Start studying Prokaryotic and Eukaryotic protein synthesis
4 stages of protein biosynthesis
What are the four stages of protein synthesis
-site at which protein synthesis occurs
-site where interaction between mRNA and tRNA occurs
-the general structure of the ribosome is conserved between prokaryotesand eukaryotes,
but there are differences in size and composition
Ribosomes have several active centers (Fig 6.27)
- binding sites of the ribosome are large
- there are different locations for several ribosomal activities
- binds mRNA
- binds initiator tRNA
- peptidyl transferase center
- responsible for translocation
- distinct P and A sites
- the tRNAs are very bulky and the problem of fitting them intoa ribosome is solved by a turn in the mRNA
- this turn causes the tRNAs to become angled which forces translocation(the moving of the tRNAs during elongation) to occur
- The 30s ribosome has several active centers, one of which isthe S1 protein (Fig 6.26)
- Initial bonding of the mRNA to the 30s subunit requiresthe S1 protein which has an affinity for single stranded nucleicacids
- The S1 protein is responsible for maintaining the single strandedstate of mRNA that is bound to it
The role of rRNA in protein synthesis
- there is evidence that rRNA interacts with mRNA and tRNA ateach stage of translation and is not just static
- the proteins are thought to be necessary to maintain the rRNAin a structure in which it can perform its catalytic functions
- recent studies suggest that the rRNA of the large subunit hasthe catalytic activity of the peptidyl transferase which fitswith the notion that the ribosome evolved from functions originallypossessed by RNA
process of protein synthesis in prokaryotes and ..
Translation is the synthesis of a polypeptide using the information in the mRNA
After transcription the mRNA moves to the cytoplasm and attaches to a ribosome.
A ribosome consists of a large subunit and a small subunit, each made up of proteins and one or more ribosomal RNAs (rRNAs)
As mRNA moves through a ribosome, codons are translated into amino acids by transfer RNA (tRNA)
1st : Initiation
Translation process is divided into 3 stages
Initiation of translation in bacteria begins when the small ribosomal subunit binds to the mRNA near its 5' end.
On the other side it the opposite.
5' on top
3' on top
Eukaryotes change RNA after transcription by adding a 5'cap to the 5' end and a poly-A tail to the 3'end of the mRNA strand to protect them from some enzymes and help them attach to the ribosome.
Transcription is the process by which the information in a strand of DNA is copied into a new molecule of messenger RNA (mRNA).
Discovered the DNA double helix model, and the proposal of two DNA strands held by hydrogen bonds.
Using a centrifuge, DNA molecules separated with different isotopes and revealed a pattern that supports the semiconservative model of DNA replication.
An isotope of an element is one that has more than the average number of neutrons for that element.
Transcription and Translation are both processes of protein synthesis in both Eukaryotic and Prokaryotic cells.
Both Eukaryotes and Prokaryotes require RNA polymerase to act on the DNA for transcription.
The end result of transcription and translation in both kinds of cells remain the same.
Each tRNA molecule has a specific anticodon at one end and a corresponding amino acid at the other.
Three stages of Protein Synthesis: Fig ..
In prokaryotes RNA processing is not necessary, but in eukaryotes the RNA must be processed before translation can occur.
3 main steps:
capping at 5' end
RNA splicing to remove introns
addition of polyA tail
Primary transcript is the initial product of transcription of an mRNA
Most genes have their protein-coding information interrupted by non coding sequences called
The 5' end of the primary transcript (pre-mRNA) is synthesized first.
The 5' cap is a modified guanine nucleotide which attaches to the 5-end of the pre-mRNA.
The cap protects the RNA from being degraded by enzymes that degrade RNA at the 5' end.
RNA splicing removes introns and exons are joined.
exons are a sequence within a primary transcript that remains in the RNA after RNA processing.
RNA splicing begins with helper proteins at intron/exon borders, defining the DNA that will be cut out.
Small nuclear ribonucleo proteins (snRNPs) recognize the splice sites.
A spliceosome is made up of proteins and RNA molecules that splices RNA by interacting with the ends of an RNA intron, releasing the intron and joining the two adjacent exons.
Synthesis of Poly-A tail
Poly-A tail is a stretch of adenine (A) nucleotides.
When a special poly-A attachment site in the pre-mRNA emerges, the transcript is cut there and the poly-A tail is attached to the exposed 3'end.
This completes the mRNA molecule which is ready to be transported to the cytoplasm.
It contains specific sequences of nucleotides that construct RNA, which codes for proteins.
Comparisons of Prokaryotic and Eukaryotic Protein Synthesis
Have 3 RNA polymerase
Have 1 RNA polymerase
Initiation more complex
Occurs in the cytoplasm
Occurs in the nucleus and moves to cytoplasm
Splicing of mRNA transcript does not occur
Splicing of mRNA transcript occurs
They are named RNA polymerase I, II, and III.
Watson & Grick
Bacterial viruses, called phage, were used to confirm that DNA was the genetic material, and not protein.
The central dogma explains that DNA codes for RNA, which codes for proteins.
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Three stages of Protein Synthesis: Fig. 6.6
This includes all the reactions that precede the formationof the first peptide bond .
Includes all the reactions from the synthesis of first peptidebond to addition of last amino acid
Encompasses the steps needed to release the completed polypeptidechain
-works on both prokaryotes and eukaryotes.
Three codons terminate protein synthesis
- 61 triplets are assigned to amino acids and the remaining threecodons are reserved for termination
- The three codons are:
- None of the termination codons are represented by a tRNA butinstead are recognized by protein factors (called release factorsin prokaryotes)
- These release factors catalyze termination and recognize specificrelease sequences:
- RF-1 recognizes UAA and UAG
- RF-2 recognizes UGA and UAA
- All release factors act at the ribosomal A site and requirethe peptidyl tRNA in the P site
- RF-1 and RF-2 recognize the termination codons and activatethe ribosome the hydrolyze the polypeptide formed off the peptidyltRNA in a reaction analogous to the usual peptide transfer exceptthe acceptor is water instead of an aminoacyl tRNA the polypeptideexits the ribosome through the polypeptide exit site
- RF-1 and RF-2 are released from the ribosome by RF-3 (a GTPbinding protein related to EF-G
- There is evidence to suggest that RF-3 utilizes the same siteas EF-G and the ternary complex to release RF-1 and RF-2 (Fig6.25)
- The termination reaction involves:
- release of the completed polypeptide from the last tRNA
- expulsion of the tRNA from the ribosome
- dissociation of the ribosome from the mRNA
- RRF (ribosome recycling factor) acts together with EF-G to causedissociation of the 50s and 30s subunits
- IF-3 is also required to remove deacetylated tRNA from the 30ssubunit and prevent reassociation
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