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12.19
The main genetic copying process
 
  Since the discovery of DNA, there has been discovered four and only copying processes amongst nucleic acids namely.  
 

DNA -> DNA (DNA replication)

DNA -> RNA (transcription)

RNA -> RNA (RNA replication)

RNA -> DNA (reverse transcription)
 
12.11.1 DNA ->DNA (DNA replication)- Giving birth  
  DNA to DNA replication occurs during the cell division and creation of other cells phase of cellular process. A cell does this by copying itself (gives birth) and thus creating a new cell within itself using the components.  
   
  DNA replication can only occur after the chromosomes- which are coiled coils have unwound to form extended lengths of helical DNA called chromatin. They do this as a result of the action of an enzyme called heliocase, which unwinds about 100 revolutions per second. When a double-helix DNA molecule replicates, the first step is for the two strands to separate. When they do so the stage is then set for each of the single separate strands to act as a template for the production of a new strand.  
  Contrary to expectation, separation does not start at one end and proceed along to the other. Because of the great length of the strand, this would take far too long. The double helix actually separates locally at many points to form open loops in which replication occurs. As this happens, each of these loops then opens progressively in both directions until the whole length of DNA has split.  
 
o about 1,000 bases are copied per second).
o Synthesis is always 5' to 3' direction as shown.
o When the end of the template molecule is reached the two progeny helices each contain one new strand and one parental strand.
 
  Linked to our need to seek answers is our never-ending need to label that which we see and think. As new discoveries are made, new words are also created . The English language grows with literally thousands of words each and every year.  
12.11.2 DNA-> RNA (transcription)  
  The RNA chain formed in this way are called messenger RNA (mRNA) because they carry the code of the gene out through pores in the nucleus of the cell to the ribosomes in the cytoplasm. Before the codes are used to select the amino acids, each mRNA length is edited to get rid of sections that are not needed and the remaining coding sequences are spliced together. It is this edited version of the mRNA that is read by the endoplasmic reticulum.  
  The cell fluid contains millions of samples of the 20 amino acids, mostly derived from food but some synthesized in the body. Before they can be joined together in the right order to form a new protein molecules, they have to be brought to the ribosomes in the correct sequence. This is done by yet another kind of RNA, transfer RNA (tRNA), which moves around in the cell picking up the 20 different amino acids and carrying them to the ribosome site. There, the mRNA and tRNA and the ribosome all work together to form the chain of protein. The ribosome is itself a tiny RNA/protein body that is moved along the strand of mRNA checking the sequence of bases, selecting amino acids from the rRNA in the right order and linking them together to form proteins.  
  o During the process, selected portions of the DNA sequence are copied into RNA either to make messenger RNA (mRNA) which encodes the information to specify a sequence of amino acids or to make ribosomal or transfer RNA (rRNA and tRNA respectively) which are part of the molecular apparatus needed to translate mRNA into protein. The RNA polymerase copies the DNA template strand ( there is a local unwinding of the DNA helix to allow this to happen).  
  o Again the synthesis of RNA is in the 5' to 3' direction so that the template strand of DNA which is copied is in the anti-parallel 3' to 5' orientation.  
  o A sequence of bases beyond the coding region of the gene specifies the termination of RNA synthesis.  
  Once created, messenger RNA (mRNA) is exported from the nucleus into the cell cytoplasm where it is translated into a corresponding amino acid sequence ( protein). The process is again similar to a computer tape being fed through a reading device which reads off the message three bases at a time. each triplet of bases specifies a particular amino acid.  
  In the cell the 'reading device' is a molecular organelle made of RNA and folded proteins called a 'ribosome'.  
  First a length of double helix separates longitudinally to form a loop, exposing the sequence of single bases that together constitute the gene and specify a protein. base triplets different from those coding for amino acids indicate where the gene starts and where it ends. A new complimentary strand is now made on the exposed bases. This takes place because the exposed based of DNA attract the constituent elements of RNA (Ribonucleic acid) from the cell cytoplasm where they float freely.  
  The RNA bases in turn attract the sugar phosphate molecules which form its backbone. These strands form at a rate of about 50 bases per second. RNA  
12.11.3 RNA -> RNA (RNA Replication)  
  Separate to Ribosomes, RNA appears able to replicate itself quickly and easily  
  This is one of the triggers associated with hormones, namely the rapid replication of certain RNA code for the stronger production of certain molecular activities.  
12.11.4 RNA -> DNA (reverse transcription)  
  Reverse transcription is when RNA material is fed back to DNA material via intervention in the copying process. A central assumption of most genetic scientists in the world is that this process cannot possibly occur- that genetic material only travels one direction. This is often called the Weissmann's Barrier whereby no genetic material may travel in reverse from RNA to DNA.  
  However, the virus and its attack on the DNA of all animals shows that reverse transcription in principle is at work (Virus take their RNA and implant it into the DNA of living cells of all higher organisms).  
  Reverse transcription is responsible for the inheritance of a number of genetic defect traits, significantly when mutation by virus or exposure reduces quality of genetic code. In other words genetic transcription attacks by virus may in part also be responsible in some way for the level of Junk DNA.  
  Let us now look at the Error rates for the genetic copying process.  
     
     
 
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