The story of DNA

 

The central dogma of molecular biology explains that DNA codes for RNA, which codes for proteins. DNA is the molecule of heredity that passes from parents to offspring. It contains the instructions for building RNA and proteins, which make up the structure of the body and carry out most of its functions.

 

What we know:

People always noticed that many traits were somhow inherited, or passed down, from parent to child, and some traits were the result of environment.  Examples of inherited traits include such things as attached or free ear lobe, cleft chin, chance of developing cancer, and widow’s peak.  Some examples of characteristics that are not inherited, but are the result of environmental factors, include being good at football, the ability to read French, being able to ride a bicycle.

It was in the 1940's that scientists were able to demonstrate the  relationship between genes and proteins, and that genes are made up of DNA.   Then, in 1953,  James Watson and Francis Crick were able to determine the structure for the DNA molecule, using an X-Ray photograph of crystalized DNA taken by Rosalind Franklin.  This photograph gave Watson and Crick the information needed to identify that DNA has a double-helix shape -  DNA is a double helix made up of two molecules stuck together like a twisted ladder (wound around each other in a spiral). This discovery was awarded the Nobel Prize in Science in 1960.  Franklin would have recieved the award had it not been for her untimely death in 1958 of cancer.

People wondered how these characteristics were passed from parent to child - what was the mechanism for  this inheritance.  Remember  that the cell was seen first in the late 1600's and some of the internal parts were identified - without knowing what they were.  The nucleus was one of the first organelles that was seen - and it was in all cells.  The scientists of teh time were able to infer that the nucleus must do something important, that it is connected with the essence of the cell and life.

More about the Nucleus:

 

Scientists were able to determine that the nucleus was somehow involved in the abilitry fo cells to divide.  There must be something in the nucleus that contained the instructions for the cell - but it took many more years for scientists to figure it out. 

Mendel and his experiments:

 

 A monk in Austria discovered the mechanism of inheritance in the 1830's by conducting experiments with pea plants.  Gregor Mendel conducted crosses (breeding one type of plant with another type of the same plant) to see how the characteristics were passed from generation to generation (we'll go into this a lot more in the section on Mendelian Genetics).  Mendel was able to identify that each parent contributed a portion of the instructions to the offspring.

Discovering DNA:

Sceintists knew that there were a few molecules that were present in the nucleus in quantity, and that a cell without a nucelus could not divide.  They were able to infer that whatever the inheritance mechanism was had to be contained inside the nucleus.  Studies and experiments were done on each of the possible candidates, and DNA, a large Nucelic Acid macromolecule was determined to be the molecule that contained the code.  This was still years before anyone knew how this molecule could possibly perform all of the necessary functions:

  • DNA has to be able to make exact copies of itself without any errors

  • DNA has to be able to control what the cell does

  • DNA has to be able to pass along these instructions to offspring.

Scientists had no idea of how any of these functions could be working inside the cell.

 

In the late 1880's, Albercht Kossel deternmined that DNA was made up of four sub-units (bases):  Adenine (A), Thymine (T), Guanine (G) and Cytosine (C).

Evidence 1: DNA Structure:

The photograph that helped determine the shape of DNA.

Watson & Crick were able to determine the shape of the DNA molecule because the photograph showed the shadow that the DNA molecule cast.  They were able to backtrack and determine the shape of the molecule from this clue. 

Franklin's photograph  helped determine that DNA is a double helix made up of two molecules stuck together like a twisted ladder (two molecules wound around each other in a spiral).

Evidence 2: Protein Structure:


Proteins are Macromolecules that are made up of smaller subunits called Amino Acids.  A protein is two or more Amino Acids connected together like a chain.  Proteins are a single strand - they do not branch.   There are only 20 different amino acids, so they act like an alphabet.  Our alphabet has 26 letters, but can write millions of different words depending on the length and sequence.  Amino Acids are the building blocks of proteins. 

Evidence to consider:

Amino acids are classified based on their properties.

  • electrical charge (Positive or Negative)

  • hydrophobicity (hydrophobic is repelled by water; or hydrophilic is attracted to water)

  • aromatic (having a hexagon ring; these Amino Acids also have an odor).

Evidence 4: The Discovery of RNA:

 

In the 1920s Pheobus Levene discovered that there was another type of nucleic acid called RNA.

He found three main differences between RNA and DNA:

  • RNA has a different sugar than DNA that makes up the molecule;

    • DNA has the sugar Deoxyribose, while RNA has the sugar Ribose.

  • RNA is only a single strand (it does not form a double helix like DNA).

  • RNA has the base “U” instead of a “T”.

Where are Nucelic Acids located in cells?

Scientists needed to determine how DNA and RNA were different, so they tried to find out where each of these molecules was located in a cell. 

This is what they did:

  1. They took 500 cells and broke their cell membranes open

  2. They used a tool to separate the different components of the cells

  3. They then tested each cellular component for the presence of DNA and RNA

The organelles where Nucleic Acids are found

How does DNA and RNA react?

Scientists found that that RNA and DNA molecules could pair up. The RNA bases could match up with the DNA bases, even though RNA did not contain the base Thymine (T).

The colors of the bases have meaning.  The blue base represents Cytosine (C), and the green base represents Guanine (G).  The red base in RNA represents Uracil (U), but in DNA it represents Thymine (T), and the yellow base represents Adenine (A).  This illustrates the way the bases pair together.

Evidence 5:  Protein Composition:

 

How do Nucleic Acids code for Proteins? 

 

By this point, scientists had determined that Nucleic Acids were where the instructions were stored, but how?  A number of scientists performed many experiments to find the answer to this critical question. Here are a few samples of the experiments conducted to get this answer.

Experiment A:

  1. Scientists were able to make RNA sequences containing only one type of base (AAAAAAAAAA, or CCCCCCCCCC, etc.).  They could then place one repeating RNA sequence into a test tube with all 20 types of amino acids

  2. Ribosomes were added to each test tube because we had already determined that ribosomes are involved in making proteins

  3. This experiment was repeated, with the following results:

Experiment B:

  1. The same experiment was performed with one difference:  the RNA sequences had only the A base, but were of a specific length.

  2. The same methods were used as before (using RNA, amino acids, and ribosomes). 

  3. Results of these experiments are shown:

Experiment C:

  1. Scientists next tried using three RNA sequences that had 5 repeats of the sequence (GUU)

  2. The same methods were used as before: using this RNA sequence, amino acids, and ribosomes in a test tube. 

  3. This was repeated with different combinations of bases as shown in the results:

So what is the relationship between DNA and proteins?

We need to use all of the information we've been given to come up with a model of how DNA is related to proteins.  This won't be easy, and you'll need to work with a partner to share ideas.  You should be able to come up with a drqwing and explanation that can tie all of the evidence together.  Remember to use the Evidence Matrix and to evaluate the strength of each peice of evidence as we did back with Sam Spade activity earlier this marking period.  

 

You can always reference the Writing Rubric or Model Criteria as you complete this assignment.

Evidence 3: Chargaff's Rule:

 

The Austrian Chemist Erwin Chargaff determined the composition of DNA from any cell of all organisms always has a 1:1 ratio of each base pair, and that the amount of guanine is equal to cytosine and the amount of adenine is equal to thymine. He was also able to determine that the percentages of each pase pair varies from one species to another; in particular in the relative amounts of A, G, T, and C bases. Such evidence of molecular diversity, which had been presumed absent from DNA, made DNA a more credible candidate for the genetic material than protein.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Chargaff's Rule

The amount of Adenine and Thymine is the same in each cells DNA. 

The amount of Cytosine and Guanine is the same in each cells DNA.

Concept Explained