As we know rbcs don't have nucleus and dna, but still they have essential rnas that are translated in rbcs so my question is from where these mrnas come and als

Abhijeet Gaurav Best Answer

Basically red blood cells lack an nucleus and most organelles to have more space for hemoglobine to increase oxygen transportation capacity. Essential mRNAs will be still present in the cytoplasma where it is translated but they will be exhausted over time. This is also the reason for the 100 days lifespan of red blood cells. Cytoplasmic extracts prepared from erythrocytes lack mRNA translation activity in vitro, in contrast to reticulocyte lysate. Both, erythrocytes as well as enucleated reticulocytes of different stages circulate in the blood stream and protein synthesis activity takes place in the latter fraction. Enucleated reticulocytes translate mRNAs that are stabilzed by specific RBPs, like alpha globin encoding mRNA or those, which were previousely silenced by RBPs, like r15-LOX mRNA. R15-LOX is required to initiate mitochondria degardation in the already enucleated cells, which switch from oxidative phosphorylation to glycolysis. The miRNA study mentioned aboth made use of a mixed population of reticulocytes and erythrocytes.  Different levels of regulation of gene expression in erythropoiesis. Cell-specific transcription factors control which genes are expressed. Transcript levels per gene differ because of transcription rate, but also due to mRNA stability. Transcript levels poorly predict protein expression. Translation factors and mRNA structure control protein synthesis efficiency. Transcript and protein expression is partly cell intrinsically regulated as a cascade of subsequent gene expression regulation (left hand arrows). In addition, signal transduction (Epo and SCF in erythroid progenitors) can modify signaling molecules to activate transcription and translation. For instance, phosphorylation of STAT5 causes dimerization and activates the transcription activity, ERK phosphorylation cause nuclear location and phosphorylation of transcription factors. PI3K activates mTOR, a critical factor in protein synthesis.