Gene Expression & Translation

Genotype gives rise to phenotype

the genetic code defines the final product and its function

Gene expression describes the entire process of converting the genetic information encoded in the genome into the corresponding functional gene product. Mostly, these products are proteins, however, gene expression can also occure from non-protein coding genes and result in the production of e.g. transfer RNA (tRNA) or small nuclear RNA (snRNA). The single steps of gene expression, including transcription, RNA splicing, translation and post-translational modification are striclty regulated to controle processes like differentiation, development or morphogenesis.

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Gene Expression

following a strictly regulated process

The process of gene expression always starts with the transcription, the production of an RNA copy from the DNA. During this process the RNA polymerase adds nucleotife after nucleotide to the growing RNA strand according to the DNA template. The only difference is the exchange of thymines (T) by uracils (U) within the RNA sequence. After the production of the primary RNA transcript (pre-RNA) in eukaryotic cells, the RNA has to undergo several modifications. Two major modifications are the 5´capping and the 3´cleavage and polyadenylation which is crucial for mRNA export into the cytosol and translation initiation. Another very important modification is the RNA splicing in which introns get removed from the RNA sequence and necessary exons get spliced together. Alternative splicing is a variation of this process where some introns or exons can be retained and create an alternativ RNA sequence from the same gene. This allows the production of different proteins from the same genomic sequence and increases the size of the proteome.

In some cases, the mature RNA also represents the final product. However, in most cases the final product is a protein which needs to be synthesized from the mature mRNA during translation.

Translation

from mRNA to protein

mRNA represents only a preliminary form and the translation template for the synthesis of one (monocistronic) or more proteins (polycistronic). mRNA contains the protein coding region flanked by the 5´and 3´untranslated region. Within the coding region each triplet of nucleotides codes for specific transfer RNA (tRNA) carrying the complementary anticodon triplet as well as the corresponding amino acid. With the help of ribosomes, the amino acids get chained together according to the order of the triplets encoded in the mRNA sequence.

The process can be divided in three phases:

Initiation: Assembly of the ribosome around the mRNA and attachment of the first tRNA at the start codon.
Elongation: after binding of the tRNAs the small ribosomal subunit verifys binding of the correct tRNA. The correct amino acid is then transferred the large ribosomal subunit which adds the amino acid to the growing amino acid chain and moves to the next triplet of the mRNA.
Temination: A stop codon induces the release of the polypeptide from the ribosome.