module 6 - genetic change Flashcards
what is a mutagen?
an agent, such as radiation or a chemical substance, which causes genetic mutation.
what is mutation?
A mutation is any change to the DNA of an organism, that is unpredictable and random. That is, it cannot be predicted which gene will be affected, and how it will be affected.
Most mutations have no effect on the organism. Some result in a phenotypic change to the
individual.
how does radiation cause mutation?
Ionising radiation such as x-rays, gamma (γ) rays, alpha (α) particles and (β) particles, can damage DNA in a number of ways:
Directly -
Ionising radiation can cause the ionisation of molecules
in the DNA, resulting in the sugar-phosphate backbone
breaking, or a nitrogenous base changing so that it no
longer pairs with its complementary base.
Indirectly -
Ionising radiation can also ionise other molecules in the
cell, sometimes producing free radicals which can
interact with other molecules to form compounds
(such as H2O2) which can damage DNA.
these can lead to development of cancer.
what is ionising radiation?
Ionising radiation refers to radiation that has enough energy to break an electron away from an atom
for example:
γ radiation
Ultraviolet light
X-rays
but not radio waves.
examples of chemical mutagens:
Reactive oxygen species
(ROS) such as free radicals
* Intercalating agents
* Heavy metals which can:
– cause the sugarphosphate backbone of
DNA to break.
– inhibit enzymes
which repair DNA.
* Deaminating agents like nitrous
acid (converts C to U)
* Base analogs which can replace
bases during DNA replication
Deaminating agents in mutation
Deaminating agents can remove an amino group (containing nitrogen) from a base, turning it
into a different base. For example, nitrous acid can convert cytosine into uracil.
Base analogs in mutation
Nucleotides other than A, T, C, G,
and U can sometimes be
incorporated by mistake into DNA,
because they are very similar.
In some cases, they are able to
pair with more than one base.
intercalating agents and heavy metals in mutation
Naturally occurring mutagens of non-biological origin
Although some mutagens are not naturally occurring, many are. Naturally occurring mutagens
can be considered in two groups: those which are of biological origin, and those of non-biological
origin. Examples of non-biological, naturally occurring, chemical mutagens include some heavy
metals like arsenic, nickel and cadmium, and chemicals in charred food. Some ionising radiation
is also naturally occurring
Naturally occurring mutagens of biological origin
Examples of naturally occurring mutagens of biological
origin include various toxins produced by living organisms,
such as alkaloids produced by plants such a bracken ferns,
and mycotoxins produced by fungi. Some viruses, and
even the bacterium Helicobacter pylori, have been shown
to cause mutations.
what are the different types of mutations?
- Point mutations
- Frameshift mutations
- Structural chromosomal mutations
- Chromosomal number mutations
Point mutations
A change to even a single base in DNA, can alter the function of the protein encoded by a gene.
The function of a protein is dependent on its shape. The shape is determined by the interaction of neighbouring amino acids. If an amino acid is changed, then the shape of the protein may
also change.
include:
Base insertion
Base substitution
Base deletion
point mutation - Base substitution
One type of point mutation is a single base substitution, in which one base is replaced by or changed to another.
Silent – no change to the amino acid sequence in the
polypeptide.
Missense – change to one amino acid in the polypeptide.
Nonsense – premature stop codon, which shortens the
polypeptide and usually results in a non-functional protein.
point mutation - Base deletion
Base deletion is another kind of point mutation is a single base deletion, where a base is deleted
from the sequence.
point mutation - Base insertion
Yet another kind of point mutation is a single base insertion, where a base is added to the sequence.
Frameshift mutations
If a single base is either inserted or deleted, it not only affects the amino acid encoded by the
base triplet in which the mutation occurred, but all following amino acids because the rest of the sequence moves in one place through the ‘reading frame’. A frameshift can also be caused if several bases are added or deleted, so long as the number of bases involved is not a multiple of 3.
Structural chromosomal mutations
A structural chromosomal mutation, also called a block mutation is a mutation which affects large
regions of a chromosome, often involving many genes.
includes:
inversion -
deletion -
duplication -
translocation -
Chromosomal number mutations
There are two kinds of mutation involving whole chromosomes. Aneuploidy describes a diploid
cell which has an extra, or missing chromosome. Polyploidy describes a cell which has three or more sets of chromosomes.
Chromosomal mutations
Chromosomal mutations can cause structural changes to the
chromosome. This can be caused by having chunks deleted,
duplicated or translocated from one chromosome to another. Chunks
can also be inverted.
Translocation structural chromosome mutations
Translocation, a type of structural chromosome mutation, involves a piece of one chromosome breaking off and attaching to another non-homologous chromosome, leading to a rearrangement of genetic material
crossing over causing mutation
Crossing-over can occur between non-homologous
chromosomes resulting in translocation mutations. A different amount of genetic material could be exchanged,
resulting in one homologous chromosome with missing genetic
material (deletion mutation) and the other homologous
chromosome with duplication of a region.
Balanced structural abnormalities
involve no gain or loss of genes,
just a rearrangement – inversions and translocations.
Unbalanced structural abnormalities
involve gain or loss of genes-
deletions and duplications.
the blue people of kentucky
huntingtons disease
Non-disjunction (numerical chromosome mutation)
occurs in meiosis (and sometimes mitosis)
resulting in incorrect numbers of chromosomes in gametes (daughter
cells in mitosis).
There are three forms of nondisjunction:
failure of a pair of homologous chromosomes to separate in meiosis I,
failure of sister chromatids to separate during meiosis II,
failure of sister chromatids to separate during mitosis.
Aneuploidy (numerical chromosome mutation)
having an abnormal number of chromosomes in a cell.
Polyploidy (numerical chromosome mutation)
cells having three or more complete sets of
chromosomes (down syndrome)
Sex chromosome abnormalities
Triple X syndrome, also called trisomy X or is characterized by the
presence of an additional X chromosome in each of a female's cells.
Although females with this condition may be taller than average, this
chromosomal change typically causes no unusual physical features. Jacob’s syndrome
XYY syndrome is a genetic condition in which a male has an extra Y
chromosome. Symptoms are usually few. They may include being
taller than average, acne, and an increased risk of learning problems.
A gene pool
is the combined genes of a population, including the
different alleles for each gene.
Variation
is created in a population by sexual reproduction and
mutations.
Allele frequency
is how often each allele for a gene occurs within a
population and is often expressed as a percentage.
Sexual selection
is a process where some traits become more
common in a population due to mating partners being selected on the
basis of them having those traits.
Gene flow
refers to changes in allele frequency due to new
individuals entering a population or from individuals exiting a
population. These individuals alter the gene pool.
Genetic drift
involves changes in allele frequency in the gene pool of
a population due to random chance.
The bottleneck effect
refers to when a chance event causes a drastic
decrease in population size, resulting in genetic drift.
The founder effect
occurs when a new population is started by a
small number of individuals who are not representative of the original
Thefounder effectis the loss ofgenetic variationthat occurs when a
newpopulationis established by a very small number of individuals
from a larger population. E.g. The blue people of Kentucky.
Genetic drift and gene flow are both mechanisms of evolution, but
they operate in different ways (comparison):
Genetic Drift
o Definition: A random change in allele frequencies in a population due to
chance events (e.g., natural disasters, random mating, or accidents).
o Effect: It tends to reduce genetic diversity, especially in small populations,
and can lead to the loss of alleles over time.
o Example: A small population of rabbits has both brown and white fur alleles.
If, by chance, most of the white-furred rabbits die in a storm, the next
generations will have mostly brown fur, not because of selection, but due to
random chance.
o Types: Includes bottleneck effect (drastic population reduction) and founder
effect (new population started by a small group).
- Gene Flow
o Definition: The transfer of alleles from one population to another through
migration and interbreeding.
o Effect: It increases genetic diversity and makes populations more similar to
each other over time.
o Example: If a group of birds from one island migrates to another and breeds
with the local population, new alleles are introduced into the gene pool.
Key Differences:
genetic drift- Decreases genetic
variation (in small
populations) - Stronger effect in small
populations
gene flow - Increases genetic variation - Can occur in any
population size
Calculating allele frequency
Calculate the allele frequency for the green pea (G) and yellow
pea (g) allele for the following population.
35 homozygous green GG
105 heterozygous green Gg
40 yellow gg
Total number of alleles =
Allele frequency G
Allele frequency g
DNA sequencing
Determining the order of nucleotide bases (A,T,CG) in a section
of DNA.
DNA is cut into small segments
Enzymes are used to make multiple copies of the segments
These are fed into a machine
DNA strands separate allowing dideoxy nucleotides to bind with
the DNA
Dideoxy nucleotides give off a colour signal that is read by the
machine
These colour signals allow the order of the nucleotide bases to
be determined.
HGP
The human genome project (HGP) involved the sequencing of
the 3 billion bases of the entire human genome and identifying
every human gene.
DNA PROFILING (fingerprinting)
Used to identify an individual and to determine if two individuals are
related to each other.
STRs – short tandem repeats (in non-coding sections of DNA)
–vary in length between individuals due to different numbers of
repeats
Steps in DNA profiling
Steps in DNA profiling
1. DNA is extracted and purified from a suitable sample, e.g.
Blood, hair etc.
2. Multiple copies of STRs in the DNA are made using PCR –
polymerase chain reaction.
3. Gel electrophoresis is used to separate the STRs based on
their differing lengths.
4. The gel results display the length for each STR for an
individual. These can be compared to other DNA samples to
determine relationships or for forensic identification of an
individual.
PCR (look at diagram on classroom)
SNPs – Single nucleotide polymorphism
Single nucleotide polymorphisms, frequently called SNPs (pronounced “snips”), are
the most common type of genetic variation among people. Each SNP represents a
difference in a single DNA building block, called a nucleotide. For example, a SNP
may replace the nucleotide cytosine (C) with the nucleotide thymine (T) in a certain
stretch of DNA.
A variation at a single base pair
Created by a base substitution mutation
Two differences between a SNP and an allele
An allele is a variant for a gene, so occurs in a coding
region. A SNP can be in coding or non-coding sections
An allele can involve multiple differences in base
sequence, whereas a SNP is only one base substitution
The maximum number of variants possible for any on SNP is
4 (4 base types)
Conservation genetics
the use of data on the DNA and genes of a population
to help guide management decisions that seek to preserve the population.
Used to assist the preservation of endangered species.
Genetic diversity is important for the long-term survival of a population.
DNA sampling can give an indication of genetic diversity.
