GENETICS
OBJECTIVES
Define gene, allele and chromosome
Describe mitosis and meiosis (names and stages not required, restrict only to number of chromosomes)
Describe the stages in mitosis and meiosis
Discuss relationship between gene and chromosome
Distinguish between phenotype and genotype, recessive and dominant
Discuss complete, incomplete and co-dominance Objective
Explain co-dominance by referring to inheritance of the ABO blood group phenotypes (A, B, AB & O)
Describe gene and chromosomal mutation
Discuss factors which may lead to mutation
Describe differences between continuous and discontinuous variation and give examples of each
Describe monohybrid inheritance
Predict results of simple crosses with ratios of 3:1 and 1:1 using terms homozygous,
heterozygous, F1 and F2 generations
Explain why observed ratios differ from expected ratios especially when there are small number of progeny
Discuss Mendelian experiments in peas and maize
Construct pedigrees for monohybrid crosses
Describe a back cross to determine the genotype of a dominant phenotype
Discuss the determination of sex in humans
Discuss sex linkage
Discuss effects of variation and competition to the survival of organisms in the environment.
Assess the importance of natural selection as a possible mechanism for evolution
1
Genetics
Genetics is the study of heredity, or inheritance. Inheritance is how characteristics are passed from generation
to generation, i.e. genes passed from parents to children. The information is carried by genes which are
carried by chromosomes.
Gene
This is the basic unit of heredity. A gene is an ordered sequence of nucleotides located in a particular position on a
chromosome. There is order in which the nucleotides are arranged.
Alleles
Alternative forms of a gene which occupy the same position or locus on a chromosome. Each allele of a gene is
inherited separately from each parent. e.g., at a locus for eye color the allele might result in blue or brown eyes.
Chromosomes
Chromosomes are found in the nucleus of a cell, they carry the genes or genetic information, they are made up of
DNA and protein. Each species has its own set of genes and its number of chromosome. Humans have 46
chromosomes in each cell.
The structure of DNA
DNA comprises of sugar – phosphate backbone and nucleotides. Each nucleotide is composed of sugar, phosphate
group and 4 nitrogenous bases Thymine, Cytosine, Adenine and Guanine.
The sugar and the phosphate group link the nucleotides to form each strand. These strands are held together by
hydrogen bonds that is present between complimentary bases. The two strands of DNA run in opposite directions.
The strands are twisted like a helix.
The order of the nitrogenous bases determines the genetic code or instructions.
2
Homologous chromosomes
Same size, shape, one came from the father and the other from the mother, each pair carries the genes for the same
characteristics. In humans there are 23 pairs of homologous chromosomes.
Sex is determined by two chromosomes. These genes are located on sex chromosomes, the X and the Y
chromosome. Females have two X chromosomes XX. Males have one X and one Y chromosome XY. The Y
chromosome is smaller than the X.
Exercise 1
Complete the paragraph about the contents of the nucleus by writing the most appropriate word in each space. Use
only words below.
alleles chromosomes
diploid
DNA gametes
genes
haploid
muscle
Chromosomes are long threads of ________________ made up of many _______________. Two or more
alternative forms of a gene, are called _______________. A __________________nucleus contains a single set of
unpaired __________________. These nuclei are found in __________________.
Exercise 2
The diagram below shows some pairs of chromosome found in a human cell, including the chromosomes that
determine sex.
(a) (i) Complete the following table by listing in pairs, using the letters, those chromosomes that match each
other.
(ii) (a) Name two chromosomes which do not match.
(b) From whom the cells were taken.
(c) Name the Phenotype.
3
Cell division
Mitosis
Mitosis produces two daughter cells with exactly the same number and type of chromosomes present in the parent
cell.
The importance of mitosis
It is important for growth of organism, replacement of cells and regeneration.
Stages of Mitosis
A good mnemonic for remembering these stages is: IPMAT
Interphase
Cells synthesizes organelles and increase in size. DNA duplicate, the chromosome now exists as a pair of
chromatids joined by centromere. Chromosomes are not visible.
Prophase
The chromatids shorten, thicken and become visible.
Metaphase
Chromatid pair align along the centre of the cell or in the middle of the cell. Centrioles produce spindle fibres that
attach to the centromere of the chromosome. Chromatid pair becomes attached by their centromeres to the spindle
fibres .
Anaphase
The spindle fibres contract and pull on the centromere, the centromeres split and the two chromatids of each
chromosome separate. Daughter chromosomes move apart led by their centromere.
4
Telophase
Chromosomes at opposite ends of the cell, uncoil, lengthen and not visible. The cell separates by a process called
cytokinesis. This creates two genetically identical cells.
Exercise 3
The diagram below shows the appearance of a pair of homologous chromosomes during mitosis.
(a) What stage of mitosis is shown in the diagram.
i.
Name structure C
ii.
State the function of structure A and B. (2)
(c) State why the chromosomes in fig are described as homologous. (2)
(d) Describe the events that occur in mitosis immediately after this stage. (3)
(e) State how a sperm cell differs from a muscle cell. (1)
5
Meiosis
Occurs within the reproductive organs and produces four daughter cells with half the number of chromosomes
The importance of meiosis
Production of gametes
It brings about Genetic Variation by Allowing for new combination of genes to occur in the gametes.
Where meiosis takes place?
It occurs during formation of gametes in the gonads such as testes and ovaries.
Stages of Meiosis
Meiosis I (the first meiotic division)
Homologous Chromosomes pair and separate
Meiosis II (the second meiotic division)
Interphase 1
Each chromosome duplicate, each chromosome now exists as a pair of chromatids joined together by a centromere.
Prophase I
Homologous pairs of chromatids come together. The chromosomes coil around each other and exchange segments
and genes. This is called crossing over. segments of chromosome from the father are switching with segments of
chromosomes from the mother increasing variation.
Metaphase I
Paired homologous chromosomes align along the centre of the cell. The random alignment pattern is called
independent assortment. This further brings genetic variation.
Anaphase I
Homologous chromosomes move apart towards opposite poles of the spindle. The spindle fibers pull the
homologous chromosomes to the opposite poles.
6
Telophase I
Chromosomes reach the poles forming two new daughter cells. The division has halfed chromosomes number of
the original.
Nuclear membranes re-form and a short resting space follows, the cell enters the metaphase 2 stage. Due to
crossing over, the chromatids are not genetically identical. The spindle fibers disintegrate and a nuclear envelope
forms around the chromatid pairs at each end.
Meiosis II
Interphase II
The cell increases in size and do not duplicate the chromosomes as have already done in interphase.
Prophase II
The centrioles duplicate again and move to the opposite poles of the cell. The nuclear envelope disintegrates and
the chromatid pairs shorten and thicken and new spindle fibres appear.
Metaphase II
The chromatid pairs line up in the centre of the cells and the spindle fibres attach to the centromeres.
Anaphase II
The chromatid pairs separate and the individual chromosomes are pulled to the opposite poles by the action of the
spindle fibers.
7
Telophase II
Four daughter cells are produced from the parent cell, with each cell now containing half the number of
chromosomes (haploid) of the original parent cell.
Difference between mitosis and meiosis
Mitosis
Meiosis
-Forms somatic/body cells
-Forms 2 diploid (2n) cells.
-Responsible for the growth of an individual.
-Completed after one division.
-No pairing of homologous chromosomes
-Forms germ/sex cells
-Forms 4 haploid (n) gametes (sex cells)
-Responsible for sexual reproduction
-completed after two divisions
-Homologous chromosomes pair in prophase 1
Exercise 4
Complete the following passage using only words from the list below.
Diploid, Gametes, Haploid, Meiosis, Mitosis and Red blood cells.
The transfer of inherited characteristics to new cells and new individuals depends on two types of cell division.
During ___________________the chromosomes are duplicated exactly and __________________ cells are
produced. However, during ______________________the chromosome sets are first duplicated and then halved
producing haploid cells_cells. These cells will become gametes. ______________________,
Exercise 5
Complete the sentences by writing the most appropriate word in each space.
Use the following words, allele, diploid, dominant gene, genotype, haploid heterozygous, homozygous,
meiosis, mitosis, phenotype and recessive.
Wing length in the fruit fly, Drosophila, is controlled by a single ________________ that has two forms, one for
long and one for short wings. The sperm and ova of fruit flies are produced by the process
of__________________. When fertilization occurs the gametes fuse to form a___________________ zygote.
When two long-winged fruit flies were crossed with each other some of the offspring were short-winged. The
_______________ of the rest of the offspring was long-winged.
The short-winged form is__________________ to the long-winged form and each of
the parents must have been_________________________.
8
Gene expression
Genotype
This refers to the genetic makeup of an organism.
Phenotype
All the visible characteristics of an individual. The phenotype involves all the structural, physiological and
functional characteristics of an individual. For example, hair color.
Locus
The particular position on homologous chromosomes of a gene.
Carrier
An individual that has one copy of a recessive allele that causes a genetic disease in individuals that are
homozygous for this allele.
F1. GENERATION -The offspring produced after one generation of breeding are known as the first filial
generation and this is usually abbreviated as F1 generation.
F2. GENERATION –The offspring resulting from the interbreeding of the F1 generation are called the second
filial generation abbreviated F2 generation
A gene is made up of two alleles
i. BB: a homozygous dominant allele. Homozygous: having two identical alleles of a gene.
ii. bb: homozygous recessive allele
iii. Bb: heterozygous dominant. Heterozygous: having two different alleles of a gene.
(where ‘B’ can be an allele/gene expression for black hair and ‘b’ could be for brown hair.
Therefore ‘BB’ and ‘Bb’ mean that an individual has black hair and a ‘bb’ individual has brown hair.
The BB/Bb/bb is the genotype and the brown/black hair is the phenotype associated with the genotypes.
Dominant allele
This allele has the same effect on the phenotype whether it is present in the homozygous or heterozygous state. A
dominant allele is often expressed over a recessive one. This is known as Complete dominance
Recessive allele
This is an allele that only has an effect on the phenotype when present in the homozygous state.
Co dominant alleles
Co dominant alleles are pairs of allele that both affect the phenotype when present together in a heterozygote.
Some genes have more than two alleles (multiple alleles).
9
The ABO blood group is a good example of co dominance and multiple alleles.
There are three alleles that control the ABO blood groups. If there are more than two allele of a gene, then they are
called multiple allele.
-
Allele IA corresponds to blood group A (genotype IAIA)
Allele IB corresponds to blood group B (genotype IBIB).
Both of these are dominant and so if IA and IB are present together they form blood group AB (genotype IAIB).
Both alleles affect the phenotype since they are both co dominant.
The allele i is recessive to both IA and IB so if you have the genotype IA i you will have blood group A and if you
have the genotype IB i you will have blood group B.
However, if you have the genotype ii then you are homozygous for i and will be of blood group O.
Below is a table to summaries which genotypes give which phenotypes.
Example
The fathers blood group is A and the mothers blood group is B, construct a genetic diagrams to show that it is
possible for parents to have a child with blood group O.
Phenotype Genotype
A IAIA or IAi
B IBIB or IBi
AB IAIB
O ii
Example
A father’s blood group is AB; the mother’s is O.
Which of the following blood groups could appear in their offspring?
The father’s genome must be iAiB and The mother’s genome will be iOiO
The combination iAiO is possible and will be expressed as group A because iA is the dominant allele
The combination iBiO is possible and will be expressed as group B because iB is the dominant allele
The offspring will all inherit either iA or iB from their father and iO from their mother. Their genomes will be
either iAiO (expressed as group A) or iBiO (expressed as Group B. Group AB is not possible.
10
Monohybrid inheritance
Inheritance is the passing of characteristics from parents to offspring, while monohybrid inheritance is inheritance
for a single characteristic.
When studying this form of inheritance, the environmental influence is usually ignored.
Monohybrid inheritance was discovered by an Austrian Monk/priest when studying the inheritance of garden pea
plant, Pisum sativum. He studied characteristics that we noticeable and easily distinguishable.
The results of his experiments resulted in him developing some laws of heredity which are as follows;
1. Genes occur impairs (one gene comes from the male parent the other from the female parent).
2. Genes can be dominant or recessive (one gene of a pair may hide the trait of the other)
3. Genes of pairs separate from each other when gametes are formed (only one gene goes into a gamete.
Example 1
A pure-breeding male black mouse is mated with a female brown mouse and they produce a litter of 12. The allele
for black fur is dominant to the allele to brown fur.
What is the expected distribution of colour and sex in their litter?
-
The allele for black fur is dominant to the brown allele, so there can be no brown mice in the litter from this
cross. All the 12 will be black. The male mouse carries the X and Y chromosomes. The female mouse carries
two X chromosomes. At meiosis, only one of each chromosome pair goes to the gametes.
11
Monohybrids cross between smooth and wrinkled seed pea plants
All the seeds are smooth
Monohybrid cross between heterozygous smooth seeds
The phenotype ratio is 3:1 and the genotype ratio is 1:2:1.
12
Exercise 5
The structure on the head of a chicken (the comb) can be of different shapes. Fig 5 shows how two different shapes
of comb ’walnut’ and ‘pea’ were inherited in an experiment. Walnut is dominant.
(i) Assuming that comb shape is controlled by a pair of allele, use Q for the dominant allele and q for the recessive
allele to show the genotype of the following chickens. S, T, U and V.
(ii) Chicken S and V were bred together. What proportion of their offspring had pea comb?
Show your working.
Test/back crossing
Crossing a suspected heterozygous with a known homozygous to determine whether it is homozygous or
heterozygous for a character under consideration.
This method is used to distinguish between homozygous and heterozygous dominant forms.
If the F2 individual is homozygous dominant, all the offspring will show the dominant character. If theF2
individual is heterozygous a 1:1 ratio of the dominant and recessive character is obtained.
13
Sex determination and linkage
Objective
Explain how the sex chromosomes control gender by referring to the inheritance of X and Y chromosomes
in humans.
Two chromosomes determine sex. These genes are located on sex chromosomes, the X and the Y chromosome.
-
Females have two X chromosomes XX.
Males have one X and one Y chromosome XY.
The female always passes on to her offspring the X chromosome from the egg (female gamete).
The male can pass on either the Y or the X chromosome from the sperm (male gamete).
In males about half of the gametes are X and half are Y whereas in females all eggs are X. During fertilization
when the Y sperm and fuses with the egg the zygote becomes a male of XY. When the X sperm fuses with egg,
they form an XX zygote which is female.
Each time a couple have a baby they have a 1 in 2 chances of producing a male.
Ratio =
1:1 (since we have 2XX (girl) and 2XY (boy). This concludes that the chances of having either a
male or female human offspring are 50/50.
Sex linkage
-This is when the genes controlling a certain character are located on the sex chromosome.
-The character is inherited along the sex; such characters are sex linked.
-Most sex-linked genes are carried on the X chromosome.
-Females have two X chromosomes, they have two copies of the sex-linked gene, males have one since they only
have one X chromosome.
-Sex linked disease affect mostly males than females.
-Females are carriers of such disorders because they carry the gene.
Examples of sex linked conditions and diseases include the following, Baldness effect in some men, Red green
colour blindness in men than in women and Hemophilia common in men than in women.
14
Hemophilia
-
The dorminant allele XH is the allele for normal blood clotting
The recessive allele Xh which is recessive and causes hemophilia failare to clot.
The table below shows the genotype and possible phenotypes for humans.
Genotype
XH XH
XH Xh
Xh Xh
XH Y
Xh Y
Phenotype
Normal blood clotting female
Carrier female
Hemophiliac female
Normal blood clotting male
Hemophiliac male
Female hemophiliac is rare because both parents must carry the gene, for woman the recessive allele can be
expressed in the homozygous recessive.
-
So whether the child has the disease or is unaffected depends on which allele the mother had passed on.
If she has passed on the recessive allele (Xh) then the male child will have hemophilia, however if she has
passed on the dominate allele (XH) then the child will be unaffected.
The chance of a female offspring having hemophilia is 0%. Finally, there is a 25% chance overall that the
offspring will be affected.
The following shows an example of how red-green colour blindness is inherited;
Let B represent a normal sight and b represent red-green colour blindness.
15
-
color blindness is common in males and less common in females
one recessive allele from female is enough to make a colour blind male since there is no corresponding allele
on the Y chromosome.
A man who is colour blind may not pass the condition to his sons but may pass it on indirectly to his grand
sons and this can be shown by a form of a pedigree analysis.
A colour blind/carrier woman does not pass the disease to her daughters if the husband is normal but she
does pass them to her sons.
Pedigree analysis
A pedigree maybe uses to establish the probability of a child having a particular disorder. It may be used to show
where the genes in question are located. (X, Y) or autosome, or whether a trait is recessive or dorminant.
Mutation
Genetics definitions of mutations
Spontaneous, permanent, sudden change in the gene or chromosome structure.
This result in the change in genotype which may be inherited by cells derived from cells by mitosis or. meiosis
Any offspring produced by an individual with a mutation, showing the new characteristics is known as a mutant.
Gene or chromosome mutations
These are mutation resulting from the change in the amount or arrangement of DNA. Changes in the number of
chromosomes are usually a result of errors during meiosis. This involve loss or gain of a single chromosome, a
condition called aneuploidy (n+1, 2n+1) or increase in the haploid set of chromosomes called euploidy (n-1, 2n1).
Down syndrome is a result of mutation where 2n=47.
Structural changes in chromosome
A. Inversion of Genes
This is where the order of a particular order of genes are reversed as seen below
16
B. Translocation of Gene
This is where information from one of two homologous chromosomes breaks and binds to the other. Usually this
sort of mutation is lethal
C. Deletion of a Gene
As the name implies, genes of a chromosome are permanently lost as they become unattached to the centromeres
and are lost forever
Factors that lead to mutations (mutagens)
1. Chemical Mutagens change the sequence of bases in a DNA gene in a number of ways;
-
Mimic/copy the correct nucleotide bases in a DNA molecule, but fail to base pair correctly during DNA
replication.
-
Remove parts of the nucleotide (such as the amino group on adenine), again causing improper base pairing
during DNA replication.
-
Add hydrocarbon groups to various nucleotides, also causing incorrect base pairing during DNA
replication.
2. Radiation lead to the mutations in the following ways.
-
It can also cause double strand breaks in the DNA molecule, which the cell's repair mechanisms cannot put
right.
-
Radiation can damage the bases in DNA molecules. If this happens in the ovaries or testes, then the altered
DNA may be passed on to the offspring.
3. Sunlight contains ultraviolet radiation (the component that causes a suntan) which, when absorbed by the DNA
causes a cross link to form between certain adjacent bases. In most normal cases the cells can repair this damage,
but unrepaired dimers of this sort cause the replicating system to skip over the mistake leaving a gap, which is
supposed to be filled in later.
17
Unprotected exposure to UV radiation by the human skin can cause serious damage and may lead to skin cancer
and extensive skin tumors.
4. Spontaneous mutations occur without exposure to any obvious mutagenic agent. Sometimes DNA nucleotides
shift without warning to a different chemical form (known as an isomer) which in turn will form a different series
of hydrogen bonds with its partner. This leads to mistakes at the time of DNA replication.
Variation
Variation within a species is the way that two animals of the same species display different characteristics and/or
behavior.
There are two types of variation; genetic and environmental.
Genetic variation
All animals of the same species will look slightly different because their genes will be different, apart from
identical twins, where the genes are identical.
Examples
blood groups, skin color, eye colour, height, colour blindness, hemophilia, baldness, sickle cell anaemia, albinism,
hairy ear lobes (pinna), long sightedness and tongue rolling.
Environmental variation is variation due to the environment.
Examples
include; scars, knowledge, goitre, night blindness and language. A plant in a good soil area might be twice as
strong in one where the soil quality is poor.
Continuous variation
- There are small differences amongst individuals of the same species and many intermediate forms.
-
This is due to the effects of many genes of small effect and contributed by the environment in some cases.
Examples include traits such as height and intelligence quotients (IQ) in humans, weight.
Discontinuous variation
- This variation shows clear, harp differences amongst individuals of a species. There are no intermediate
forms. Examples are the inheritance of blood groups A, B, AB or O, sickle cell anaemia, tongue rolling,
sex determination, red green colour blindness, and hemophilia.
-
Mutations are the basis of discontinuous variation. The environment cannot affect this type of variation.
Difference between Continuous variation and Discontinuous variation
Continuous variation
There are small differences or intermediates in
phenotype.
modified by the environment
Controlled by few genes
Discontinuous variation
There are Clear sharp differences, no
intermediate in phenotype
Not modified by the environment
Controlled by many genes
18
Natural selection
This is a process where organisms that are better adapted to an environment will survive and reproduce. This
means that the advantageous alleles of this variant organism are passed on to offspring.
Importance of natural selection as a possible mechanism of evolution
-
It ensures survival of certain population with good heritable characters.
Production of resistant individuals, for example sickle cell anaemia is found in African regions which are
malaria infested region. The carriers of the sickle cell trait give some protection against malaria even though
those that have the disease die.
Give rise to new species, for example the polymorphism in the peppered moth. Species can be formed by
reproductive and geographical isolation.
Mechanisms of Natural selection
Population stabilization
A condition, in which the continual increase in number of any particular species is retarded naturally by death as a
result of competition for resources, members with less competitive features are outcompeted and die.
Directional selection
This is due to environment conditions which impose change; environmental factors such as disease, climate.
Disruptive selection
This depends on short listed environmental conditions such as drought outbreak
Some examples of Natural selection
Houseflies and some mosquitoes are resistant to DDT, and this gives them a better chance of survival in the
presence of DDT
Drug resistance in some strains of plasmodium occurs in East and West Africa due to frequent use of
pyrimethamine.
Artificial selection
This is the selecting and mating of individuals with good and useful traits by man to produce offspring with
improved qualities. This involves getting desired varieties and combining them. This is also known as selective
breeding.
Methods of Artificial selection
Inbreeding
This is the mating of two closely related species i.e. Mating of offspring of the same parent.
Disadvantage of inbreeding
- It increases the chances of homozysity and hence leads to rise of inherited recessive genetic disorders.
19
Cross breeding
mating unrelated individuals of the same species. i.e. crossing two genetically different varieties to form a hybrid
with greater survival values than both parents/
Advantage of cross breeding
offspring shows improved qualities such as
1. Cows that produce more milk.
2. Chickens that produce more eggs.
3. Wheat plants that produce lots of grains.
Other methods include removing all individuals showing undesirable characteristics, preventing such individuals
from mating with individuals with desirable traits and sterilization
Main steps involved in selective breeding
1.
2.
3.
4.
Decide which characteristics are important enough to select.
Choose the parents that that shows the desired characteristics.
Choose the best offspring from the parents to produce the next generation.
Repeat the process continuously.
Advantages of artificial selection
Variety of crops and animals have been bred to have favorable characteristics such as
-
Large scale production, large milk production in cows, Higher yield in crops.
Drought resistant varieties
Better nutritional values in crops
Better response of plants to fertilizers
Rapid growth
Differences between artificial and natural selection
Artificial selection
Natural selection
Humans are the agents of selection
It is much quicker
Does not offer the plant or animal any
advantages in its natural environment
Selection depends on the natural environment
Takes time
Offer the plant/animal advantages to survive
in its natural habitat.
20
Genetic Engineering
Genetic engineering is changing the genetic material of an organism by removing, changing or inserting
individual genes from another organism
The organism receiving the genetic material is said to be ‘genetically modified’, or is described as a ‘transgenic
organism’
The DNA of the organism that now contains DNA from another organism as well is known as ‘recombinant
DNA’
There are many examples of genetically modified organisms, including:




The gene for human insulin has been inserted into bacteria which then produce human insulin which can
be collected and purified for medical use for diabetics
Crop plants, such as wheat and maize, have been genetically modified to contain a gene from a bacterium
that produces a poison that kills insects, making them resistant to insect pests such as caterpillars
Crop plants have also been genetically modified to make them resistant to certain herbicides (chemicals
that kill plants), meaning that when the herbicide is sprayed on the crop it only kills weeds and does not
affect the crop plant
Some crops have been genetically modified to produce additional vitamins, eg ‘golden rice’ contains
genes from another plant and a bacterium which make the rice grains produce a chemical that is turned
into vitamin A in the human body, which could help prevent deficiency diseases in certain areas of the
world
Process of Genetic Engineering



The gene that is to be inserted is located in the original organism (for example, this could be the gene for
human insulin)
Restriction enzymes are used to isolate the required gene, leaving it with ‘sticky ends’ (a short section of
unpaired bases)
A bacterial plasmid is cut by the same restriction enzyme leaving it with corresponding sticky
ends (plasmids are circles of DNA found inside bacterial cells)
21
Restriction enzymes cut DNA strands at specific sequences to form ‘sticky ends’


The plasmid and the isolated gene are joined together by DNA ligase enzyme
If two pieces of DNA have matching sticky ends (because they have been cut by the same restriction
enzyme), DNA ligase will link them to form a single, unbroken molecule of DNA
22
DNA ligase is used to join two separate pieces of DNA together




The genetically engineered plasmid is inserted into a bacterial cell
When the bacteria reproduce the plasmids are copied as well and so a recombinant plasmid can quickly
be spread as the bacteria multiply and they will then all express the gene and make the human protein
The genetically engineered bacteria can be placed in a fermenter to reproduce quickly in controlled
conditions and make large quantities of the human protein
Bacteria are extremely useful for genetic engineering purposes because:
o They contain the same genetic code as the organisms we are taking the genes from, meaning they
can easily ‘read’ it and produce the same proteins
o There are no ethical concerns over their manipulation and growth (unlike if animals were used,
as they can feel pain and distress)
o The presence of plasmids in bacteria, separate from the main bacterial chromosome, makes
them easy to remove and manipulate to insert genes into them and then place back inside the
bacterial cells
Process of Genetic Engineering



The gene that is to be inserted is located in the original organism (for example, this could be the gene for
human insulin)
Restriction enzymes are used to isolate the required gene, leaving it with ‘sticky ends’ (a short section of
unpaired bases)
A bacterial plasmid is cut by the same restriction enzyme leaving it with corresponding sticky
ends (plasmids are circles of DNA found inside bacterial cells)
23
24
Restriction enzymes cut DNA strands at specific sequences to form ‘sticky ends’


The plasmid and the isolated gene are joined together by DNA ligase enzyme
If two pieces of DNA have matching sticky ends (because they have been cut by the same
restriction enzyme), DNA ligase will link them to form a single, unbroken molecule of DNA
DNA ligase is used to join two separate pieces of DNA together




The genetically engineered plasmid is inserted into a bacterial cell
When the bacteria reproduce the plasmids are copied as well and so a recombinant plasmid
can quickly be spread as the bacteria multiply and they will then all express the gene and
make the human protein
The genetically engineered bacteria can be placed in a fermenter to reproduce quickly in
controlled conditions and make large quantities of the human protein
Bacteria are extremely useful for genetic engineering purposes because:
o They contain the same genetic code as the organisms we are taking the genes from,
meaning they can easily ‘read’ it and produce the same proteins
o There are no ethical concerns over their manipulation and growth (unlike if animals
were used, as they can feel pain and distress)
o The presence of plasmids in bacteria, separate from the main bacterial chromosome,
makes them easy to remove and manipulate to insert genes into them and then place
back inside the bacterial cells
25