Question

describe the process of Eukaryotic transcription in detail please.

Thank you.

Answer 1

Eukaryotic transcription is the elaborate process that eukaryotic cells use to copy genetic information stored in DNA into units of transportable complementary RNA replica. Gene transcription occurs in both eukaryotic and prokaryotic cells.

• Transcription is the process by which the information in a strand of DNA is copied into a new molecule of RNA.
• It is the first step of gene expression, in which a particular segment of DNA is copied into RNA (especially mRNA) by the enzyme RNA polymerase.
• It results in a complementary, antiparallel RNA strand called a primary transcript.

Transcription in Eukaryotes

Transcription occurs in eukaryotes in a way that is similar to prokaryotes with reference to the basic steps involved. However, some major differences between them include:

• Initiation is more complex.
• Termination does not involve stem-loop structures.
• Transcription is carried out by three enzymes (RNA polymerases I, II and III).
• The regulation of transcription is more extensive than prokaryotes.

• Enzyme(s) Involved in Eukaryotic Transcription

  Unlike prokaryotes where all RNA is synthesized by a single RNA polymerase, the nucleus of a eukaryotic cell has three RNA polymerases responsible for transcribing different types of RNA.

• RNA polymerase I (RNA Pol I) is located in the nucleolus and transcribes the 28S, 18S, and 5.8S rRNA genes.
• RNA polymerase II (RNA Pol II) is located in the nucleoplasm and transcribes protein-coding genes, to yield pre-mRNA, and also the genes encoding small nucleolar RNAs (snoRNAs) involved in rRNA processing and small nuclear RNAs (snRNAs) involved in mRNA processing, except for U6 snRNA.
• RNA polymerase III (RNA Pol III) is also located in the nucleoplasm. It transcribes the genes for tRNA, 5S rRNA, U6 snRNA, and the 7S RNA associated with the signal recognition particle (SRP) involved in the translocation of proteins across the endoplasmic reticulum membrane.
• Each of the three eukaryotic RNA polymerases contains 12 or more subunits and so these are large complex enzymes.
• The genes encoding some of the subunits of each eukaryotic enzyme show DNA sequence similarities to genes encoding subunits of the core enzyme of E. coli RNA polymerase.
• However, four to seven other subunits of each eukaryotic RNA polymerase are unique in that they show no similarity either with bacterial RNA polymerase subunits or with the subunits of other eukaryotic RNA polymerases.

Steps in Eukaryotic Transcription

Eukaryotic transcription is carried out in the nucleus of the cell by one of three RNA polymerases, depending on the RNA being transcribed, and proceeds in three sequential stages:

1. Initiation
2. Elongation
3. Termination.

Step 1: Initiation

Initiation is the beginning of transcription. It occurs when the enzyme RNA polymerase binds to a region of a gene called the promoter. This signals the DNA to unwind so the enzyme can ‘‘read’’ the bases in one of the DNA strands. The enzyme is now ready to make a strand of mRNA with a complementary sequence of bases.

Step 2: Elongation

Elongation is the addition of nucleotides to the mRNA strand. RNA polymerase reads the unwound DNA strand and builds the mRNA molecule, using complementary base pairs. There is a brief time during this process when the newly formed RNA is bound to the unwound DNA. During this process, an adenine (A) in the DNA binds to an uracil (U) in the RNA.

Step 3: Termination

Termination is the ending of transcription, and occurs when RNA polymerase crosses a stop (termination) sequence in the gene. The mRNA strand is complete, and it detaches from DNA.