Dna transcription

very easy to understand transcription topic within a very less time.

1. TOPIC : TRANSCRIPTION Dr. Shree Lakshmi.BN 1yr MvSc Dept.VPT LUVAS, Hisar,Haryana

2. Transcription Cells are governed by a cellular chain of command DNA  RNA  protein Transcription Is the synthesis of RNA under the direction of DNA Produces messenger RNA (mRNA) For this purpose, one of the 2 strands of DNA serves as a template(anti sense or non coding strand) and produces working copies of RNA molecules. Other DNA strand referred as coding strand used since with the exception of T for U

3. Transcription is selective Primary transcript : product formed in transcription ,are inactive. which undergo alterations like splicing,terminal addition,base modifications…etc known as post transcriptional modifications to produce functionally active RNA molecules.

4. Transcription in prokaryotes There is a single prokaryotic RNA polymerase that synthesizes all types of RNA in the cell. The core polymerase responsible for making the RNA molecule has the subunit structure (α2ββ`). A protein factor called sigma (σ) is required for the initiation of transcription at a promoter. Sigma factor is released immediately after-initiation of transcription. Termination of transcription sometimes requires a protein called rho (ρ) factor.

5. Synthesis of an RNA Transcript The stages of transcription are Initiation Elongation Termination Promoter Transcription unit RNA polymerase Start point 5  3  3  5  3  5  5  3  5  3  3  5  5  3  3  5  5  5  Rewound RNA RNA transcript 3  3  Completed RNA transcript Unwound DNA RNA transcript Template strand of DNA DNA 1 Initiation. After RNA polymerase binds to the promoter, the DNA strands unwind, and the polymerase initiates RNA synthesis at the start point on the template strand. 2 Elongation. The polymerase moves downstream, unwinding the DNA and elongating the RNA transcript 5   3  . In the wake of transcription, the DNA strands re-form a double helix. 3 Termination. Eventually, the RNA transcript is released, and the polymerase detaches from the DNA.

6. Initiation The following events occur during the expression of a prokaryotic gene: With the help of sigma factor, RNA polymerase recognizes and binds to the promoter, region. The bacterial promoter contains two "consensus" sequences, called the Pribnow box (or TATA box) and the -35 sequence. The promoter identifies the start site for transcription and orients the enzyme on the template strand.

7. Elongation RNA polymerase locates genes in DNA by searching for promoter regions . The promoter is the binding site for transcription factors and RNA polymerase. RNA polymerase moves along the template strand in the 3' to 5' direction as it synthesizes the RNA product in the 5' to 3' direction using NTPs (ATP, GTP, CTP, UTP) as substrates. RNA polymerase does not proofread its work. The RNA product is complementary and antiparallel to the template strand. The coding (non-template) strand is not used during transcription. It is identical in sequence to the RNA molecule, except that RNA contains uracil instead of the thymine found in DNA.

8. Termination There are two kinds of transcription terminators commonly found in prokaryotic genes: Rho-independent termination occurs when the newly formed RNA folds back on itself to form a GC-rich hairpin loop closely followed by 6-8 U residues due to the presence of Palindromes. These two structural features of the newly synthesized RNA promote dissociation of the RNA from the DNA template. Rho-dependent termination requires participation of rho factor. This protein binds to the newly formed RNA and moves toward the RNA polymerase that has paused at a termination site. Rho then displaces RNA polymerase from the 3' end of the RNA.

9. Transcription in eukaryotes RNA polymerases There are three eukaryotic RNA polymerases RNA polymerase I is located in the nucleolus and synthesizes 28S, 18S, and 5.8S rRNAs . RNA polymerase II is located in the nucleoplasm and synthesizes hnRNA /mRNA and some snRNA . RNA polymerase III is located in the nucleoplasm and synthesizes tRNA , some snRNA , and 5S rRNA . Transcription factors (such as TFIIB for RNA polymerase II) help to initiate transcription.

10. Promoter sites Transcription of a typical eukaryotic gene occurs as follows: 1. With the help of proteins called transcription factors, RNA polymerase II recognizes and binds to the promoter region. The basal promoter region of eukaryotic genes usually has two consensus sequences called the TATA box (also called Hogness box) and the CAAT box. 2. RNA polymerase II separates the strands of the DNA over a short region to initiate transcription and read the DNA sequence. The template strand is read in the 3' to 5' direction as the RNA product (the primary transcript) is synthesized in the 5' to 3' direction. Both exons and introns are transcribed.

11. Initiation A large number of transcription factors interact with eukaryotic promoter regions. TFIID,TFIIA,TEIIB,TFIIF,TFIIE,TFIIH

12. Transcription Enhancers and Silencers Both are Binding Sites for Transcription Factors (TF’s) Enhancers: Increase the amount of Transcription from a nearby promoter (core + upstream elements) Silencers: Decrease amount of Transcription from nearby promoters Initially Defined as being “Position and orientation independent” Found upstream, within, or downstream of genes Function in either orientation (not always true)

13. What Are Enhancers? Enhancer = “non-promoter DNA elements that stimulate transcription” First found in eukaryotes and then found in bacteria Binding sites for transcription factors (= activators or enhancer binding proteins). NOT the binding sites for RNA polymerase Can function over long distances (100 - 1000 bp ) away from +1 sites (upstream or downstream) They are also tissue-specific (rely on tissue-specific DNA-binding proteins for activity). Sometimes a DNA element can act as an enhancer or a silencer depending on what is bound to it.

14. Heterogenous nuclear RNA( hnRNA ) The primary mRNA transcript produced by RNA Polymerase II in eukaryotes is often referred to as heterogeneous nuclear RNA. This is then processed to produce mRNA needed for protein synthesis.

15. Post transcriptional modifications Transcription of RNA processing occur in the nucleus. After this, the messenger RNA moves to the cytoplasm for translation. The cell adds a protective cap to one end, and a tail of A’s to the other end. These both function to protect the RNA from enzymes that would degrade Most of the genome consists of non-coding regions called introns Non-coding regions may have specific chromosomal functions or have regulatory purposes Introns also allow for alternative RNA splicing Thus, an RNA copy of a gene is converted into messenger RNA by doing 2 things Add protective bases to the ends Cut out the introns

16. Transcription Overview

17. Difference Between Prokaryotic and Eukaryotic Transcription Prokaryotes Location Transcription Transcription of mRNA Type of mRNA Occurs in cell cytoplasm Transcription happen simultaneously. mRNA is transcribed directly from template DNA molecule. The type of RNA polymerase does not vary with the bacterial type. Occurs in cell nucleus Transcription differ in space and time (transcription – nucleus, translation – cytoplasm) Initially a pre-mRNA molecule (primary transcript) is formed and then processed to yield a mature mRNA The type of RNA varies with the organisms. eukaryotes

18. Prokaryotes eukaryotes RNA Polymerase Subunits Promoter Recognition Type of Transcription A single type of RNA polymerase, which has a core enzyme and other subunits, is involved . Prokaryotic RNA polymerase consists of five subunits. In prokaryotes , holoenzyme (RNA polymerase + sigma factor) recognizes and binds directly to the promoter. In prokaryotic , DNA is not bound to the histone proteins. Therefore, transcription occurs directly. Type of RNA polymerase varies according to the type of RNA that is transcribed. Eukaryotic RNA polymerase consists of 10 – 17 subunits. In eukaryotes , promoter recognition cannot be carried out by RNA polymerase alone. a complex of histone proteins and DNA should be accessible, before the transcription.

19. Promoter Transcription Terminators Binding to the RNA Molecule no such differentiation can be seen. Prokaryotic cells possess two types of transcription terminators; Rho-dependent terminators and Rho-independent terminators. Rho factor binds to the growing RNA molecule in the prokaryotic transcription. Eukaryotic DNA that is identified by the RNA polymerase II has two parts of the promoter known as core promoter and regulatory promoter. In eukaryotes transcription , the three RNA polymerases use different mechanisms for the termination. Termination factor in eukaryotes binds to the template DNA molecule.

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