View source for Ribosome

A <span class="GRcorrect" id="GRmark_9dc60e7f130d4155596335e721cb0c620fff4334_ribosome:0">ribosome</span><ref>http://www.proteopedia.org/wiki/index.php/Ribosome</ref> is the particle upon which [[MRNA|<span class="GRcorrect" id="GRmark_9dc60e7f130d4155596335e721cb0c620fff4334_mRNA:1">mRNA</span>]] from [[DNA|DNA]] [[Transcription|transcription]] is translated <span class="GRcorrect" id="GRmark_9dc60e7f130d4155596335e721cb0c620fff4334_to:2">to</span> a [[Polypeptide|polypeptide]] with a specific [[Amino acid|amino acid]] sequence defined by the genetic code. The size of the ribosome in eukaryotes and prokaryotes are slightly different. Prokaryotes has a 70S ribosome that is comprised of 2 subunits: 

30S unit: This is <span class="GRcorrect" id="GRmark_d861d4b34ca9b79dbc03e95dfb30f9ca27de25f4_the_smaller:0">the smaller</span> unit which consists of 21 [[Proteins|proteins]] and a [[16S RNA molecule|16S RNA molecule]] . It is involve in initiating translation by selecting a AUG or alternative start codon via the 30S ribosomal subunit. This acts as the starting point for the rest of the translation process.

50S unit: This is the larger unit which consists of 34 [[Proteins|proteins]] and 2 RNA molecules, 23S and 5S&nbsp;<ref>Berg JM, Tymoczko JL, Stryer L: Biochemistry 6th (2007), WH Freeman and Company, New York. Pg 866</ref> 

The 2 units together form the complete <span class="GRcorrect" id="GRmark_e760584e4eba5d3d58eae5fae8ca4ad652a27b1f_ribosome:0">ribosome</span>, known as the 70S unit.&nbsp;S refers to the [[Svedberg unit|Svedberg unit]], which is a measure of <span class="GRcorrect" id="GRmark_96d844fa4bbfc266f2b4e27c98ea05820e2c549c_rate:0">rate</span> at which a compound moves when centrifuged. It is used as a measure of <span class="GRcorrect" id="GRmark_389205ecea2e29261df4f874d1aa43c856605342_size:0">size</span> of a molecule but is not directly proportional to molecular weight&nbsp;<ref>Berg JM, Tymoczko JL, Stryer L: Biochemistry 6th (2007), WH Freeman and Company, New York. Pg 76</ref>. 


As <span class="GRcorrect" id="GRmark_65f64a25737b7eb88e852c799f53be9badabe402_ribosomes:0">ribosomes</span> contain [[RNA|RNA]] (also referred to as [[RRNA|ribosomal RNA]]-[[RRNA|rRNA]]) and [[Proteins|proteins]], they are also referred to as [[Ribonucleoproteins|<span class="GRcorrect" id="GRmark_65f64a25737b7eb88e852c799f53be9badabe402_ribonucleoproteins:1">ribonucleoproteins</span>]]. <span class="GRcorrect" id="GRmark_8e0807bdeb00f693a2b3ff4df39083c8f6b8616d_Ribosomes:0">Ribosomes</span> translate [[MRNA|<span class="GRcorrect" id="GRmark_8e0807bdeb00f693a2b3ff4df39083c8f6b8616d_mRNA:1">mRNA</span>]] in triplets ([[Codon|codons]]) by aligning complementary triplets found in [[TRNA|<span class="GRcorrect" id="GRmark_8e0807bdeb00f693a2b3ff4df39083c8f6b8616d_tRNA:2">tRNA</span>]] molecules ([[Anticodons|<span class="GRcorrect" id="GRmark_8e0807bdeb00f693a2b3ff4df39083c8f6b8616d_anticodons:3">anticodons</span>]]). Each [[TRNA|<span class="GRcorrect" id="GRmark_437e284f3f6df97f8e040feb9642e5c897845e48_tRNA:0">tRNA</span>]] is assigned a specific <span class="GRcorrect" id="GRmark_437e284f3f6df97f8e040feb9642e5c897845e48_anticodon:1">anticodon</span> and [[Amino acid|amino acid]] and therefore translation leads to the formation of a [[Protein|protein]] by forming [[Peptide bonds|peptide bonds]] between adjacently aligned [[Amino acids|amino acids]]&nbsp;<ref>Berg JM, Tymoczko JL, Stryer L: Biochemistry 6th (2007), WH Freeman and Company, New York. Pg 872</ref>. 

<span class="GRcorrect" id="GRmark_ec45ac57dfee8cc2d989956dac54fd1cb0309221_Ribosomes:0">Ribosomes</span> have three [[TRNA|<span class="GRcorrect" id="GRmark_ec45ac57dfee8cc2d989956dac54fd1cb0309221_tRNA:1">tRNA</span>]] binding sites in the 30S subunit - the A-site ([[Aminoacyl|<span class="GRcorrect" id="GRmark_ec45ac57dfee8cc2d989956dac54fd1cb0309221_aminoacyl:2">aminoacyl</span>]] site), the P-site ([[Peptidyl|<span class="GRcorrect" id="GRmark_ec45ac57dfee8cc2d989956dac54fd1cb0309221_peptidyl:3">peptidyl</span>]] site) and the E-site (empty <span class="GRcorrect" id="GRmark_ec45ac57dfee8cc2d989956dac54fd1cb0309221_site:4">site</span>) - which allow peptide bonds to form between adjacent [[Amino acids|amino acids]]. They are in order A, P, E from the 3' to 5' end or the [[MRNA|<span class="GRcorrect" id="GRmark_054ae5e1911bb73b49215e6cf1a43c42ef82844d_mRNA:0">mRNA</span>]] strand and are involved in the elongation process of [[Translation|translation]]. The first charged <span class="GRcorrect" id="GRmark_b5cc18ec775790c795a4ff5327c0b918e5d6d443_tRNA:0">tRNA</span> attaches to the start [[Codon|<span class="GRcorrect" id="GRmark_b5cc18ec775790c795a4ff5327c0b918e5d6d443_codon:1">codon</span>]] in the P-site and is joined by a second, charged <span class="GRcorrect" id="GRmark_b5cc18ec775790c795a4ff5327c0b918e5d6d443_tRNA:2">tRNA</span> molecule adjacent <span class="GRcorrect" id="GRmark_b5cc18ec775790c795a4ff5327c0b918e5d6d443_in:3">in</span> the A-site. A peptide bond forms between the two amino acids and the first amino acid is released from its <span class="GRcorrect" id="GRmark_0941461df366394d516f7784f688a980f396f761_tRNA:0">tRNA</span>. The uncharged tRNA moves along into the E-site, whilst the second charged <span class="GRcorrect" id="GRmark_39277c069b21ea7b74aeb555af1fc9b7274d0582_tRNA:0">tRNA</span> moves from the A-site and into the P-site. The A-site becomes occupied by the next charged <span class="GRcorrect" id="GRmark_0a5185250d67a73fd7d2e3371954f2700ac2ffc2_tRNA:0">tRNA</span> molecule. This process continues and the polypeptide continues to grow until <span class="GRcorrect" id="GRmark_8fef6d71e364b3ea75182c205e5510b7c96d0ecc_complete:0">complete</span> and termination occurs<ref>Berg JM, Tymoczko JL, Stryer L. Biochemistry 7th ed.(2012), WH Freeman and Company, New York. Pg 934</ref>. 

<span class="GRcorrect" id="GRmark_eee34daa01d8c9d1dfa292612be356b0685be94e_Ribosomes:0">Ribosomes</span> are small structures found in all living cells. They can be free in the [[Cytoplasm|cytoplasm]] or attached to [[Endoplasmic Reticulum|<span class="GRcorrect" id="GRmark_0062e68f559c8a01447529b317c70ce14adb617a_endoplasmic_reticulum:0">endoplasmic reticulum</span>]] (ER), making [[Rough Endoplasmic Reticulum|Rough ER]]. They can differ in size and number, according to whether they are found in [[Bacteria|bacteria]], [[Archaea|archaea]] or in [[Eukaryotes|eukaryotes]]. There are a large number of <span class="GRcorrect" id="GRmark_5ea977443520bc63f141bea390cedbfc612df216_ribosomes:0">ribosomes</span> in cells. In [[Eukaryotes|eukaryotes]], there can be millions in one cell alone. As <span class="GRcorrect" id="GRmark_c377955229c5c5c0636721e7a3a8021aae870a8c_ribosomes:0">ribosomes</span> are so small, (it has a diameter of 25-30 <span class="GRcorrect" id="GRmark_c377955229c5c5c0636721e7a3a8021aae870a8c_nm:1">nm</span> approximately) <ref>Becker, Wayne M., Kleinsmith, Lewis J., Hardin, Jeff., Bertoni, Gregory Paul. (2009) The World of the Cell, 7th Edition, San Francisco: Pearson Benjamin Cummings. P95.</ref>, an [[Electron Microscope|electron microscope]] is needed to see it. <span class="GRcorrect" id="GRmark_b07549eb6715ca6843190586e047fbfd90180c7d_Ribosomes:0">Ribosomes</span> are made up of two subunits, one larger than the other. The two subunits join together when attached to [[MRNA|<span class="GRcorrect" id="GRmark_c08097484ae7a5e403a05cac33f00739ce08260d_mRNA:0">mRNA</span>]] <span class="GRcorrect" id="GRmark_c08097484ae7a5e403a05cac33f00739ce08260d_to_make:1">to make</span> a [[Protein|protein]] in [[Protein synthesis|protein synthesis]]. <span class="GRcorrect" id="GRmark_97e775bdb74994eb4745066e865bd73d05ec16ab_Ribosomes:0">Ribosomes</span> are also found in [[Mitochondria|mitochondria]] and [[Chloroplasts|chloroplasts]] and carry out [[Protein synthesis|protein synthesis]], specifically for these [[Organelles|organelles]]. 

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