SEARCHING FOR THE ORIGIN OF LIFE
THE ORIGIN OF INFORMATION
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Question from Chapter 9
Is there any way the information needed to make the first living cell could organise itself onto DNA, and then reorganise itself to make new and different life forms?
Let's consider another familiar example of encoded information - a STOP sign.
Stop or Arret?
We have seen that the information communicated by a code is not due to Natural Properties of the individual parts of the code. This is confirmed by the fact we can convey the same information with different code symbols. Look at two different ways the information for "stop" can be put into code.
The information in the English word "stop" is not due to the Natural Properties of the parts S, T, O, or P, any more than the French word for stop is due to the properties of A, R, E or T. This is confirmed by the fact a red light can convey the same meaning as "Stop" without any letters and words. Individual code symbols and combination have only the meaning we invent for them.
Now consider the information stored in a living cell.
The information in DNA is coded on four different bases, adenine, thymine, cytosine and guanine, which are usually written as four letters: A, T, C and G. DNA is made of two strands with the bases facing one another so that they form pairs across the strands. Due to the precise shapes and chemical properties of the bases, A and T always pair with one another, and C and G always pair with one another. Therefore, the two strands are said to be complementary copies. For example:
If one strand is: GCATGACCGAA
The opposite strand is: CGTACTGGCTT
When a cell divides the two strands are pulled apart and each one acts as a template for the nucleotides with the complementary bases to be lined up and joined together to make the new opposite strands.
Could the bases A, T, C and G in DNA organise themselves into a code?
Because the bases in DNA attach to one another in predictable base-pairs, i.e. A always to T and C always to G, it has been suggested the bases could self organise into code. However, the pair bonding property of the bases merely enables the already existing arrangement to be copied accurately from one strand of DNA to another.
The information is in the sequence of the bases along the strands, not in the base pairing across the strands. Although each base is connected to a complementary base on the opposite strand, it is not connected to the bases on either side. Indeed, if that happens, e.g. if two adjacent thymine molecules bond together, it is a serious problem because it disrupts the information copying processes.
From DNA to RNA
DNA stays within the protected environment of the cell nucleus, so the information it contains has to be copied onto RNA, which is then exported to the cytoplasm of the cell in order to be used for making proteins. The principle of complementary base pairing is used to copy DNA information onto RNA, but there is one difference. In RNA the bases are adenine, uracil, cytosine and thymine, or A, U, C and G. In RNA code U acts like T - it pairs with A. Therefore, when an RNA copy is made of DNA information it consists of complementary letters, with U being complementary for A. For example:
If the DNA code is: GCATGACCGAA
The RNA code is: CGUACUGGCUU
Now, think about again how we had different words meaning "stop". DNA and RNA both have three letter code words that mean "stop" so that the ribosome knows when to stop making a protein." DNA spells this instruction with any one of the words ATT, ATC or ACT. These are copied onto RNA using complementary base pairing, but with U instead of T, so RNA code for "stop" is UAA, UAG or UGA. Even though the code symbols are different, the same information is accurately transferred from DNA to RNA so the process stops at the right place for each protein made by a cell.
Information is conveyed from DNA to proteins in the following sequence:
DNA → Information copied → RNA → Information used → Proteins
But is DNA the ultimate source of the information?
We have already seen that DNA does not function by itself. It cannot assemble itself or copy itself. Is the source of information in the DNA polymerase proteins, which assemble new DNA molecules and can also edit and correct any miscopied DNA information? Is it on the messenger RNA that takes the DNA information to the ribosomes. Is it in the ribosomes that read the messenger RNA and assemble the proteins? Ribosomes are a complex combination of protein and RNA molecules. Are any of these the ultimate source of information?
Consider again where the information is in a STOP sign? It is not in the letters on the sign. A French ARRET sign and an English STOP sign display different symbols but mean the same thing. Is the information in the minds of drivers, in the Road Traffic Authority who put the sign up, or the traffic police who fine drivers who do not STOP? If a driver does not STOP because there are no police around, has the information gone away?
The Origin of Information
The information in road signs did not originate in the signs, even though the signs convey it. The information originated in human minds, and the meaning of the code is defined by its creator. Only the creator has the authority to give the code its meaning. Try telling a policemen who is writing you an infringement notice for failing to stop that you have decided STOP really means "slow down," so that is what you did. You will soon find out who has the authority to give STOP its meaning.
In the cell we have a system where DNA, several types of RNA and many proteins combine to make DNA polymerase proteins, which then read DNA, help make a copy and edit any mistakes. In each of these substances the information is encoded by different chemicals. None of these chemicals are the source of the information, any more than the letters in the road signs are the source of road sign information. Words written on road signs, or anywhere else, only convey information that had its origin in the mind of the creator of the code words. Coded information in living cells has the same characteristics as coded information invented and used by human beings, i.e. the code symbols convey the information, but they did not invent it. They behave like a system that has been deliberately created by an outside intelligence.
DNA and Evolution
After the original DNA information came into being, evolution could only occur if the information on DNA was changed and increased. Is there any way this can happen by natural processes?
Changes in DNA do regularly occur - they are called mutations. If evolution by natural processes occurred then mutations must be able to change DNA information in a meaningful way.
How do mutations change DNA information, and what are the results of these changes?
We will look at mutations and what they have been observed to do to DNA information in Chapter 11.
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