4. The structure of DNA

Syllabus reference (October 2002 version)
4. The structure of DNA can be changed and such changes may be reflected in the phenotype of the affected organism	Students learn to: describe the process of DNA replication and explain its significance    outline, using a simple model, the process by which DNA controls the production of polypeptides    explain the relationship between proteins and polypeptides    explain how mutations in DNA may lead to the generation of new alleles    discuss evidence for the mutagenic nature of radiation    explain how an understanding of the source of variation in organisms has provided support for DarwinÕs theory of evolution by natural selection    describe the concept of punctuated equilibrium in evolution and how it differs from the gradual process proposed by Darwin	Students: perform a first-hand investigation or process information from secondary sources to develop a simple model for polypeptide synthesis    analyse information from secondary sources to outline the evidence that led to Beadle and TatumÕs Ôone gene – one proteinÕ hypothesis and to explain why this was altered to the Ôone gene – one polypeptideÕ hypothesis    process information to construct a flow chart that shows that changes in DNA sequences can result in changes in cell activity    process and analyse information from secondary sources to explain a modern example of ÔnaturalÕ selection    process information from secondary sources to describe and analyse the relative importance of the work of: James Watson Francis Crick Rosalind Franklin Maurice Wilkins in determining the structure of DNA and the impact of collaboration and communication in scientific research

Extract from Biology Stage 6 Syllabus (Amended October 2002). © Board of Studies, NSW. 
[Edit: 10 Jun 10]

describe the process of DNA replication and explain its significance

The process

• Recall from Blueprint of Life, Subsection 3, that DNA is a double-stranded molecule twisted into a helix with each strand comprised of a sugar-phosphate backbone and attached bases - adenine (A), thymine (T), cytosine (C) and guanine (G) – connected to a complementary strand by pairing the bases, A-T and G-C.
• A unit made up of a deoxyribose sugar, a phosphate molecule and one of four nitrogen bases (adenine, thymine, guanine and cytosine) is called a nucleotide. Thus, DNA consists of nucleotide units. The nitrogen bases that are part of each nucleotide are the rungs of the double helix of DNA. They only combine in one way, adenine binds to thymine and guanine binds to cytosine.
• Along the DNA molecule, there are long sequences of genetic code made up of these bases. The replication of these long sequences accounts for the replication of the genetic code of an organism.
• The process of DNA replication consists of the following steps.

Step 1:	The DNA double helix is unwound by an enzyme.
Step 2:	The DNA unzips forming two single strands
Step 3:	Nucleotides are added to the single strands resulting in two identical strands of DNA

The two double stranded molecules are the chromatids.

The significance

• The significance of this process is the genetic information is passed on from generation to generation. During sexual reproduction, the genetic code is copied and then half of the genetic information passes into each of the sex cells (ovum or sperm). When fertilisation occurs the new organism has half the genetic material from each parent.
• The DNA in a cell contains the genetic information to make an entire organism. When a cell divides it takes with it an exact copy of the genetic code of that organism.

To see an animation of this process go to the site below.

DNA Workshop  PBS Online, the site of the Corporation for Public Broadcasting, USA.

 

outline, using a simple model, the process by which DNA controls the production of polypeptides

• Polypeptide synthesis involves a type of nucleic acid, called RNA (ribonucleic acid). RNA is the intermediary between DNA and polypeptide synthesis. It is a single strand of nucleotide bases. It has ribose sugar and the nitrogen base, thymine, is replaced by uracil which bonds with adenine.

There are two types of RNA that are involved in polypeptide synthesis, messenger RNA (mRNA) and transfer RNA (tRNA).

In the nucleus, the double stranded DNA molecules unzip and the DNA code is transcribed into the single stranded mRNA molecule. The mRNA moves out of the nucleus into the cytoplasm and attaches to a ribosome. In the cytoplasm, the mRNA is translated into amino acids.

At the ribosome, the messenger RNA lines up forming a template. A group of three bases, called a codon, codes for a specific amino acid. There are codes that start and stop the chain formation. AUG is the starting point for translation.

tRNA has an anticodon (a non-amino acid forming codon) on one end and an amino acid on the other. A polypeptide is formed as each amino acid is added from tRNA to a chain following the sequence on the mRNA.

 

explain the relationship between proteins and polypeptides

A protein is made up of one or more polypeptides. A polypeptide is made up of a chain of many amino acids.

 

perform a first-hand investigation or process information from secondary sources to develop a simple model for polypeptide synthesis

• You could choose to perform a first hand investigation using beads and plasticine to model polypeptide synthesis. Include in your model the transfer of information from DNA to messenger RNA (mRNA). You will also need to make a model of transfer RNA and of several amino acids. Alternatively, you could process information from an animation, such as those indicated below. When doing this activity, evaluate the relevance of first-hand and secondary information in relation to the understanding of the process of polypeptide synthesis.
• You could test your understanding of protein synthesis by attempting to explain it to a friend in your class. 
  
  Protein synthesis  Access Excellence, USA

 

process information to construct a flow chart that shows that changes in DNA sequences can result in changes in cell activity

• If there is a simple substitution for a single base pair on a strand of DNA such as a G-C replaced by A-T, then this will result in a different amino acid codon forming a different polypeptide. If one base pair is lost from the sequence there will be a shift along the DNA molecule producing different polypeptides.The flow chart below shows the reaction if thymine is lost from the start of a DNA sequence. 
  
   
• Cell activity is controlled by enzymes. Enzymes are formed from chains of polypeptides. If the chain of amino acids forming the polypeptide is not in the right sequence, then the enzyme formed will not be functional. In this case, there is a premature stop.

For another example, go to the following webpage.

What does DNA code for?    genome.org, Wellcome Trust Sanger Institute.

 

analyse information from secondary sources to outline the evidence that led to Beadle and TatumÕs Ôone gene – one proteinÕ hypothesis and to explain why this was altered to the Ôone gene – one polypeptideÕ hypothesis

• Analyse secondary information to identify trends and patterns as well as contradictions in data and consider if Beadle and Tatum would have got the same result if they worked on more complex enzymes.

The following web sites are a good source of secondary information.

The One Gene / One Enzyme Hypothesis  Access Excellence. 
Experimental methods of Beadle and Tatum  University of Newfoundland.

 

Sample

Beadle and Tatum used bread mould to investigate nutritional mutations. Using X-rays, they produced mould that was unable to produce a specific amino acid.  The mould was unable to grow unless the amino acid was added. They showed that genes controlled biochemical processes. Their hypothesis was that for each gene there was one enzyme or protein. The enzymes that they studied consisted of one polypeptide but many enzymes consist of chains of polypeptides. Therefore, the hypothesis has been changed to the Òone – gene one – polypeptideÓ hypothesis.

 

explain how mutations in DNA may lead to the generation of new alleles

• Any change in the base sequence in DNA results in changes to the polypeptides that are produced and is a source of new alleles.
• To produce changes in alleles, the mutation must occur in the sex cells of the organism which are then passed on to the next generation.

 

discuss evidence for the mutagenic nature of radiation

• There is much evidence for the mutagenic nature of radiation. Environmental factors that may increase the rate of mutation include X-rays, radiation from atomic bombs and ultraviolet light.

A mutagen is a natural or human-made agent (physical or chemical) which can alter the structure or sequence of DNA. Mutagens can be carcinogens (cancer causing) or teratogens (birth defects causing).

Radiation was the first mutagenic agent known. Its effects on genes were first noticed in the 1920's.When X-rays were first discovered, they were thought to be harmless and were a great novelty. You could even buy an ÒX-ray machineÓ for your home for entertainment. Most of the first generation of scientists who worked with radiation died of cancer. Famous examples are Marie Curie and her daughter who both died of leukaemia.

Herman Muller received the Nobel Prize in 1946 for showing that genes had the ability to mutate when exposed to X-rays. Beadle and Tatum used X-rays to produce mutations in bread mould in the formulation of their Òone – gene one – polypeptideÓ hypothesis.

The atomic bombs dropped on Hiroshima and Nagasaki also increased the evidence for mutations caused by radiation. There was a tenfold increase in cancer deaths directly after the bombs were dropped.

Mutagens may cause death in the individual but unless they affect the sex cells the effect is not passed on to the next generation.

Sites with more information: 
Mutation, Mutagens, and DNA Repair  Kansas State University, USA 

Mutations  Professor John Blamire, Science at a Distance, Brooklyn College, City University of New York, USA

How does radiation effect humans?  ACHRE Report, Department of Energy, USA

process and analyse information from secondary sources to explain  a modern example of ÔnaturalÕ selection

• Process information to best illustrate trends and patterns. Modern examples include antibiotic resistance of bacteria and the insecticide resistance of insects. Look for similarities in the way that bacteria have become antibiotic resistant and insects have become insecticide resistant. 
• Analyse the information looking for a cause and effect relationship between the organism and the selecting pressure, such as antibiotic or insecticide.

Here is a starting point

Antibiotic resistance  World Health Organisation

And here is an example of a way science may be able to slow or stop the evolution of mosquitoes resistance to insecticides.
Evolution-proof insecticides may stall malaria forever  Andrew Read The Pennsylvania State University, Center for Infectious Disease Dynamics, April 2009

Sample explanation

Some organisms, such as bacteria and insects, produce large numbers of offspring.

Amongst large numbers of bacteria offspring, some individuals may carry genes that give them resistance to antibiotics. These individuals are then able to survive and reproduce with reduced competition from other members of the same species. Each generation will produce a higher percentage of individuals containing the resistant genes. This has been the story for antibiotics since they were first used. The initial use of an antibiotic results in good protection from bacteria. Over time the chemicals become less and less effective. A case study provides a good example of how natural selection occurs. A similar situation occurs in the resistance of insects to insecticides.

Selecting those individuals that are able to survive and reproduce increases the frequencies of those genes in the population. This is Òsurvival of the fittestÓ where the fittest are those that have a natural resistance to a selecting factor, which in the case of bacteria described above, is antibiotics.

(You are required to process and analyse one example, not just have a general knowledge of these principles. You should be able to quote at least two different sources of information)

 

explain how an understanding of the source of variation in organisms has provided support for DarwinÕs theory of evolution by natural selection

• One of the foundation pillars for the theory of evolution is the variation that occurs among individual members of a species. The basis of this variation is the genetic makeup of the individuals in a species. It is this variation that selection acts upon. Mutation of DNA provides a source of new variations thus supporting Darwin's theory of evolution.

 

process information from secondary sources to describe and analyse the relative importance of the work of:

á       James Watson

á       Francis Crick

á       Rosalind Franklin

á       Maurice Wilkins

in determining the structure of DNA and the impact of the quality of collaboration and communication on their scientific research

• Process information on the four scientists and pay particular attention to the information that comes from the mass media compared with the information that comes from scientific journals. For many years, there was no acknowledgement of the work of Rosalind Franklin, and to this day, newspaper articles only use the names of Watson and Crick in the discovery of the double helix of DNA.

Here are some good starting points.

Watson  Access excellence 
Crick  PBS Online, the site of the Corporation for Public Broadcasting, USA 
The story of the discovery of DNA structure  PBS Online, the site of the Corporation for Public Broadcasting, USA 
Rosalind Franklin  PBS Online, the site of the Corporation for Public Broadcasting, USA 
Maurice Wilkins  The new Zealand Edge, nzedge.com

Sample description

Scientific discoveries are rarely the work of one person but tend to result from teams of people bringing together different skills. These teams may be working together or may be scattered all over the world working independently in different laboratories. Determining the structure of DNA is a good case study exemplifying the role of collaboration and effective communication in scientific research.

The four people in this story worked at two different places. Rosalind Franklin and Maurice Wilkins were from King's College London and James Watson and Francis Crick were from Cambridge University. Rosalind Franklin was a woman working in a field that was male dominated. You will see as the story progresses that she didn't get equal recognition for her contributions. Her work on X-ray diffraction showed that DNA had the characteristics of a helix. She wished to gather more evidence of this result but Maurice Wilkins showed her results to Watson and Crick without her permission or knowledge.

This information was enough to encourage Watson and Crick to develop their model of the double helix for the structure of DNA.

Rosalind Franklin died of cancer in 1958 at the age of 37.  Watson, Crick and Wilkins received the Nobel Prize for their work in 1962.

describe the concept of punctuated equilibrium in evolution and how it differs from the gradual process proposed by Darwin

• Punctuated equilibrium differs from Darwin's gradual evolution in that evolution is seen as long periods where there is little change in organisms, followed by a shorter period where there e rapid changes. Evolution is a sudden process rather than slow gradual change. The evidence for this comes from the fossil record where there are mass extinctions of organisms followed by the appearance of new species.

Punctuated equilibrium  PBS, USA