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Distinguishing Characteristics:
Ribosomes are typically composed of two subunits: a large
subunit and a small subunit. Ribosomal subunits are
synthesized by the nucleolus. These two subunits join
together when the ribosome attaches to messenger RNA
(mRNA) during protein synthesis. Ribosomes along with
another RNA molecule, transfer RNA (tRNA), help to translate
the protein-coding genes in mRNA into proteins.
Ribosomes:
In Journey into the Cell, we looked at the structure of the two major
types of cells: prokaryotic and eukaryotic cells. Now we turn our
attention to the protein assemblers of a eukaryotic cell, theribosomes.
Ribosomes are cell organelles that consist ofRNA and proteins. They
are responsible for assembling the proteins of the cell. Depending on
the protein production level of a particular cell, ribosomes may number
in the millions
http://biology.about.com/od/cellanatomy/p/nucleus.htmhttp://biology.about.com/od/cellularprocesses/ss/protein-synthesis-translation_2.htmhttp://biology.about.com/od/molecularbiology/ss/rna_2.htmhttp://biology.about.com/od/geneticsglossary/g/Genes.htmhttp://biology.about.com/library/weekly/aa031600a.htmhttp://www.daviddarling.info/encyclopedia/R/ribosome.htmlhttp://biology.about.com/od/molecularbiology/ss/rna.htmhttp://biology.about.com/od/molecularbiology/ss/protein-structure.htmhttp://biology.about.com/od/molecularbiology/ss/protein-structure.htmhttp://biology.about.com/od/molecularbiology/ss/rna.htmhttp://www.daviddarling.info/encyclopedia/R/ribosome.htmlhttp://biology.about.com/library/weekly/aa031600a.htmhttp://biology.about.com/od/geneticsglossary/g/Genes.htmhttp://biology.about.com/od/molecularbiology/ss/rna_2.htmhttp://biology.about.com/od/cellularprocesses/ss/protein-synthesis-translation_2.htmhttp://biology.about.com/od/cellanatomy/p/nucleus.htm7/30/2019 Suba Biology
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Location in the Cell:
There are two places that ribosomes usually exist in the cell:
suspended in the cytosol and bound to the endoplasmic
reticulum. These ribosomes are called free ribosomes and boundribosomes respectively. In both cases, the ribosomes usually
form aggregates called polysomes or polyribosomes during
protein synthesis.
Free ribosomes usually make proteins that will function in the
cytosol (fluid component of thecytoplasm), while bound
ribosomes usually make proteins that are exported from the cell
or included in the cell's membranes. Interestingly enough, free
ribosomes and bound ribosomes are interchangeable and thecell can change their numbers according to metabolic needs.
http://biology.about.com/od/cellanatomy/ss/endoplasmic-reticulum.htmhttp://biology.about.com/od/cellanatomy/ss/endoplasmic-reticulum.htmhttp://biology.about.com/od/biologydictionary/g/cytoplasm.htmhttp://biology.about.com/od/cellanatomy/ss/cell-membrane.htmhttp://biology.about.com/od/cellanatomy/ss/cell-membrane.htmhttp://biology.about.com/od/biologydictionary/g/cytoplasm.htmhttp://biology.about.com/od/cellanatomy/ss/endoplasmic-reticulum.htmhttp://biology.about.com/od/cellanatomy/ss/endoplasmic-reticulum.htm7/30/2019 Suba Biology
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Protein Assembly:
Protein synthesis occurs by the processes
oftranscription and translation. In transcription, the genetic
code contained within DNA is transcribed into an RNA versionof the code known as messenger RNA (mRNA). In translation,
a growing amino acid chain, also called a polypeptide chain, is
produced. Ribosomal RNA helps to link amino acids together
to produce the polypeptide chain. The polypeptide chainundergoes several modifications before becoming a fully
functioning protein. Proteins are very important biological
polymers in our cells as they are involved in virtually all cell
functions.
http://biology.about.com/od/cellularprocesses/ss/Dna-Transcription.htmhttp://biology.about.com/od/cellularprocesses/ss/protein-synthesis-translation.htmhttp://biology.about.com/od/genetics/ss/genetic-code.htmhttp://biology.about.com/od/genetics/ss/genetic-code.htmhttp://biology.about.com/od/geneticsglossary/g/DNA.htmhttp://biology.about.com/od/molecularbiology/ss/rna.htmhttp://biology.about.com/od/molecularbiology/ss/amino-acid.htmhttp://biology.about.com/od/molecularbiology/a/aa101904a.htmhttp://biology.about.com/od/molecularbiology/ss/polymers.htmhttp://biology.about.com/od/molecularbiology/ss/polymers.htmhttp://biology.about.com/od/molecularbiology/ss/polymers.htmhttp://biology.about.com/od/molecularbiology/ss/polymers.htmhttp://biology.about.com/od/molecularbiology/a/aa101904a.htmhttp://biology.about.com/od/molecularbiology/ss/amino-acid.htmhttp://biology.about.com/od/molecularbiology/ss/rna.htmhttp://biology.about.com/od/geneticsglossary/g/DNA.htmhttp://biology.about.com/od/genetics/ss/genetic-code.htmhttp://biology.about.com/od/genetics/ss/genetic-code.htmhttp://biology.about.com/od/cellularprocesses/ss/protein-synthesis-translation.htmhttp://biology.about.com/od/cellularprocesses/ss/Dna-Transcription.htmhttp://en.wikipedia.org/wiki/Methionine7/30/2019 Suba Biology
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- ,codon on the mRNA and recruits the large ribosomal subunit. The
ribosome contains three RNA binding sites, designated A, P and E. The A
site binds an aminoacyl-tRNA; the P site binds a peptidyl-tRNA (a tRNA
bound to the peptide being synthesized); and the E site binds a free tRNA
before it exits the ribosome. Protein synthesis begins at a start codon AUGnear the 5' end of the mRNA. mRNA binds to the P site of the ribosome
first. The ribosome is able to identify the start codon by use of the Shine-
Dalgarno sequence of the mRNA in prokaryotes and Kozak box in
eukaryotes.
Figure 3 : Translation of mRNA (1) by a ribosome (2)(shown
as small and large subunits) into a polypeptide chain (3). The ribosome
begins at the start codon of mRNA (AUG) and ends at the stop codon
(UAG).
In Figure 3, both ribosomal subunits (small and large) assemble at the start
codon (towards the 5' end of the mRNA). The ribosome uses tRNA thatmatches the current codon (triplet) on the mRNA to append an amino
acid to the polypeptide chain. This is done for each triplet on the mRNA,
while the ribosome moves towards the 3' end of the mRNA. Usually in
bacterial cells, several ribosomes are working parallel on a single mRNA,
forming what is called apolyribosome orpolysome.See also
http://en.wikipedia.org/wiki/Start_codonhttp://en.wikipedia.org/wiki/Shine-Dalgarno_sequencehttp://en.wikipedia.org/wiki/Shine-Dalgarno_sequencehttp://en.wikipedia.org/wiki/Kozak_consensus_sequencehttp://en.wikipedia.org/wiki/TRNAhttp://en.wikipedia.org/wiki/Amino_acidhttp://en.wikipedia.org/wiki/Amino_acidhttp://en.wikipedia.org/wiki/Polysomehttp://en.wikipedia.org/wiki/Polysomehttp://en.wikipedia.org/wiki/Amino_acidhttp://en.wikipedia.org/wiki/Amino_acidhttp://en.wikipedia.org/wiki/TRNAhttp://en.wikipedia.org/wiki/File:Ribosomer_i_arbete.pnghttp://en.wikipedia.org/wiki/Kozak_consensus_sequencehttp://en.wikipedia.org/wiki/Kozak_consensus_sequencehttp://en.wikipedia.org/wiki/Kozak_consensus_sequencehttp://en.wikipedia.org/wiki/Shine-Dalgarno_sequencehttp://en.wikipedia.org/wiki/Shine-Dalgarno_sequencehttp://en.wikipedia.org/wiki/Shine-Dalgarno_sequencehttp://en.wikipedia.org/wiki/Shine-Dalgarno_sequencehttp://en.wikipedia.org/wiki/Shine-Dalgarno_sequencehttp://en.wikipedia.org/wiki/Start_codonhttp://en.wikipedia.org/wiki/Methionine7/30/2019 Suba Biology
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The ribosome (from ribonucleic acid and the Greek soma, meaning "body") is a
large and complex molecular machine, found within all living cells, that serves as
the primary site ofbiological protein synthesis (translation). Ribosomes link amino
acids together in the order specified by messenger RNA (mRNA) molecules.
Ribosomes consist of two major subunits
the small ribosomal subunit reads themRNA, while the large subunit joins amino acids to form a polypeptide chain. Each
subunit is composed of one or more ribosomal RNA (rRNA) molecules and a variety
of proteins.
The sequence ofDNA encoding for a protein may be copied many times
into messenger RNA (mRNA) chains of a similar sequence. Ribosomes can bind to
an mRNA chain and use it as a template for determining the correct sequence of
amino acids in a particular protein. Amino acids are selected, collected and carried
to the ribosome by transfer RNA (tRNA molecules), which enter one part of the
ribosome and bind to the messenger RNA chain. The attached amino acids are
then linked together by another part of the ribosome. Once the protein is
produced, it can then 'fold' to produce a specific functional three-dimensionalstructure.
A ribosome is made from complexes of RNAs and proteins and is therefore
a ribonucleoprotein. Each ribosome is divided into two subunits: the smaller
subunit binds to the mRNA pattern, while the larger subunit binds to the tRNA and
the amino acids. When a ribosome finishes reading an mRNA molecule, these twosubunits split apart. Ribosomes are ribozymes, because the catalyticpeptidyl
http://en.wikipedia.org/wiki/Ribonucleic_acidhttp://en.wikipedia.org/wiki/Ribonucleic_acidhttp://en.wikipedia.org/wiki/Translation_(biology)http://en.wikipedia.org/wiki/Amino_acidshttp://en.wikipedia.org/wiki/Amino_acidshttp://en.wikipedia.org/wiki/Messenger_RNAhttp://en.wikipedia.org/wiki/Polypeptidehttp://en.wikipedia.org/wiki/Ribosomal_RNAhttp://en.wikipedia.org/wiki/DNAhttp://en.wikipedia.org/wiki/Messenger_RNAhttp://en.wikipedia.org/wiki/Transfer_RNAhttp://en.wikipedia.org/wiki/Protein_foldinghttp://en.wikipedia.org/wiki/Ribonucleoproteinhttp://en.wikipedia.org/wiki/Ribozymehttp://en.wikipedia.org/wiki/Catalysishttp://en.wikipedia.org/wiki/Peptidyl_transferasehttp://en.wikipedia.org/wiki/Ribosomal_RNAhttp://en.wikipedia.org/wiki/Peptidyl_transferasehttp://en.wikipedia.org/wiki/Peptidyl_transferasehttp://en.wikipedia.org/wiki/Peptidyl_transferasehttp://en.wikipedia.org/wiki/Catalysishttp://en.wikipedia.org/wiki/Ribozymehttp://en.wikipedia.org/wiki/Ribonucleoproteinhttp://en.wikipedia.org/wiki/Protein_foldinghttp://en.wikipedia.org/wiki/Transfer_RNAhttp://en.wikipedia.org/wiki/Messenger_RNAhttp://en.wikipedia.org/wiki/DNAhttp://en.wikipedia.org/wiki/Ribosomal_RNAhttp://en.wikipedia.org/wiki/Polypeptidehttp://en.wikipedia.org/wiki/Messenger_RNAhttp://en.wikipedia.org/wiki/Amino_acidshttp://en.wikipedia.org/wiki/Amino_acidshttp://en.wikipedia.org/wiki/Translation_(biology)http://en.wikipedia.org/wiki/Ribonucleic_acidhttp://en.wikipedia.org/wiki/Ribonucleic_acid7/30/2019 Suba Biology
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Metallic bonding constitutes the electrostatic attractive forcesbetween the delocalized electrons, called conduction electrons, gathered in
an electron cloud and the positively charged metal ions. Understood as the
sharing of "free" electrons among a lattice of positively charged ions(cations), metallic bonding is sometimes compared with that ofmolten
salts; however, this simplistic view[which?] holds true for very
few[which?]metals. In a more quantum-mechanical view, the conduction
electrons divide their density equally over all atoms that function as neutral
(non-charged) entities.
[citation needed]
Metallic bonding accounts formany physical properties of metals, such
asstrength, malleability, ductility, thermal and electrical
conductivity, opacity, and luster.[1][2][3][4]
Although the term "metallic bond" is often used in contrast to the term
"covalent bond", it is preferable[by whom?] to use the term metallic bonding,
because this type of bonding is collective in nature and a single "metallic
bond" does not exist. Metallic bond is not the only type ofchemical
bonding a metal can exhibit, even as a simple substance. For example,
elemental gallium consists of covalently-bound pairs of atoms in both liquid
and solid statethese pairs form a crystal lattice with metallic bonding
between them. Another example of a metal
metal covalent bondis mercurous ion H 2+2
http://en.wikipedia.org/wiki/Delocalized_electronhttp://en.wikipedia.org/wiki/Conduction_electronshttp://en.wikipedia.org/wiki/Crystal_structurehttp://en.wikipedia.org/wiki/Cationshttp://en.wikipedia.org/wiki/Molten_salthttp://en.wikipedia.org/wiki/Molten_salthttp://en.wikipedia.org/wiki/Wikipedia:Avoid_weasel_wordshttp://en.wikipedia.org/wiki/Wikipedia:Avoid_weasel_wordshttp://en.wikipedia.org/wiki/Metalhttp://en.wikipedia.org/wiki/Quantum-mechanicalhttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Physical_propertieshttp://en.wikipedia.org/wiki/Strength_of_materialshttp://en.wikipedia.org/wiki/Malleabilityhttp://en.wikipedia.org/wiki/Ductilityhttp://en.wikipedia.org/wiki/Thermal_conductivityhttp://en.wikipedia.org/wiki/Electrical_conductivityhttp://en.wikipedia.org/wiki/Electrical_conductivityhttp://en.wikipedia.org/wiki/Opacity_(optics)http://en.wikipedia.org/wiki/Lustre_(mineralogy)http://en.wikipedia.org/wiki/Metallic_bondhttp://en.wikipedia.org/wiki/Covalent_bondhttp://en.wikipedia.org/wiki/Wikipedia:Avoid_weasel_wordshttp://en.wikipedia.org/wiki/Chemical_bondhttp://en.wikipedia.org/wiki/Chemical_bondhttp://en.wikipedia.org/wiki/Galliumhttp://en.wikipedia.org/wiki/Crystal_latticehttp://en.wikipedia.org/wiki/Mercurous_ionhttp://en.wikipedia.org/wiki/Mercurous_ionhttp://en.wikipedia.org/wiki/Mercurous_ionhttp://en.wikipedia.org/wiki/Mercurous_ionhttp://en.wikipedia.org/wiki/Crystal_latticehttp://en.wikipedia.org/wiki/Galliumhttp://en.wikipedia.org/wiki/Chemical_bondhttp://en.wikipedia.org/wiki/Chemical_bondhttp://en.wikipedia.org/wiki/Wikipedia:Avoid_weasel_wordshttp://en.wikipedia.org/wiki/Covalent_bondhttp://en.wikipedia.org/wiki/Metallic_bondhttp://en.wikipedia.org/wiki/Metallic_bondhttp://en.wikipedia.org/wiki/Metallic_bondhttp://en.wikipedia.org/wiki/Metallic_bondhttp://en.wikipedia.org/wiki/Lustre_(mineralogy)http://en.wikipedia.org/wiki/Opacity_(optics)http://en.wikipedia.org/wiki/Electrical_conductivityhttp://en.wikipedia.org/wiki/Electrical_conductivityhttp://en.wikipedia.org/wiki/Thermal_conductivityhttp://en.wikipedia.org/wiki/Ductilityhttp://en.wikipedia.org/wiki/Malleabilityhttp://en.wikipedia.org/wiki/Strength_of_materialshttp://en.wikipedia.org/wiki/Physical_propertieshttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Quantum-mechanicalhttp://en.wikipedia.org/wiki/Quantum-mechanicalhttp://en.wikipedia.org/wiki/Quantum-mechanicalhttp://en.wikipedia.org/wiki/Metalhttp://en.wikipedia.org/wiki/Wikipedia:Avoid_weasel_wordshttp://en.wikipedia.org/wiki/Wikipedia:Avoid_weasel_wordshttp://en.wikipedia.org/wiki/Molten_salthttp://en.wikipedia.org/wiki/Molten_salthttp://en.wikipedia.org/wiki/Cationshttp://en.wikipedia.org/wiki/Crystal_structurehttp://en.wikipedia.org/wiki/Conduction_electronshttp://en.wikipedia.org/wiki/Delocalized_electron7/30/2019 Suba Biology
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Strength of the bond:The atoms in metals have a strong attractive force between them. Much energy is
required to overcome it. Therefore, metals often have high boiling points,
with tungsten (5828 K) being extremely high. A remarkable exception are the
elements of the zinc group: Zn, Cd, and Hg. Their electron configuration ends in...ns2 and this comes to resemble a noble gas configuration like that ofheliummore
and more when going down in the periodic table because the energy distance to
the empty np orbitals becomes larger. These metals are therefore relatively
volatile, and are avoided in ultra-high vacuum systems.
Otherwise, metallic bonding can be very strong, even in molten metals, such
as Gallium. Even though gallium will melt from the heat of one's hand just above
room temperature, its boiling point is not far from that of copper. Molten gallium is
therefore a very nonvolatile liquid thanks to its strong metallic bonding.
The latter also exemplifies that metallic bonding due to its delocalization in all
directions is often not very particular about the directionality of the bonding.
There is typically a preference for close packing of the atoms, such as face or body
centered cubic arrangements, but in the case of liquid gallium the stacking is not
regular, at least not at long range and bond angles are easily changed.
Given high enough cooling rates and appropriate alloy composition, metallic
bonding can occur even in glasses with an amorphous structure.
Much biochemistry is mediated by the weak interaction of metal ions andbiomolecules. Such interactions and their associated conformational chan e has
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Solubility and compound formation:Metals are insoluble in water or organic solvents unless they
undergo a reaction with them. Typically this is an oxidation reaction
that robs the metal atoms of their itinerant electrons, destroying the
metallic bonding. However metals are often readily soluble in each
other while retaining the metallic character of their bonding. Gold,
for example, dissolves easily in mercury, even at room temperature.
Even in solid metals, the solubility can be extensive. If the structures
of the two metals are the same, there can even be complete solidsolubility, as in the case ofelectrum, the alloys of silver and gold. At
times, however, two metals will form alloys with different structures
than either of the two parents. One could call these materials metal
compounds, but, because materials with metallic bonding aretypically not molecular, Dalton's law of integral proportions is not
valid and often a range of stoichiometric ratios can be achieved. It is
better to abandon such concepts as 'pure substance' or 'solute' is
such cases and speak ofphases instead. The study of such phases
has traditionally been more the domain ofmetallurgy than
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