Describe the process of transcription and …

Process of Translation

The protein synthesis occurs in two steps, namely transcription and translation. During transcription, the information from DNA is encoded into mRNA. During translation, the mRNA works with a ribosome and tRNA to synthesize proteins. 

Transcription takes place in the nucleus. During transcription, DNA partially unwinds by the enzyme helicase. This results in the single nucleotide chain to be copied. RNA polymerase reads the DNA strand from the 3' to 5' direction and synthesizes the complementary strand of messenger RNA in the 5' to 3' direction. The DNA strand which is transcribed is called as the template strand. The nucleotides on the RNA strand are complementary to the nucleotides on the DNA strand. DNA cytosine pairs with RNA guanine, DNA guanine pairs with RNA cytosine, DNA thymine pairs with RNA adenine and DNA adenine pairs with RNA uracil. After mRNA is synthesized, it is transported to the cytoplasm where it binds with ribosomes. Ribosomes are the sites of protein synthesis. Ribosomes have three important binding sites, one for mRNA and two (A site and P site) for tRNA. The start codon methionine occupies the P site and the second codon occupies the A site. The tRNA molecule whose anticodon is complementary to the mRNA forms a base pair with the mRNA in the A site. A peptide bond is formed between the amino acid attached to the tRNA in the A site and the methionine in the P site. The ribosome then slides down the mRNA in such a way that the tRNA in the A site moves to the P site and a new codon occupies the A site. This process continues until one of the three stop codons occupies the A site. At that point, the protein chain connected to the tRNA in the P site is released and the translation is complete.

Transcription

Transcription is the process of synthesis of RNA from DNA template. A segment of DNA gets copied into mRNA during the process. The process of transcription starts at the promoter region of the template DNA and terminates at the terminator region. The segment of DNA between these two regions is known as transcription unit. The transcription requires RNA polymerase enzyme, a DNA template, four types of ribonucleotides, and certain cofactors such as Mg2+.

The three important events that occur during the process of transcription are as follows.

(i) Initiation

(ii) Elongation

(iii) Termination

The DNA-dependent RNA polymerase and certain initiation factors (σ) bind at the double stranded DNA at the promoter region of the template strand and initiate the process of transcription. RNA polymerase moves along the DNA and leads to the unwinding of DNA duplex into two separate strands. Then, one of the strands, called sense strand, acts as template for mRNA synthesis. The enzyme, RNA polymerase, utilizes nucleoside triphosphates (dNTPs) as raw material and polymerizes them to form mRNA according to the complementary bases present on the template DNA. This process of opening of helix and elongation of polynucleotide chain continues until the enzyme reaches the terminator region. As RNA polymerase reaches the terminator region, the newly synthesized mRNA transcripted along with enzyme is released. Another factor called terminator factor (ρ) is required for the termination of the transcription.