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7/30/2019 Suba Biology

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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.htm


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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.htm


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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/Methionine


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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/Methionine


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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_acid


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http://en.wikipedia.org/wiki/File:Ribosomer_i_arbete.png


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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_electron


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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
http://en.wikipedia.org/wiki/Tungstenhttp://en.wikipedia.org/wiki/Group_12_elementhttp://en.wikipedia.org/wiki/Heliumhttp://en.wikipedia.org/wiki/Ultra-high_vacuumhttp://en.wikipedia.org/wiki/Galliumhttp://en.wikipedia.org/wiki/Metallic_glasshttp://en.wikipedia.org/wiki/Conformational_changehttp://en.wikipedia.org/wiki/Conformational_changehttp://en.wikipedia.org/wiki/Metallic_glasshttp://en.wikipedia.org/wiki/Galliumhttp://en.wikipedia.org/wiki/Ultra-high_vacuumhttp://en.wikipedia.org/wiki/Ultra-high_vacuumhttp://en.wikipedia.org/wiki/Ultra-high_vacuumhttp://en.wikipedia.org/wiki/Heliumhttp://en.wikipedia.org/wiki/Group_12_elementhttp://en.wikipedia.org/wiki/Tungsten


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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
http://en.wikipedia.org/wiki/Solid_solubilityhttp://en.wikipedia.org/wiki/Solid_solubilityhttp://en.wikipedia.org/wiki/Electrumhttp://en.wikipedia.org/wiki/Intermetallicshttp://en.wikipedia.org/wiki/Intermetallicshttp://en.wikipedia.org/wiki/Law_of_definite_proportionshttp://en.wikipedia.org/wiki/Phase_(matter)http://en.wikipedia.org/wiki/Metallurgyhttp://en.wikipedia.org/wiki/Chemistryhttp://en.wikipedia.org/wiki/Metallurgyhttp://en.wikipedia.org/wiki/Phase_(matter)http://en.wikipedia.org/wiki/Law_of_definite_proportionshttp://en.wikipedia.org/wiki/Intermetallicshttp://en.wikipedia.org/wiki/Intermetallicshttp://en.wikipedia.org/wiki/Electrumhttp://en.wikipedia.org/wiki/Solid_solubilityhttp://en.wikipedia.org/wiki/Solid_solubility


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