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1. :(2)Rho-dependent

1.1. occur in sequence that are C-rich and G-poor. Rho binds to the C-rich terminator sequence in the transcript upstream then moves along the transcript until it reaches the RNA polymerase,Then new RNA strand is then release Ds-DNA helix reforms and RNA polymerase and Rho dissociate from the DNA

2. 2-Transcription in Prokaryotes

2.1. 1-Initiation

2.1.1. 1.2-These sequences generally are found at-35 and -10 , formerly called the Pribnow box, at the promoter

2.1.2. 1.1-The sigma factor of RNA polymerase bind into specific region of promoter (up stream sequence)

2.2. 2.1- the RNA polymerase begins to move along the DNA and the sigma factor is released

2.3. 2-Elongation

2.3.1. 2.2 the core RNA polymerase moves along, untwisting the DNA double helix .and the new mRNA synthesized and displaced as the DNA helix reforms.

2.4. 3- Termination

2.4.1. 3.1-There are two way to termination the process (1) Rho-independent (2)Rho-dependent 2.3-RNA polymerase has two proofreading activities (3’ – 5’):(1)Removing one nucleotide and adding correct one (2)Removing several nucleotides and adding new ones

2.4.2. :(1)Rho-independent RNA polymerase transcribes the terminator sequence which is part of the initial RNAcoding sequence of the gene the inverted repeat arrangement make the RNA fold into hairpin structure, so the RNA Pol slow down then stop.

3. 3-Transcription in Eukaryotes

3.1. • Gene :

3.1.1. 1- Intron

3.1.2. 2- Exon

3.2. 4-There are 4 main types of RNA: (a)mRNA (b)tRNA (c)rRNA (d)snRNA .

3.3. • Eukaryotic mRNAs parts :

3.3.1. 1- The leader sequence (5’untranslated region)

3.3.2. 2- The coding sequence

3.3.3. 1- Transcription of the gene by RNA polymerase II

3.3.4. 3- The trailer sequence (3’untranslated region)

3.4. • Production of mRNAs

3.4.1. modify pre-RNA into mRNA by RNA processing

3.5. • Processing of Pre-mRNA to Mature mRNA :

3.5.1. 2- Addition of the 5’ cap

3.5.2. 3- Addition of the poly(A) tail

3.5.3. 4- Splicing to remove introns

3.6. • Splicing

3.6.1. Mediated by spliceosomes consisting of : 1- small nuclear ribonucleoprotein particles (snRNPs) The snRNPs consist of snRNAs associated with proteins 2- Pre mRNA

3.6.2. Self-Splicing of Introns

3.7. • RNA world hypothesis

3.7.1. - Can carry out the necessary catalytic reactions required for life in the presumably primitive cells of the time

3.7.2. - Can store the genetic information at the same time

4. 1-Overview of the transcription process

4.1. 1-Transcription is the first step of gene expression- central dogma of molecular biology.

4.2. 2-Some regions in the DNA called "genes" are going to transcript into single RNA strand.

4.3. 3-The gene consist of three parts :(a)Promoter (b)RNA-coding sequence (c)Terminator