Transcription is the process by which the genetic information from one strand of the DNA is copied to another strand of the RNA. Here also complementary base pairing is done and the complementarity principle governs the process of transcription also. There is just a small difference here that instead of uracil, thymine pairs with uracil. This is the general definition of transcription in biology. In the process of DNA replication, the whole of the DNA is replicated which means that both the strands of the DNA are replicated but here in this process, only one strand is transcripted. The reason behind this is that:
The RNA molecule would have different sequences if both the strands act as templates. This would result in a difference in codes for the proteins because the sequence of amino acids would be different. From this, we can say that one strand will code for more than one protein and hence different codes would be there for the proteins. But, this is not possible so due to this only the strand is transcripted.
If both the strands of the DNA are transcribed then it would result in two complementary strands of the RNA and as these two strands are complementary to each other, they will come together, and hence the translation process will be hampered.
We will learn more about the transcription factor and the difference between prokaryotic and eukaryotic transcription.
We will learn about the transcription factor here. For the smooth running of the transcription process, there are certain factors and units that are responsible for this process. They are very essential for the transcription process and hold utmost importance. The transcription unit is the segment of the DNA that takes part in the process of transcription. The three main components or the general transcription factors of the transcription are:
The template strand is also known as the non-coding strand. The enzyme that is responsible for catalyzing the template strand works in 5’ to 3’ polarity. It is able to recognize only such strands. Therefore it recognizes only one strand. The template strand has 3’ to 5’ polarity and the other strand which is known as the complementary strand has the polarity of 5’ to 3’. This strand is also known as a sense strand or non-template strand. On the 5’ end of the DNA, we have the presence of the promoter sequences. These sequences help in recognizing the polarity and also attract the enzyme to start the process of transcription. This sequence helps in providing a binding site for the RNA polymerase transcription enzyme. The sequences that help in recognizing the template strand are known as recognition sequences. Also on the 3’ end, there are certain sequences that are known as terminator sequences. As the name suggests, these sequences help in the process of termination of the transcription process.
Types of RNA
There are three types of RNA that are present. They are:
mRNA: It is known as messenger RNA. It comprises 5% of the total RNA in the cell. It is the longest RNA. It is also known as the template messenger or the nuclear messenger. It helps in carrying the genetic information that is given by the DNA.
rRNA: It is known as ribosomal RNA. It comprises 80% of total RNA. It is smaller than the mRNA. It plays a catalytic and structural role in the process of translation.
tRNA: It is known as the transfer RNA. It is the smallest RNA and is present in 15% of the total RNA content of the cell. It is also known as the soluble RNA and adapter RNA and it helps in carrying the amino acids. For the synthesis of proteins in the cell, all three types of RNA are needed.
This is not a difference between prokaryotic and eukaryotic transcription and all these RNAs are the same.
Transcription Unit and the Gene
Gene is the functional unit of the inheritance. On the DNA, these genes are located. The cistron is known as the functional unit of the gene. It is just a sequence of DNA that helps in coding the polypeptide. In eukaryotes, a monocistronic gene is present and a polycistronic gene is present in prokaryotes. Both of these are different in gene synthesis. Exons are known as the coding sequences and introns are known as the non-coding sequences. The introns are not present in the mature RNA because they are removed in the process of splicing. This is the difference between prokaryotic and eukaryotic transcription.
Transcription in Prokaryotes
This process takes place in the cytoplasm of the cell. It is done with the help of transcription enzymes. The enzyme that is responsible for the process is DNA-dependent RNA polymerase. This enzyme helps in transcribing all the types of RNA. The RNA polymerase is made up of different polypeptides and is a holoenzyme. The stages of transcription were:
Initiation: This is done with the help of sigma factors. The sigma factors are also known as the initiation factors. They get bound to the promoter site and then start the process of transcription.
Elongation: As the name suggests, in this process the core enzyme that is the RNA polymerase takes part and helps in the elongation of the strand.
Termination: For the termination of transcription, rho factors are required.
Some points can be summarised from the above process, that is:
No processing is required for the RNA to be active.
The process of transcription and translation takes place in the same compartment so they have the same site of transcription.
The process of transcription and translation can begin at almost the same time in the bacteria.
Transcription of Eukaryotes
The transcription of eukaryotes is almost similar to that of the transcription in prokaryotes. There are some modifications that take place in eukaryotes. They are capping and tailing. In capping, methyl guanosine triphosphate is added at the 5’ end of the DNA. This is an important step because it is essential for the formation of the mRNA-ribosome complex. If no cap is present then it is difficult to identify the DNA. In the tailing process, around 200-300 base pairs are added at the 3’ end of the DNA. The introns are removed from the DNA by the process of splicing as these are not coding sequences and are not essential for the DNA. These processes also help in transcription regulation.
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After the process of transcription and then translation the RNA is converted to a genetic code. This genetic code helps in coding the protein. So from this, we can say that DNA carries the information of the proteins because it is the DNA only that gets converted into RNA by the process of transcription, and then that RNA is converted into proteins with the help of genetic code. These proteins are required in almost all the functions of the body. So this way the sequence of the amino acids is important because specific amino acid sequences help in the coding of specific proteins. By studying the above information we can state that it is the genetic code that helps in holding a relationship between the DNA and the RNA.
Some Features of the Genetic Code are:
Three nitrogenous bases help in making the genetic code. Total there are 61 codons and these codons code for proteins. The codons are present in a triplet form.
We can see that only one codon helps in coding one amino acid so this means that only one amino acid can be coded from one coded and thus the genetic codes are unambiguous and specific in nature.
These codons are present in a continuous fashion and there are no punctuation marks or full stops in between these codons.
There are some specific start codons and stop codons that are present in the genetic codes and these help in starting and stopping the formation of genetic codes.
AUG is one of the start codons that help in forming or start the forming of the amino acid chain.