(f)
Examples of the
different functions of proteins
See also
Cell division, cell cycle, mitosis, meiosis, sexual/asexual reproduction,
binary fission
(a) How was the
structure of DNA discovered?
DNA (deoxyribonucleic acid) was first isolated,
from white blood cells, by the Swiss scientist Friedrich Miescher in
1869.
Before the crucial X-ray photograph of Rosalind
Franklin, chemical analysis showed that four bases seem to occur in
ratios of 1 : 1 for one pair and 1 : 1 for another pair.
What were the roles of the scientists Watson, Crick,
Franklin and Wilkins?
In the 1950s, Rosalind Franklin working for
Maurice Wilkins examined strands of DNA using a technique called X-ray
diffraction analysis.
The sample under investigation, e.g. a DNA
crystal strands, is bombarded with X-rays and the layers of atoms behave
like a diffraction grating and scatter the X-rays in particular pattern that
depends on the 3D arrangement of atoms in the molecule. The path of the
scattered X-rays is recorded on a photographic plate.
Rosalind Franklin died tragically young from
cancer, and never received the Nobel Prize she would have undoubtedly
received, BUT, in one of the last things she wrote in her laboratory
notebook, she speculated that DNA had a helix structure.
Later Frances Crick and James Watson
gathered together this X-ray data (Crick had access to Rosalind Franklin's
'classic' X-ray photograph of crystallised DNA, characteristic of a helical
structure) with other information ...
e.g. the chemical analysis of DNA,
particularly the ratios of the four bases (adenine, cytosine, guanine and
thymine), the shape of the four base molecules ...
and then built a model and deduced what we
recognise today as the double helix structure of DNA - brilliant insight,
more Nobel Prize winners along with Maurice Wilkins.
The important thing is that the experimental
observations from chemical and structural analysis fitted the evidence based
model.
 |
 |
|
Rosalind Franklin, Physicist and
Biologist, tragically dying young from a combination of pneumonia
and advanced cancer. All the other three scientists mentioned above
received the ultimate accolade of a Nobel prize. Rosalind
Franklin would also have received a Nobel prize, if she had
not died so tragically young.. |
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(b)
The structure of nucleotides and DNA - deoxyribonucleic acid
Introduction to DNA and its function
DNA
(deoxyribonucleic acid) is a large molecule essential for life and cell
replication and is another example of a natural polymer.
Its
structure was worked out in the 1950s, notably by the Nobel Prize winners
Crick and Watson, though several other notable scientists made important
contributions.
DNA, a natural polymer is made up of
string of repeating units (the monomer) called nucleotides.
The DNA forms two linked strands
coiled together in the shape of a double helix (more DNA
structure later).
DNA molecules hold all of an organism's genetic
material, that is all the chemical instructions for individual cells and
complex organisms to grow and develop.
All the instructions that are needed for an
organism to grow, develop and reproduce is encoded in the DNA.
The contents of your DNA directly determines what
your inherited characteristics are.
The DNA is organised into long coiled up sections
called chromosomes in the cell nucleus, and within the chromosomal DNA there are shorter
sections called genes.
Each chromosome consists of many short sections of
DNA called genes, one or more of which codes for one
characteristic of an organism e.g. blood group, eye colour or hair
colour.
The genes have the codes for making all the
different proteins, many of which are enzymes and how to make large
important molecules like haemoglobin.
Your total DNA, that is the full contents of
your genes, is called the genome.
Chromosomes normally come in pairs and the
both have the same type of genes in the same order along their
very long length (in the molecular sense!).
Reminder- one chromosome from each parent makes up
the pair, and human diploid cells have 46 chromosomes (23 pairs).
Apart from the nucleus, certain other parts of the
cell can contain DNA e.g. mitochondria (sites of respiration).
Bacteria can contain free rings/strings of DNA
called plasmids.
Plasmids are not part of a bacteria's
chromosome, and do not help the functioning of the cell, but
e.g. they contain genes that help them develop resistance
against antibiotics, and so help in their survival.
DNA encodes genetic instructions for the development and
functioning of living organisms and viruses.
e.g. every protein molecule
needed by a living organism down to individual cell level is
synthesised by other molecules reading the genetic DNA code and combining the right
amino acids in the right order.
This means every different protein has its own
specific number and precise order of amino acids.
After synthesis, the protein molecule (polymer
chain of amino acids) folds into its own specific unique shape to
perform its own unique function e.g. an enzyme to catalyse a
specific biochemical reaction.
A section of DNA that codes for a particular
protein is called a gene and it is the order of the bases
in a gene that determines the order of amino acids in the
protein, hence its structure and function.
The DNA not only codes for all the necessary
proteins, it also determines what type a cell becomes e.g. blood
cell, brain cell, muscle cell, skin cell etc.
Proteins are polymers of amino acids. DNA is a
polymer of nucleotides.
So amino acids and nucleotides are monomers.
Every protein has a specific structure for a
particular function including enzymes, and most be encoded in DNA.
The
structure of nucleotides and the DNA molecule
Most DNA molecules consist of two polymer chains, made
from four different monomers called nucleotides, connected together in the
form of a double helix.
Unlike man-made poly(ethene), from the
monomer ethene etc. DNA is a naturally occurring polymer - long molecular
chains of joined up monomer (single) molecules.
The nucleotide is the small
basic molecular unit - the monomer from which the polymer is formed.
Nucleotides form the building blocks of DNA
(deoxyribonucleic acid) and RNA
(ribonucleic acid). An individual nucleotide consists of three molecular bits
combined together - the same phosphate group, a variable base (adenine, cytosine, guanine
or
thymine), and the same pentose sugar (pentose just means having a
ring of 5 atoms). The phosphate group and base are attached to the sugar
(see left diagram of a single nucleotide).
The DNA (and RNA) polymer chain is formed by a large
number of phosphate-sugar linkages. The base is a sort 'branch' off the main
chain, but this helps it to intermolecular bond with a base of another
opposite strand of DNA.
The
result is full DNA molecule consists of two 'molecular' strands coiled together to form a
double helix,
but how is this helix held together?
The two polymer strands of DNA are cross-linked by a series of
complementary base pairs joined together by weak intermolecular bonds - cross links (base-pairing
bonds shown here as
on the diagram):
There are four bases in DNA holding the
structure together and the same two bases are always paired together -
known as
complementary base pairing.
This is shown on the right diagram, holding the
two strands of DNA together.
Adenine (A) with thymine (T)
AT,
and cytosine (C) with guanine (G)
CG.
Whererepresents
the weak (but crucial) intermolecular attractive bonding force between the pairs of
bases. This weaker intermolecular bond is actually called a hydrogen bond, but you
might not need to know
any more detail at GCSE level.
These cross linking complementary base pair bonds
hold the DNA molecules tightly together giving it the necessary stability
to perform their genetic roles - but not to tightly, that they cannot be
'unzipped' - a necessary process in cell replication!
Here complementary means 'matching pairs'.
A with T and C with G are the linked complimentary base
pairs.
The double helix structure is shown in the diagram
above on
the right, illustrating how the DNA is held together by the
cross-linking
hydrogen bonds between the bases to hold together the double helix together.
A short section of DNA is illustrated in more detail
below.
A more detailed diagram of a very short section of a double-helix DNA
molecule showing the two different base pairings holding the two molecular
strands together.
It is the order of the bases in the DNA strands of
a gene that decides the order of amino acids in a protein.
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(c) Details of protein synthesis
DNA code,
bases, genes and the triplet code
As already mentioned, DNA polymer molecules contain
the genetic codes that determine which proteins are synthesised.
These synthesised proteins control how all the
cells in an organism function, in other words the DNA controls the
production of all proteins - protein synthesis in the ribosomes, one of
the sub-cellular structures in the cytoplasm of cells.
A short section of DNA that codes for a
particular protein is known as a gene.
This means each gene codes for a particular
set of amino acids that form a protein.
It is the order of the bases in the gene
that determines the order of the amino acids in the protein.
Every gene has a different sequence of
bases to code for all the proteins an organism requires.
Only 20 different amino acids are used to
synthesise all the thousands of different proteins.
It is the genes of the DNA that tells the
ribosomes in cells the correct order to assemble the amino acids
to make a specific protein.
A ribosome, a tiny structure in the cytoplasm, is essentially a protein factory
that makes everything from enzymes, keratin, muscle fibre cells, red
blood cells etc. and all based on the DNA codes.
Every protein, a polymer chain of amino acids, has
a unique structure based on a specific number of amino acids
AND a specific sequence of amino acids.
Each protein also has a specific
3D shape, essential for it to carry out its particular function e.g. an
enzyme or type of tissue.
The order of bases in a gene of the DNA determines
the order of amino acids which will combine to form a specific the protein,
which in turn, will perform a specific function in the living organism.
Every amino acid is coded for by a sequence of three
bases in the gene, known as a triplet code (illustrated by
diagram below for three 'fictitious' amino acids).
Every gene contains a different sequence of
bases so it can code for a particular protein.
The order of bases on an organism's DNA is called
the genetic code
of the genome.
The genome is the whole of an organism's genetic material.
See
Introduction to the GENOME and gene expression
- considering chromosomes, alleles, genotype, phenotype, variations
Examples of triplet base codes for amino acids
Example of three triplet codes based on the four
bases: adenine A, thymine T, cytosine C, guanine G along the DNA molecule.
The triplet base codes are called
codons.
Triplet code and amino acid: CCA is for proline, TCG
is for serine and AGA for arginine
The amino acids are joined together to make the
various proteins dependent on the order of the bases in the gene.
The diagram above shows how the triplet codes on DNA
work.
A sequence of three bases (e.g. CCA) on a single strand of DNA codes
for a particular amino acid. A sequence of three triplet codes will code for
three amino acids in that particular sequence on that part of the gene.
Using letters to represent the sequence of bases
on a strand of DNA is an example of a scientific model.
Reminder: The double helix
structure of DNA is another spatial scientific model and this model
must be tried and tested in the laboratory and all observations must
back up any hypothesis to become a workable scientific model.
The cell chemistry allows the reading of the
genetic triplet codes (sequence of bases) on the DNA code to
eventually join these three amino acids together in the precise
order dictated by the DNA code. In fact for any protein you are
actually dealing with sequences of dozens-hundreds of triplet codes for
a particular protein.
In the next section (after notes on non-coding
DNA) we look at how we get from DNA
triplet codes to the actual production of a protein and unfortunately
its a bit more complicated than the above diagram suggests!
Notes on non-coding DNA
There are parts of
the DNA strands that do NOT code for any amino acids, hence do NOT
code for proteins.
However, some of these non-coding sections switch genes on and off, in other words, they control whether or
not a gene is expressed to make a protein.
Therefore some of these non-coding regions
of the DNA are involved in protein synthesis.
Before transcription can occur (involves
reading the DNA code), the RNA
polymerase (enzyme) has to bind to a non-coding section of DNA adjacent to
the specific gene (coding for a specific protein).
If a mutation has occurred in this section of
the DNA it can affect the ability of the RNA polymerase to bind to
it - it might be harder or easier (or no effect).
The quantity and accuracy of how much mRNA is
transcribed depends on how well this binding takes place - and
therefore affects how well the protein is produced.
Therefore the production of the protein may be
affected, and, depending on its function, that specific
phenotype
may also be affected.
This means that genetic variants in non-coding
regions of DNA can affect the phenotypes exhibited by an organism,
despite the fact that these non-coding sections of DNA done code for
proteins themselves.
Example of a mutation in a triplet code
The original triplet code and amino acid sequence was:
CCA for proline, TCG for serine and AGA for arginine.
If just one base has changed 'mutated', e.g. middle
triplet from TCG to TGG, the 2nd amino acid is this sequence
is changed.
The sequence is now: CCA for proline,
TGG for threonine
and AGA for arginine.
original sequence of triplet codes
The mutation causing the protein to have a slightly
different structure can have consequences.
(i) It may not affect the function of the protein
at all.
(ii) It may enhance the function of the protein.
(iii)
BUT, it may adversely affect the function or activity of the
protein e.g. the enzyme might not work as efficiently or maybe not at
all due to a change in shape.
Effectively the gene is changed to a
genetic variant of that gene, known as an allele, and can
result in a different gene expression - a different phenotype.
These genetic changes in the DNA structure can involve
substituting one base for another, deletion of a base or addition of a base.
All of these change the triplet code sequence.
Mutations can also occur in non-coding sections of
the DNA.
See
An introduction to genetic
variation and the formation and consequence of mutations
The
formation of mRNA and the
actual synthesis of proteins in cytoplasmic ribosomes
DNA is found in a cell's nucleus and cannot
move from it through the nucleus membrane because of the large size of its
molecules.
Therefore there must be a means of getting the genetic
information from the nucleus to the tiny structures, called ribosomes in
the cytoplasm, in which the
proteins are synthesised.
This is achieved using a molecule called messenger
ribonucleic acid (mRNA, a type of RNA) i.e. how the cell gets the
code from the nucleus to the ribosomes - the mRNA is a sort of
'messenger'.
mRNA is shorter than DNA and a single strand
molecule, but still another polymer of nucleotides, but small enough to
exit through the membrane of the nucleus.
The mRNA is the code used in the ribosomes to
connect the amino acids together in the right order to assemble the
protein molecule.
Note that there is an important difference between DNA and
RNA.
In RNA the base thymine (T) is replaced by the base
uracil (U),
so the base pairings in RNA are C-G (as in DNA) but A-U in
RNA
(not A-T as in DNA).
As illustrated above, the DNA contains the gene's
triple coding system for the amino acids to needed to be combined to form a
specific protein - with specific molecular properties to perform a
particular chemical function in an organism.
The process of
TRANSCRIPTION - transferring the genetic code
The
mRNA is made by copying the DNA base sequence of a gene - the process of
transcription.
In the nucleus, using enzymes, the two strands of the
DNA double helix unzip and become a template for the production of
mRNA (messenger ribonucleic acid).
The enzyme RNA polymerase binds to the
non-coding DNA in front of a gene sequence of bases.
The two DNA strands of the double helix unzip and the RNA
polymerase moves along one of the strands of the DNA (see diagram on right).
Therefore the RNA polymerase uses the DNA coding of a gene as a
template to make the mRNA.
Note: In the mRNA molecule, the base uracil
(U) replaces the base thymine (T) in pairing up with adenine (A).
By pairing up the complementary bases on the DNA and
RNA, the correct sequential nucleotides in the nucleus are brought together to form a
complementary strand of mRNA, a step in the overall process called
transcription taking place in the nucleus.
This means the mRNA is complimentary to the gene.
The smaller mRNA molecule can now
migrate out of
the cell nucleus into the cytoplasm and attach themselves to a ribosome
(the actual protein 'factory'!).
The process of TRANSLATION - building the amino acid chain of the protein
In the cytoplasmic ribosomes, the mRNA now
itself acts as a template of triplet codes for amino acids to be joined
together in the correct sequence for a specific protein.
In order for this to happen, the amino acids in the cytoplasm are drawn into the
ribosome complex and assembled in order to match the complementary
triplet codes.
The correct amino acids are brought to the ribosomes
by a carrier molecule called transfer ribonucleic acid (tRNA).
The amino acids are then joined together, by enzymes,
in the correct order to make a particular protein in the ribosome.
The order of the amino acids connected together in
the ribosome will match the order of the base triplets (called
codons) on the mRNA molecule.
The complimentary triplet base sequence on the
tRNA structure is called the anticodon.
This production of the protein, dictated by the
complementary triplet codes on the mRNA, is called the
translation
stage, and this takes place in the cytoplasm.
So, the RNA and appropriate enzymes in the
ribosome, join the amino acids together to form the protein - a
polypeptide - meaning a polymer formed from the amino acid monomer
units.
Immediately after its synthesis, the protein adopts its own unique 3D
structure - its specific shape.
Translation
The above diagram shows translation in more detail,
including the role of another type of RNA - transfer ribonucleic acid (tRNA)
which brings the amino acids together onto the mRNA.
- Points to consider when studying the
translation diagram above
- The joining together of the amino acids on the mRNA is done using
transfer ribonucleic acid (tRNA).
- These relatively short molecules of tRNA
actually bring the amino acids together to match the mRNA triplet codes.
- In
other words the triplet codes of tRNA and mRNA are also complementary.
- Note that In RNA (mRNA or tRNA) the base thymine (T) has been
replaced by the base uracil (U), so complimentary base pairing is now U-A (not
A-T), but C-G retained and its still all about matching complimentary
base pairs.
- The sequence of events is as follows:
- The attachment of the mRNA to the ribosome
- The mRNA has exited from the nucleus and
docks into a ribosome
- The coding
by triplets of bases (codons) in the mRNA for specific amino acids
- The triplet base codes for particular
amino acids and their joining up sequence can now be read from the mRNA
molecules.
- The
transfer of amino acids to the ribosome by tRNA (transfer ribonucleic acid)
- After the mRNA joins onto a ribosome,
molecules of transfer RNA (tRNA) bring the amino acid that matches the
code on the mRNA, the complimentary base codes of the mRNA and tRNA
ensure that all proteins are synthesised with their specific protein
sequence, so all proteins are completely reproducible.
- The tRNA is then 'empty' and free to collect
another set of amino acids for the ribosome to join up.
- The linking of amino acids to
form polypeptides
- The ribosome then acts as the catalytic
site for linking the amino acids together to synthesise a specific
protein.
- This second process is called translation
because the triplet base code sequence is read and
translated into the amino acid sequence of a protein.
- A sequence of amino acids joined
together in a chain is called a polypeptide, a natural polymer or
macromolecule.
- All of these reaction are catalysed by
enzymes.
More
on variants in non-coding DNA
A mutation changes the base sequence in a DNA
molecule in a gene.
This produces a genetic variant that can lead to
changes in an organisms phenotype - gene expression characteristics.
Variants in non-coding sections of the DNA
molecule can also affect the phenotype of an organism, despite the fact
that the non-coding DNA does not code for proteins.
This can happen because before transcription can
take place, RNA polymerase needs to bind to a section of non-coding DNA
in front of a gene sequence of bases.
If a mutation occurs in the region of DNA, then it
can affect the ability of RNA polymerase to bind to it - it might have
no effect, promote binding or inhibit binding - there are always several
possibilities in these sorts of situations - including driving
evolution!
Depending on how well RNA polymerase can bind to
this non-coding section of DNA will affect how much mRNA is transcribed
in the transcription process - therefore how much of the protein is
synthesised.
Therefore the structure and function of the
protein is changed and the final phenotype of the organism can be
affected.
See also
An introduction to genetic
variation and the formation and consequence of mutations
SUMMARY of
protein synthesis
So, to summarise, you start with DNA in the nucleus,
then to complementary mRNA in the nucleus (transcription stage), mRNA moves
into the cytoplasm and then the amino acids are joined together in the
ribosomes via the complementary triplet codes (translation stage).
The
diagram 'sketch' below also 'attempts' to summarise what is actually a very
complicated process!
The diagram below puts protein synthesis in
perspective of starting with the genome of a cell's nucleus.
The genome is the whole of an organism's genetic material.
You are now ready for the following sections, in
order:
Introduction to the GENOME, gene expression -
considering chromosomes, alleles, genotype, phenotype, variations
An introduction to genetic
variation and the formation and consequence of mutations
gcse biology revision notes
Introduction to the inheritance of characteristics and
genetic diagrams (including Punnett squares)
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(d) How to
extract DNA from plant cells - a simple experiment
This section is illustrated by the extraction of DNA
from strawberries.
Humans have 23 pairs of chromosomes (46 in total).
The modern common strawberry has 8 pairs of each of the 7 chromosomes (56 in
total) and is a rich 'school/college experiment' source of DNA.
1. You need an appropriate source
e.g. split peas, strawberries, kiwi fruit, onions or bananas etc.
However, it is reported that the 'white
strands' of DNA from fruit, may actually contain pectin too!
Any green leaves or stalk should be removed
from the strawberry.
You can use a plastic bag or beaker in the
first steps, you then need a test tube and filter funnel and filter
paper.
2. The starting plant material is well
mashed, but avoid creating air bubbles in the mash.
Squishing!
You can crush the strawberries in a plastic
bag for a few minutes.
3. The mash is scraped into a beaker
containing a solution of detergent and salt (the DNA extraction
liquid).
You can add this mixture to the plastic bag or
beaker of crushed strawberry.
With a plastic bag, its easier to do further
effective crushing if you add the detergent/salt solution to the
bag.
The detergent further helps to break down the cell
membranes to release the DNA from the cell nuclei.
The salt makes the DNA strands stick together.
4. The resulting mixture is filtered
into a test tube/2nd clean beaker. I used a hand held coffee filter paper!
Filtering!
In school/college you can use a normal filter funnel and filter
paper, as in your chemistry lessons!
This removes the froth and the bigger
insoluble bits of the plant cells.
Kitchen style!
The filtrate was transferred from the 2nd
clean beaker into a test tube (if not already in a test tube).
5. Some ice-cooled alcohol is
carefully added to the filtered mixture down the side of the test tube.
Adding alcohol, mixture goes cloudy
6. A band of white precipitated DNA
strands should form - DNA is not soluble in cold alcohol.
White coagulated mass of DNA
7. You can then extract the DNA from
this band with a glass rod.
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(e)
A
summary of DNA replication
A more detailed diagram of the base sequence
replication using the DNA template.
1. The DNA double helix molecules splits
in two, and the two strands then act as templates.
2. Freely moving nucleotides can be
matched up to form the weak bonds between the complimentary base pairs.
3. Two identical strands of DNA
produced, both identical in their original sequence of bases.
Reminders:
The full DNA molecule consists of two 'molecular' strands coiled together to form a double helix.
The two polymer strands of DNA are cross-linked by a series of
complementary base pairs joined together by weak bonds - cross links
There are four bases in DNA holding the
structure together and the same two bases are always paired together -
known as
complementary base pairing.
Complementary means 'matching pairs'.
A with T and C with G links.
i.e. adenine (A) with thymine (T) AT,
and cytosine (C) with guanine (G) CG.
These cross linking complementary base pair bonds
hold the DNA strands tightly together giving it the necessary stability
to perform their genetic roles.
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(f) Examples of the
different functions of proteins
As already mentioned, protein molecules adopt a
very specific folded 3D shape in order to be able to carry out their
specific function - so what are these functions?
Every protein has its own unique shape to
for its specific structure and function in an organism.
These proteins may end up in muscle cells,
brain cells, enzyme catalysts, haemoglobin molecules etc.
BUT, note that proteins, particularly enzymes,
are involved in building up non-protein molecules e.g. fats, cell
walls, glycogen etc.
(a) Enzymes are biological catalysts that
control most chemical reactions in living organisms.
Reminder:
Enzymes have active sites of a specific shape that
connect with substrate molecules, and this allows them to catalyse a
specific chemical reaction in the biochemistry of living organisms.
Enzymes are physiologically active proteins.
The 'key and lock' mechanism of how an enzyme
works - the correct 3D structure is crucial and that depends on the
sequence and interconnection of the amino acids.
The crucial 3D protein structure of an enzyme,
and denaturing effect e.g. from a high temperature or wrong pH.
(b) Many tissues are built of proteins e.g.
collagen a strong structural protein (triple helix of polypeptides )that strengthens connective tissue
like ligaments and cartilage of muscle systems of the joints.
Elongated muscle cells made from the protein
actin - forming the filaments in muscles.
The strong fibres of our hair are made from
the fibrous protein keratin.
These particular tissues are partially built
from structural proteins.
(c) Carrier molecules like haemoglobin
(conveys oxygen to cells) are also protein molecules.
(d) Some antibodies are protein molecules.
The shape of the protein must match the
antigen of a pathogen e.g. of a virus.
These particular antibodies are
physiologically active proteins.
See notes on
Keeping healthy - defence against
pathogens and infections
(e) Many hormones are protein molecules
e.g. insulin, the hormone released into the blood from the pancreas,
controls blood sugar levels.
The shape of the insulin molecule must match
the shape of its receptor molecule.
These particular hormones are physiologically
active proteins.
(Note there are many hormones which are NOT
proteins e.g. some hormones involved in the menstrual cycle.)
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Sub-index of Genetics Notes - from DNA to GM and lots
in between!
DNA and RNA structure and Protein Synthesis and an
experiment to extract DNA gcse
biology revision notes
An introduction to genetic
variation and the formation and consequence of mutations
gcse biology revision notes
Introduction to the inheritance of characteristics and
genetic diagrams (including Punnett squares) including technical terms, Mendel's work and inherited
genetic disorder, genetic testing gcse biology revision notes
The human GENOME project - gene expression, chromosomes, alleles, genotype, phenotype, variations,
uses of genetic testing including 'pros and cons' gcse biology revision notes
Inherited characteristics and human sexual
reproduction, genetic fingerprinting and its uses gcse biology
Genetic
engineering: uses - making insulin, medical applications, GM crops & food
security gcse biology
More complicated genetics: Sex-linked genetic
disorders, inheritance of blood groups gcse biology revision
See also section on
Cloning -
tissue culture of plants and animals gcse biology revision notes page
Enter specific biology words or courses e.g. topic, module, exam board,
structure, concept, animal or plant feature, 'phrase', homework question! anything
of biological interest!