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RNAs involved in protein biosynthesis

Protein Biosynthesis

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Process of protein biosynthesis

Figure 2. The eukaryotic secretory pathway. Secretory proteins enter the membrane trafficking system by (a) translocation from the cytoplasm into the endoplasmic reticulum (ER) (b) In the ER, proteins are packaged into ER to Golgi complex (GC) carrier vesicles that (c) deliver proteins to the GC. As proteins progress through the GC they become (d) glycosylated. Once the (e) reach the /rans-Golgi network (TGN), they (f) are sorted into secretory vesicles in which proteins are (g) transported to the plasma membrane (PM). At the cell surface secretory proteins are (h) released into the extracellular environment.

The nucleolus and ribosomes form part of the proteinsynthesizing machinery of the cell

Proteins are assembled on ribosomes,
and ribosomes are made of
several dozen proteins as well as rRNA.
Transcription
RNA molecules
are produced by
copying part of the nucleotide sequence of DNA
into a
complementary sequence in RNA
.
This process is called
transcription.
Transcription
requires an
enzyme
known as
RNA polymerase
, which is
very similar
to the DNA polymerase in
DNA replication.
During
transcription,
RNA polymerase
binds to DNA and separates the DNA strands.

This processrequires the use of (membrane proteins).

Discrete segments of DNA, called genes, encode the instructions for making proteins.

When E. coli makes a protein that is destined for export, it is made as a preprotein containing a signal sequence (4). In most proteins, these signal sequences are a stretch of contiguous amino acids at the amino terminus of a protein, with a positively charged amino terminus, at least six sequential hydrophobic amino acids, and a cleavage site for the signal peptidase that removes the signal after translocation. The presence of the signal sequence in a newly synthesized protein causes a protein chaperone to bind to it, retarding its folding. There appear to be at least two major chaperone systems in E. coli. One involves the tetrameric cytosolic protein secB, which binds to newly synthesized proteins that contain a signal sequence. It binds throughout the length of the newly synthesized polypeptide chain, keeping it from adopting its native conformation (2). A different class of chaperones binds to the nascent chain as the secreted protein is being synthesized on the ribosome. An example of such a chaperone is a ribonucleoprotein, the signal recognition protein (SRP). In E. coli SRP consists of a 4.5 S RNA molecule and a single protein, Ffh, which is a GTPase; that is, it hydrolyzes guanosine triphosphate (5).

Much of the research on protein export by bacteria has utilized Escherichia coli, a gram-negative bacterium. In E. coli there are at least two different ATP-driven transport systems, one that uses the sec A system (see Sec Mutants/Proteins) to pump proteins into the periplasmic space (2), and a second that uses a member of the ABC (ATP-binding cassette) family of proteins to export proteins across both E. coli membranes simultaneously (3).

The proton channel and rotating stalk are shown in blue.

In the example below, a ligand molecule (e.g., acetylcholine)binds to the membrane protein.

As secreted proteins pass through the Golgi complex, they can be modified by glycosylation, sulfation, or proteolysis. Three major types of glycosylation occur, N-linked, O-linked, and glycosaminoglycan addition. In mammalian cells, the mannose-rich, asparagine-linked oligosaccharides that had been added in the ER are first trimmed in the Golgi complex and then rebuilt. Mannoses are first removed and then replaced by other sugars. On a stump of the original mannose tree, a multichain oligosaccharide is generated by the orderly addition of the sugars N-acetylglucosamine, galactose, and sialic acid (N-acetylneuraminic acid). The previous form of glycosylation is referred to as N-glycosylation, since the sugars are added to an asparagine residue. If a sugar chain is added to a serine or threonine residue, it is referred to as O-glycosylation. The first sugar added is usually N-acetylgalactosamine. The oligosaccharide chains are branched but are usually shorter and more variable than N-linked oligosaccharides.

Together with a protein partner called myosin, actin filaments make possible the muscle contractions necessary for everything from your action on a sports field to the automatic beating of your heart.

Now the transport protein, or carrier, isready to start the cycle again.
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  • Protein folding in the endoplasmic reticulum Protein synthesis

    Microtubules consists of a strong protein called tubulin and they are the 'heavy lifters' of the cytoskeleton.

  • Protein Synthesis Animation Video - YouTube

    Some ER has ribosomes on the surface (rough endoplasmic reticulum) --the cell's protein-making machinery.

  • Protein Synthesis Steps by edgar lavery - issuu

    Proteins that arrive there are sorted, packaged and transported to various destinations in the cell.

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Ribosomes and Protein Synthesis | Biology I


Translation takes place on
ribosomes.
During
translation
,
the cell uses
information from mRNA
to produce
proteins.
Before
translation occurs,
mRNA is transcribed from DNA
in the
nucleus
and
released
into the
cytoplasm.
Translation
begins

when an
mRNA molecule
in the cytoplasm
attaches
to a
ribosome
.

Ribosomes and Protein Synthesis

But we then need something to
"read"

these instructions and
put them to use
, and that is the job of a tiny factory called the
ribosome.
The
decoding
of an
mRNA message
into a polypeptide chain
(protein)
is known as
translation.

Describe the different steps in protein synthesis; ..

Each three-letter
"word" in mRNA
is known as a
codon.
A
codon
consists of
three
consecutive nucleotides that specify a
single amino acid
that is to be
added
to the
polypeptide.
For example, consider the following RNA sequence:
UCGCACGGU

It would be read three bases at a time:
UCG-CAC-GGU

And these three codons represent the different amino acids:
UCG- CAC- GGU
Serine-Histidine-Glycine
Because there are
four different bases,
there are
64
possible three-based codons (4x4x4 =64)
This wheel shows all 64 possible codons of the genetic code.

Some amino acids
can be specified by
more than one codon.
You start at the center of the circle and move outwards to find the amino acid.
There is also one codon,
AUG
, that can either specify for the
amino acid methionine
or serve as the

"start" codon
for protein synthesis.
There are also
three "stop" codons
that do
not
code for
any amino acid.

of the protein synthesis machinery are comparable from bacteria …

Our genetic identity is "coded" in the sense that four building blocks, called nucleotides, string together to spell out a biochemical message—the manufacturing instructions for a protein.

Two major steps are involved in protein synthesis; (i) ..

Mammalian cells export proteins, such as interleukin-1b, basic fibroblast growth factor, and thioredoxin, that do not have a classical signal sequence (31). In addition, drugs such as monensin and brefeldin A that normally block classical secretion are without effect on these proteins. Their secretion has been linked to the heat-shock response. Little is currently known about the molecular mechanisms that are involved.

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