DNA. Is a molecule that encodes the genetic instructions used in the development and functioning of all known living organisms and many viruses.

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2 Is a molecule that encodes the genetic instructions used in the development and functioning of all known living organisms and many viruses. Genetic information is encoded as a sequence of nucleotides (guanine, adenine, thymine, and cytosine) recorded using the letters G, A, T, and C. Most molecules are double-stranded helices, consisting of two long polymers of simple units called nucleotides, molecules with backbones made of alternating sugars (deoxyribose) and phosphate groups (related to phosphoric acid), with the nucleobases (G, A, T, C) attached to the sugars.

3 is well-suited for biological information storage, since the backbone is resistant to cleavage and the doublestranded structure provides the molecule with a built-in duplicate of the encoded information. The amounts of A = T, G = C, and purines = pyrimidines [Chargaff s Rule]. is a double-stranded helix with antiparallel strands [Watson and Crick]. Nucleotides in each strand are linked by 5-3 phosphodiester bonds Bases on opposite strands are linked by hydrogen bonding: A with T, and G with C

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5 Transcription Transcription Process by which a sequence is copied to produce a complementary RNA. In other words, it is the transfer of genetic information from into RNA. Like replication, but making RNA. Beginning of the process that ultimately leads to the translation of the genetic code (via mrna) into a protein.

6 Transcription The enzyme used in transcription is RNA polymerase. There are several forms of RNA polymerase. In eukaryotes, most genes are transcribed RNA polymerase types : RNA polymerase I : it synthesizes the precursor of the 28s,18s, and 5.8s r-rna in the nucleolus RNA polymerase II : it synthesizes the precursor of m-rna in addition to srrna RNA polymerase III : it produces the small RNA including trna 5s ribosomal RNA and some snrna

7 Transcription Unlike replication, transcription does not need to build on a primer. Instead, transcription starts at a region of called a promoter. For proteincoding genes, the promoter is located a few bases 5 to (upstream from) the first base that is transcribed into RNA. Promoter sequences are very similar to each other, but not identical. If many promoters are compared, a consensus sequence can be derived. All promoters would be similar to this consensus sequence, but not necessarily identical.

8 Process of Transcription Transcription The process of transcribing a typical gene in an eukaryotic cell is divided into three phases : initiation, elongation and termination. Initiation Eukaryotic RNA polymerase does not directly recognize the core promoter sequences. Instead, a collection of proteins called transcription factors mediate the binding of RNA polymerase and the initiation of transcription. Only after certain transcription factors are attached to the promoter does the RNA polymerase bind to it. The completed assembly of transcription factors and RNA polymerase bind to the promoter, forming a transcription initiation complex. Transcription in the archaea domain is similar to transcription in eukaryotes.

9 Transcription Initiation

10 Transcription Elongation One strand of the, the template strand (or noncoding strand), is used as a template for RNA synthesis. As transcription proceeds, RNA polymerase traverses the template strand and uses base pairing complementarity with the template to create an RNA copy. Although RNA polymerase traverses the template strand from 3' 5', the coding (non-template) strand and newly formed RNA can also be used as reference points, so transcription can be described as occurring 5' 3'. This produces an RNA molecule from 5' 3', an exact copy of the coding strand (except that thymines are replaced with uracils, and the nucleotides are composed of a ribose (5-carbon) sugar where has deoxyribose (one less oxygen atom) in its sugar-phosphate backbone).[citation needed]

11 Transcription Elongation Unlike replication, mrna transcription can involve multiple RNA polymerases on a single template and multiple rounds of transcription (amplification of particular mrna), so many mrna molecules can be rapidly produced from a single copy of a gene.[citation needed] Elongation also involves a proofreading mechanism that can replace incorrectly incorporated bases. In eukaryotes, this may correspond with short pauses during transcription that allow appropriate RNA editing factors to bind. These pauses may be intrinsic to the RNA polymerase or due to chromatin structure

12 Transcription Termination Eukaryotic protein genes contain a poly-a signal located downstream of the last exon. This signal is used to add a series of adenylate residues during RNA processing. Transcription often terminates at kb downstream of the poly-a signal, but the mechanism is unclear.

13 Transcription The role of regulatory transcription factors In eukaryotes, the association between and histones prevents access of the polymerase and general transcription factors to the promoter. Histone acetylation catalyzed by HATs can relieve the binding between and histones. Although a subunit of TFIID (TAF250 in human) has the HAT activity, participation of other HATs can make transcription more efficient. The following rules apply to most (but not all) cases: 1. Binding of activators to the enhancer element recruits HATs to relieve association between histones and, thereby enhancing transcription. 2. Binding of repressors to the silencer element recruits histone deacetylases (denoted by HDs or HDACs) to tighten association between histones and.

14 Transcription Eukaryotic mrna posttranscriptional modification The mrna molecule synthesized in eukaryotic nuclei by RNA polymerase II is a collection of the precursor molecules of mrna called as heterogeneous nuclear RNA (hnrna). The primary transcription are extensively modified in the nucleus after transcription. these modification usually include : 1_5 > capping : this process is the first of the processing reaction for hnrna the cap is a 7-methylguanosine attached ( backward) to the 5 terminl end of the mrna, forming an unusual 5 _5 triphosphate linkage. the creation of the guanosine triphosphate part of the cap requires the nuclear enzyme guanylyltransferase. methylation of this terminal guanine occurs in the cytosol, and is catalyzed by guanine -7- methyltransferase.the presence of this 7-methyl guanosine triphosphate cap is very essential in starting the mrna translation later on ( i.e. protein synthesis )

15 Protein synthesis (Translation) Translation is the RNA-directed synthesis of a polypeptide Translation involves: Messenger RNA (mrna): Carries information specifying amino acid sequences of proteins from lo ribosomes. Ribosomal RNA (rrna): Plays catalytic (ribozyme) rotes and structural roles in ribosomes. Transfer RNA (trna): Serves as adapter molecule in protein synthesis; translates mrna codons into amino acids. Genetic coding codons : Genetic information is encoded as a sequence of nonoverlapping base triplets, or codons

16 This is a molecule of messenger RNA. It was made in the nucleus by transcription from a molecule. A ribosome on the rough endoplasmic reticulum attaches to the mrna molecule

17 Another trna molecule comes into place, bringing a second amino Acid. Another trna molecule brings the next amino acid into place.

18 A peptide bond joins the second and third amino acids to form a Polyoeotide chain The process continues, the poly peptide chain get longer. This continues until termination (stop) codon is reached, the poly peptide is then completed

19 References: World Wide Web ((Internet)) Student Contribution: Zaid Thabit: Collection from references about Transcription. Zubaida Hamza: Collection from references about Translation. Fajer Bashir: Made the PowerPoint presentation.