Molecular Biology Lec 2 Dr Mohammed Hussein M
Molecular Biology Lec. 2 Dr. Mohammed Hussein M. B. Ch. B, MSC, DCH (UK), MRCPCH
Transcription and RNA Processing
Transcription
What is Transcription? • The First Stage in the expression of genetic information. • Is the transcription of the information in the base sequence of a ds. DNA molecule to form the base sequence of a ss. RNA.
Types of RNA 1. Ribosomal RNA (r. RNA), which is the most abundant type of RNA in the cell. is used as a structural component of the 2. Transfer RNAIt (t. RNA) ribosome. 3. Messenger RNA (m. RNA) 2. Transfer RNA (t. RNA), which is the second most abundant type of Its function nuclear is to carry amino acids or to pre-m. RNA) the ribosome, 4. RNA. Heterogeneous RNA (hn. RNA where they will be linked together during protein synthesis. 5. Small nuclear (sn. RNA) 3. Messenger RNA (m. RNA), which carries the information specifying the amino acid sequence of a protein to the 6. Ribozymes ribosome. 4. Heterogeneous nuclear RNA (hn. RNA or pre-m. RNA), which is found only in the nucleus of eukaryotic cells. It represents precursors of m. RNA. 5. Small nuclear RNA (sn. RNA), which is also only found in the nucleus of eukaryotes. One of its major functions is to participate in splicing (removal of introns) m. RNA. 6. Ribozymes, which are RNA molecules with enzymatic activity. They are found in both prokaryotes and eukaryotes.
The 3 main types of RNA
RNA Polymerases • Are enzymes that synthesis RNA • In eukaryotic cells, there are 3 types 1. RNA polymerase I synthesizes r. RNA 2. RNA polymerase II synthesizes m. RNA 3. RNA polymerase III synthesizes t. RNA
Gene • Physically a gene consists of a sequence of DNA that encodes a specific protein. • It is now estimated that the human genome contains 20 000– 25 000 genes
Exons & Introns Exon = Expressed Intron= Interval
Transcription
Promoter & Terminator
RNA polymerase locates genes in DNA (template strand) by searching for promoter regions 3՜ 5՜
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 (nucleoside triphosphates) as ATP, GTP, ………etc. 3՜ 5՜
The coding (antitemplate) 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. Coding (antitemplate) strand Template strand
Sample Questions • During RNA synthesis, the DNA template sequence TAGC would be transcribed to produce ……………. sequences? 1. 2. 3. 4. 5. ATCG GCTA CGTA AUCG GCUA
Answer • • • Our DNA template is 5՜ TAGC 3՜ This will transcribed to 3՜ AUCG 5՜ in m. RNA As all base sequences should be written in 5՜ to 3՜ So the sequences should write as 5՜ GCUA 3՜ Or just GCUA
Sample Questions • During RNA synthesis, the DNA template sequence TAGC would be transcribed to produce ……………. sequences? 1. 2. 3. 4. 5. ATCG GCTA CGTA AUCG GCUA
Sample Questions • The base sequence of a gene is CTGCGC. • What is the base sequence of the m. RNA produced upon transcription? 1. 2. 3. 4. 5. GCGCTG CUGCGCUG CAGCGC GUCGCG
Answer • Our DNA coding strand is 5՜ CTGCGC 3՜ • So the template strand must be 3՜ GACGCG 5՜ • This will transcribed to 5՜ CUGCGC 3՜ in m. RNA • NOTE: The m. RNA sequence must be identical to the code strand sequences (except U for T)
Production of Messenger RNA 1. Production of pre-m. RNA ( precursors of m. RNA) 2. Processing of pre-m. RNA to mature m. RNA
Production of pre-m. RNA
Processing of Pre-Messenger RNA 1. 7 -methylguanosine cap is added to the 5՜ end while the RNA molecule is still being synthesized. The cap structure serves as a ribosome-binding site and also helps to protect the m. RNA chain from degradation. 2. Poly-A tail is attached to the 3՜ end. In this process, an endonuclease cuts the molecule on the 3՜ side of the sequence AAUAAA (poly-A addition signal), then poly-A polymerase adds the poly-A tail (about 200 As) to the new 3՜ end. The poly-A tail protects the message against rapid degradation and aids in its transport to the cytoplasm. 3. Introns are removed from hn. RNA by splicing, accomplished by spliceosomes, which are complexes of sn. RNA and protein.
- Slides: 31