Fig : 1-1.5.1: Steps involved in Gene Expression

• RNA Synthesis involves the transcription of a gene into RNA molecule and it occurs in the nucleus.
  
  
• RNA Processing comprises of post transcriptional modification/alterations of the RNA molecule and its chemical structure required for the RNA to be translated into protein or non-coding RNA (rRNA, tRNA, miRNA). RNA splicing (deletion of introns and combination of exons), 5' capping, polyadenylation etc are commonly occurred RNA processing steps in eukaryotes. However, prokaryotic organisms do not have a well developed RNA processing mechinary.
  
  
• Degradation of RNA is the controlled turnover of RNA molecules and should not be viewed simply as a mean of getting rid of unwanted RNAs. It determines the makeup of the transcriptome and is considered as an important step in genome expression. Different ribonucleases (RNases) plays the prime role in this process and multiple cofactors like small RNA (siRNA, miRNA etc), molecular cheparons (Lsm1-7, Lsm2-8, Hfq etc) regulate this process.
  
  
• Protein synthesis is initiated after the assembly of the translation initiation complex near the 5' termini of a mRNA molecule. It involves translation of RNA molecules into proteins.
  
  
• Protein folding and protein processing may occur together after protein synthesis. Post translation events like folding involve the protein attaining its correct three dimentional configuration. Processing (phosphorylation, glycosylation, carboxylation etc) involves the modification of the protein by addition of chemical groups and removal of one or more functional units of the protein.

1-1.5.2 Types of gene expression:

• Constitutive expression: Housekeeping genes are essential and necessary for sustaining life, and are therefore continuously expressed. gapdh (glyceraldehydes 3 phosphate dehydrogenase), sdha (succinate dehydrogenase) etc are human housekeeping genes which are expressed throughout the development.
  
  
• Induction and repression: The expression levels of some genes fluctuate in response to external signals. Also, under a certain situation, some genes show higher expression level, while others show lower expression levels. The former is called induced expression and the latter is called repressed expression.