Genetics Lecture 6 PDF
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Mansoura University
Dr. El-Sawy
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Summary
This document provides a lecture on post-transcriptional modifications of RNA, focusing on mRNA, tRNA, and rRNA. It explains the processes of capping, tailing, and splicing, and describes the role of the spliceosome in these processes. The document includes diagrams to illustrate the concepts.
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Genetics Post Transcriptional Modification LECTURE (6) DR. El-Sawy 1 Genetics...
Genetics Post Transcriptional Modification LECTURE (6) DR. El-Sawy 1 Genetics Post Transcriptional Modification Nearly all eukaryotic RNA (mRNA, rRNA and tRNA) primary transcripts undergo extensive processing. mRNA formed and released from DNA template is known as primary transcript which undergoes extensive processing to become the mature mRNA. Processing occurs primarily within the nucleus. Processing of mRNA : Tailing (addition of Splicing Capping : poly-adenylate tail) : (Removal of intron) : Addition of 7 Addition of (20-250) Cutting out the intron & methyl guanosine adenine nucleotide joining the ends of Def cap to 5 end of at 3 end by poly-A- exons. mRNA. polymerase enzyme. 1. Protect 5ʼ end of 1. Protect 3ʼ end of The primary transcripts mRNA from attack mRNA from attack by are very long. by 5' → 3' 3' → 5' The primary transcript Function exonucleases. exonucleases. contains coding regions 2. Required for 2. Facilitate translation. (exons) interrupted efficient translation with noncoding regions initiation. (introns). Spliceosome is the molecular complex that performs the splicing process. Splicing is an energy requiring process. DR. El-Sawy 2 Genetics Post Transcriptional Modification DR. El-Sawy 3 Genetics Post Transcriptional Modification tRNA is made from large precursor molecule that must be modified. 1. Sequences at both ends are removed. 2. If present, intron in anticodon loop is removed by nuclease. 3. Modifications of bases at specific positions to produce unusual bases characteristic of tRNA 4. Attachment of the characteristic three bases CCA terminal at 3’ end by nucleotidyl transferase in the cytoplasm. DR. El-Sawy 4 Genetics Post Transcriptional Modification In eukaryotes, the ribosomes are composed of: Four rRNA species (18S, 28S, 5.8S and 5S) Several proteins. S means Svedberg unit. In eukaryotes: 1. Three rRNA molecules (18S, 28S, 5.8S): Transcribed as a part of single large 45S precursor molecule. 45S precursor molecule is then processed to the three species of rRNA in the nucleolus. 2. Eukaryotic 5S rRNA: synthesized and modified separately. DR. El-Sawy 5
