Transcription and Translation PDF

TRANSCRIPTI ON & TRANSLATION Dr. Talal Al-Qaisi DEFINITION :- Transcription is synthesis of single stranded RNA from a double stranded DNA template. Its produces messenger RNA ( mRNA). Translation is the 1st stage of protein biosynthesis from RNA. In this process formation of a poly...

TRANSCRIPTI ON & TRANSLATION Dr. Talal Al-Qaisi DEFINITION :- Transcription is synthesis of single stranded RNA from a double stranded DNA template. Its produces messenger RNA ( mRNA). Translation is the 1st stage of protein biosynthesis from RNA. In this process formation of a polypeptide by using mRNA as a template. It occurs in ribosomes. Transcription and Translation both process are the part of gene expression. TYPES OF RNA RNA molecule plays a variety of roles in the cell. The major types of RNA are 1.Ribosomal RNA (rRNA), which is the most abundant type of RNA in the cell, it is used as a structural component of the ribosome. Ribosomal RNA associates with ribosomal protein to form the complete ribosome. 2.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. 3.Messenger RNA (mRNA), which carries the information specifying the amino acid 10/21/2024 3  Cont… In a eukaryotic cell the nuclear envelope separates transcription from translation. Extensive RNA processing occurs in the nucleus.  TRANSCRIPTION INTRODUCTION :- Transcription is the synthesis of mRNA from a DNA template which occurs in 5’-3’ direction. During transcription , a DNA sequence is read by an RNA polymerase which produce a complementary and antiparallel RNA strand. Transcription is the first step leading to gene expression.  Cont… The stretch of DNA transcribed into an RNA molecule is called as transcription unit which encoded at least one gene. The result of the transcription is a mRNA which will then be used to create that protein via the process of translation  Transcription Prokaryotes Vs Eukaryotes. Prokaryotic transcription occurring in cytoplasm alongside translation and eukaryotic transcription occurring only in the nucleus where it is separated from the cytoplasm by the nuclear membrane. Eukaryotic DNA not currently used in stored as heterochromatin around histones to from nucleosomes and must be unwound as euchromatin to be transcribed. STAGES OF TRANSCRIPTION :- There are three stages involved in transcription :- A. INITIATION B. ELONGATION C. TERMINATION  INITIATION:- RNA polymerase binds to specific DNA region and initiate transcription called as promoter site. RNA polymerase is the enzyme responsible for transcription. It have 5 subunits:-2αsubunit,βsubunit,β’subunit,ω subunit. After polymerase is bound to the promoter DNA, the two DNA strands unwind and the enzyme starts transcribing the template strand.  Cont… The position of the first synthesized base of the RNA is called the start site.  FIGURE 3 10/21/2024 14 ELONGATION:- RNA polymerase moves along DNA template and sequentially synthesizes the RNA chain. DNA is unwinding ahead of the moving polymerase and the helix is reformed behind it. It unwinds 10-20 DNA bases at a time. RNA polymerase add nucleotides in the 5’-3’ direction. The new section of RNA ‘peels away’ as the double helix reforms. TERMINATION:- Transcription stops when RNA polymerase reaches a section of DNA called the terminator. Terminator sequence = AAUAAA. Next, the RNA strand is released and RNA polymerase dissociates from the DNA. The RNA strand will go through more processing.  RNA Processing :- The orignal transcript from the DNA is called as pre-m RNA. It contains transcript of both intron and exons. Intron:- it is non-coding sections of nucleic acid found between coding regions. Exons:-coding regions of nucleic acids Pre-mRNA never leaves the cell’s nucleus.  Cont… The introns are excised and exons are joined together to form mRNA. The introns are removed by a process called splicing to produce messenger RNA (mRNA)  TRANSLATION Translation is a process in which the formation of polypeptide (PROTEIN) by decoding mRNA produced in transcription. It occurs in ribosome which are present in cytoplasm. It begins after mRNA enters in cytoplasm. It uses tRNA as the interpreter of mRNA. PHASES IN TRANSLATION :- Translation proceed in four phases:- 1. INITIATION 2. ELONGATION 3. TRANSLOCATION 4. TERMINATION INITIATION :- The initiation stage of translation brings together mRNA, tRNA bearing the first amino acid of the polypeptide, and two subunits of a ribosome The components involved are the large and small subunits of ribosome, mRNA, initiator tRNA in its charged form and three factors ( IF1.IF2,IF3) and GTP. The tRNA has a amino acid linked to it is term as Charged tRNA.  Cont… IF1 and IF2 bind to free 30S subunit. IF3 complexed with GTP then bind to the small subunit. It will assist the charged initiator tRNA to bind. The assembled ribosome has 2 tRNA binding sites. These are called A-site (acceptor) for aminoacyl and P- site(donor)for polypeptide. The A-site is where incoming aminoacyl- tRNA moleules bind and P-site where the growing polypeptide chain usually found.  Cont… One major outcome of initiation is the placement of initiator tRNA in the P- site. Start codon :- AUG Start anticodon :- UAC The small ribosomal subunit attaches to 5’ end of mRNA. ELONGATION:- In this amino acid are added one by one to the first amino acid called as amino acid delivery. In codon recognition , mRNA codon in the A site forms hydrogen bond with the tRNA anticodon. In peptide bond formation, the ribosome catalyzes the formation of the peptide bond between the amino acids. The polypeptide extending from the P-site moves to A-site to attach to the new Amino Acid. In elongation process three elongation factors TRANSLOCATION :- The t-RNA with the polypeptide chain in the A site is translocated to the P site. tRNA at the P site moves to the E site and leaves the ribosome. The ribosome moves down the mRNA in the 5’-3’ direction. TERMINATION:- Protein factors called release factors interact with the stop codons and cause release of the completed polypeptide chain. Stop codon – UAA,UAG,UGA. RF1 recognises the codons UAA and UAG. RF2 recognises UAA and UGA. RF3 helps either RF1 and RF2 to carry out the reaction. Translation in Eukaryotes:- INITIATION:- Eukaryote have atleast 9 initiation factor. Eukaryotic initiator t RNA does not become formylated as in prokaryotes. ELONGATION:- The factors eEF1a,eEF1b,eEFz are involved in elongation. TERMINATION:- Eukaryotes have only one release factor eRF which recognize all the stop codons.