Protein biosynthesis refers to the process whereby biological cells generate new proteins;
it is balanced by the loss of cellular proteins via degradation or export.
Translation, the assembly of proteins by ribosomes, is an essential part of the biosynthetic pathway,
along with generation of messenger RNA, aminoacylation of transfer RNA, co-translational transport,
and post-translational modification.
Protein biosynthesis is strictly regulated at multiple steps, and error-checking mechanisms
are in place.
The cistron DNA is transcribed into a variety of RNA intermediates.
The last version is used as a template in synthesis of a polypeptide chain.
Protein will often be synthesized directly from genes by translating mRNA.
When a protein must be available on short notice or in large quantities, a protein precursor
is produced.
A proprotein is an inactive protein containing one or more inhibitory peptides that can be
activated when the inhibitory sequence is removed by proteolysis during posttranslational
modification.
A preprotein is a form that contains a signal sequence that specifies its insertion into
or through membranes, i.e., targets them for secretion.
The signal peptide is cleaved off in the endoplasmic reticulum.
Preproproteins have both sequences still present.
In protein synthesis, a succession of tRNA molecules charged with appropriate amino acids
are brought together with an mRNA molecule and matched up by base-pairing through the
anti-codons of the tRNA with successive codons of the mRNA.
The amino acids are then linked together to extend the growing protein chain, and the
tRNAs, no longer carrying amino acids, are released.
This whole complex of processes is carried out by the ribosome, formed of two main chains
of RNA, called ribosomal RNA, and more than 50 different proteins.
The ribosome latches onto the end of an mRNA molecule and moves along it, capturing loaded
tRNA molecules and joining together their amino acids to form a new protein chain.
Protein biosynthesis, although very similar, is different for prokaryotes and eukaryotes.
Transcription
In transcription an mRNA chain is generated, with one strand of the DNA double helix in
the genome as a template.
This strand is called the template strand.
Transcription can be divided into 3 stages: initiation, elongation, and termination, each
regulated by a large number of proteins such as transcription factors and coactivators
that ensure that the correct gene is transcribed.
Transcription occurs in the cell nucleus, where the DNA is held.
The DNA structure of the cell is made up of two helixes made up of sugar and phosphate
held together by hydrogen bonds between the bases of opposite strands.
The sugar and the phosphate in each strand are joined together by stronger phosphodiester
covalent bonds.
The DNA is "unzipped" by the enzyme helicase, leaving the single nucleotide chain open to
be copied.
RNA polymerase reads the DNA strand from the 3-prime end to the 5-prime end, while it synthesizes
a single strand of messenger RNA in the 5'-to-3' direction.
The general RNA structure is very similar to the DNA structure, but in RNA the nucleotide
uracil takes the place that thymine occupies in DNA.
The single strand of mRNA leaves the nucleus through nuclear pores, and migrates into the
cytoplasm.
The first product of transcription differs in prokaryotic cells from that of eukaryotic
cells, as in prokaryotic cells the product is mRNA, which needs no post-transcriptional
modification, whereas, in eukaryotic cells, the first product is called primary transcript,
that needs post-transcriptional modification to give hnRNA.
hnRNA then undergoes splicing of introns via spliceosomes to produce the final mRNA..
Translation
The synthesis of proteins from RNA is known as translation.
Translation occurs in the cytoplasm, where the ribosomes are located.
Ribosomes are made of a small and large subunit that surround the mRNA.
In translation, messenger RNA is decoded to produce a specific polypeptide according to
the rules specified by the trinucleotide genetic code.
This uses an mRNA sequence as a template to guide the synthesis of a chain of amino acids
that form a protein.
Translation proceeds in four phases: activation, initiation, elongation, and termination.
In activation, the correct amino acid is joined to the correct transfer RNA.
While this is not, in the technical sense, a step in translation, it is required for
translation to proceed.
The AA is joined by its carboxyl group to the 3' OH of the tRNA by an ester bond.
When the tRNA has an amino acid linked to it, it is termed "charged".
Initiation involves the small subunit of the ribosome binding to 5' end of mRNA with the
help of initiation factors, other proteins that assist the process.
Elongation occurs when the next aminoacyl-tRNA in line binds to the ribosome along with GTP
and an elongation factor.
Termination of the polypeptide happens when the A site of the ribosome faces a stop codon.
When this happens, no tRNA can recognize it, but releasing factor can recognize nonsense
codons and causes the release of the polypeptide chain.
The capacity of disabling or inhibiting translation in protein biosynthesis is used by some antibiotics
such as anisomycin, cycloheximide, chloramphenicol, tetracycline, streptomycin, erythromycin,
puromycin, etc.
Events following protein translation
The events following biosynthesis include post-translational modification and protein
folding.
During and after synthesis, polypeptide chains often fold to assume, so called, native secondary
and tertiary structures.
This is known as protein folding.
See also Cistron
Genetic code lac operon
Operon Peptide synthesis
Central dogma of molecular biology References
External links Interactive Java simulation of transcription
initiation.
From Center for Models of Life at the Niels Bohr Institute.
A website focused primarily on Protein Synthesis.