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DNA replication: also known as semi-conservative replication, is the process by which DNA is essentially doubled. It is an important process that takes place within the dividing cell.

DNA Structure:DNA is made up of millions of nucleotides. These are molecules composed of a deoxyribose sugar, with a phosphate and a base (or

nucleobase) attached to it. These nucleotides are attached to each other in strands via phosphodiester bonds to form a ‘sugar-phosphate backbone’. The bond formed is between the third carbon atom on the deoxyribose sugar of one nucleotide (henceforth known as the 3’) and the fifth carbon atom of another sugar on the next nucleotide (known as the 5’).

Stages of DNA replication:

DNA replication can be thought of in three stages;

  • Initiation
  • Elongation • Termination
  • Initiation:DNA synthesis is initiated at particular points within the DNA

    strand known as ‘origins’, which are specific coding regions. These origins are targeted by initiator proteins, which go on to recruit more proteins that help aid the replication process, forming a replication complex around the DNA origin. There are multiple origin sites, and when replication of DNA begins, these sites are referred to as replication forks.

    Within the replication complex is the enzyme DNA Helicase, which unwinds the double helix and exposes each of the two strands, so that they can be used as a template for replication. It does this by hydrolysing the ATP used to form the bonds between the nucleobases, therefore breaking the bond holding the two strands together.DNA Primase is another enzyme that is important in DNA

    replication. It synthesises a small RNA primer, which acts as a ‘kick-starter’ for DNA Polymerase. DNA Polymerase is the enzyme that is ultimately responsible for the creation and expansion of the new strands of DNA.

    Elongation:Once the DNA Polymerase has attached to the original,

    unzipped two strands of DNA (i.e. the template strands), it is able to start synthesising the new DNA to match the templates. It is essential to note that DNA polymerase is only able to extend the primer by adding free nucleotides to the 3’ end.

    Termination:The process of expanding the new DNA strands continues

    until there is either no more DNA template left to replicate (i.e. at the end of the chromosome), or two replication forks meet and subsequently terminate. The meeting of two replication forks is not regulated and happens randomly along the course of the chromosome.

    Transcription describes the process by which the genetic information contained within DNA is re-written into messenger RNA (mRNA) by RNA polymerase. This mRNA then exits the nucleus, where it provides the basis for the translation of DNA. By controlling the production of mRNA in the nucleus, the cell regulates the rate of gene expression.

    Stages of Transcription:

    The process of transcription can be broadly categorised into 3 main stages: İnitiation,Elongation and Termination

    Initiation:Transcription is catalysed by the enzyme RNA polymerase. It attaches to and moves along the DNA molecule until it recognises a promoter sequence,

    which indicates the starting point of transcription. There may be multiple promoter sequences in a DNA molecule

    Elongation:One DNA strand (the template strand) is read in a 3′ to 5′ direction and so provides the template for the new mRNA molecule. The other DNA strand

    is referred to as the coding strand. This is because the base sequence of the new mRNA is identical to it, except for the replacement of thiamine bases with uracil

    Termination:Elongation will continue until the RNA polymerase encounters a stop sequence. At this point, transcription stops and the RNA polymerase releases the DNA template

    Translation is a process by which the genetic code contained within a messenger RNA (mRNA) molecule is decoded to produce a specific sequence of amino acids in a polypeptide chain.It occurs in the cytoplasm following transcription and, like transcription, has three stages: initiation, elongation and termination. In this article we will look at the components and stages of DNA translation.

    Components of Translation

    The key components required for translation are:

    mRNA, ribosomes and transfer RNA (tRNA)

    Aminoacyl-tRNA synthetases are enzymes that link amino acids to their corresponding tRNA molecules. The resulting complex is charged and is referred to as an aminoacyl-tRNA.

    Initiation:For translation to begin, the start codon 5’AUG must be recognised. This is a codon specific to the amino acid methionine, which is nearly always the

    firstamino acid in a polypeptide chain.At the 5’ cap of mRNA, the small 40s subunit of the ribosome binds. Subsequently, the larger 60s subunit binds to complete the initiation complex. The next step (elongation) can now commence

    Elongation:The ribosome has two tRNA binding sites; the P site which holds the peptide chain and the A site which accepts the tRNA.While Methionine-tRNA

    occupies the P site, the aminoacyl-tRNA that is complementary to the next codon binds to the A site, using energy yielded from the hydrolysis of GTP.

    Termination:One of the three stop codons enters the A site. No tRNA molecules bind to these codons so the peptide and tRNA in the P site become

    hydrolysed releasing the polypeptide into the cytoplasm.The small and large subunits of the ribosome dissociate ready for the next round of translation.


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