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TRANSCRIPTION IN EUKARYOTES, SANDHYA A 191822016 - Coggle Diagram
TRANSCRIPTION IN EUKARYOTES
DEFINITION
Transcription is the process by which the information in a strand of DNA is copied into a new molecule of RNA.
It is the first step of gene expression, in which a particular segment of DNA is copied into RNA (especially mRNA) by the enzyme RNA polymerase.
ENZYMES INVOLVED
The genes encoding some of the subunits of each eukaryotic enzyme show DNA sequence similarities to genes encoding subunits of the core enzyme
RNA polymerase II (RNA Pol II)
RNA polymerase I (RNA Pol I)
Each of the three eukaryotic RNA polymerases contains 12 or more subunits and so these are large complex enzymes.
MAIN FACTORS/FEATURES
Transcription in eukaryotes occurs within the nucleus and mRNA moves out of the nucleus into the cytoplasm for translation
The initiation of RNA synthesis by RNA polymerase is directed by the presence of a promoter site on the 5’ side of the transcriptional start site.
The RNA polymerase transcribes one strand, the antisense (-) strand, of the DNA template.
RNA synthesis does not require a primer.
RNA synthesis occurs in the 5’ → 3’ direction with the RNA polymerase catalyzing a nucleophilic attack by the 3-OH of the growing RNA chain on the alpha-phosphorus atom on an incoming ribonucleoside 5-triphosphate.
mRNA in eukaryotes is processed from the primary RNA transcript, a process called maturation.
PROCESS/MECHANISM
PRE-INITIATION
TATA Binding Protein (TBP) is a subunit of TFIID and binds to the promoter, creating a sharp bend.
TBP-TFIIA interact; TBP-TFIIB interact; TFIIB-TFIIF interact & TFIIF recruits RNA Pol II; TFIIE joins the group and recruits TFIIH
Subunits within TFIIH that have ATPase and helicase activity create negative superhelical tension in the DNA.
INITIATION
This produces a truncated polypeptide chain.
Many cycles of abortive initiation may occur before actually producing a growing polypeptide chain.
Before entering elongation phase, The polymerase may terminate prematurely.
Promoter melting requires hydrolysis by ATP and is mediated by TFIIH. TFIIH pulls the double stranded DNA into the cleft of RNA Polymerase and helps in transition from closed to open state. The two strands get separated.
Negative superhelical tension causes approximately one turn of DNA to unwind and form the transcription bubble.
This helps in providing a scrunching kind of motion.
ELONGATION
The polypeptide chain is elongated with the help of Elongation Factors.
RNA Pol conveniently adds nucleotides to the 3’ end. The template strand for this is known as the sense strand and the other anti-sense strand.
There are different classes of elongation factors. Some factors can increase the overall rate of transcribing, some can help the polymerase through transient pausing sites, and some can assist the polymerase to transcribe through chromatin
TERMINATION
TYPE 1
Factor dependent requires Termination Factors along with RNA Pol I.
FACTOR DEPENDENT
TYPE 2
FACTOR INDEPENDENT
Factor Independent termination can be done by RNA Pol III. A stretch of Thymines along a hair pin loop causes disintegration of complexes.
RNA PROCESSING
CLEAVAGE
Larger RNA precursors are cleaved to form smaller RNAs.
Primary transcript is cleaved by ribonuclease-P (an RNA enzyme) to form 5-7 tRNA precursors.
CAPPING AND TAILING
Initially at the 5′ end a cap (consisting of 7-methyl guanosine or 7 mG) and a tail of poly A at the 3′ end are added.
The cap is a chemically modified molecule of guanosine triphosphate (GTP).
SPLICING
The eukaryotic primary mRNAs are made up of two types of segments; non-coding introns and the coding exons.
The introns are removed by a process called RNA splicing where ATP is used to cut the RNA, releasing the introns and joining two adjacent exons to produce mature mRNA.
NUCLEOTIDE MODIFICATIONS
They are most common in tRNA-methylation (e.g., methyl cytosine, methyl guanosine), deamination (e.g., inosine from adenine), dihydrouracil, pseudouracil, etc.
SIGNIFICANCE
Transcription of DNA is the method for regulating gene expression.
It occurs in preparation for and is necessary for protein translation.
DIFFERENCE FROM PROKARYOTIC TRANSCRIPTION
Initiation is more complex
Termination does not involve stem-loop structures.
Transcription is carried out by three enzymes (RNA polymerases I, II and III).
The regulation of transcription is more extensive than prokaryotes.
SANDHYA A 191822016