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From DNA to RNA

Transcription is the name of the step in which DNA is copied into RNA. Do you need a little reminder?


Table of Contents

  1. What do we need for transcription?
  2. First step: transcription into a pre RNA
  3. Modification of this pre RNA
  4. The final product
  5. To go further

What do we need for transcription?

  1. DNA (Let's just state the obvious...)

    2. DNA is a long chain of bases, as shown elsewhere.

    In eukaryotes, the genome is divided into :

    • Non-coding areas... between genes.
    • Genesask Dr Chromo! : Each gene is divided into several exonsask Dr Chromo!, separated by non coding sequences,
      • Intronsask Dr Chromo! (not coding)
      • Exonsask Dr Chromo! (coding)
    • Promotersask Dr Chromo!, and regulation sequences.
  2. RNA polymerases

      3. RNA polymerases are enzymes that will synthesise different kinds of RNA.

  3. Other factors

      4. E.g. factor sigma: this stabilises the polymerase at its specific site, to help polymerisation to start. These other factors may be proteins or other kinds of molecules.


First Steps

Transcription of DNA into RNA is different in prokaryotesask Dr Chromo!, and in eukaryotesask Dr Chromo!. Bacteria are prokaryotes. Transcription in bacteria has been studied in detail: it is also simpler than in mammals like humans.



Template DNAComplementary DNARNA Polymerase Template DNA

Principal events in transcription

1) The RNA polymerase has a natural affinity for DNA, and will fix itself to DNA, and start polymerization.

a) If the RNA polymerase attaches to any DNA sequence, it will soon detach, and the tiny RNA generated by the polymerase will be hydrolysed.

b) If the RNA polymerase attaches to a special sequence called a promoterask Dr Chromo!, an additional small protein, the factor sigma, will also attach to the polymerase and lock it on the DNA. The factor 'sigma' will only attach itself to the complex DNA / RNA polymerase when the RNA polymerase is attached to a promoter. Another hypothesis is that the factor sigma attaches to RNApol anyway and the enzyme is then able to slide along the DNA until it finds a promoter. It prevents detaching and speeds up promoter location, and decreases the affinity of RNApol for general regions of DNA.

2) Polymerisation:

Once the polymerase is attached to DNA (as shown in the figure) , and locked on the DNA by the fator sigma, it catalyses the addition of nucleotides from 5' to 3'... (a nucleotide is attached to the 3' OH of its preceding nucleotide).

After some nucleotides have been added, the factor 'sigma' detaches, and can be reused for starting more RNA synthesis.

Stop signals in the DNA indicate the end of the gene to be transcribed. It is a signal for the termination of transcriptionm although the polymerase often goes on transcribing for a while before detaching.

This long RNA is called pre mRNA, and needs further modification before being transported into the cytoplasm and translated into protein.

Modifications

One of the features of prokariotic RNA, is its very short life (minutes). In contrast, more stable mRNAs are found in eukaryotes. For example, the mRNA for haemoglobin in the red cell, is active for hours and even days, after the loss of the nucleus.

RNA is stable after a few modification steps.

  1. Capping: most mRNAs have their starting end blocked by the addition of a cap (7-methylguanosine). The cap structure has no free phosphates, and then is protected from attack by phosphatases, or nucleases
  2. Addition of a poly-A tail : this polyadenylic segment is 100 to 200 nucleotides long and is added sequentially after transcription is completed by the action of a poly A synthetase that uses ATP as substrate.
  3. Methylation: when a methyl group is added to nucleotides.
  4. Cleavage of big RNAs: when the polymerase arrives at the end of the RNA, it does not terminate transcription abruptly, but goes on transcribing DNA (sometimes 1500 nucleotides). The transcript is cut and polyadenylated while it is still transcribed.
  5. Splicing: eukaryotic genes are frequently interrupted by intervening sequences (introns). These sequences must be removed and the molecule precisely rejoined in the correct sequence, because an error of one nucleotide would cause the rest of the message to be read incorrectly.

The Final Product

The final product is a messenger RNA. It is used as a template for the synthesis of protein by ribosomes.

The polymerisation of RNA by the RNA Polymerase is regulated. Some proteins are synthesized in specific cells. Regulation of translation is explained in the chapter about the "What controls genes".



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