A. Transcription:
During transcription, information of only one strand of DNA is copied into RNA. This strand of DNA acts as template.
Enzyme RNA polymerase catalyses the formation of RNA transcript.
DNA is located in the nucleoid of Prokaryotes and in nucleus of Eukaryotes.
DNA transcription takes place in nucleus in eukaryotes whereas translation occurs in cytoplasm.
DNA transfers information to m-RNA which then moves to ribosomes.
Transcription occurs in the nucleus during G1 and G2 phases of cell cycle.
DNA has promotor and terminator sites. Transcription starts at promotor site and stops at terminator site.
Actually the process of transcription, in both Prokaryotes and Eukaryotes, involves three stages viz. Initiation, Elongation and Termination.
Transcription Unit:
Each transcribed segment of DNA is called transcription unit.
It consists of
i. Promotor
ii. The structural gene
iii. A terminator.
Two strands of DNA in the structural gene show following features :
i. Promoter
The promotor is located towards 5' end of structural gene i.e. upstream. It is a DNA sequence that provides binding site for enzyme RNA polymerase.
RNA polymerase binds to specific Promoter. In prokaryotes, the enzyme recognizes the promotor by its sigma factor subunit.
ii. Structural genes
The two strands of DNA have opposite polarity. DNA dependent RNA polymerase catalyses polymerisation in 5'→3' direction. So the DNA strand having 3'→5' polarity acts as template strand. The other strand of DNA having 5'→3' polarity is complementary to template strand. The sequence of bases in this strand, is same as in RNA (where Thymine is replaced by Uracil). It is the actual coding strand. The information on this strand of DNA is copied on mRNA. This is called sense strand.
iii. Terminator
The terminator is located at 3' end of coding strand i.e. downstream. It defines the end of the transcription process.
After binding to promoter, RNA polymerase moves along the DNA and causes local unwinding of DNA duplex into two chains in the region of the gene.
Exposed ATCG bases project into nucleoplasm.
Only one strand functions as template (antisense strand) and the other strand is complementary which is actiually a coding strand (sense strand). The ribonucleoside tri phosphates join to bases of DNA template chain.
As transcription proceeds, the hybrid DNA-RNA molecule dissociates and makes mRNA molecule free.
When RNA polymerase reaches the terminator signal on the DNA, it leaves DNA and fully formed mRNA (primary transcript) is released.
As the mRNA grows, the transcribed region of DNA molecule becomes spirally coiled and attains (regains) double helical form.
In bacteria, m-RNA does not require any processing because it has no introns.
Prokaryotes posses only one type of RNA polymerase. In eukaryotes, there are three types RNA polymerases.
- RNA polymerase-I transcribes r-RNA.
- RNA polymerase-II transcribes m-RNA (primary transcript) and heterogeneous nuclear RNA (or hnRNA).
- RNA polymerase-III transcribes t-RNA and small nuclear RNA (snRNA).
Transcription unit and the gene:
The DNA sequence coding for m-RNA/t-RNA or r- RNA is defined as a gene.
Cistron is a segment of DNA coding for a polypeptide. A single structural gene in transcription unit is said to be monocistronic where as a long segment of DNA having set of various structural genes in one transcription unit is referred as polycistronic.
Structral genes in eukaryotes have interrupted non-coding sequences (introns). The coding sequences or express sequences are defined are exons. Only exons appear in procesed mRNA in Eukaryotes.
Processing of hnRNA:
In eukaryotes, forms of RNA transcribed from DNA are called primary transcripts.
Such transcripts undergo changes called processing or maturation before becoming functional.
Primary transcript is non functional and contains both exons and introns. During processing only introns are removed by the process called splicing.
Exons are joined in a definite sequence (order) by DNA ligase enzyme.
Heterogeneous nuclear RNA, undergoes the process of capping and tailing.
In capping, methylated guanosine tri phosphate is added to 5' end of hnRNA.
In tailing, polyadenylation take place at 3'end.
It is the fully processed hnRNA, now called m-RNA. For translation m-RNA is transported out of the nucleus through nuclear pore.
