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Genetics
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Allele: a specific form of a gene occupying the same gene locus or position as other alleles of
that gene, but differing from other alleles by small variations in its base sequence
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Gene mutation: a change in the sequence of bases in a gene
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Spontaneous
errors in copying causes mutations as well as factors in the environment known as mutagens
(substances that cause mutation)
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The changed base sequence may
produce a different amino acid sequence in the protein translated
...

The expression of the mutated gene may or may not be beneficial to the survival of the
organism; some are beneficial, detrimental and some are neutral
...
The consequence of
changing one base could mean that a different amino acid is placed in the growing
polypeptide chain
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Deletion: A single letter of the base sequence is deleted
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Sickle Cell Anaemia:
The consequence of changing just one base in the Haemoglobin beta chain gene could mean
that a different amino acid is placed in the growing polypeptide chain
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Normal HB Gene


Faulty HB Gene


Sickle cell anaemia results from a change to the 6th codon for the beta chain of haemoglobin
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mRNA: The mRNA sequence changes from GAG to GUG at the 6th codon position
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The amino acid change alters the structure of haemoglobin, causing it to form insoluble
fibrous strands
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The formation of fibrous haemoglobin strands changes the shape of the red blood cell to a
sickle shape
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The sickle cells are also destroyed more rapidly
than normal cells, leading to a low red blood cell count (anaemia)
...

• The presence of mutant beta-chains in a haemoglobin molecule modifies its
biochemical properties
...

• The erythrocyte will then change from a biconcave shape into a sickle shape and can
block small capillaries, blocking blood flow and is less efficient at transporting oxygen
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Malaria and Sickle Cell Anaemia:
Malaria is a parasitic disease caused by a plasmodium cell
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The plasmodium invades red blood cells and causes them to lyse
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As a result, sickle cell is more prevalent in areas where malaria is endemic
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The HGP showed that humans share the majority of their
sequence, with short nucleotide polymorphisms contributing to diversity
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• Screening – this has allowed for the production of specific gene probes to detect
sufferers and carriers of genetic diseases
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Feature
Prokaryotes
Eukaryotes
DNA circular or linear
Circular
Linear
Location of chromosomes
Nucleoid area
Nucleus
Naked or with associated Naked
Associated with histone
proteins
proteins
Plasmids present
Yes
No
Number of chromosomes
One
Two or more

Histone Proteins:
Eukaryotic chromosomes are found inside the nucleus and consist of linear DNA wrapped
around histone proteins
...
Chromatin will supercoil and condense during prophase to form
chromosomes that can be visualised under a microscope
...
Chromatin:
Chromosomes:
Chromatin:
• Tightly packaged DNA
• Unwound DNA
• Found only during cell division
• Found throughout interphase
• DNA is not being used for
• DNA is being used for
macromolecule synthesis
macromolecule synthesis

Homologous Chromosomes:
Eukaryotic chromosomes have two chromosome types
...
They are the same length and carry the same sequence of
genes at the same locations along their length although the version of that gene may be
different
...

Most of your body cells contain two full sets of chromosomes – one from your mother, and
one from your father
...

Although they carry the same set of genes in the same position, there can be small
differences
...
It can be a difference of just a few bases in the DNA sequence when
comparing the alleles
...
g
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Haploid: a haploid nucleus contains one chromosome of each homologous pair
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g
...

Somatic cell: a diploid body cell that is not a gamete
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• The process involves chemically inducing cell division, then halting it during
prophase/metaphase when chromosomes are condensed and thus visible
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• The dividing cells are photographed during prophase/metaphase
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• From looking at the sex chromosome the sex of an individual can be determined
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• It involves harvesting cells (usually from amniotic fluid for a foetus or white blood cells
for adults)
...
It can be labelled with
the radioactive form of hydrogen called tritium
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If cells are allowed to replicated in the presence of titrated
thymidine, the DNA becomes radioactive
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• He cultured them with titrated thymidine
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• This would release the DNA
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• After developing the film black dots could be seen where the bacterial chromosome
had been
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• The circle could be measured to determine the length of the DNA molecule
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3: Meiosis

Reduction Division:
Meiosis is called reduction division because the daughter cells have only half the number of
chromosomes as the original parent cell
...
During sexual
reproduction gametes fuse together (fertilisation) and form a zygote that will have the same
number of chromosomes as a somatic (typical body) cell
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Meiosis is preceded by interphase,
which includes the replication of DNA (S phase) to create chromosomes with genetically
identical sister chromatids joined at the centromere
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Meiosis 1 separates the homologous pairs of chromosomes
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Meiosis 2 separates the sister chromatids
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• Replicated chromosomes pair up in their
homologous pairs to form a bivalent (this is called
synapsis)
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• Supercoiling/condensation completed
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• Nuclear envelope breaks down
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• Microtubules attach to centromeres
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Anaphase I:
• Centromeres do not split
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• Homologous chromosomes pulled to opposite
poles
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• Microtubules break down
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Cytokinesis I:
• First cell division into two diploid cells
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•
Microtubules start to form
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Metaphase II:
•
Microtubule network completed
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•
Chromosomes move to equator
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•
Microtubules contract
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Telophase II:
•
Chromosomes uncoil
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•
Nuclear envelope reforms
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Crossing Over:


Mitosis and Meiosis Differences:
Mitosis:
• One division
• The number of chromosomes remains the same
• Homologous chromosomes do not pair up
• Chiasmata do not form and crossing over never occurs
• Daughter cells are genetically identical
• Two daughter cells are formed
Meiosis:
• Two divisions
• The number of chromosomes is halved
• Homologous chromosomes pair up to form bivalents
• Chiasmata form and crossing over occurs
• Daughter cells are genetically different from the parent cells
• Four daughter cells are formed








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Phenotype: the characteristics of an organism; a characteristic may be an external feature,
such as the colour of flower petals, or internal, such as sickle-cell anaemia
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Recessive allele: an allele that only has an effect on the phenotype when in the homozygous
state; a recessive allele is always given the lower case of the same letter given to the dominant
allele
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Locus: the specific position of a gene on a homologous chromosome; a gene locus is fixed for
a species
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Heterozygous: having two different alleles at a gene locus
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Carrier: an individual with one copy of a recessive allele that causes a genetic disease in
individuals that are homozygous for this allele
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Discovery of Principles of Inheritance:
• Study of inheritance began in 19th century with the work of a monk named Gregor
Mendel who lived in what is now the Czech Republic
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• He carried out hundreds of experiments using pea plants that he grew in the
monastery
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• He then self-pollinated these F1 plants and grew seeds from the cross to produce the
F2 generation
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Genetic Crosses:
A genetic cross is a way of finding the expected ratio genetic characteristics of potential
offspring based on the genetic characteristics of the prospective parents
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Punnett Square:


Co-Dominance:
Some genes have more than two alleles for a given trait
...
So, the phenotype is a mixture of the two alleles
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Blood Groups:
• The ABO blood group system is an example of both a multiple alleles and
codominance
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• These produce four different phenotypes: A, B, AB
and O
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• They are both dominant to IO
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The Y chromosome contains the genes for developing male
sex characteristics – hence the father is always responsible for determining gender
...
If the
male sperm contains a Y chromosome the growing embryo will develop into a boy
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Because the X and Y chromosomes are of a different size, they cannot undergo crossing over
during meiosis
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Sex Linked Genes
• The Y chromosome is much shorter than the X chromosome and contains only a few
genes
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•
•
•

•

This includes the genes for haemophilia and red-green colour blindness
...

This means that a male with the recessive allele for colour blindness on the X
chromosome will be colour blind since there is no dominant allele on the Y
chromosome to mask it
...
Both of these conditions are therefore much more
common in males than females
...
Often genetic traits which show sex
linkage affect one gender more than the other
...
Sex-linked conditions are usually X-linked, as very few genes exist on the shorter
Y chromosome
...
Males only have one X chromosome
and therefore only one allele so cannot be homozygous or heterozygous
...
The
gene loci for these conditions are found on the non-homologous region of the X chromosome
(they are not present on the Y chromosome)
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• If this pigment absorbs light a message is sent to the brain and we see that colour
...

• The ability to make the pigment is a dominant allele, the recessive allele will not allow
the pigment to be made
...
For people who are colour blind, the two colours look the same
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Haemophilia:
Haemophilia is a blood disorder
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If even one of these genes has an allele that does not code for
the proper protein, the entire process of blood clotting can be disturbed and even very small
wounds will not clot, risk bleeding to death
...
The
gene that codes for Factor VIII, and important protein involved in blood clotting, is a sexlinked gene found on the X chromosome
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Haemophiliacs are unable
to make Factor VIII and can lose a lot of blood from even small injuries or bruises
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Pedigree Charts:



Mutations:
Mutations are spontaneous, permanent changes in the base sequence of DNA – they can
occur at any time in any organism
...

Ionising radiation from nuclear plants, ultraviolet light (UV), X-rays and mutagenic chemicals
such as formaldehyde, benzene and tobacco tar are mutagens, which increase mutation rate
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If two parents are unaffected,
all offspring must be unaffected (homozygous recessive)
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Autosomal Recessive:
If two parents show a trait, all children must also show the train (homozygous recessive)
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Genetic Disorders:
Disorder
Chromosome
Dominant or recessive
Huntington’s
4
Dominant
Haemophilia
X
Recessive
Colour blindness
X
Recessive
Cystic fibrosis
7
Recessive


Mutagens:
A mutagen refers to any agent that causes or increases the frequency of mutations by
triggering changes in the genetic material of an organism
...
Additionally, the mutations caused can often lead to cancer, in which
case the mutagens are also called carcinogens
...

Humans can be exposed to various types of mutagens in the form of radiation in their
environment or chemicals from their workplace, food, or cigarette smoke
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5: Genetic Modification and Biotechnology

DNA Profiling:
At a crime scene, forensics scientists check for fingerprints because a person’s fingerprint is
unique and can be used to identify them
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DNA profiling involves comparing samples of DNA from different sources
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The arrangement of the bands can be compared with other samples to see which bands are
the same
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DNA fingerprinting is a less accurate term for DNA profiling
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Polymerase Chain Reaction:
Polymerase chain reaction is a laboratory technique using a machine called a thermocycler
that takes a very small quantity of DNA and copies all the nucleic acids in it to makes millions
of copies of the DNA
...

PCR occurs in a thermal cycler and involves three
steps:
1
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2
...

3
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One cycle of PCR yields two identical copies of the
DNA sequence
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Gel Electrophoresis:
Gel electrophoresis is a method used to separate
fragments of DNA on the basis of size and the electric
charge they carry
...

Any DNA sample usually contains long molecules that
are too large to be used for profiling
...

These fragments have a small negative charge and will
move through a gel towards a positive electrode
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Other regions that are noncoding are called introns
...
These repetitive regions of ‘junk
DNA’ in introns are now called STRs; Short Tandem
Repeats
...
It is the number of times
that these core sequences are repeated that
produces the variations in individuals
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Uses of DNA Profiling:
• Paternity – sometimes a woman is unsure who the father might be, or the men
disputes being the father or wants to know if he is the father
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• Forensic investigations – often tiny samples of DNA can be found such as a drop of
blood or saliva, a hair follicle, scrapings under a finger nail, and the DNA can be
amplified using the PCR before profiling
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• Determining if rare birds and other animals being sold as captive bred have actually
been captive bred or have been stolen from the wild
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• Determining the contents of manufactured foods, especially those containing meat
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Gene transfer is possible because the genetic code is universal
...


Golden Rice:
A new variety of golden-coloured rice has been genetically modified so that it contains high
levels of a substance called beta-carotene
...
Three genes had to be introduced into ride
so it could produce beta-carotene
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This golden ride is a valuable dietary supplement for people whose diet is low in vitamin A
and who might otherwise suffer vision problems or blindness
...
Some
interesting and controversial issues have arisen about the ownership of the rights to the
seeds, the flavour of the rice and the publication of research data from experimental studies
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Many year ago, insulin was obtained from
a cow or a pig pancreas but the process was difficult and the insulin was likely to be
contaminated
...
coli bacteria
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Obtaining the desired human insulin gene in the form of piece of DNA
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Attaching this DNA to vector, which will carry it into the host cell (E
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3
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coli bacteria so that they translate the DNA and make insulin, which is
collected
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This toxic protein is lethal to certain types of larvae,
particularly the European corn borer which would otherwise eat the crop
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While monarch butterfly larva feed exclusively on milkweed, wind-borne pollen
from Bt corn may dust nearby milkweeds
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Some scientists suggested that these results may lack validity as they do not accurately reflect
natural conditions:
• There were higher amounts of Bt pollen on the leaves than would be found naturally
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A second study was conducted comparing the survivor rates of monarch butterflies based on
proximity to Bt corn fields:
• There was no significant increase in mortality when monarch larvae were placed in or
near an actual Bt corn field
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Advantages of Genetic Modification:
• As our population increases and more people need feeding, modifying plants and
animals to increase yield or to be able to grow in places where they previously could
not, will provide more food
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• Crop plants that are disease resistant not only increase yields but also reduce the need
for applying potentially harmful pesticides
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• Plants can be nutritionally enhanced
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• There is concern that people consuming genetically modified plants and animals could
be harmed
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Plants or animals could ‘escape’ into the environment and their genes might become
incorporated into wild populations, with unknown effects
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• GM seeds/plants may be more expensive, preventing poorer farmers from buying
them
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• More genetically modified organisms might lead to a reduction in natural biodiversity
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Cloning is also very
widespread in agriculture and horticulture and has been used for many years to propagate
new plants from cuttings, taken from roots, stems or leaves
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Clones of farmed animals can be produced from early embryos, which are divided into two or
more parts in a Petri dish soon after fertilisation
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In this way farmers can produce
more animals with desirable characteristics, such as high milk yield or meat production
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Clones from Differentiated Animal Cells (Adults):


This is called somatic cell nuclear transfer, and is a method by which cloned embryos can be
produced using differentiated adult cells
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• An unfertilised egg is removed from a female adult and its haploid nucleus is removed
to produced an enucleated egg cell
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• An electric current is then delivered to stimulate the egg to divide and develop into an
embryo
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This method of using differentiated cells to generate cloned embryos can be used for two
main purposes:
• Reproductive cloning: if the embryo is implanted into the uterus of a surrogate, a new
cloned organism will develop
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