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Chapter 7: DNA And The Genetic Code
7.1 Evidence that the nucleus contains
the hereditary material
- Hammerling used an unusually large single-celled
alga to section & isolate the nucleus and proved that
the nucleus contain the hereditary material DNA.
- Evidence that DNA ( DeoxyRibosenucleic Acid )
is the hereditary material:
1. Chromosome analysis
2. Metabolic stability of DNA
3. Constancy of DNA in a cell
4. Correlation between mutagens and their effects
on DNA
5. Experiments on bacterial transformation
(Griffith 1928)
6. Experiments to identify the transforming principle
7. Transduction experiments
7.2 Evidence that DNA is the hereditary material
7.2.1 Chromosome Analysis
- Chromosomes occur only when cell division occurs
proteins
- Chromosomes are made of _____
DNA and _____________
- DNA analysis shows that its bases carry a genetic code
for heredity
7.2.2 Metabolic stability of DNA
- Unlike protein, DNA molecules show very remarkable stability,
e.g. when using radioactive isotopes,
their disappearance is very slow
7.2 Evidence that DNA is the hereditary material
7.2.3 Constancy of DNA within a cell
- analysis showed that the amount of DNA remains constant
for all cells within a species except for the gametes (why?)
- prior to cell division, amount of DNA per cell doubles
Gametes have only half of the chromosome number as their
7.2.4
Correlation between mutagens and their effects on DNA
parents.
- Mutagens are agents which cause mutations in living organisms
A Mutation is an alteration to an organism's characteristics
which is inherited
- Examples of mutagens: X-rays, nitrous acids, dyes
Mutagens alter the structure of DNA in some way,
e.g.UV ray on bases (pyrimidine)
7.2.5 Experiments on bacterial transformation - Griffith, 1928
- Bacterium Pneumococcus exists in two forms:
harmful form with shiny, smooth colonies (S- strain)
safe form with dull, rough colonies (R-strain)
Explanation:
The code for the toxin was transferred from the head harmful form
to the living safe variety. Living safe form can make the toxin and
pneumonia results - The substance (or DNA) was able to transform
one strain of pneumococcus into another:
The transforming principle
7.2.6 Experiments to identify The Transforming Principle
- McCarty & McCleod (1944) isolated and purified different
substances from the dead & harmful types of pneumococcus
and found DNA to be capable of bringing about the
transforming. The ability stopped when DNAase was added.
7.2.7 Transduction experiments
- Using T2 bacteriophage attacking E. Coli with radioactive
substances:
35S in protein of one phage and 32P in DNA of another phage
- Culture injected with radioactive DNA contained radioactive
bacteria, while that injected with radioactive protein did not
- DNA was the hereditary material, with further proof from
electron microscope studies
7.3 Nucleic Acids
7.3.1 Structure of Nucleotides
Each nucleotide consists of 3 parts:
1. Phosphoric acid
2. Pentose sugar
3. Organic base:
7.3 Nucleic Acids
7.3.1 Structure of Nucleotides
Each nucleotide consists of 3 parts:
1. Phosphoric acid
2. Pentose sugar
3. Organic base:
Pyrimidines - single ringed, six-sided; cytosine, thymine, uracil
Purines - 5 sided & 6 sided double ringed base; adenine & guanine
Purines
Pyrimidines
Dinucleotide: 2 nucleotides joined together
Polynucleotide: > 2 nucleotides joined together
Ribonucleic Acid (RNA)
RNA is a single-stranded polymer of
nucleotides where the pentose is ribose
& the bases are adenine, guanine,
cytosine and uracil.
Three types of RNA:
Ribosomal (rRNA), Transfer (tRNA),
and Messenger (mRNA)
Function: for protein synthesis
Ribosomal RNA:
- a large, complex molecule with single & double helices
- it is manufactured by DNA but found in cytoplasm, making up
of half the mass of ribosmes
- base sequence is similar in all organisms
Transfer RNA:
- small, single stranded, manufactured by DNA
- clover-leaf shape, one end with CCA to attach amino acid
- at least 20 types
- anticodon with 3 specific bases for specific amino acid
during protein synthesis
Messenger RNA:
- long, single-stranded helix
- manufactured in nucleus, a mirror copy of part of DNA
strand
- immense variety
- enters cytoplasm as a template for protein synthesis
- easily broken down & short-lived
Some important nucleotides:
Molecule
Abbreviation Function
Some important nucleotides:
Molecule
Abbreviation Function
deoxyribonucleic DNA
contain the genetic information of
acid
cells
Some important nucleotides:
Molecule
Abbreviation Function
deoxyribonucleic DNA
contain the genetic information of
acid
cells
Ribonucleic acid RNA
protein synthesis
Some important nucleotides:
Molecule
Abbreviation Function
deoxyribonucleic DNA
contain the genetic information of
acid
cells
Ribonucleic acid RNA
protein synthesis
Adenosine
coenzymes for energy release
Some important nucleotides:
Molecule
Abbreviation
deoxyribonucleic DNA
acid
Ribonucleic acid RNA
Adenosine
monophosphate AMP
diphosphate
ADP
triphosphate
ATP
Function
contain the genetic information of
cells
protein synthesis
coenzymes for energy release
Some important nucleotides:
Molecule
Abbreviation Function
deoxyribonucleic DNA
contain the genetic information of
acid
cells
Ribonucleic acid RNA
protein synthesis
Adenosine
coenzymes for energy release
monophosphate AMP
diphosphate
ADP
triphosphate
ATP
Nicotinamide
Electron carriers in transferring
adenine dinucleotide NAD hydrogen atoms in respiratory Krebs
Flavine adenine
cycle
dinucleotide
FAD
Some important nucleotides:
Molecule
Abbreviation Function
deoxyribonucleic DNA
contain the genetic information of
acid
cells
Ribonucleic acid RNA
protein synthesis
Adenosine
coenzymes for energy release
monophosphate AMP
diphosphate
ADP
triphosphate
ATP
Nicotinamide
Electron carriers in transferring
adenine dinucleotide NAD hydrogen atoms in respiratory Krebs
Flavine adenine
cycle
dinucleotide
FAD
Nicotinamide
Electron carrier for accepting
adenine dinucleotide
electrons from chlorophyll molecule
phosphate
NADP
in photolysis of water
Some important nucleotides:
Molecule
Abbreviation Function
deoxyribonucleic DNA
contain the genetic information of
acid
cells
Ribonucleic acid RNA
protein synthesis
Adenosine
coenzymes for energy release
monophosphate AMP
diphosphate
ADP
triphosphate
ATP
Nicotinamide
Electron carriers in transferring
adenine dinucleotide NAD hydrogen atoms in respiratory Krebs
Flavine adenine
cycle
dinucleotide
FAD
Nicotinamide
Electron carrier for accepting
adenine dinucleotide
electrons from chlorophyll molecule
phosphate
NADP
in photolysis of water
Coenzyme A
CoA
coenzyme for respiratory Krebs cycle
7.3.3 Deoxyribonucleic Acid (DNA)
A double stranded polymer of nucleotides, with deoxyribose sugar
and organic bases: adenine, guanine, cytosine, thymine (no uracil);
each chain is extremely long with millions of nucleotide units
Facts about DNA:
1 It is very long, made up nucleotides
2 It contains 4 organic bases: adenine, guanine, cytosine, thymine
3 Amount of guanine is equal to that of cytosine
4 Amount of adenine is equal to that of thymine
5 It is in the form of a helix maintained by hydrogen bonding
Watson and Crick (1953) suggested a molecular structure for
DNA:
- a double helix of two nucleotide strands linked together by pairs
of organic bases which are joined together by hydrogen bonds
- C pairs with G by 3 hydrogen bonds;
A pairs with T by 2 hydrogen bonds;
consistent with the known ratio of bases in molecule and allowed
for an identical separation of strands throughout the molecule
Watson and Crick (1953) suggested a molecular structure for
DNA:
- a double helix of two nucleotide strands linked together by pairs
of organic bases which are joined together by hydrogen bonds
- C pairs with G by 3 hydrogen bonds;
A pairs with T by 2 hydrogen bonds;
consistent with the known ratio of bases in molecule and allowed
for an identical separation of strands throughout the molecule
- two strands twist around each other in an antiparallel direction
Watson and Crick (1953) suggested a molecular structure for
DNA:
- a double helix of two nucleotide strands linked together by pairs
of organic bases which are joined together by hydrogen bonds
- C pairs with G by 3 hydrogen bonds;
A pairs with T by 2 hydrogen bonds;
consistent with the known ratio of bases in molecule and allowed
for an identical separation of strands throughout the molecule
- two strands twist around each other in an antiparallel direction
- DNA's extreme length permits a long sequence of bases which
can vary indefinitely for its immense store of genetic information
- Double stranded structures allow its semi-conservative
replication during cell division to give two identical daughter
cells
Differences between RNA
and DNA
1 single stranded
1 double stranded
Differences between RNA
and DNA
1 single stranded
1 double stranded
2 smaller molecule mass
2 larger molecule mass
Differences between RNA
and DNA
1 single stranded
1 double stranded
2 smaller molecule mass
2 larger molecule mass
3 may be single or double helix
3 always double helix
Differences between RNA
and DNA
1 single stranded
1 double stranded
2 smaller molecule mass
2 larger molecule mass
3 may be single or double helix
3 always double helix
4 Pentose: oxyribose
4 Pentose: deoxyribose
Differences between RNA
and DNA
1 single stranded
1 double stranded
2 smaller molecule mass
2 larger molecule mass
3 may be single or double helix
3 always double helix
4 Pentose: oxyribose
4 Pentose: deoxyribose
5 Organic bases: A, G, C, U
5 Organic bases: A, G, C, T
Differences between RNA
and DNA
1 single stranded
1 double stranded
2 smaller molecule mass
2 larger molecule mass
3 may be single or double helix
3 always double helix
4 Pentose: oxyribose
4 Pentose: deoxyribose
5 Organic bases: A, G, C, U
5 Organic bases: A, G, C, T
6 Ratio of A, U to C, G varies
6 Ratio of A, T to C, G is 1
Differences between RNA
and DNA
1 single stranded
1 double stranded
2 smaller molecule mass
2 larger molecule mass
3 may be single or double helix
3 always double helix
4 Pentose: oxyribose
4 Pentose: deoxyribose
5 Organic bases: A, G, C, U
5 Organic bases: A, G, C, T
6 Ratio of A, U to C, G varies
6 Ratio of A, T to C, G is 1
7 made in nucleus but found
throughout the cell
7 found almost entirely in nucleus
Differences between RNA
and DNA
1 single stranded
1 double stranded
2 smaller molecule mass
2 larger molecule mass
3 may be single or double helix
3 always double helix
4 Pentose: oxyribose
4 Pentose: deoxyribose
5 Organic bases: A, G, C, U
5 Organic bases: A, G, C, T
6 Ratio of A, U to C, G varies
6 Ratio of A, T to C, G is 1
7 made in nucleus but found
throughout the cell
7 found almost entirely in nucleus
8 Amount varies from cell to cell
8 Amt constant for cells of a species
Differences between RNA
and DNA
1 single stranded
1 double stranded
2 smaller molecule mass
2 larger molecule mass
3 may be single or double helix
3 always double helix
4 Pentose: oxyribose
4 Pentose: deoxyribose
5 Organic bases: A, G, C, U
5 Organic bases: A, G, C, T
6 Ratio of A, U to C, G varies
6 Ratio of A, T to C, G is 1
7 made in nucleus but found
throughout the cell
7 found almost entirely in nucleus
8 Amount varies from cell to cell
9 Chemically less stable
8 Amt constant for cells of a species
9 Chemically very stable
Differences between RNA
and DNA
1 single stranded
1 double stranded
2 smaller molecule mass
2 larger molecule mass
3 may be single or double helix
3 always double helix
4 Pentose: oxyribose
4 Pentose: deoxyribose
5 Organic bases: A, G, C, U
5 Organic bases: A, G, C, T
6 Ratio of A, U to C, G varies
6 Ratio of A, T to C, G is 1
7 made in nucleus but found
throughout the cell
7 found almost entirely in nucleus
8 Amount varies from cell to cell
8 Amt constant for cells of a species
9 Chemically less stable
9 Chemically very stable
10 exit temporarily
10 permanent
Differences between RNA
and DNA
1 single stranded
1 double stranded
2 smaller molecule mass
2 larger molecule mass
3 may be single or double helix
3 always double helix
4 Pentose: oxyribose
4 Pentose: deoxyribose
5 Organic bases: A, G, C, U
5 Organic bases: A, G, C, T
6 Ratio of A, U to C, G varies
6 Ratio of A, T to C, G is 1
7 made in nucleus but found
throughout the cell
7 found almost entirely in nucleus
8 Amount varies from cell to cell
8 Amt constant for cells of a species
9 Chemically less stable
9 Chemically very stable
10 exit temporarily
10 permanent
11 3 forms: rRNA, tRNA, mRNA
11 one form but indefinite variety
7.4 DNA Replication - semi-conservative method
7.4 DNA Replication - semi-conservative method
2. Two strands of DNA separate
by DNA polymerase
7.4 DNA Replication - semi-conservative method
3. Free nucleotides are attracted to
their complementary bases
7.4 DNA Replication - semi-conservative method
4. New nucleotides line up
and join together, with
unpaired bases continue to
attract their complementary
bases
7.4 DNA Replication - semi-conservative method
5. Finally all the nucleotides are joined to form a complete
polynucleotide chain to form two identical strands of DNA
7.4 DNA Replication - semi-conservative method
1. A representative portion of DNA about to undergo replication
2. Two strands of DNA separate by DNA polymerase
3. Free nucleotides are attracted to their complementary bases
4.New nucleotides line up and join together, with unpaired bases
continue to attract their complementary bases
5.Finally all the nucleotides are joined to form a complete
polynucleotide chain to form two identical strands of DNA
7.4 DNA Replication - semi-conservative method
Experiments by Meselsohn and Stahl using
labelled 15N:
1. DNA extracted from E. coli grown in a
medium containing normal nitrogen (14N);
All DNA is of the light type.
2. DNA extracted from E. coli grown in a
medium containing heavy nitrogen (15N)
and then transferred to a medium containing
normal nitrogen (14N);
The weight was intermediate between the
heavy and light DNA.
3. DNA extracted from E. coli grown in a
medium containing heavy nitrogen (15N)
1
2
3
7.5 The Genetic Code
DNA  enzymes  proteins which determine an
organism's characteristics
triplet code: GUA, CGC, AAA, GGG, AUG, etc.
Characteristics of the genetic code: degenerate code
stop/nonsense code
non-overlapping
universal
Totally: 64 combinations
eg. valine (GU*) where * can be any base
UAA, UAG, UGA code for no amino acids but stop/nonsense command
CUGAGCUAG is read as CUG-AGC-UAG
The codes are precisely the same for all organisms.
1 (a) Explain the features of the genetic code (6 marks) (98-II-3)
2. Distinguish between transcription and translation. (2 marks) 95-I-4(a)
Describe how the information carried on DNA is used in protein
synthesis. (If you wish, you may use labelled diagrams to answer
this question.) (10 marks) 80-II-5(a)
4. (a) Briefly illustrate the structure of the DNA molecule. (5 marks)
81-II-1
Diagram to show the double helix, sugar phosphate
backbone, nucleotide bases (names of bases should
be mentioned).
4. (a) Briefly illustrate the structure of the DNA molecule. (5 marks)
81-II-1
(b) How does the DNA molecule function as the carrier of genetic information?
(The role of the messenger RNAs should also be mentioned.) (15 marks)
Order of nucleotide bases determine specificity of the gene
The triplet code
Involvement of messenger RNA
Order of amino acids determine specificity of polypeptide chain,
order is dictated by order of the codons
Phenotype of organism depends on specific spectrum of proteins
(e.g. enzymes)
DNA molecule can duplicate itself
DNA molecule carried in germ cells
max 15 marks
7.6 Protein Synthesis
Four main stages in the formation of a protein:
1 Synthesis of amino acids
2 Transcription (formation of mRNA)
3 Amino acid activation
4 Translation
7.6.1 Synthesis of Amino Acids
In plants, synthesis of amino acids occurs in mitochondria &
chloroplasts:
a. Absorption of nitrates from soil
b. Reduction of nitrate to amino group (NH2)
c. Combination of amino groups with a carbohydrate skeleton
d. Transfer amino groups from one carbohydrate skeleton to another
non-essential amino acids
In animals, most of the amino acids _______________________
can be synthesized in their bodies;
essential amino acids
about 9 amino acids _____________________
must be supplied
from the diet.
7.6.2 Transcription (formation of messenger RNA)
- Transcription is the process by which a complementary mRNA
cistron/gene of the DNA
copy is made of the specific region (=___________)
molecule which codes for a polypeptide chain:
1. Unwinding of a portion of a cistron - by breaking of H-bonds
2. One strand acts as a template for the formation of mRNA:
each base along one strand attracts its complementary RNA
uracil instead of ________
thymine
nucleotide, C - G ; but A - _______
3. Enzyme RNA polymerase moves along the DNA adding
complementary RNA nucleotide at a time to the newly unwind
portion of DNA. A number of mRNA molecules may be
formed before RNA polymerase leaves the DNA which then
closes up reforming its double helix structure.
4. Each mRNA contains a sequence of triplet codes that have been
determined by the DNA. mRNA goes to the ribosomes in the
cytoplasm through the nuclear pore.
7.6.3Amino Acid Activation
Activation is the process
by which amino acids
combine with tRNA using
energy from ATP
Each type of tRNA binds
with a specific amino acid,
therefore there are at least
20
____kinds
of tRNA
Each tRNA possesses a specific anticodon (triplet of bases) for a
particular amino acid AND a free end which terminates in the
triplet CCA for the attachment of individual a.a. to form a
polypeptide chain
7.6.4 Translation
Translation is the means by which a specific sequence of amino
acids is formed in accordance with the codons on the mRNA.
Polysome is a group of ribosomes which becomes attached to
the mRNA
7.6.4 Translation
Complementary anticodon of a tRNAamino acid complex is attached to the
1st codon on the mRNA;
2nd codon likewise attracts its
complementary anticodon
Ribosome thus holds mRNA and tRNA
a.a. complex until the 2 a.a. form a
peptide bond between them; then
ribosome moves along to a 3rd codonanticodon complex until the 3rd a.a. is
formed with the 2nd, and so on to form a
complete polypeptide chain
7.6.4 Translation
Second and subsequent ribosomes may
pass along mRNA immediately behind
the 1st so that many identical
polypeptide chains can be produced
simultaneously
Free tRNA moves back to cytoplasm
& combine with another a.a.
A non-sense code orders the casting of
the complete polypeptide chain;
polypeptide chain may undergo *spiral
configuration to give its secondary
structure, *folding to give a tertiary
structure, or combine with other
polypeptide chains to give a quaternary
structure. *by H-bonding or disulphide bonds
One gene specifies one polypeptide
1 (b) Describe in detail the cellular processes that are necessary in
the transfer and decoding of genetic information for polypeptide
synthesis.
(12 marks)
(98-II-3)
Transcription
(8M)
Translation
(4M)
1 (b) Describe in detail the cellular processes that are necessary in
the transfer and decoding of genetic information for polypeptide
synthesis.
(12 marks)
(98-II-3)
(c) In general, what additional processes are necessary for the
formation of the three-dimensional structure of proteins after
polypeptide synthesis?
(2 marks)
7.7 Genetic Engineering
Technology which allows genes to be
manipulated, altered and transferred from
organism to organism, even to transform
DNA itself
Use of rapidly reproducing organisms
(bacteria) as chemical factories producing
useful, often life-saving, substances,
e.g. hormones, antibodies, vitamins
Use of plasmid vector in
gene cloning
Use of plasmid vector
in gene cloning
7.7.1 Recombinant DNA Technology
Methods for isolating portion of human DNA responsible for
producing insulin and combining with bacterial DNA in such a
way that the micro-organism will continually produce insulin
because of the recombinant DNA present
Traditional methods of obtaining insulin:
Extracts from animals
Disadvantages of traditional methods:
Too expensive
Other extracts from animals or humans, e.g. thyroxine, may
cause antibody production, and risks of infectious disease, e.g.
human HIV in haemophiliacs
7.7.2 Techniques used to manipulate DNA
1. Cutting of DNA into small sections using restriction
endonucleases
2. Production of copies of DNA using either plasmids or
reverse transcriptase
3. Joining together portions of DNA using DNA ligase
7.7.2 Techniques used to manipulate DNA
1. Cutting of DNA into small sections using restriction
endonucleases
2. Production of copies of DNA using either plasmids or
reverse transcriptase
3. Joining together portions of DNA using DNA ligase
7.7.2 Techniques used to manipulate DNA
1. Cutting of DNA into small sections using restriction
endonucleases
2. Production of copies of DNA using either plasmids or
reverse transcriptase
3. Joining together portions of DNA using DNA ligase
7.7.3 Gene Cloning
multiple copies of a specific gene are produced which may then
be used to manufacture large quantities of valuable products
- involves the following stages:
1. Identification of that gene
2. Isolation of that gene
3. Insertion of the gene into a vector
4. Insertion of the vector into a host cell
5. Multiplication of the host cell
6. Synthesis of the required product by the host cell
7. Separation of the product from the host cell
8. Purification of the product
7.7.3 Gene Cloning
The bacteria produced can be grown in industrial fermenters
using a specific nutrient medium under strictly controlled
conditions.
The bacteria can then be collected and the insulin is extracted
from them by suitable methods.
7.7.4 Insertion of vector into a host cell
The final destination of a particular gene may be a
crop plant and the bacterium chosen has the ability
to infect plants.
For example, Agrobacterium tumefaciens invades
plant cells by incorporating its large tumourinducing plasmid into the genome of the host
cells.
Agrobacterium tumefaciens has a number of
different strains which can infect a wide variety of
plants. The bacterium is very useful in transferring
genes into new organisms.
7.7.4 Insertion of vector into a host cell
The desired gene is not transferred on its own, but along with
a second gene (the gene marker).
For example, an antibiotic resistant gene could be cultured by
growing Agrobacterium on a media containing an
antibiotic.
Those Agrobacterium with the desired gene will survive
because they also possess the marker gene (antibiotic
resistant).
The Agrobacterium with the new gene can now be cultured to
provide a large population.
They may then be used to infect host plants, which will
incorporate the new gene into their own genome.
7.7.5 Application of genetic engineering
This techniques can make a range of materials used to treat
diseases and disorders. Other examples include:
In medicine
• Growth hormone
Erythropoietin (controls red blood cell production)
• Calcitonin (regulates calcium level in blood)
Transferring a normal gene for thalassamia (a disease with
abnormal haemoglobin) into patients suffering by the
disease
In agriculture

Transfer genes which produce toxins with
insecticidal properties to higher plants (potato &
cotton): “built-in’ resistance to certain insect
species to save time & money on insecticides and
avoid killing harmless or beneficial insect species
on the field

Transfer genes nitrogen-fixing bacteria to
cereal crops: less need to apply expensive nitrogen
fertilizers and reduces the pollution problems of
‘leaching’
Other possibilities
Ø
Transfer genes providing resistance to all diseases,
Ø
Develop plants with more efficient rates of photosynthesis
Ø
Control weeds
Ø
Develops oil-digesting bacteria to clear up oil spillages
• Other possibilities
• Transfer genes providing resistance to all diseases,
• Develop plants with more efficient rates of photosynthesis
• Control weeds
• Develops oil-digesting bacteria to clear up oil spillages
7.7.6 Implications of genetic engineering
But there are ethical as well as practical
problems to be overcome before many of these
ideas can be brought to reality!
Practical issues:
1.
It is impossible to predict with complete accuracy what will
happen if genetically engineered organisms are released into the
environment. Our natural habitats might be damaged.
2.
Organisms designed for use in one environment may escape to
other environments with harmful consequences.
3.
Advantageous genes added to domestic animals or cops may be
transferred to their competitors, making them even greater
potential dangers.
4.
The escape of a pathogenic bacterium into a susceptible
population could end in considerable damage to the species.
5.
Human characteristics and behaviour could be modified but in
the wrong hands this could be used by individuals, groups or
governments to achieve certain goals, control opposition or
gain ultimate power.
Ethical issues:
1. Is it right to replace a defective gene with a normal
one?
2. Is the answer the same which causes the bearer pain
as it is where the gene has a merely cosmetic effect?
3. Who decides what is normal and what is defective?
4. A defective gene actually may give advantage, e.g.
sickle-cell anaemia in Africa.
5. Reducing the variety of genes (unwanted genes):
hindering evolution?
6. Increasing the variety of genes (beneficial genes):
favouring evolution?
7. Fetal abnormality: What criteria, if any, should be
applied before deciding ‘abortion’?
The challenge is to develop regulations and
safeguards within normal boundaries which
permit genetic engineering to be used in a
safe and effective way to the benefit of both
individuals in particular and humans in
general.