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Key mechanisms that are involved in expressing a eukaryotic gene as a protein

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  1. Introduction
  2. Phosphodiester bonds between free nucleotides
  3. The mma molecule
  4. The polypwptide chain
  5. Conclusion

The information contained within the DNA, in the form of a nucleotide sequence, is responsible for the production of the phenotype of an individual. The dna contains specific sequenmces of nucleotides, called genes, that code for specific chasracteristics. The basic concept of how this occurs was described by Francis Crick as the central dogma of mollecular biology and genetics; that a gene codes for a specific intermediate mollecule, that in turn codes for the synthesis of a specific protein.

Thus the first step in expressing a eukaryotic gene is producing the intermediate mollecule, a process called transcription. The DNA cannot be decoded by the cell directly to prodice a phenotype. The process of decoding goes through intermediate mollecules, called RNAs. Thus transcription is the copying of the nucleotide sequence information from a DNA strand into an RNA strand.

The DNA sequences are transcribed into mRNA by DNA dependent RNA polymerases, they are DNA dependant since they use DNA as a template. There are four phases to transcription; RNA polymerase binding, initiation, elongation, and termination. All four stages are highly regulated by the cell.

Transcription of a gene initiates via a region of DNA to the coding region, called a promoter. A promoter consists of specific short DNA sequence motifs. It defines the site of transcript initiation, and the direction of transcription, i.e., which DNA strand is used as the template. It was found that E.coli promoters consist of two main DNA sequence elements; -35 and ?10 regions. The first nucleotide in the RNA transcript is defined as +1, and the ?10 and ?35 DNA elements are measured relative to it .

[...] Eukaryotic promotors are functionaly similar to those in E.coli but are more variable, as wiith promoters for rna pol II, and can consist of twon sequence ellements, the UPE and the CORE, as found in promotors for rna pol I. Promoters for rna pol II are internall. Gernall transcription factors have an anologous function to sigma subunits, in eukaryotic cells. GTFs are mostlly rna polymerase specific multi protein complexes. The only diference between the transcription processes is that in eukaryotic cells during elongation the GTF is jetisoned and replaced by an elongation factor complex which aids processivity of the p;olymerase. [...]

[...] In the cytoplasm the mrna mollecule travels to the r.e.r, where it is read at the attatched ribosomes, and a polypeptide chain is sysnthesised from amino acids attatched to tRNA mollecules thhat are broguht together as the mrna runs between the two subunits of the ribosome. This process is called translation. A ribosome is a ribonucleoprotein particle comprising a small and a large sububit. Rna constitutes approx 2/3 of the mass of a ribosome, and it is currently believed that rna plays a major role in ribosome function, with pproteins playing a minor role. [...]

[...] This was proved by using a synthetic polynucleotide as the template in a cell free protein synthesising system and seeing which amino acid was encorperated first into the polypeptide chain. The directions of transcription and translation are the same and therefore mrna can be translated whilst its being synmthesised. The mrna mollecule contains an initiator sequence that determins at what point protein synthesis begins. This is usualy an AUG codon. This sequmnce can be up to 25 nucleotides in from the end of the mrna mollcule. [...]

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