Transcription in eukaryotes

HNGU PATAN.
m.sc sem 3 botany
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1. Transcription in Eukaryotes

2. Contents Introduction Mechanism of transcription Similarities between prokaryotic and eukaryotic transcription Difference between prokaryotic and eukaryotic transcription

3. Introduction Transcription has been defined in various ways. Some definitions of transcription are given here. The synthesis of RNA from a single strand of a DNA molecule in the presence of enzyme RNA polymerase is called transcription . In other words, the process of formation of a messenger RNA molecule using a DNA molecule as a template is referred to as transcription .

4. 1. Synthesis :- RNA is synthesized from a DNA template. The RNA is processed in to messenger RNA ( m RNA) which is then used for synthesis of a protein. The RNA thus synthesized is called m RNA , because it carries a genetic message from the DNA to the protein synthesizing machinery of the cell. The main difference between RNA and DNA sequence is the presence of U, or uracil in RNA instead of the T of thymine of DNA.

5. 2. Template used :- The RNA is synthesized from a single strand or template of a DNA molecule. The stretch of DNA that is transcribed into an RNA molecule is called a transcription unit. A transaction unit codes the sequences that is translated into protein. It also directs and regulates protein synthesis.

6. The DNA strand which is used in RNA synthesis is called template strand ; because it provides the template for ordering the sequence of nucleotides in an RNA transcript. The DNA strand which does not take part in DNA synthesis is called coding strand because it's nucleotide sequence is the same as that of the newly created RNA transcript.

7. 3.Enzyme involved :- The process of transcription is catalyzed by the specific enzyme called RNA polymerase. DNA sequence is enzymatically copied by RNA polymerase to produce a complement tarry nucleotide RNA strand. In eukaryotes , there are three classes of RNA polymerase I, II and III which are involved in the transcription of all protein genes.

8. 4. Genetic information copied : In this process the genetic information coded in DNA is copied into a molecule of RNA. The genetic information is transcribed or copied from DNA to RNA. In other words it results in the transfer of genetic information from DNA into RNA.

9. 5 . first step :- The expression of a gene consists of two major steps vise ; transcription and translation. Thus transaction is the first step in the process of gene regulation or protein synthesis.

10. 6. Direction of synthesis :- As in DNA replication , RNA is synthesized in the 5' –3' direction. The DNA template strand is read 3'–5' by RNA polymerase and the new RNA strand is synthesized in the 5' –3' direction. RNA polymerase bind to the 3' end of a gene on the DNA template strand and travels to ward the 5' end. The regulatory sequence that is before or 5' of the coding sequence is called 5' untranslated region and sequence found following or 3' of the coding sequence is called 3' untranslated region.

11. Mechanism of Transcription The mechanism of transcription consists of three major steps , viz ; 1. Initiation 2. Elongation 3. Termination

12. 1. Initiation Pre - initiation :- In initiation of transcription does not require a prime to start. RNA polymerase simply binds to the DNA and along with other cofactors . the DNA to create an initiation bubble so that the RNA polymerase has access to the single stranded DNA template. However, RNA polymerase does require a promoter like sequence.

13. Proximal (core) promoters :- TATA promoters are found around –30bp to the start site of transcription. Not all genes have TATA box promoters and there exists TATA -less promoters and there exists promoter consensus sequence is TATA (A/T) A (A/T).

14. Initiation : – In eukaryotes and archaea , transcription initiation is Far more complex. The main difference is that eukaryotic polymerases do not recognize directly their core promoter sequence. In eukaryotes, a collection of proteins called transcription factors mediate the binding of RNA polymerase and the initiation of transcription. Only after attachment of certain transcription factors to the promoter , the RNA polymerase binds to it.

15. The complete assembly of transcription factors and RNA polymerase bind to the promoter, called transcription initiation complex. Initiation starts as soon as the complex is opened and the first phosphodiester bond is formed. This is the end of Initiation. RNA pol II does not contain a subunit similar to the prokaryotic factor, which can recognize the promoter and unwind the DNA double helix.

16. Con ... In eukaryotes, these two functions are carried out by a set of proteins called general transacritpton factors. The RNA pol II is associated with six general transcription factors, designated as TFIIA , TFIIB, TFIID, TFIIE, TFIIF and TFIIH where ' TF ' strands for transcription factors and ' II ' for the RNA pol II. TFIID consists of TBP ( TATA - box binding protein ) and TAFS ( TBP associated factors ). The role of TBP is the core promoter.

17. Con.. TAFs may assist TBP in this process. In human cells TAFs are formed by 12 subunits. One of them , TAFs 250 ( with molecular weight 250KD ) , has the histone acetyl transfers activity which can relieve the binding between DNA and histone in the nucleosome. The transcription factor which catalyzes DNA melting is TFIIH. However, before TFIIH can unwind DNA the RNA pol III at least five general transcription factors have to form a pre - initation complex.

18. Promoter clearance :- After the first bond is synthesized the RNA polymerase must clear the promoter. During this time there is a tendency to release the RNA transcript and produce truncated transcripts. This is called abortive imitation and is common for both eukaryotes and prokaryotes. Once the transcript reaches approximately 23 nucleotides it no longer slips and elongation can occur. This is an ATP dependent process.

19. 2.Elongation For RNA synthesis one strand of DNA known as the template strand or non coding strand is used as a template. As transcription proceeds RNA polymerase traverses the template strand and uses base pairing complementarity with the DNA template to create an RNA copy. Although RNA polymerase traverses the template strand from 3' –5' the coding strand is usually used as the reference point 50 transcription is said to go from 5'–3'.

20. This produces an RNA molecule from 5' –3' an exact copy of the coding strand ( except that thymines are replaced uracil's and the nucleotides are composed of a ribose sugar where DNA has deoxyribose in its sugar phosphate back bone. After pre - initiation complex is assembled at the promoter TFIIH can use its helicase activity to unwind DNA. This requires energy released from ATP hydrolysis. The DNA melting starts from about 10bp.

21. Then RNA pol II uses nucleoside triphosphate to synthesize a RNA transcript. During RNA elongation TFIIF remains attached to the RNA polymerase but all of the other transcription factors have dissociated from PIC. The carboxyl terminal domain (CTD ) of the largest subunit of RNA pol II is critical for elongation. In the imitation phase CTD is un -phosphorylated but during elongation it has to be phosphorylated. This domain contains many proline, serine residues .

22. 3. Termination In eukaryotic transcription the mechanism of termination is not very clear. In other words, it is not well understood. It involves cleavage of the new transcript followed by template independent addition of as at its new 3' and in a process called polyadenylation. Eukaryotic protein genes contain a poly -A signal located down stream of the last exon. This signal is used to add a series of adenylate residues RNA processing. Transcription often terminates at 0.5 - 2 kb downstream of the poly A signal.

23. Similarities between prokaryotic and eukaryotic transcription 1. In both groups DNA acts as the template for RNA synthesis. 2. In both groups transcription produces RNA molecule. 3. Chemical composition of transcript is similar in both groups. 4. Transcription is facilitated by the enzyme RNA polymerase in both groups. 5. In both groups one strand of the DNA duplex acts as the template.

24. Difference between prokaryotic and eukaryotic transcription No. Prokaryotic Eukaryotic 1 . Coupled transcription translation is the rule Coupled transcription translation is not possible 2 . Occurs in the cytoplasm Occurs in the nucleus 3 . There is not definite phase for its occurrence Take place in the G₁ and G₂ phase of cell cycle 4 . A single RNA polymerase synthesis all the three types of RNA (mRNA, tRNA , rRNA ) The RNA polymerase I , II and III synthesis rRNA , mRNA and tRNA respectively 5 . RNAs are related and processed in the cytoplasm RNA are released and processes in the nucleus 6 . RNA polymerase are complexes of five polypeptides RNA polymerases are complex of 10-15 polypeptides 7. Transcriptional units has one or more genes Transcriptional unit has only one gene

25. References Molecular biology by :- P . S . Verma and V . K . Agraval https:// en . m . Wikipedia . Org

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