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:

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

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Drosophila was caused by a change in one protein. He concluded that genes must influence heredity chemically.

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.

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Blueprint of Life – HSC BIOLOGY

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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

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Blueprint of Life – HSC BIOLOGY

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.

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 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.

Messenger RNA (mRNA) Ribonucleic acid which carries information from DNA in the nucleus to ribosomes in the cytoplasm.

Transfer RNA (tRNA) 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.

Ribosomes 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.

Enzymes Acts as a catalyst in all chemical processes.

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Stages in polypeptide production1 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

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Transcription Factors all bind onto the

Blueprint of Life – HSC BIOLOGY

nucleus into the cytoplasm. Nucleotides from the introns are reused for further use.

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.

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Complementary mRNA is a replica of the DNA code

Blueprint of Life – HSC BIOLOGY

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.

b) Develop a simple model for protein synthesis.

You may make your model out of any materials you like.

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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

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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.

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.

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Blueprint of Life – HSC BIOLOGY

Bibliography

Websites

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>.

http://biology.about.com/od/cellularprocesses/ss/protein-synthesis-translation.htm Regina Bailey 2011

Videos

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

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/

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