AREA OF STUDY 1. – MOLECULAR GENETICS AND HEREDITY
Molecular genetics
The study of the inheritance of traits within a species is known as Heredity. Classical genetics as
a study of heredity commenced with the work of Gregor Mendel in the 19th century and deals
with the mechanisms and patterns of inheritance through the coded chemical instructions
(genes which are the units of inheritance) from one generation to the next. Early studies focused
on single genes. Now that we have gained much more knowledge of the biochemical
mechanism for the inheritance of traits our attention has turned more to the Genome – the
entire set of genes found in the cells of an organism and the matter in which they interact and
are expressed.
Chromosomes, Genes and Alleles
This section, repeats a little of what we considered in Unit 3, but it is knowledge critical to
understanding what follows in the later sessions this week.
Chromosomes are the structures – made up of the chemical compound deoxyribonucleic acid
(DNA) – which carry our genes. In humans there are 46 chromosomes arranged in 23 pairs. Of
these 23 pairs, 22 are referred to as autosomal and one pair is referred to as the sex
chromosomes. In humans XX is female and XY is male. Diploid (2n) = 46 chromosomes and
Haploid (n) = 23 chromosomes.
In eukaryotes, each chromosome is made of two strands of DNA joined in a double helix, each
chromosome therefore carries two alleles for each gene, one which has been inherited from the
mother the other from the father. These two alleles may be the same, or may be different. A
normal individual can only carry two alleles for any single trait at a single gene locus, but in a
population there may be many possible alleles which allows for variation within the species.
Prokaryotes have only a single strand of DNA and therefore an individual organism can only
carry one allele for that trait.
Gene Classification
Genes are sections of a chromosome that contain information for the production of a particular
protein. They are classified as either structural, regulatory or homoeotic:
Structural genes – produce proteins that become part of the structure and function of the
organism
Regulatory genes – produce proteins that control the action of other genes
Homoeotic genes – produce proteins that play a role in embryonic development of the
individual organism
Gene Activity
Genes are switched on or off during our lives. Some are important during childhood and some
during adolescence. Also in different cells certain genes will be switched on and others off. For
example, genes to make epidermal tissue are of little value for eye tissue cells
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DNA Structure
The chemical responsible for inheritance of features is called Deoxyribonucleic Acid (DNA). It is
comprised of four nucleotides. The nucleotides are made of a sugar group, a phosphate group
and a Nitrogen base. Two strands of bases joined by weak Hydrogen bonds between the
Nitrogen bases make a double helix chain. The DNA double helix was described by James
Watson and Francis Crick in 1953 and earned them the Nobel Prize for Chemistry.
Features of the DNA structure:
 Basic unit: nucleotide, which comprises a:
o Sugar unit (S)– deoxyribose sugar
o Phosphate group (P)
o Nitrogen base - one of four chemicals: adenine (A), thymine (T), guanine (G),
and cytosine (C) they pair in the following combinations A – T and C – G.
Complementary base pairs: A only pairs with T; C only pairs with G
Hydrogen bonds between the complementary nitrogen bases hold the two strands together
A has a double bond to T
C has a triple bond to G
Overall the molecule has a negative charge due to the presence of the phosphate group.
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Protein Synthesis
Transcription is the process by which the DNA strands are unzipped, either for the full length of
the strand, or just for the segment coding for a protein and copied into mRNA.
1.
RNA polymerase attaches to a region of DNA and unwinds the double stranded DNA
and exposes the bases of the template strand.
2.
The base sequence of the DNA template guides the building of a complementary
copy of the mRNA sequence.
3.
The result is a single-stranded molecule called pre-mRNA.
4.
The introns are removed to produce mRNA
5.
The final mRNA molecule is chemically capped and a poly-A tail added to produce
the operational mRNA.
Before the pre mRNA leaves the nucleus it undergoes a number of modifications to become
mRNA. A methylated cap (a modified guanine nucleotide that has a methyl and phosphate
group bonded to it) is added to the 5’ end as soon as the mRNA leaves the DNA template.
The Introns are removed by RNA splicing. The introns are regions of base sequences not
translated into amino acid sequences (non-coding regions of DNA). Exons are the coding regions
that contain the information for protein formation.
Translation
The mRNA passes through the nuclear membrane into the cytoplasm and attaches to submicroscopic organelles known as ribosomes. Amino acids are brought to the ribosome by tRNA
which corresponds to the triplet code on the mRNA.
The Amino Acids are joined together at the ribosome to make a protein.
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Comparison of DNA and RNA
Sugar unit
Nitrogen bases
Double/single stranded
Location
DNA
Deoxyribose
A, T, G, C
Double
Nucleus
RNA
Oxyribose
A, U, G, C
Single
Mostly Cytoplasm
The outcome of transcription and translation is a functional protein.
DNA complimentary strand
DNA template strand
mRNA
tRNA
amino acid sequence
ATG GTC GCC GGC AGA TGA (not copied)
TAC CAG CGG CCG TCT ACT (copied)
AUG GUC GCC GGC AGA UGA
UAC CAG CGG CCG UCU ACU
Start, Val, Ala, Gly, Arg, Stop (a very short protein)
Genetic Code
The main features of the genetic code are:
 Pieces of information in the genetic code consist of triplets or three-base sequences.
 The code is non-overlapping (that is; bases are read three at a time in discrete triplets).
 The code is universal (same in all organisms, except a few protozoa and bacteria).
 The code is said to be redundant or degenerate since, more than one triplet of bases
codes for one particular amino acid.
 The information encoded in DNA are instructions to assemble amino acid sub-units into
proteins
 The information is unambiguous (one codon codes for only one amino acid)
 The information also includes a START instruction and a STOP instruction
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Cell reproduction
The cell cycle is the process by which cells are replaced or more are grown as the normal
process of maintenance and repair of the organism. It is a continuous process made up of five
stages:
1. G1 phase – cell growth prior to DNA replication (Interphase)
2. S phase – DNA replication (see below) (Interphase)
3. G2 phase – cell prepares for division into two (interphase)
4. M phase – mitosis – the nucleus divides (see below)
5.C phase – cytokinesis – the division of the cell
DNA Replication (S phase) is the term used to describe the process by which the cell copies its
genetic material. All mammals begin life as a single zygote which is the result of a fusion of
gametes (sex cells containing only one set of chromosomes), but as adults we are made of
trillions of cells. This requires much cell growth. The replication process occurs in 4 steps:
1. Two strands of DNA untwist and unwind
2. Two strands separate - Hydrogen bonds (which are weak) are broken
3. Free nucleotides which are abundant in the cytoplasm come in and pair up with their
complementary base. DNA polymerases (enzymes) help the nucleotides become attached to the
exposed bases.
4. DNA ligase (another enzyme) joins the new stretches of nucleotides into a new strand. The
new strand is built in the opposite direction from the original strand. The site where the new
strands are being built is known as the replicating fork.
Replication is semi-conservative which means that each new molecule contains one existing
parent strand and one new daughter strand
During Mitosis the two chromatids (the two strands of a single chromosome joined together at
the centromere) separate. The nuclear membrane breaks down and the chromatids move to
opposite poles of the cell. The nuclear membrane reforms and the cytoplasm divides to produce
two new cells (C phase). Mitosis occurs in all organisms that are actively growing. During this
process one cell divides and produces 2 daughter cells that are identical to the parent cell. These
cells have two complete sets of chromosomes are therefore described as diploid or 2n cells.
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Meiosis is a similar process of cell division, however, it produces 4 daughter cells that contain
half the genetic information of the parent or one copy of each homologous pair (haploid cells n). Gametes – sperm and eggs – are made in special organs called gonads – testes and ovaries,
by Meiosis. The first Division separates the homologous pairs and the second division pulls the
chromatids apart so that the 4 haploid cells may form.
Division I (a normal mitotic division)
Interphase I – chromatids uncoil
Prophase I – DNA replication, potential for single gene mutations, the chromosomes pair
Metaphase I – the chromosomes line-up randomly at the equator (this is called independent
assortment). During this crossing over may occur.
Anaphase I – the chromosomes move to the poles.
Telophase I – 2 daughter cells form (diploid)
Homologous pairs of chromosomes are required for meiosis to produce fertile gametes.
Division II
Prophase II – chromosomes disentangle and move to equator
Metaphase II – chromosomes line up at equator
Anaphase II – chromosomes move to the poles
Telophase II – 4 cells with 23 chromosomes are formed (haploid)
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Comparison of Mitosis and Meiosis
MITOSIS
Number of replications
One
Number of divisions
One
Number of daughter cells Two
produced
Chromosome
number Diploid
(haploid /diploid)
Type of cells produced
Somatic Cells
Variation
Minimal
MEIOSIS
One
Two
Four
Haploid
Gametes
Highly Possible
Apoptosis is the programmed death of cells. Cells cannot live forever and each type of cell has a
preprogrammed age. Interestingly, cancerous cells are a problem because apoptosis does not
occur and they continue to grow and spread throughout the body.
Karyotpes
It is possible to sample a person’s genetic information and make a chromosome map called a
karyotype. This can be done by chorionic villus sampling (a fine needle is passed up the vagina
and through the cervix to collect tissue in the fluid from around a developing embryo),
amniocentesis (a fine needle is passed through the abdominal wall, wall of the uterus and
samples fluid from around the developing embryo) and tissue sampling – sperm, mucous, skin,
hair, mouth swabs supply cells containing a nucleus which will contain the genetic information
of the individual.
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The dotted line represents the position of the centromere.
A homologous pair of chromosomes is a matching pair of same shape and structure. In human
males that is 22 pairs of homologous chromosomes and one non-homologous pair (XY) and in
human females there are 23 homologous pairs.
In mammals sex is determined by the fathers sperm as he can pass on an X or a Y chromosome,
while the mother can only pass on X chromosomes.
X
X
X XX XX
Y XY XY
In birds and some reptiles and amphibians the situation is reversed and the mother determines
the sex of the offspring.
Z W
Z ZZ ZW
Z ZZ ZW
Males are therefore ZZ and females ZW.
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Inheritance at one gene locus
Monohybrid Autosomal Crosses
A monohybrid autosomal cross involves alleles of only one gene at a single locus. If the parents’
genotypes are known, the predicted ratio of offspring can be calculated using a Punnett Square.
Mother’s Allele 1 (A)
Mother’s Allele 2 (a)
Father’s Allele 1 (A)
AA
Aa
Father’s Allele 2 (a)
Aa
aa
A Punnett Square shows the chance of each type of offspring occurring and are often required
on your exam (when in doubt include one anyway). In the example above, the two heterozygous
parents give a genotypic ratio of 1 homozygous dominant: 2 heterozygous: 1 homozygous
recessive or a phenotypic ratio of 3 “Big A’s” : 1 “little a”.
Complete Dominance
In many situations the pattern of inheritance is described as being completely dominant. That is
that the phenotypic expression of one allele completely masks the phenotypic expression of the
other allele. The former allele is said to be dominant and the latter is described as recessive.
These are described with letters of the alphabet. Capital letters are used to denote the
dominant allele and the recessive alleles are given a lower case letter, eg:
B – big nosed allele dominant to b – small nosed allele.
BB (homozygous dominant individual – would have a big nose)
Bb (heterozygous individual – would have a big nose)
bb (homozygous recessive individual – would have a small nose)
We can use Punnett squares to describe how complete dominance works. If a homozygous
dominant male mates with a homozygous recessive female then:
B B
b Bb Bb
b Bb Bb
All offspring have big noses regardless of gender.
If two of the heterozygous individuals are then mated, then
B
b
B BB Bb
b Bb bb
This gives the genotypic ratio of 1 dominant homozygote: 2 heterozygotes: 1 recessive
homozygote. The phenotypic ratio is 3 big nosed individuals: 1 small nosed individual. These are
the standard ratios for a heterozygous cross.
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Co-Dominance and Multiple Alleles
For some traits there are more than two alleles. A classic example of this is the ABO blood
system. There are three alleles for this trait, but an individual only ever has two of them.
IA – antigen A produced
IB – antigen B produced
i – no antigen produced
Because there are more than two alleles present greater number of genotypes and hence
phenotypes are possible.
The ABO blood system is also an example of co-dominance as both the IA and IB alleles are
expressed in the heterozygote (AB Blood group). However the third allele i, is recessive to both
IA and IB.
Phenotype
Genotype
A Type Blood
IA IA , IA i
B Type Blood
IB IB , IB i
AB Type Blood IA IB
O Type Blood i i
Another example is the inheritance of flower colour in peas; if
W for white flowers
R for red flowers, then
WW – white
RW – pink
RR – red
We can use Punnett squares to describe how incomplete dominance works. If a homozygous
white flower mates with a homozygous red flower then:
W
W
R RW RW
R RW RW
If two of the heterozygous individuals are then mated, then
R
W
R RR RW
W RW WW
The phenotypic ratio and the genotypic ratio are the same - 1 red homozygote: 2 pink
heterozygotes: 1 white homozygote.
Lethal Alleles
Lethal alleles often cause death of the embryo or the offspring prior to reproductive age.
Therefore not all alleles have equal chance of survival. In most cases lethal alleles are recessive
so that individuals have a low probability of death, however, there are cases when the lethal
gene can be dominant. In this case the probability of death for the individual is high.
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Sex Linkage
A special type of monohybrid inheritance is called sex linkage. Genes carried on either of the
chromosomes responsible for sex (X or Y) are said to be sex-linked. A trait can be said to be Xlinked inheritance involving genes carried on the X chromosome or Y-linked inheritance
involving genes carried on the Y chromosome.
One common example in humans is the inheritance of Colour blindness. Colour blindness is a
recessive trait linked to the X chromosome and is more common in males than females. If we
cross a Colour blind male with a heterozygous normal for colour vision female then we would
have the following Punnett Square and outcomes.
B
X
Xb
Xb
Y
B b
X X XB Y
Xb Xb X b Y
This would result in one normal female and one colour blind female and one normal male and
one colour blind male.
Test Crosses
A test cross is used to find out the genotype of an organism. One crosses an unknown individual
with an individual known to be homozygous recessive (said to be pure breeding for the recessive
trait) and one records the phenotypes from the cross.
For example, you may wish to work out the genotype of a black eyed dog. You may think that
brown eyes are recessive to black eyes.
So B – black eyes, b – brown eyes
You believed that you owned a true breeding black eyed dog. You would mate your dog with a
brown eyed dog known to be true breeding.
If your hypothesis was correct then the first mating would produce all black dogs as shown in
the Punnett Square below. This is the test cross.
b
b
B
Bb
Bb
B
Bb
Bb
To prove this you would then need to breed two of the puppies produced. This is not correctly
described as a test cross, but is used to confirm the test cross.
B
b
B
BB
Bb
b
Bb
bb
This would give you the phenotypic ratio of 3 black to 1 brown and would be considered as
evidence for your theory.
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Pedigree Analysis
Pedigrees are used to show the inheritance of a trait or disease in a family and are often used in
combination with Punnett Squares to investigate how a certain trait has been passed on or
could be passed on.
EXAM TIPS
There are particular modes of inheritance that can easily be observed when looking at a
pedigree. One usually needs three generations of a pedigree to be able to make reasonable
inferences from the data.
Autosomal recessive inheritance
- for an individual to express an autosomal recessive trait both copies of the allele must
present
- if both parents affected all offspring will also be affected
- recessive traits tend to skip generations and few individuals are affected
Autosomal dominant inheritance
- for an autosomal dominant trait to be expressed only one copy of an allele is required
- usually present in each generation, many affected individuals
X-linked recessive inheritance
- males only require one allele to express the trait, therefore more males than females
will show the trait
- females require both alleles, often females don’t show the trait but act as carriers
passing to a majority of their male offspring
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X-linked dominant inheritance
- any individual with the trait must have a parent with the trait
- females may be heterozygous and show the trait
- much harder to detect
Inheritance involving two gene loci
Independent Assortment
In the case of the inheritance of two genes on two separate chromosomes (independent
assortment) the phenotypes from the two heterozygous individuals are in the ratio 9 dominant
for both genes: 3 dominant for one gene: 3 dominant for the other gene: 1 recessive for both
genes.
You can demonstrate this with a Punnet square. Two traits A and B with the possible alleles A
and a and B and b. Both parents are heterozygous for both traits, that is AaBb. The possible
gametes are AB, Ab, aB, ab.
AB
AB
Ab
AABB
AABb
aB
AaBB
Ab
AaBb
Ab
AABb
aB
AaBB
Aabb
AaBb
aaBB
AaBb
Aabb
aaBb
ab
AaBb
Aabb
aaBb
aabb
If one of the parents is homozygous for both traits and the other is heterozygous, then the
Punnett Square offers a different outcome.
AB
AB
AABB
Ab
AABb
aB
AaBB
ab
AaBb
All the offspring will look the same displaying both the dominant phenotypes, although they all
have different genotypes.
Variation
Variation is the degree of difference that exists between members of a population. The greater
the degree of variation, the more genetically healthy is a population, ie more able to respond to
change in the environment.
If there is only one gene with two alleles for a particular trait, you either express the trait or you
don’t. Example: the allele for tongue rolling is dominant to the allele for not being able to roll
your tongue. That means within a population there is very little variation, you either can roll
your tongue or you can’t. This is known as discrete variation.
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If there is more than one gene for a particular trait and if each gene has multiple alleles, such as
those that control height, or skin colour, there is much greater variation within the population
and it usually follows a normal distribution pattern. This is known as continuous variation.
Continuous
Variation
Discrete
Variation
number of
individuals
number of
individuals
The Sources of Variation are:
 Multiple Alleles and Poly Genes – as discussed above the more alleles present for a
particular gene and the more genes involved in a certain trait, then the greater the
source of variation.
 Crossing Over in Meiosis – during meiosis it is possible for chromatids to become
entangle and to break and reform differently causing variation by rearranging the
normal groups of alleles present in the gametes
 Environment – has an influence over the phenotypic expression of the genotype, thus
increasing variation within a species, eg. Diet and nutrition affect the size of individuals.
 Independent Assortment – normal fertilization sees the alleles being assorted
independently – thus sexual reproduction assists with the increase in variation.
 Random Fertilisation – plants are more able to hybridise by mixing gametes between
like species and so further variation occurs.
 Mutations
Mutations
Mutation – is a change in the DNA sequence (this can have a variety of effects)
During DNA replication or when a cell is undergoing mitosis and meiosis, errors can occasionally
occur which result in a mutation. Mutations occur naturally. However the mutation rate can be
increased by factors which cause instability in DNA. These include:
 high temperatures
 certain chemicals known as mutagens, such as asbestos and many chemicals in cigarette
smoke
 radiation – particularly high energy forms such as those in X rays and in nuclear reactors.
Mutations can occur in:
 germline cells – this means that they can be passed to the next generation
 somatic cells – these mutations are confined to the organism the mutation occurs in and
are therefore not passed on.
Mutations cause:
 new alleles in the population – so a source of further variation
 new phenotypes which can be acted on by natural selection
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Types of mutations:
 Mutations can be changes in just one single base (gene mutations) OR
 they can be changes where thousands of bases are involved (chromosome mutations)
There are four types of gene mutations.
 Point Mutations – are those gene mutations that occur at a single base
 Deletion of a base – the removal of a complete base
 Addition of a base – the addition of a complete base
Deletion and addition mutations cause frameshift mutations (when the reading frame is
changed). Some frameshift mutations involve a stop codon being produced - this is known as a
nonsense mutation.

The substitution of a base. This mutation only affects one codon, and a different amino
acid may be incorporated into the protein. If so, the phenotype could be severely
affected. This type of mutation is known as a missense mutation.
Some substitution mutations produce no change to the protein being produced. If AGA, which
codes for serine, is altered by substitution to AGG then the amino acid serine will still be coded
for. This is known as a silent mutation. For example, AGA is transcribed to UCU, and AGG to
UCC.

Inversion mutations. This is when two or more bases are reversed; it may produce a
slightly different functional protein. eg, GAG becomes AGG and causes a change from
leucine to serine.
Template DNA TAC TTC ACG GCA ATC CGA
Deletion
TAC TCA CGG CAA TCC GA?
Addition
TTA CTT CAC GGC AAT CCG A??
Substitution
TAT TTC ACG GCA ATC CGA
Inversion
TAC TTC AGC GCA ATC CGA
Chromosomal Mutations
There are four types of chromosomal mutations. These mutations usually affect much more of
the chromosome than gene mutations and so their effect is usually much greater.




Deletion – a region of the chromosome is deleted all together
Duplication – a region of the chromosome is duplicated
Inversion – a region of the chromosome is turned around
Translocation – a region of a DNA from one chromosome is added to another
chromosome
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All of these changes, whether to large sections of the chromosome, or to single bases can
change the protein being coded for so that it no longer works at all, works – but does something
else, or it may have no effect at all.
Genotype and Phenotype
The Genotype of an individual is the alleles an individual carries for a particular gene. For
example: Alleles for eye colour (autosomal inheritance) can be denoted as B – brown or b –
blue. Therefore a Homozygous individual carries two identical alleles for a particular trait eg BB,
bb. Heterozygous individuals carry two different alleles for a particular trait eg Bb
The Phenotype of an individual is the visible characteristics an organism displays for a particular
trait, for example eye colour, the ability to roll your tongue or the presence or absence of a
disease. These arise from their genotype as well as the influences of the environment.
Environmental influences on Phenotype
phenotype = genetics + environment
Some characteristics are predominantly determined by genetics, such as the ability to roll you
tongue, you either can or can’t do it. Non tongue rollers will never be able to roll their tongue,
no matter how much they practice.
Other characteristics are determined by both your environment and your genetics. You may
have fair skin, but when exposed to sunlight it changes colour to a light brown colour. You may
possess the alleles for tallness inherited from one or both parents, but because of poor nutrition
or disease during a critical stage of your development, you may not achieve the potential height
that you alleles code for.
Gene Technology and its Applications
REVERSE TRANSCRIPTASE
This is an enzyme used to make copy DNA (single stranded DNA) from messenger RNA. Often
mRNA can be obtained, which contains no introns, only the coding regions of a gene. This is
particularly useful for inserting genes into other organisms as many organisms will only accept
small amounts of DNA.
A Poly – A tail is added to the mRNA, this then acts as the region for the primer to bind to.
Reverse transcriptase, a naturally occurring enzyme, catalyses the addition of new bases to the
cDNA. Once this is made, the mRNA is removed and DNA polymerase is added to build the
complementary strand of DNA. The result is double stranded DNA created from the mRNA base.
These enzymes were discovered in viruses. The enzyme is encoded and used by reversetranscribing viruses, which use the enzyme during the process of replication. Reversetranscribing RNA viruses, such as retroviruses, use the enzyme to reverse-transcribe their RNA
genomes into DNA, which is then integrated into the host genome and replicated along with it.
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HIV infects humans with the use of this enzyme. Without reverse transcriptase, the viral genome
would not be able to incorporate into the host cell, resulting in the failure of the ability to
replicate. Understanding and then replicating this process has allowed medical scientists to find
treatments, cures and vaccines for retroviruses.
ENDONUCLEASES (RESTRICTION ENZYMES)
Restriction enzymes are used to cut pieces of DNA. They have specific recognition sequences
where they cut. They can produce either sticky ends or blunt ends. They can be used to insert
pieces of foreign DNA into bacteria or virus. The bacterial DNA and the gene to be inserted in
the bacteria are cut with the same restriction enzyme, to ensure their ends match.
Blunt ends
Sticky ends
Restriction enzymes are naturally occurring proteins often found in bacteria. They are named
after the bacteria and numbered in order of their discovery. For example:
EcoRI – the first restriction enzyme isolated from the bacteria Escherichia coli
AluI – the first restriction enzyme isolated from the bacteria Arthrobacter luteus
RECOMBINING DNA
DNA ligase is used to join fragments of DNA together. Once DNA has been cut by a restriction
enzyme and the new piece of DNA is ready to insert, DNA ligase is used to join the nucleotides
together. DNA ligase occurs naturally in all organisms and is used to zip up the DNA strands after
they have been copied in natural protein synthesis.
VECTORS
Vectors are commonly used to transport DNA from one organism to another
There are two main types of vectors: plasmids and viruses.
Plasmids:
These are small circular parts of DNA that have come from bacteria. Genes can be inserted into
these plasmids (using restriction enzymes and DNA ligase) and transported to another organism
or put back into the bacteria, once it has been modified. Plasmids are commonly used by genetic
engineers. For example, Insulin is now produced in vast quantities using bacteria. The human
gene for the production of insulin is inserted into a plasmid. The bacteria are then grown with
the insulin gene and caused to produce insulin which is harvested and sold to insulin diabetics.
In the past insulin was taken from the pancreas of dead pigs, and sometimes sheep and cattle. It
was expensive, some people had allergic reactions to it and others had religious, moral or
ethical objections to using the animal insulin.
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Plasmids which have been modified (genetically engineered) are easily taken up by bacteria.
These cells are then said to have been transformed.
Viruses:
Viruses by nature infect other cells and insert their DNA or RNA in the process. Genetic
engineers can insert foreign DNA into viruses and target cells. An attempt was made to use
viruses as a vector to treat Cystic Fibrosis. Individuals with this disease produce a very thick
mucous in their respiratory system (as well as thick mucous at other mucous membranes and
other symptoms) due to a point mutation. The viruses were transformed to carry the correct
gene coding for the protein that makes the mucous and inhaled by the sufferer. A number of
problems occurred with this gene therapy and it was withdrawn from sale.
POLYMERASE CHAIN REACTION (PCR)
PCR is a method used to amplify DNA (make many copies of it). Often the sample off body tissue
you have only contains a small amount of DNA. PCR is used to make many millions of copies
quickly. PCR is an artificial way of DNA replication. PCR is a simple 3-stage process that is
repeated 25 – 30 times.
Stage 1 Denaturation. The DNA is heated to 95oC and the strands separate as the H bonds are
broken
Stage 2 Attach primers. The solution is cooled to 50oC and primers (nucleotide bases) bind to
both strands of the DNA.
Stage 3 Extension. Raise the temperature to 72oC the new strands are completed.
Repeat as often as required.
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GENE PROBES
A gene probe is a small piece of single stranded DNA or RNA that has been radioactively labelled
or labelled with a flourescent marker. Gene probes are used to locate target sequences of DNA;
that is, a known nucleotide sequence that is unique to the gene in question. They tend to be 20
– 40 nucleotides in length, but can be up to 1000 and are complementary for a known sequence.
The sequence must be complementary to the target sequence to be able to find the correct
piece of DNA in the sample being studied. This is commonly used when testing an individual for
the presence of the gene responsible for an inherited disease.
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Headstart Revision Classes
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ELECTROPHORESIS
This is a method for separating DNA fragments according to size. The DNA sample is put into
wells in a gel. The gel floats in a buffer solution and a current is passed through the solution. The
DNA fragments, being slightly negative in charge moves toward the positive end of the gel.
Smaller fragments move faster through the gel than larger ones, with fragments of the same
size being aligned as shown in the diagram below.
DNA PROFILES
There are regions in DNA which vary in length in individuals, as they contain repeated sequences
of DNA eg GAGAGAGA. These regions are inherited, just like alleles, thus individuals are unique.
The size of the repeated sequences can be shown on an electrophoresis gel and this is known as
a DNA Profile. There are two types:
STR – short tandem repeats – section of non-coding DNA of between 2 and 5 bases. Eg. the
dinucleotide GA is repeated many times to form the STR. These are inherited and an individual
has a distinctive number based upon heredity.
VNTR – variable nucleotide tandem repeats – also a section of non-coding DNA, but they are
longer than 5 bases.
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Headstart Revision Classes
Unit 4 Biology
DNA SEQUENCING
Using Gel Electrophoresis it is possible to establish the order of each nucleotide in a strand of
DNA (that is the sequence of the nucleotides). This can be used to map the location of genes as
in a species based Genome project or to study a particular mutation or disorder.
Manual Sequencing – this is often called the Sanger method and uses gel electrophoresis. The
gel is left to run for 1 – 3 hours. Small samples are sequenced by a trained technician analyzing
the location of the different nucleotides.
Automatic Sequencing – when a large amount of DNA needs to be sequenced this can be done
reasonably quickly by machine. The four nucleotides are fluorescently labeled and a laser
scanner reads the single lane of a gel sending the information to a computer which reconstructs
the base sequence. These machines can be left to run for 10 hours at a time.
MOLECULAR CLONING
Cloning means to make an identical copy. By using these techniques it has become possible for
scientists to remove sequences of nucleotides which are known to code for a particular protein,
insert them in another organism and have the gene function to produce a particular effect
An example of this, described above, is the addition of the human insulin gene to a bacterial
plasmid so that the bacteria can be used to produce insulin for use by diabetics. To achieve this
scientists needed to sequence the gene for human insulin, find restriction enzymes that would
cut the DNA in the appropriate places, find a gene probe that could be used to remove the
insulin gene and then transform the bacterial plasmid.
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Headstart Revision Classes
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APPLICATIONS
MEDICINE
Treatment of Cancer: Most Cancers are caused by sporadic mutations of somatic cells and can
not be passed on, but some are regularly seen in families suggesting a change in the gametes for
that family. Gene technologies can be used to assess the possibilities of particular cancers
occurring, assessing the mutation that has caused a cancer and applying this information to the
treatment of the cancer. Medical scientists are working on gene therapies to treat cancers, such
as injecting genes for toxic proteins into the cancerous cells.
Pharmaceuticals: The production of drugs (like insulin as discussed earlier) using
transformations of bacterial plasmids with genes from other organisms which code for a
particular drug.
Gene Therapy: Some diseases are caused by the lack of a functioning gene. If the gene can be
isolated, inserted into a vector (such as a virus) and then the vector can be used to introduce a
functioning copy of the gene into the affected cells. This was attempted with Cystic Fibrosis as
discussed earlier.
Stem Cell research: The discovery of totipotent embryonic stem cells (cells that can be caused to
become any other cell) and omnipotent adult stem cells (cells that can still be caused to become
a lot of other types of cells) has raised the possibility of using a patient’s own cells to grow
specific tissue. For example, injecting stem cells into damaged regions of the brain and growing
new nerve tissue or growing skin tissue to be used as a graft.
MALNUTRITION
Malnutrition is caused by both the lack of food and the lack of food with sufficient nutrition
value. Many crop species, including rice, wheat and sorghum – common food staples in many
cultures, are being transformed to become richer sources of nutrients such as Vitamin A. The
vitamin enriched crops are then grown by the community to ensure that they receive both food
and food of a reasonable nutritional quality.
AGRICULTURE
Many agricultural crop species are being transformed to confer protection against diseases such
as cotton, canola, corn and wheat. These transformed crops often have a genetic resistance to
pesticides introduced so that the farmer can apply pesticides more efficiently and not worry
about the effect on his crop.
CONSERVATION
For a species to be viable, it requires genetic diversity. Researchers are using the DNA profiles of
many endangered species to better manage the existing populations. They can use this
information to ensure individuals with different profiles are mated and that individuals with very
similar profiles are not mated. Also, they are able to identify populations that are distinct
enough to be considered either sub-species or alternative species. The decision then has to be
made whether to combine the genetic information and reverse the evolutionary process or to
keep the populations distinct.
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Headstart Revision Classes
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LEGAL USES
As you are all no doubt aware from watching television, these techniques can be used in the
solution of crimes. People leave their DNA behind them wherever they go. The presence of a
person’s DNA at a crime scene indicates that they were present at the location. The police still
need to prove that the person was there at the time the crime took place and were involved in
that crime. DNA profiling also allows cases of paternity to be solved.
SUGGESTED ACTIVITIES
1. Arrange 20 concepts or ideas from this part of the course and list them in order of know
well to know least well.
2. Prepare flash cards for each of these 20 concepts/ideas. Idea on one side and
explanation on the other.
3. Select 20 concepts or ideas from this section of the course and create a mix and match
activity. That is, you have a list of 20 concepts or ideas and a list of 20 definitions that
have been mismatched. You then share your list with someone else and try to solve
each other’s lists.
4. Write your own quiz. Prepare 20 multiple choice or True/False questions for a friend.
Have your friend prepare 20 questions for you. Try each other’s questions.
5. Take 5 of these processes and create a poster for each that explains the process and its
potential use. Remember a poster is a graphical representation of the information –
more drawings than text.
6. Choose the 5 you are least comfortable with – you might like to list the processes and
put them in rank order from know well to know least well first.
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