M.B.Ch.B, MSC, DCH (UK), MRCPCH 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 dsDNA molecule to form the base sequence of a ssRNA.

Types of RNA 1. Ribosomal RNA (rRNA) 2. Transfer RNA (tRNA) Ribosomal RNA (rRNA), which is the most abundant type of RNA in the cell. It is used as a structural component of the ribosome. Transfer RNA (tRNA), which is the second most abundant type of RNA. Its function is to carry amino acids to the ribosome, where they will be linked together during protein synthesis. Messenger RNA (mRNA), which carries the information specifying the amino acid sequence of a protein to the ribosome. Heterogeneous nuclear RNA (hnRNA or pre-mRNA), which is found only in the nucleus of eukaryotic cells. It represents precursors of mRNA. Small nuclear RNA (snRNA), which is also only found in the nucleus of eukaryotes. One of its major functions is to participate in splicing (removal of introns) mRNA. Ribozymes, which are RNA molecules with enzymatic activity. They are found in both prokaryotes and eukaryotes. 2. Transfer RNA (tRNA) 3. Messenger RNA (mRNA) 4. Heterogeneous nuclear RNA (hnRNA or pre-mRNA) 5. Small nuclear RNA (snRNA) 6. Ribozymes

The 3 main types of RNA

RNA Polymerases Are enzymes that synthesis RNA In eukaryotic cells, there are 3 types RNA polymerase I synthesizes rRNA RNA polymerase II synthesizes mRNA RNA polymerase III synthesizes tRNA

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՜

Coding (antitemplate) strand 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 ATCG GCTA CGTA AUCG GCUA During RNA synthesis, the DNA template sequence TAGC would be transcribed to produce ………………………. sequences? ATCG GCTA CGTA AUCG GCUA

Answer Our DNA template is 5՜ TAGC 3՜ This will transcribed to 3՜AUCG 5՜ in mRNA 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 ATCG GCTA CGTA AUCG GCUA During RNA synthesis, the DNA template sequence TAGC would be transcribed to produce ………………………. sequences? ATCG GCTA CGTA AUCG GCUA

Sample Questions GCGCTG CUGCGC GCGCUG CAGCGC GUCGCG The base sequence of a gene is CTGCGC. What is the base sequence of the mRNA produced upon transcription? GCGCTG CUGCGC GCGCUG 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 mRNA NOTE: The mRNA sequence must be identical to the code strand sequences (except U for T)

Production of Messenger RNA Production of pre-mRNA (precursors of mRNA) Processing of pre-mRNA to mature mRNA

Production of pre-mRNA

Processing of Pre-Messenger RNA 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 mRNA chain from degradation. 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. Introns are removed from hnRNA by splicing, accomplished by spliceosomes, which are complexes of snRNA and protein.