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Blueprint of Life – HSC BIOLOGY
1a) Outline the experimental evidence that led to Beadle and Tatum’s ‘one gene-one protein’
hypothesis
George Wells Beadle was an American scientist and Edward Lawrie Tatum was an American
geneticist. They both proposed the ‘one gene-one protein’ hypothesis which states:
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All biochemical processes in all living organisms are under genetic control
All biochemical reactions in an organism are resolvable into separate steps
Each step is under the control of a single gene.
Mutation of a single gene results in the loss of function of the appropriate enzyme
But of course, this hypothesis was based on other ideas and theories to consolidate it.
Here are a list of experiments that led to Beadle and Tatum’s ‘one gene-one protein’ hypothesis.
The Double Helix
In the 1940's, members of the
scientific community were aware
that DNA was most likely the
molecule of life, even though many
were sceptical since it was so
"simple”. They also knew that DNA
included different amounts of the
four bases adenine, thymine,
guanine and cytosine (usually
abbreviated A, T, G and C where A
would only pair up with T and G
would only pair up with C), but
nobody had the slightest idea of
what the molecule might look like.
Beadle and
Morgan
In 1931, George Beadle was
studying the inheritance in the fruit
fly (Drosophila) with Thomas
Morgan. The results of the
experiment indicated that the eye
colour of the fly is the result of a
long series of chemical reactions
and that also genes affected the
results. Beadle found that mutant
eye colour in Drosophila was caused
by a change in one protein. He
concluded that genes must
influence heredity chemically.
Michelle Qiu 2011
Blueprint of Life – HSC BIOLOGY
Beadle and Tatum
In the 1940’s, Beadle and Tatum discovered a connection of genes to proteins. They
published their results of their experiments with bread mould which provided
evidence of a link between genes and proteins.
Neurospora crassa was the experimental organism. It was fungus that was grown,
containing only the bare minimum of nutrients necessary. They used x-rays to
produce millions of mutated strains of the mould. The strains couldn’t produce the
essential nutrients such as amino acids or vitamins due to the absence of an
enzyme. After inducing mutations in the mould using radiation, some of the
progeny were unable to grow. By growing them with different combinations of
nutrients, Beadle and Tatum were able to establish which enzyme was lacking in
each mutant strain. Each genetic mutation was at a specific site on the mould’s
chromosomes. By breeding the lab specimens with wild specimens, it was
concluded that the mutation was transmitted in a simple Mendelian inheritance. It
was assumed that the ability to synthesize the appropriate amino acid was caused
by the loss of a single enzyme. The work was supported by similar evidence found in
humans, plants, and the fruit fly, Drosophila.
In conclusion, different sites were related with each enzyme which led to the ‘one
gene- one enzyme’ hypothesis.
Michelle Qiu 2011
Blueprint of Life – HSC BIOLOGY
Michelle Qiu 2011
Blueprint of Life – HSC BIOLOGY
b) Why was this later altered to the ‘one gene-one polypeptide’ hypothesis?
Beadle and Tatum’s ‘one gene-one protein’ hypothesis was later altered to the ‘one gene-one
polypeptide’ hypothesis due to the fact that many proteins are not enzymes. Some proteins require
more than one polypeptide chain. The modification was due to a research conducted in 1949 on
sickle cell anaemia. Sickle cell disease was caused by a single gene mutation, which was
heterozygous in sickle cell trait individuals and homozygous in individuals with full sickle cell
anaemia. It was noted that the haemoglobin from normal individuals and that from sickle cell
anaemic individuals migrated differently. There was a physical difference in the haemoglobin types
and supporting the single gene mutation. A normal haemoglobin molecule is made of four different
polypeptide chains (two identical alpha chains and two identical beta chains). When the section
which had the differences was isolated and analysed, the only difference was in one amino
acid. From this, it can be seen that haemoglobin is composed of two independent gene products,
each of which is a separate polypeptide is a single chain of many amino acids linked according to
type, number and sequence of amino acids. The gene is a section of DNA that determines the
amino acid sequence of a polypeptide. One gene codes for one polypeptide and several
polypeptides may be required for a functional protein or enzyme.
c) Discuss how Beadle and Tatum’s work showed that biochemical processes that occur in cells are
related to macroscopic changes in the organism.
Beadle and Tatum’s work showed that biochemical processes that occur in cells are related to
macroscopic changes in the organism with their bread mould experiment. It suggested that genes
direct the synthesis of enzymes that control metabolic processes. It showed that a specific gene
coded for a specific enzyme. Enzymes are protein molecules that act as a biological catalyst. They
speed up rate of reactions and they only catalyse one type of reaction. E.g. Keratin refers to a
family of fibrous structural proteins and is the key structural material in making the outer layer of
human skin, hair and nails. Keratin is also present in reptiles, birds, and amphibians and it
determines how hard the shells/beaks/horns are or how soft wool/feathers of the organisms should
be. Another example is collagen which is a group of naturally occurring proteins found in animals. It
determines the form of tissues such as the tendon, ligament and skin, cartilage, bone, and blood
vessels.
Therefore, the biochemical processes such as functions of proteins and nucleic acids, that occur in
cells are related to macroscopic changes in the organism, which is shown through the phenotypes
of hair colour, strength of cartilage, elongated fibrous tissues, etc.
Michelle Qiu 2011
Blueprint of Life – HSC BIOLOGY
2a) List the sequence of events that occur in protein synthesis
Our body has to manufacture over 100000 different proteins. Our brain sends a message to the
body to produce more protein when needed through the process of transcription and translation.
These proteins are needed to keep track of the information about our eyes, hair colour, skin colour,
size of our hands etc. This is why protein synthesis is an essential function for the survival of the
human body.
Production of protein involves:
DNA
Messenger RNA (mRNA)
Transfer RNA (tRNA)
Ribosomes
Enzymes
A nucleic acid that contains the genetic
instructions used in the development of all living
organisms. The gene on the DNA strand
provides information required to make
polypeptides.
Ribonucleic acid which carries information from
DNA in the nucleus to ribosomes in the
cytoplasm.
Brings amino acids to the ribosome to be linked
to build a polypeptide chain. It contains a triplet
of bases (codon) which corresponds to the
codon on the mRNA.
Acts as a site for polypeptide synthesis in the
cytoplasm. Contains 3 binding sites which hold
the mRNA strand and 2 tRNA molecules
together, linking amino acids to make the
polypeptide chain.
Acts as a catalyst in all chemical processes.
Michelle Qiu 2011
Blueprint of Life – HSC BIOLOGY
Stages in polypeptide production
1 Transcription
Transcription is the process of creating a complementary RNA copy of a sequence of DNA (Both
nucleotides) in order to transfer information. This is the initial step into producing a polypeptide
chain. A template of the DNA’s information is copied which is then used in translation.
First, the several complexes known as transcription factors (TBP, TFIIA, and TRIIB) all align and
attaches to the DNA. The enzyme RNA polymerase then bounds onto the DNA grabbing the
genetic information while ATP is then added.
The double DNA strand in the nucleus then unwinds in the area of the gene containing the
information about the protein to be made. RNA polymerase then moves along the strand, linking
complementary RNA nucleotides to form an mRNA strand. A nucleotide triplet is called a codon.
There is a ‘start’ codon and a ‘stop’ codon which controls the length of the mRNA strand.
When the RNA copy is complete, the mRNA strand is modified so that it consists only of the base
sequence that will code for the protein. This is because most genes contain non-coding regions
known as introns. The regions coding for protein are exons. While still in the nucleus, the introns are
cut out (spliced) from the strand and the exons combine. The modified mRNA then moves from the
nucleus into the cytoplasm. Nucleotides from the introns are reused for further use.
Transcription
Factors all bind
onto the DNA
strand.
Complementary mRNA
is a replica of the DNA
code except Uracil
replaces Thymine.
Michelle Qiu 2011
Blueprint of Life – HSC BIOLOGY
2 Translation
Translation is the synthesis of a protein from a mRNA template. Translation is broken up into 3
stages starting with initiation, then elongation, and finally, termination. The mRNA which travels
out of the nucleus binds to a ribosome at the end with the start codon AUG. A tRNA which carries
the amino acid methionine at one end and the anticodon UAC at the other end binds to the start
codon on the mRNA within the ribosome. When it's time for the next amino acid to be positioned in
the growing protein, a new codon on the mRNA molecule is exposed, and the complementary
three-base anticodon of a tRNA molecule positions itself opposite the codon. This brings another
amino acid into position, and that amino acid links to the previous amino acids.
When the first tRNA is released from the ribosome, the ribosome moves further down the mRNA
molecule and exposes another codon. This will attract another tRNA molecule with its anticodon.
Two tRNAs are temporarily bound within the ribosome and their amino acids link. A polypeptide
chain then forms which is known as elongation. Termination is achieved when a stop codon is
reached. The release factor enters the A site and the polypeptide chain is released into the
cytoplasm.
A polypeptide chain is only the primary structure of a protein. Each protein has a particular shape
formed by the twisting and folding of the polypeptide chain. If DNA sequences are changed by
mutation the protein production will change.
Michelle Qiu 2011
Blueprint of Life – HSC BIOLOGY
b) Develop a simple model for protein synthesis.
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You may make your model out of any materials you like.
c) Describe your method and justify how your model demonstrates protein synthesis.
First, I gathered all the theory of protein synthesis to consolidate my understanding through various
websites. I found YouTube extremely helpful since it visually gives me an idea of the transcription
and translation processes. I also found the textbook helpful since it is straight forward and
approaches the syllabus directly. I had a little idea of what I needed to buy so I went shopping at
Riot (craft shop) in Parramatta and bought a packet of coloured wooden sticks, pipe cleaners,
terracotta, pom-pom balls and cardboard. I was going to use the sticks and pipe cleaners for the
DNA model, terracotta for the ribosome and pom-pom balls for the polypeptide chain. First I tried
to glue the sticks together but they were too weak and I knew that if I twisted it for the 3D effect,
they would just fall off. I thought of beads next but then string would be too loose. Mum then
thought of wire so I bought a reel of wire at Lincraft and beads. I represented the base ‘A’ as Pink, ‘T’
as white, ‘G’ as blue and ‘C’ as red. After I made the DNA model, the DNA when it’s split and the
mRNA, I started to make the ribosome. I moulded the terracotta to be a bit round on the outer
surface and I made them into sub units to see the process inside. I also made the release factor and
the RNA polymerase out of terracotta because I wanted them to be different to the pom-poms.
After that I made the tRNA and the polypeptide chain which were pretty simple. I scrunched up pipe
cleaners for tRNA and I sewed on the pom-pom balls for the polypeptide chain.
I chose beads on wire and pipe cleaners because they were malleable which was necessary for
twisting the model. They also came in different colours which I could use to represent my bases. I
chose pom-pom balls because they were round and were coloured differently to show that the
amino acids were unlike. I chose terracotta because I couldn’t find anything that was shaped as a
ribosome.
My model demonstrates protein synthesis as it shows the transcription and translation process
through a 3D structure. It starts with DNA in the nucleus which splits due to RNA polymerase which
copies the information and replicated it into mRNA. This is small enough to migrate out of the
nucleus. I show my mRNA travelling across the nucleus. Then I show the translation process in the
ribosome where tRNA occupies sites and produces a polypeptide chain. It is colour co-ordinated to
show more accuracy in the process. It visually represents protein synthesis so it is more
unambiguous as to words on a page.
Michelle Qiu 2011
Blueprint of Life – HSC BIOLOGY
d) Evaluate your model, making a judgement based on its strengths and weaknesses
My model represents my understanding of protein synthesis. I don’t think I have missed anything in
particular. The problem I encountered first was the stability of the wooden sticks. I overcame that
problem by searching for alternatives. Another problem I had was how I was meant to stick the
ribosome onto the cell. I’m leaving it detached because I don’t think it is necessary for it to be
attached as I will be moving the model around. Moulding the ribosome was a little troublesome as I
kept creating gaps or slits that made it look unattractive. I tried to minimise it by kneading it as
much as possible. Overall my model was not a disappointment but if I started earlier or if I had more
time, maybe I could have thought of better idea to represent protein synthesis. I could have made it
more appealing if I had more time. My main problem was time but it didn’t affect me too much. It
was surprisingly quite fun making the model and enhanced my knowledge of what protein synthesis
involves.
Michelle Qiu 2011
Blueprint of Life – HSC BIOLOGY
Bibliography
Websites
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http://www.accessexcellence.org/RC/AB/BC/One_Gene_One_Enzyme.php
http://en.wikipedia.org/wiki/One_gene-one_enzyme_hypothesis
http://hsc.csu.edu.au/biology/core/blueprint/9_3_2/BIO934NET.htm#net9
http://www.mun.ca/biology/scarr/Beadle_%26_Tatum_experiment.htm
http://www2.estrellamountain.edu/faculty/farabee/biobk/BioBookPROTSYn.html
http://en.wikipedia.org/wiki/George_Wells_Beadle
http://www.mun.ca/biology/scarr/Beadle_%26_Tatum_experiment.htm
http://www.genomenewsnetwork.org/resources/timeline/1941_Beadle_Tatum.php
http://www.bookrags.com/research/one-gene-one-enzyme-wog/
http://en.wikipedia.org/wiki/Edward_Lawrie_Tatum
http://en.wikipedia.org/wiki/Keratin
http://en.wikipedia.org/wiki/Collagen
http://en.wikipedia.org/wiki/George_Wells_Beadle
CliffsNotes.com. Protein Synthesis. 1 May 2011
<http://www.cliffsnotes.com/study_guide/topicArticleId-8741,articleId-8621.html>.
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http://biology.about.com/od/cellularprocesses/ss/protein-synthesis-translation.htm
Regina Bailey 2011
Videos
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http://www.youtube.com/watch?v=Jqx4Y0OjWW4&feature=related
http://www.youtube.com/watch?v=ztPkv7wc3yU&feature=related
http://www.youtube.com/watch?v=WsofH466lqk
http://www.youtube.com/watch?v=5bLEDd-PSTQ&feature=related
http://www.youtube.com/watch?v=-zb6r1MMTkc&feature=related
Books
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Diane Alford and Jennifer Hill (2010). Excel HSC Biology. Printed in Singapore by Green Giant Press:
Vivienne Petris Joannou.
Judith Brotherton and Kate Mudie (2010). Biology . 3rd ed. Australia: Pearson Australia. p88-91.
KISS booklet (Copyright 2005-2006) and class notes
http://www.neilstoolbox.com/bibliography-creator/
Michelle Qiu 2011