Overview of mRNA 5' Capping
In this video, we explore the detailed process of 5' capping of mRNA, a crucial step in RNA processing. The capping occurs co-transcriptionally, where a 7-methylguanylate (m7G) cap is added to the 5' end of the mRNA strand synthesized by RNA polymerase. This process involves several key enzymes and steps:
Key Steps in 5' Capping
- Transcription Process: RNA polymerase synthesizes mRNA from the DNA template, creating a strand that grows from the 5' to 3' end. For a deeper understanding of this process, you can refer to our summary on Understanding DNA Transcription: A Comprehensive Guide.
- Capping Initiation: The capping process begins shortly after transcription starts, with the addition of a 7-methylguanylate cap linked by a 5'-5' triphosphate bridge.
- Enzymatic Action: The enzyme phosphohydrolase removes the terminal gamma phosphate from the mRNA, leaving two phosphates.
- Guanosine Addition: GTP (guanosine triphosphate) donates a guanosine monophosphate (GMP) residue to the 5' end of the mRNA.
- Methylation: A methyl group is added to the guanosine residue at the seventh position, facilitated by a methyltransferase enzyme.
Cap Structures
- Cap 0: Contains only the m7G cap with a single methylation at the guanosine residue.
- Cap 1: Features methylation at the guanosine and the first ribose sugar of the mRNA.
- Cap 2: Includes methylation at the guanosine and both the first and second ribose sugars.
Functions of mRNA Capping
- Exonuclease Protection: The cap protects mRNA from degradation, which is crucial for maintaining mRNA stability. For more on RNA stability, check out The Essential Roles of RNA in Genetics and Protein Synthesis.
- Nuclear Export: Facilitates the transport of mRNA from the nucleus to the cytoplasm.
- Translation Promotion: Aids in ribosome binding, enhancing translation efficiency. To learn more about the translation process, see our summary on Understanding Translation: The Process of Protein Synthesis Made Simple.
In the next video, we will discuss the molecular mechanisms of mRNA capping in greater detail, including structural diagrams. If you found this video helpful, please give it a thumbs up, consider supporting on Patreon, and subscribe for more content!
in the previous video we discussed about the overview of mrna processing now in this video we'll be discussing
about the pi prime capping of mrna in detail before we start our lecture
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now getting back to our capping process we see when we have a transcription process where dna as shown in the
diagram is being worked upon by rna polymerase this rna polymerase enzyme works on 3
prime to 5 prime end of dna strand and synthesizes a newly formed strand in 5 prime to three prime
end what we call as mrna strand and to this five prime end the capping occurs
it is the seven methyl gonoscene cap shown in the diagram which is added just after the transcription starts
so we can say capping is the co-transcriptional process this five prime cap is linked by a five
five triphosphate bridge and we have three different caps for five prime end as
cab zero cap and cap two these caps will be discussed in detail later in this video now if we
see the structure of mrna molecule we see at five prime end we have three phosphates alpha
phosphate beta and gamma phosphate then we have nitrogenous bases with sugars and
phosphates repeatedly to start a process of capping we have an enzyme called the
phosphohydrolase enzyme this phosphohydralase enzyme works on fibroprime end of mrna molecule
and it removes the terminal karma phosphate as shown in the diagram then we get a structure like this with
only two phosphates furthermore we have gtp molecule in this reaction it's having phosphates as alpha
beta and gamma shown in the diagram now for this reaction to occur we have gone on a transparent enzyme
that transports gmp residue from gtp here we see gmp that's gonosine monophosphate is getting attached to the
5 prime end of mrna leaving back us 2 phosphates in the next step mrna molecule adds
methyl group to the gonosine residue we see it's the acetomat molecule which takes part in this reaction
it has methyl group and by the help of methyltransplaced enzyme this methyl group from s aromat is
transported to the gonoscene residue so it acts as a methyl group donor here so we see guanosine is methylated on the
seventh position directly after capping and ultimately we get the m7g that's cap bound to the mrna at five
prime end which forms at the mature cap at five prime end of mrna
now let's see what are the concepts of cap zero cap1 and capital structures of mrna capping
the first normal structure which is m7g phosphate phosphate phosphate means methyl group is added to the seventh
position of gonosine only so what does this mean it means in this structure only one methylation has taken
place that's on the gonosine residue so this structure is termed as cab zero structure
now if you proceed further we have enzyme that modifies this mrna molecule further
we have two or methyltransplaced enzyme which transports methyl group to the mrna thereby giving us modified
structures of mrna cap it's when two hydroxy group on first tribal sugar is methylated
we call it cap1 whereas when methylation occurs at 2 hydroxy group on the first
two ribose sugars of mrna molecule as shown in the diagram we call it cap2 structure
here we see cab0 has only one methylated site that's at ganoshin residue then on cap1 we have methylation at
gaunosine and first ribose sugar of nitrogenous base as shown in the diagram and when we look
at the cab2 we have three methylated sites at gonoscene
and at first and second sugars of nitrogenous bases as shown in the diagram moreover one
thing to remember here is that this cap has diverse of functions but the major functions include exonuclease
protection nuclear export regulation and translation promotion
as it aids in ribosome binding so this is all about 5 prime capping or the mrna capping
and its modified structures in the next video we'll be discussing its mechanism at molecular level structural diagrams
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