Gene Expression

Gene Expression
Definition
All functions in the body of an organism are controlled by genes. A function expressed or performed by a gene is called gene expression.
Process of Gene Expression
The process of gene expression occurs in two phases
1. Transcription
2. Translation
1. Transcription
Definition
The process in which an RNA copy of the DNA sequence encoding the gene is produced with the help of an enzyme RNA polymerase is called Transcription.
Step of Transcription

• Transcription is initiated when a special enzyme called RNA polymerase binds to a particular sequence of nucleotide on one of the RNA strands. This strands is known as template strands or Antisense strands while the other strand is called coding or sense strand.
• RNA polymerase proceeds to assemble to assemble a single strand of RNA with a nucleotide sequence complementary to that of the DNA pairing Adenine to Uracil and Guanine to Cytosine and vice versa.
• Only one strand of DNA is transcribed and when the RNA polymerase reaches a specific stop sequence at the far end of the gene, it disengages itself from the DNA and release the newly assembled RNA chain.
• This RNA chain is called the primary RNA transcript (copy) of the DNA nucleotide sequence of the gene or simply mRNA.

Translation
Definition
The process of formation of the polypeptide chains using the messenger RNA is called Translation.
Steps of Translation
1. Binding of mRNA
2. tRNA Binds Amino Acids
3. Reading Or Decoding of mRNA
4. Polypeptide Chain Synthesis
1. Binding of mRNA
The process of translation begins with the binding of one end of the mRNA with a rRNA on a ribosome.
2. tRNA Binds Amino Acids
A tRNA molecule possessing the complementary three nucleotide sequence or anticodon, binds to the exposed codon on the mRNA, because this tRNA molecule bind with a particular amino acid and put amino acids at correct place on the elongated polypeptide chain.
3. Reading Or Decoding of mRNA
The ribosome then starts to move along the mRNA molecules in increment of three nucleotides, adding a specific amino acid at each step through tRNA.
4. Polypeptide Chain Synthesis
It continues until it reaches the stop sequence, after which it stops the process. It then disengages itself from the mRNA and releases the newly assembled polypeptide.