ADVERTISEMENTS:
In this article we will discuss about:- 1. Meaning of Transcription 2. Mechanism of Transcription 3. Transcription in Eukaryotes.
Meaning of Transcription:
Transcription is the process by which a complementary RNA strand is synthesised from a specific region of DNA. The three different types of RNA are transcribed from different regions of DNA. The genes that transcribe mRNA are called structural genes, while other regions of DNA that code for rRNA and tRNA are called determinants for DNA.
1. mRNA:
ADVERTISEMENTS:
The specific region of DNA which codes for mRNA is known as cistron. The cistron usually codes for a specific polypeptide. The synthesised mRNA strands passes out of the nucleus via the nuclear pores found in the nuclear membrane. An organism is capable of synthesising different types of mRNA. The size and length of the mRNA molecule is directly related to the size of the protein molecules.
Two types of mRNA are recognized:
i. Monocistronic:
The mRNA that carries the code of a single cistron.
ADVERTISEMENTS:
ii. Polycistronic:
When a mRNA carries a code from several adjacent DNA cistron it is known as polycistronic.
2. rRNA:
The prokaryotic cells contain three kinds of rRNA molecules namely 23SrRNA, 16SrRNA and 5SrRNA. The 23SrRNA and. 5SrRNA are found in the 50S ribosomal subunit, while the 16SrRNA occur in the 30S subunit. Eukaryotic cells also have three kinds of rRNA molecules namely 28SrRNA, 18SrRNA and 5SrRNA.
The 28SrRNA and 5SrRNA occur in the 60s ribosomal subunit, while the 18SrRNA occurs in the 40S subunit. The 18S and 28S are transcribed by the nucleolar organiser region (NOR) of chromosomes. The nucleolus acts as a site for synthesis and maturation of the rRNA molecules. 5SrRNA is transcribed by DNA located outside the NOR. (Table 3).
3. tRNA:
About 30-40 different tRNA genes are found in E.coli. In eukaryotes, about 60 genes are found that codes for different tRNA molecules.
Mechanism of Transcription (Fig. 7a):
The main enzyme involved in the process of transcription is DNA dependent RNA polymerase. The RNA polymerase is made of several subunits in both prokaryotes and eukaryotes. The RNA polymerase is similar to the DNA polymerase in that it also adds nucleotides to the 3′ end of the growing polynucleotide chain. In prokaryotes, only one type of RNA polymerase is found, while in eukaryotes, there are three different forms of RNA polymerase.
a. RNA polymerase I – for the synthesis of rRNA
b. RNA polymerase II – for the synthesis of mRNA
c. RNA polymerase III – for the synthesis of tRNA
The process of transcription requires a promoter region, structural gene and a terminator region.
ADVERTISEMENTS:
It is completed in three stages:
a. Initiation:
During initiation the RNA polymerase binds to a specific region of the DNA known as the promoter region. In bacteria, this requires an initiation factor known as sigma factor. The RNA polymerase-sigma complex binds to the promoter and initiates transcription of a specific strand of DNA. The DNA strand unwinds by the action of this complex. No primer is required for RNA synthesis.
b. Elongation:
ADVERTISEMENTS:
Elongation is the formation of a RNA strand by the DNA-dependent RNA polymerase enzyme. This enzyme catalyse the polymerisation in the 5′ – 3′ direction. Therefore the DNA strand that has the polarity in the 3’ – 5′ acts as a template and is known as the template strand. The other strand with the 5′ – 3′ polarity is called the coding strand.
c. Termination:
When the RNA polymerase encounters sequences known as terminators in the DNA, it triggers the termination of the transcription.
The differences in replication and transcription are:
ADVERTISEMENTS:
i. In replication, new DNA molecules are formed, while in transcription RNA molecules are synthesised.
ii. Uracil base is inducted in RNA in place of thymine base.
iii. RNA transcripts do not remain connected with DNA template, but separate out as single strands and move out of the nucleus in the cytoplasm and participate in the synthesis of protein.
iv. Several copies of RNA transcripts are released from each DNA template
Transcription in Eukaryotes:
Transcription in eukaryotes is similar to prokaryotes, but there are some striking differences, which are as follows:
a. Transcription in eukaryotes occurs within the nucleus under the direction of three separate forms of RNA polymerase.
ADVERTISEMENTS:
b. mRNA must move out into the cytoplasm before translation.
c. The initiation and regulation of transcription is more extensive than prokaryotes. In addition to promoter, other control units or enhancers may be located at positions away from the point of initiation of transcription.
d. mRNA in eukaryotes is processed from the primary RNA transcript by a process called maturation. In the primary RNA transcript, at the 5′ end a cap consisting of 7-methyl guanosine or 7 mG and at the 3′ end a tail of poly A are found. The cap is a chemically modified molecule of guanosine triphosphate (GTP) (Fig. 7b). The additional sequences on the mRNA that are not translated are referred to as untranslated regions or UTR.
e. The primary eukaryotic mRNA transcript is much longer and localised in the nucleus, when it is also called heterogeneous nuclear RNA (HnRNA) or pre-mRNA.
f. The eukaryotic mRNA are made up of two types of segments-non-coding introns and the coding exons. The introns are removed by a process called RNA splicing. Of a pair of small nuclear ribonucleo protein (SnRNPs pronounced ‘snurps’), one binds to 5′ splice site and the other to 3′ splice site.
ADVERTISEMENTS:
A spliceosome is formed because of interaction between SnRNPs and other proteins. This spliceosome uses energy of ATP to cut the RNA to release the introns and joins two adjacent exons to produce mature mRNA. Besides, these two post- transcriptional modifications, RNA editing may also take place before translation begins (Fig. 7c).
Informosomes:
In eukaryotes, mRNA does not enter the cytoplasm as naked RNA strands. The mRNA strands are associated with certain proteins. Spirin (1965) coined the term informosomes to this mRNA-protein complex. This happens when there is a delay in translation as in the case of an embryo. In an embryo, several genes are expressed only at a later stage during organogenesis. The proteins protect the mRNA from the degrading action of ribonucleases, which are enzymes that digest mRNA.
