SEARCHING FOR THE ORIGIN OF LIFE
MAINTAINING DNA INFORMATION
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Question from Chapter 11
How do cells prevent their DNA information from being changed, and what are the implications for evolutionary theory?
Stability of DNA
One most impressive property of DNA is its stability. Some cells in the body which differentiate into specialist cells early in life, e.g. neurones (nerve cells) and skeletal muscle cells, exist without dividing for the term of your natural life. This means that they make no new copies of DNA and therefore use the same DNA for many decades. The DNA strands in the nerve cells of a 100 year old person have been there for 100 years. This stability is due to the very complex protective folding and packaging of the DNA strands, as well as many surveillance and repair mechanisms. DNA seems built to last.
Living organisms also have many mechanisms which enable them to resist change to genetic information. These include:
DNA is double stranded. When a cell divides each new cell receives DNA containing one of the original strands plus a newly made reciprocal copy. Its information is coded twice using a matching reciprocal code on each strand, e.g. A (Adenine) on one strand is coded by T (Thymine) on the other strand; G on one strand is always matched with C.
DNA uses a code which has 4 chemical letters T, C, A, G which are used 3 at a time. Each combination of 3 code letters is called a codon. Four different letters used 3 at a time means there are 64 possible combinations of letters. Since only 20 amino acids are used by DNA via RNA to make protein, many amino acids have more than one way of being 'spelt'. (Some amino acids, such as Argenine, Leucine and Serine, have 6 different codons.) This enables plenty of ways to back up information.
Copy Check and Edit
We have already seen that DNA polymerase proteins check and edit the new DNA strands made when DNA is being copied for new cells. Even when a cell is not dividing there are a number of mechanisms that keep the DNA information intact.
DNA is being constantly surveyed for damage and cells have "repair crews" that enable them to correct some types of DNA damage, e.g. ultra-violet (UV) radiation. In normal DNA the bases on opposite strands are linked together by weak bonds. UV radiation can break these weak bonds. Usually they re-form. But if two thymines (T) are side by side on one strand, they have a tendency to form a sideways bond with each other instead of rejoining with the Adenines other strand. This sideways bond is called a thymine dimer and it prevents the DNA code from being read in that place. The cell has a group of proteins that constantly survey the DNA and when they find one of these thymine dimers they cut out the piece of the strand containing the dimer. Then using the sequence of bases on the opposite strand they rebuild the deleted piece. The new strand is an accurate copy of the original information because the opposite strand contains the matching code in the reciprocal form.
If the DNA of a cell is found to be irreparably damaged the whole cell will be destroyed by a process called apoptosis. The DNA is first broken up into small fragments and the cell will then be broken up into a number of small fragments which are engulfed by special scavenging cells. The destroyed cell is then replaced with a new cell made by cell division of an undamaged cell.
The environment also acts as a filter to eliminate DNA changes. When altered DNA has an effect, the changes are almost always damaging so an individual with changed DNA is less fit to survive and reproduce. The environment constantly acts against such individuals and changed DNA information is constantly removed from the population. Before the advent of modern medicine, most human beings with diseases such as muscular dystrophy, haemophilia and cystic fibrosis would not have survived into adult life. Their damaged DNA was removed from the population, before it could be passed on.
Passing on Mutations
Occasionally DNA letter changes are not repaired or eliminated and the altered information is copied next time the DNA replicates. Therefore, over many generation mutations can accumulate. In bacteria, the observed mutation rate is approximately one mutation in every 105 or 106 replications. In a sexually reproducing organism only mutations occurring in the DNA of an egg or sperm cell will be passed on to subsequent generations, so the rate of passing on mutations is much lower. Mutations occurring in the rest of the body cells, called "somatic mutations", may cause disease during the lifetime of that individual, but they disappear at death.
Although mutations may change existing DNA information they do not explain how the information came to exist in the first place. That could only happen if the bases could organise themselves into a code.
New Evolution Theory
Many scientists have abandoned the idea of organisms gradually evolving by mutations slowly adding new information to DNA, which is then maintained in the population by the process of natural selection. They have proposed a new theory of evolution called punctuated equilibrium. This theory states life forms are observed to be basically stable i.e. in equilibrium. Any evolution or changes have been sudden and large, i.e. the creature's stability or equilibrium has been punctuated.
Is there evidence for Equilibrium?
In Shark Bay, Western Australia, there are living algal mats called Stromatolites. In many parts of the world there are fossil Stromatolites which appear to be similar to living ones. Evolutionists claim fossil Stromatolites are among the oldest known fossils. Stromatolite DNA must have stayed the same, i.e. been in equilibrium in order for living algal mats to still be similar to the oldest known fossil Stromatolites. Stromatolites with their obviously stable DNA are an excellent example of organisms in equilibrium which are producing their own kind.
Is there evidence DNA equilibrium can be punctuated?
All observations on the effect of natural processes on DNA show that, at best, DNA stays the same, at worst it loses information over time. To date, there is no observed genetic mechanism which shows the obvious equilibrium of any known life form can be naturally punctuated to produce sudden evolutionary gains to DNA information.
The usual excuse for not observing punctuated equilibrium is that we are living in a period of stability, and because evolution happens over such vast periods of time we are unlikely to observe any "punctuations".
Stromatolites are an example of organisms known as "living fossils". This term was used by Charles Darwin to describe living things that are the same as their fossils. Some other examples are cockroaches, spiders, crocodiles, salamanders, tuatara lizards and the Wollemi Pine. No matter how old you may believe the oldest fossils of these creatures to be, they have stayed the same throughout that period. They have not evolved.
Evolution and Time
The process of evolution is claimed to have happened over millions of years, so the fact that we have not observed one kind of living thing change into a new and more complex living things is usually put down to the fact we are only able to directly observe biological processes over decades. Whether they believe in slow gradual evolution or punctuated equilibrium, evolutionists' basic assumption is that vast amounts of time is the key to producing life from non-life and new more complex living things from simple living things.
Would any amount of time enable information to be generated from nothing, and then increase it, either gradually or large leaps?
To answer this we need to consider how information is affected as natural properties act through time. We will do this in the Chapter 13.
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