is degenerate, silent mutations can occur in certain bases within a
rd
codon, such as the third base, and the specific amino acid is not changed. 3 nucleotide base.
- Missense mutations: base substitutions for which an amino acid does change sickle cell disease, it involves a
mutation in the B-globin gene which alters the polypeptide sequence such that the 6 th amino acid is changed from
glutamic acid to valine the structure +function of the B-globin gene are altered. less likely to alter protein
function since only one amino acid is changed within a polypeptide neutral mutation. A missense mutation
that substitutes an amino acid with a chemistry similar to that of the original amino acid is likely to be neutral
(G A, or CT).
A. Example of a neutral mutation: silent mutations.
- Nonsense mutations: involves a change from a codon that specifies an amino acid to a stop codon. This change
terminates the translation of the polypeptide, producing a truncated polypeptide.
- Frameshift mutations: the addition/deletion of a number of nucleotides that are not divisible by 3. Because the
codons are read as ‘3’, this type of mutation shifts the reading frame. The translation of the mRNA then results in
a completely different amino acid sequence downstream from the mutation.
Gene mutations can occur outside of a coding sequence and influence gene expression. A mutation can occur within a non-
coding sequence, thereby affecting gene expression. For example, a mutation may alter the sequence within the core
promotor of a gene.
- Up promotor mutations: mutations in the promotor that increase the transcription rate.
- Down promotor mutations: mutations in the promotor, making them less like the consensus sequence,
decreasing the affinity for transcription factors and decreasing the transcription rate.
Mutations in the lac operator site, prevent the binding of the lac repressor (protein). This causes the lac operon to be
constitutively expressed, even in the absence of lactose. In E.coli, it might be a disadvantage due to the energy waste for
expressing this lac operon even though the proteins are not needed. Mutations in the 5’-UTR of the ferritin mRNA may
alter the sequence of the IRE, thereby affecting the translation of the mRNA. Mutations in eukaryotic genes can alter splice
recognition sequences, thereby affecting the order/number of exons contained within an mRNA. Gene mutations are also
given names that describe how they effect the wild-type genotype and phenotype. A mutation may change a wild-type
(prevalent allele) by altering the DNA sequence of a gene.
- Mutant allele: alleles that are not wild-type. These alleles can be found in a frequency lesser than wildtype
alleles when a mutation is rare in a population, the result is referred to as a mutant allele.
- Reversion: changes a mutant allele back to a wild-type allele.
A neutral mutation does not alter protein function, so it does not affect survival or reproductive success.
- Deleterious mutation: mutation that decreases the chances of survival and reproduction.
Example: lethal mutation death of a cell/organism.
- Beneficial mutation: mutation that increases the chancer of survival and reproduction.
- Conditional mutants: the phenotype is affected only under a defined set of conditions.
Example: temperature-sensitive mutant (ts). A bacterium that has ts mutation grows normally in one temperature
range – the permissive range – but exhibits defective growth at a different temperature range – the
nonpermissive range.
A second mutation sometimes affects the phenotypic expression of a first mutation.
, Chapter 19 Mutations
- Suppressors/suppressor mutations: second-site mutations. This type of mutation acts to suppress (to put an end
to) the phenotypic effects of another mutation. A suppressor mutation differs from a reversion, because it occurs
at a different site in the DNA from the first mutation.
A Intragenic suppressors: the second mutation site is within the same gene as the first mutation site it
produces a change in protein structure that compensates for the abnormality in protein structure caused by the
first mutation.
B intergenic suppressors: the second mutation (suppressor mutation) occurs in a different gene than the one
gene containing the first mutation. These mutations involve a change in the expression of one gene that
compensates for the loss-of-function mutation affecting the other gene.
1. Intragenic: A first mutation may cause one protein to be partially/completely defective. An intergenic
suppressor in a different protein-encoding gene might overcome this defect by altering the structure of a
second protein so that it can take over the functional role of the defective protein.
2. Common pathway: When a first mutation effects the activity of a protein, a intergenic suppressor mutation
could alter the function of a 2nd protein involved in this pathway, thereby compensating for the defect in the
first protein.
3. Transcription factor: When a first mutation causes a protein to be defective, an intergenic suppressor
mutation may occur in a gene that encodes a transcription factor. The mutant transcription factor activates
the gene that can compensate for the loss-of-function mutation in the first gene.
4. Multimeric protein: a mutation in a gene encoding one protein subunit that inhibits function may be
suppressed by a mutation in a gene that encodes a different subunit.
A change in chromosome structure can affect the expression of a gene. An inversion/translocation have no phenotypic
consequences. However, chromosomal rearrangement can affect the phenotypic expression. A chromosomal
rearrangement may affect a gene because of a chromosomal breakpoint – a region where two chromosome pieces break
apart and re-join with other chromosome pieces – occurs within a gene. A breakpoint in the middle of a gene is likely to
inhibit the gene function because it separates the gene in two pieces.
In other cases, a gene may be left intact, but its expression is altered when it is moved to another chromosomal location
position effect. Position effects alter gene expression due to:
1. The moved gene gets located next to regulatory sequences for a different gene (silencers/enhancers) which can
influence the expression of the re-located gene.
a. A chromosomal rearrangement may reposition a gene from an euchromatic region (less condensed and
active) to a heterochromatin region (condensed), leading to the turn-off of the gene.
2. A 2nd type of position effect may produce a variegated phenotype in which the expression of the gene is variable.
Mutation can occur in germ-line/somatic cells. For multicellular organisms, the timing of the mutations is also important
with regard to the severity of the genetic effect and whether the mutations are passed from parent to offspring.
1. Germ-line mutations: germ-line cells that give rise to gametes (egg cells/sperm cells). A germ-line mutation
can occur directly in a sperm/egg cell or it can occur in a precursor cell that produces the gametes. If a mutant
gamete participates in fertilization, all cells of the resulting offspring will contain the mutation. The mutation will
be passed along the future generation of offspring.
2. Somatic mutations: somatic cells of the body excluding germ-line cells and gametes. A somatic mutation
during the embryonic stage has occurred within a single embryonic cell. As the embryo grows, this single cell is
the precursor for many cells of the adult organism. Therefore, in the adult, a portion of the body contains the
mutations. In general, the earlier the mutation occurs during development, the larger the effected region is.
Genetic mosaic: an individual that has somatic regions that differ genotypically from each other.
Genetic variation exists in a population as a matter of random chance, and natural selection results in differential
reproductive success of organisms that are better adapted to their environment. Those individuals who, by chance, happen
to have mutations that turn out to be beneficial will be more likely to survive and pass these genes to their offspring
random mutation theory by the Lederberg’s (replica plating). Mutation is a random – it can occur in any gene and does not
require the exposure of an organism to an environmental condition that causes specific mutations to happen. Some genes
are larger than other genes, which provides a greater chance of mutation. The relative locations of the genes within a
chromosome may cause some genes to be more susceptible to mutation than others.
- Hot spots: certain regions of a gene that are more likely to mutate than other regions.
- Spontaneous mutations: changes in DNA structure that result from a natural biological/chemical processes
- Induced mutations: mutations caused by environmental agents.
Spontaneous mutations
Causes of spontaneous mutations:
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