Question

What is it called the enzyme that catalyzes the transcription in the prokaryotes? and what features does it have in the process? Also explain which subunits the enzyme consists of, and what functions the sigma (σ) subunit has in the process?

What is a gene and how does the enzyme that catalyzes transcription find the start of the gene?

What is the DNA chain/thread read in the transcript?

Do the eukaryotes also have only one enzyme that catalyzes the transcription of RNA?

How are the eukaryotic primary mRNA transcripts processed before they can be transported from the cell nucleus to the ribosomes in the cytosol?

Answer 1

Gene is the basic and functional unit of inheritance and is described by the sequence of the nucleotides in DNA or RNA, which codes for the product of genes.

DNA template

DNA dependent RNA polymerase catalyse the polymerisation only in one direction that is 5' to 3'. The DNA strand that has the polarity 3' to 5' acts as template, and is called template strand or non-coding strand. The other strand with polarity 5' to 3' and the sequence same as RNA, except thymine at the place of uracil, is displaced during transcription. And this strand is called coding strand or sense strand or non-template strand. Promoter sequences are present upstream towards the 5' end of the structural gene of transcription unit. It is a DNA sequence that provides binding site for RNA polymerase. Certain short sequence is present within the promoter sites, known as recognition sites. The terminator sequence is present at 3' end of coding strand and it usually defines the end of transcription.

Transcription in prokaryotes

It occurs in cytoplasm with the help of enzyme DNA dependent RNA polymerase which is only of one type and transcribe all types of RNA including mRNA, tRNA and rRNA. RNA polymerase enzyme is a holoenzyme that is made of polypeptides (2'). The enzyme without subunit is reffered to as core enzyme. The process of transcription is done in # steps :

1) Initiation - It is catalysed by (sigma) factor or initiation factor. It binds to the promoter site of DNA and confers specificity. In the absence of sigma factor, transcription starts non-specifically by core enzyme at any base on DNA. This sigma factor helps in finding the start of transcription or gene.

2) Elongation - The RNA polymerase (core enzyme) is only capable of catalysing the process of elangation.

3) Termination - Rho factor () is required for termination of transcription.

In prokaryotes, mRNA does not require ant processing to become active.Transcription in eukaryotes

There are three types of transcripting enzymes that is RNA ploymerases in the nucleus in addition to RNA polymerase found in the organelles. There is clear cut division of labour. Functions of different polymerases in eukaryotes are :

1) RNA polymerase | - 5.8S, 18S, 28S rRNA synthesis

2) RNA polymerase || - hnRNA synthesis

3) RNA polymerase ||| - tRNA, ScRNA, 5S rRNA and SnRNA synthesis

The primary transcript hnRNA is converted into functional mRNA after 3 steps of processing :

1) Capping - Capping at 5' end occurs rapidly after the strat of transcription. An usual nucleotide that is methyl guanosine triphosphate is added to the 5'-end of hnRNA. It is catalysed by guanyl transferase. Cap is essential for formation of mRNA-ribosome complex. Translation is not possible if cap is lacking because cap is identified by 18SrRNA of ribosome unit.

2) Tailing - Tailing is the addition of adenylate residues about 200-300 at 3' end in a template-independent manner on newly formed hnRNA with the help of Poly A polymerase.

3) Splicing - Splicing is the process of removal of introns and joining of exons in a defined order. Introns are removed by small nuclear RNA (SnRNA) and protein complex called small nuclear ribonucleoproteins or SnRNPs.

The fully processed hnRNA is now called mRNA and it is transported out of the nucleus for translation.