In What Direction Does Rna Polymerase Read a Template Strand

C2005/F2401 '10  -- Lecture # 13 -- RNA & Poly peptide Synthesis

�  2010 Deborah Mowshowitz and Lawrence Chasin Department of Biological Sciences Columbia University New York, NY . Last edited 10/26/2010 09:32 AM

Handouts: 13A -- code table & tRNA structure.
13B -- Protein Synthesis
12B -- from last lecture -- Deoxyribonucleic acid synthesis vs RNA synthesis

Note: For this lecture, fig. and table numbers in the sixth & seventh ed. of Becker are all the same. In the 5th ed, translation is in ch. 20 instead of 22, simply the fig. and table #'southward are the same.

I. DNA synthesis vs RNA synthesis. The easiest style to go over RNA synthesis, given that nosotros've discussed Dna synthesis at length, is to compare Deoxyribonucleic acid and RNA synthesis. Run into handout 12-B.

A. What is the same? Run into Lecture 12.

B. What'south Different?

1. Enzymes

• Growth of Dna chain is catalyzed past Deoxyribonucleic acid polymerase (and associated enzymes)

• Growth of RNA chain is catalyzed past RNA polymerase.

2. Choice of Substrate. If you put all 8 XTP'southward in a test tube, what practise you get, Dna or RNA? Enzyme (Dna vs RNA pol) is responsible for which nucleotides used.

• RNA pol. uses ribonucleoside triphosphates (containing U, not T).

• DNA politician uses deoxyribonucleoside triphosphates (containing T, non U).

3. Products

• Deoxyribonucleic acid is long and double stranded

• RNA is brusk and unmarried stranded

4. Option of which part of Template to apply

• Template = short section, ane strand at a fourth dimension (for RNA synth.) vs all of both strands (for Dna synth.)

• Why? Because starts and stops are different. Starts & stops = sequences in DNA recognized by the enzymes = places where replication or transcription starts (or ends). These must exist unlike for the two enzymes.

• Names of commencement sequences = section where polymerase binds
      Starts for DNA synthesis =  Origins. DNA pol. recognizes (binds to) starting time signals for replication called origins (ori's).
      Starts for RNA synthesis = Promoters. RNA pol. recognizes (binds to) start signals for transcription called promoters (P'southward).

Encounter problem 7- 6

C.  Template Details

1. I Strand is Template for RNA polymerase. For whatsoever one gene or region, RNA polymerase uses Crick or Watson, merely non both, as template. RNA that is made is complementary (and antiparallel) to the template strand. Note that an entire strand is non used as template throughout.  The "Watson" strand of DNA is used as template in some sections and the "Crick" strand in others.

2. Continuous vs. discontinuous synthesis.

• DNA synthesis:  Replication fork moves downward Dna making complements to both strands; 1 new strand is made continuously and one discontinuously. Ligase is needed for synthesis of lagging strand.

• RNA synthesis: RNA polymerase moves downward Dna making complement to 1 strand or the other (in any detail region). Therefore RNA synthesis is continuous and doesn't demand ligase.

 3. Terminology
a. Transcribed Strand. Strand used as template is chosen the transcribed or template strand or the antisense strand (in that region). This strand is complementary to the RNA that is made.

  b. Sense Strand. Strand that is not transcribed (in that region) is called the sense strand or coding strand. The base of operations sequence of this strand is identical to the RNA that is made (except that the RNA has U and the sense strand has T).

c. An entire Dna strand (going the length of a whole molecule) is not all "sense" or "antisense." "Watson" may exist sense in one section and "Crick" may exist sense in the other (as in the movie on handout 12-B). The terms "sense" and "transcribed" strand are defined for each department of the Dna that is transcribed every bit a unit (usually a gene or minor number of genes).

d. Sense RNA. The usual RNA transcribed from the DNA is said to be "sense." (Sense RNA matches the sense strand of the DNA.) The complementary RNA, if it exists, is said to be "antisense." Some applied uses of "antisense RNA" are below.

e. Why this terminology? The sense strand (not the template) actually contains the data used to line upwards amino acids to brand proteins. (Bold the factor codes for a peptide.) When a DNA sequence is published, it is usually the sense strand that is given. Why? If the factor codes for a poly peptide, the amino acid sequence of the protein is much easier to effigy out using the sense strand  -- yous just consult the code table (details next fourth dimension).

f. Boosted notes FYI on terminology:

            (one). Becker (and some others) call the sense strand the coding strand, meaning the "strand coding for protein."  I prefer the term "sense strand" since coding strand could mean "coding for poly peptide" or "coding for mRNA." (The term "coding strand" is nigh e'er used the way Becker uses information technology, to mean "coding for poly peptide.")

            (2). The terms "template strand" or "transcribed strand" tin can as well be interpreted in more than one way, but these terms are virtually ever used to mean the strand acting every bit template for RNA synthesis (= the strand that is transcribed from, not the strand that is existence made, during transcription). The template or transcribed strand is not the strand equivalent to the mRNA  -- the template strand is the strand complementary to the mRNA.

4. Directions: Suppose y'all accept a double stranded DNA template. If demand to copy "Crick," RNA polymerase volition go 1 mode (say right to left -- actual direction will depend on which terminate of template is 5' end); if need to copy "Watson" RNA polymerase will demand to become the other way (say left to correct). What determines where RNA polymerases starts & which way information technology goes? This is discussed beneath.

See problems seven-3, vii-four, 7-8 & 7-9.

D.  Details for Starts and Stops  (see picture below = bottom of handout 12B)

• First sequences every bit binding sites. A starting time signal for transcription or replication is a sequence in the Deoxyribonucleic acid recognized by the appropriate polymerase = binding site for that polymerase
  

• Names of first sequences
      Starts for Deoxyribonucleic acid synthesis =  Origins. DNA pol. recognizes (binds to) commencement signals for replication called origins (ori'southward).
      Starts for RNA synthesis = Promoters. RNA political leader. recognizes (binds to) commencement signals for transcription called promoters (P's).
  

• Promoter Details:

  1. Promoters determine the management of transcription.  Promoter and enzyme are asymmetric; therefore once enzyme binds, the catalytic end of RNA pol. is "facing" in one management, and that determines the direction of transcription (and therefore which strand will exist template).

  2. The promoter will exist a double stranded sequence at the end of the gene where RNA polymerase starts (= on 3' end of template strand = on five' end of sense strand). Going along the sense strand, the way the gene is usually written (5' to 3', left to right) the promoter is  "upstream" of the cistron.
  

• How many starts? There are more P'southward than ori's in prokaryotic Deoxyribonucleic acid. (Merely demand 1 ori per prok. DNA; need one P per mRNA made.)
  
• Stop (Termination) Signals. Special sequences in DNA may not be needed for DNA pol. -- enzyme may only go until it reaches the end. You exercise need some sort of machinery to end synthesis of each RNA. In prokaryotes in that location are special sequences (often called terminators) that cause the terminate of transcription. The mech. for ending transcription is somewhat different in eukaryotes and prokaryotes.  (We'll exercise euk. details side by side term.)

  Notes:
  (one) End signals for translation (finish codons) are different than the stop signals for transcription (terminators). Encounter Sadava table (non fig.) xiv.two (12.1). Translational stops are not recognized by the transcription (or replication) machinery. Each set of enzymes (for translation, transcription, or replication) recognizes only its own corresponding start and end sequences. (More on this when nosotros get to operons.)
  (two) The process of starting and stopping macromolecular synthesis is often more circuitous than we discuss. See texts for details.

Two. Sense & Antisense

A. Why apply merely one strand in any one region?

one. The office statement: Messenger RNA must be single stranded to fit in a ribosome and be translated. If RNA complementary to mRNA were present, what would happen? The "sense" mRNA and the "anti-sense" complementary RNA would hybridize. The resulting double stranded RNA wouldn't be translated. So even though the gene was present, and transcribed, it's protein product wouldn't be fabricated. This is what would happen if both strands were transcribed.

2. The evolutionary argument: If both strands are used to make mRNA, yous can't optimize ane without messing up the other, and vice versa. If natural selection favors the sequence of i strand so that it has optimal office or coding action, that automatically determines the sequence of the other strand. Natural pick can't simultaneously select for the optimal sequences of both strands (if each strand has an independent part).

 B. Uses of "anti-sense" mRNA

i. What good is anti-sense RNA? Factor therapy (calculation Dna) should permit you lot to replace a defective gene that is making an ineffective product. But what practise y'all do about a gene that is making too much production, or making it when it shouldn't? In other words, how practise yous silence an over-agile gene? This is an important question, considering inappropriate or over expression of genes is idea to be a major factor in affliction, for example, in assuasive cancer cells to multiply when they shouldn't. Utilize of anti-sense engineering should allow you lot to silence an over-active, or inappropriately active, gene. (Usually short double stranded RNA is added instead of single stranded antisense RNA, as explained below. See Becker Figs. 23-35 & 23-36 or Sadava fig.18.8 (xvi.14).

2. How to get anti-sense RNA into cells?  There are iii ways to do it:

a. Antisense mRNA tin exist added to cells . Since RNA is easily degraded, modified RNA's, more resistant to hydrolysis, are used instead of ordinary RNA's.

b. Antisense mRNA can be fabricated in the cell from a second copy of the factor. The 2d copy is added past genetic technology methods; it is inverted (relative to the promoter), so that the 2d re-create of the gene is transcribed in the opposite orientation from the original re-create. Inverting a gene relative to its promoter is equivalent to moving the promoter to the opposite finish of the gene (and turning it effectually) thereby reversing the management of transcription.  The original copy is transcribed from the usual template ("transcribed") strand to make mRNA; the 2nd re-create is transcribed from the complementary ("sense") strand to make anti-sense RNA. The two RNA's hybridize to each other and neither RNA is translated.

            c. Double Stranded (ds) RNA tin can generate antisense RNA  -- Come across Becker fig. 23-35 (sixth or seventh ed; not in 5th).

• ds RNA tin be added to cell (or cell can brand some ds RNA from its Dna either naturally -- encounter ** below -- or because of genetic applied science, every bit above)
• Cells have normal enzymes that cutting up long ds RNA into short ds pieces, called curt interfering RNA (siRNA)
• Other enzymes degrade the 'sense' strand of the brusk ds RNA
• The remaining short piece of antisense RNA hybridizes to mRNA and blocks translation, and/or triggers degradation of the mRNA by cell enzymes.
• This miracle is called RNA interference or RNAi.

** Cells can as well make their ain 'normal' double stranded RNA. (It is made every bit a single strand, but doubles back on itself to form a relatively short hairpin.) The hairpin is and so cut up by enzymes to generate a short RNA that blocks translation as above. These short antisense RNA's are called microRNAs instead of interfering RNAs.  See Becker fig. 23-36 (sixth or 7th ed; non in fifth).

three. Why RNAi &/or microRNA? Why do cells have enzymes to do it and labs use it?

    a. RNAi is used by cells as a defense against many viruses. (The replication of many viruses generates long double stranded RNA.)

    b. Regulation of translation in multicellular organisms. This is the office of microRNAs. Precursor RNAs are made that fold dorsum on themselves to form hairpins. The double stranded hairpins are processed by the prison cell enzymes used in RNAi to make very brusque 'antisense' RNAs (here called microRNAs). The microRNAs hybridize to mRNAs and inhibit translation. This blazon of regulation seems to be very important during development in normal muticellular organisms.

    The 2009 Horowitz prize was awarded (by Columbia U.) to 2 of the discoverers of microRNAs. The awardees gave lectures last November. For more info on the prize, the lectures, and the awardees inquiry, go to http://www.cumc.columbia.edu/horwitz/

    c. RNAi is used in laboratories to block production ('knock down' expression) of specific proteins. Very short double stranded RNAs are added to cells, or the cells are genetically engineered to produce the double stranded RNAs. It is easier and more than effective to block translation with RNAi (curt ds RNA) than with antisense RNA (longer, ss RNA).  RNAi has been used extensively (in lab experiments) to silence specific eukaryotic genes and see what happens (in society to make up one's mind the function of the genes).

d. Therapeutic uses. Many possible uses are currently being tested, and promising results have been obtained for treatment of macular degeneration. For a review of possible therapeutic uses of RNAi click here. (You may demand to use a CU computer to reach this site.) Boosted info is on the Nova/PBS site.

    The 2006 Nobel prize in physiology and medicine was awarded to Burn & Mello for the discovery of RNA interference.  For more info on RNAi, try the Nova/PBS site  or the Ambion site. For a diagram of how information technology works, click here.

 To check your agreement of antisense, come across problem vii-16, function C.

III.  Proofreading. This was introduced terminal time. Hither is a review and a longer description. This will non be discussed in class at length, since the major points have already been made.

 1. What is proof reading?

    DNA pol. tin support and hydrolyze (break) phosphodiester bonds it has just made (if the incorrect base of operations was put in). This is called proof reading. (In some older texts it is called editing, but the term 'editing' is now usually reserved for a different process.) When Deoxyribonucleic acid politician. proof reads, it catalyzes the following reaction:

rxn A: chain (north+i units long) + H₂O ↔ concatenation (north units long) + XMP

2. Reminder: Proofreading is not the same every bit catalyzing the reverse of the polymerization reaction.

iii. Deoxyribonucleic acid polymerase tin can proof read, but RNA politico. probably does not

    DNA polymerase has 3' to 5' exo activity but information technology is generally assumed that RNA political leader. does not -- one time RNA polymerase catalyzes formation of a phosphodiester bail, the bond tin can non exist hydrolyzed past RNA politician. (But come across ** below.) Proof reading allows Dna polymerase to back up and remove bases (really nucleotides) that were inserted by error. If a One thousand is added at the finish of a growing chain where an A should accept been (opposite a T in the template), the enzyme can back upwardly and break off the G. And then it can try again to add the correct base (in this case an A). This allows Dna polymerase to keep the error rate low, as befits an enzyme that replicates the archival re-create of the genetic information. See Sadava fig. 13.21 A (11.22 A).  It is by and large assumed that RNA pol. does not need to proofread, because RNA molecules are working copies that tin tolerate a few errors (and can be replaced by new copies transcribed from the Dna).

**Notation: There is some evidence that some RNA polymerases tin can backup and proofread (although by a somewhat dissimilar mechanism). How wide spread this is, and important it is in reducing errors (compared to Dna proofreading) is not settled.  It is well known that the fault rate in Dna synthesis is significantly lower than the mistake rate in RNA synthesis. (The divergence is at least one order of magnitude, and may be much larger.)

If you are interested in the experimental evidence for RNA proofreading, click here. Since RNA proofreading is not well established, we will ignore information technology.

4. Proof reading and the ability to beginning bondage are linked

Table on handout 12-B says DNA polymerase proofreads and cannot starting time new bondage; RNA polymerase does non  proofread and can start new chains. These properties are linked, because the structure of Dna polymerase that allows proofreading  prevents information technology from starting new chains. Since DNA polymerase can add on to pre-existing chains, but cannot start them itself, information technology requires a primer (or primase) to go started.

 The remaining two sections, 5 & 6, are FYI only. They are here in case you are interested; you will not exist asked questions most these details. If you want to know more, consult an advanced text.

  5. FYI. How does proof reading work?

   Every polymerase has a substrate bounden site that includes the template, the last nucleotide added to the growing chain and the next dXTP to be added. With DNA polymerase, both bases, the one just added and the 1 well-nigh to be added, are checked each round to exist sure the bases match their complements in the template. First, the concluding base added-template match is "rechecked" before the chain grows any longer. If the concluding base added turns out to have been the wrong 1 (perhaps it was in the wrong tautomeric form temporarily and mispaired with the template?), and so the enzyme backs upwardly and removes the last base before trying to add another. One time the enzyme checks that the last base added is ok, it checks the lucifer betwixt the base of operations to exist added and the template. If at that place is a match, the enzyme catalyzes formation of the phosphodiester bond. Then each base of operations - template match is checked twice -- one time when the base is about to be added to the growing chain and once before the side by side base is added to information technology.
    RNA pol. also holds two nucleotides that are about to be linked by a phosphodiester bond and the template. But RNA pol. only checks the pairing betwixt the base to be added and its complement in the template. So if the last base of operations put in was incorrect, then exist it. No backing upwardly or corrections.

six. FYI. Why does proof reading affect ability to start chains?

    DNA polymerase can not offset bondage because the substrate binding site of DNA polymerase must agree both a nucleotide already office of a chain (the i only added) as well equally the side by side nucleotide to be put in. There must be a phosphodiester bond that is already fabricated, so the 3' to five' exonuclease will accept something to hydrolyze, only in case of a mismatch. At the first of a concatenation, at that place is no nucleotide already fastened to the end of a chain -- there is no chain. There are only 2, unattached nucleotides. So Dna pol. can't get started.
    We assume that RNA pol. tin start chains because its substrate bounden site does not need to hold a nucleotide that is already attached to a concatenation. Information technology can concur 2 nucleotides and hook them upwardly.

An example of proof reading (which y'all should exist able to practise) is in problem 6-fourteen, part B-iv.

Reminder: All kinds of RNA (tRNA, mRNA & rRNA) are made in the aforementioned style from a Dna template. Product of transcription can be a tRNA, mRNA or rRNA. RNA is NOT used as template to make more RNA.  So how do all three types of RNA "brand protein?" That'southward the adjacent question.

See problem vii-nine.

IV. Details of Protein Synthesis/Translation

    What are the big issues? Same as for all non repeating polymers = Order, energy and enzymes!! We'll focus on order first.

    A. How is mRNA read?

one. It's read in triplets going 5' to iii' . Reading starts at a fixed signal and then mRNA is read one triplet or codon at a time in the five' to iii' direction.

2. Code table See handout 13A or texts for lawmaking table. Note that table lists codons  = triplets found in the mRNA (Not complements of codons) and corresponding amino acids. One codon specifies i amino acid.  For example, CUA means leucine; UUU ways phenylalanine, AUG means methionine.

three. Punctuation. Note that some codons signify "stop", not an amino acrid. AUG does double duty as both "beginning" and "methionine." Translation starts at an AUG, and ends when it reaches the first stop codon afterward the AUG. How the proper AUG is called is different for prokaryotes and eukaryotes. (See the texts if you are interested in the details.) More than specifics on stops & starts next time.

4. Leaders & Trailers. The region before the first AUG is not translated. It is called a leader, or five'UTR (united nations-translated-region) or 5'UTS (un-translated sequence). Translation by and large stops before the end of the mRNA (at a cease codon -- UAG, UAA or UGA). The untranslated region after the stop codon is called a trailer, or three' UTR or iii' UTS.

five. Reading Frames. At that place is more than 1 manner to read a nucleic acid sequence in non-overlapping groups of three, depending on where you start. The different ways are called unlike reading frames. If you start with the first, quaternary, or 7th.... base you get one reading frame; if you get-go with the 2nd, 5th, or eighth.... you lot become the second, and then on. At that place are 3 possible reading frames.

To be sure you understand how to use the code tabular array, try problem 7-12, parts A & B.

B. Structure/Function of tRNA   For a video of the course demonstration see video (windows media file) by Peter Sloane at http://www.columbia.edu/cu/biology/courses/c2005/lectures/tRNA.wmv

1. Adapter Role -- how does cell know AUG is met and CUA is leu? You have the text  or handout with the lawmaking table, only cell doesn't.

 a. Transfer RNA (tRNA) = adaptor . Cell uses  tRNA to lucifer the codon in the mRNA (say AUG or CUA) with the corresponding amino acrid (met or leu, respectively).

b. Loading Enzymes. Adaptor must carry the correct amino acid. Cell uses loading enzymes to put the correct amino acids on to their respective tRNA'south. More details side by side time.

2 . Structure of tRNA (see handout 13A & texts for pictures)

a. Size: About 75 bases long (relatively small). Consists of RNA chain folded back on itself.

b. Many different ones. Actual number of dif. tRNA'southward is more than 20 (#of dif. amino acids) and less than 64 (# of dif. codons). More verbal estimate of # of unlike tRNA's to follow in side by side lecture.

c. 2 headed molecule : tRNA has 2 critical parts

• one part (in middle of chain) is complementary to codon (= anticodon)

• 1 part (on 3' finish) is acceptor end --  picks upwardly the appropriate amino acid with the help of the appropriate enzyme.

• when tRNA is folded in 3D, acceptor end and anticodon are at contrary ends of molecule

d. Full general features of structure

Secondary Structure: Each tRNA molecule is doubled back on itself to grade a cloverleaf with double stranded sections.  Sequences of unlike tRNA's differ, but all are self complementary in certain regions. Every tRNA molecule has aforementioned bones "secondary structure" = cloverleaf.

Tertiary Structure: Cloverleaf is folded into an L shaped "tertiary" construction, which has anticodon at one terminate and acceptor for its amino acid at the other. (Come across handout, Becker fig. 22-3, or Sadava fig. 14.12 (12.8),  for secondary and tertiary structures.) The final folded  tRNA molecule is virtually one codon broad. That mode 2 tRNAs can attached to neighboring codons without bumping into each other.

Important reminder: The code table lists the codons, NOT the anticodons. The anticodon in the tRNA is the complement of the triplet shown in the table.

Run into trouble vii-18.

3. How is tRNA used to line up amino acids (AA)? two AA at a time are held in identify by tRNAs (for forming peptide bond) -- see handout 13B. Why 2? considering a ribosome can hold merely 2 loaded tRNAs at a fourth dimension that are hydrogen bonded to mRNA. (Come across details beneath.)

4.  tRNA/mRNA pairing is antiparallel -- All nucleic acids pair in an antiparallel fashion. So if mRNA is written in usual way (five' → 3'), and so tRNA is lined up in the reverse way, 3' → 5'. (With the amino acrid or chain on its left, 3' end.) Anticodon is often written 3' → five' to brand this clear. For ex., if codon is CGG, anticodon is usually written 3' GCC five' not CCG (or it is written upside downwardly equally on handout 13A).

        five. How are the tRNA and AA connected? The AA is fastened to the iii' end of its respective tRNA by a ester bond betwixt the COOH terminate of the AA and the two' or 3' OH on the final ribose (at the three' end). This leaves the amino of the AA free.

        vi. Loading of tRNA. How exercise you become the right AA on the respective tRNA in the first identify, and/or how do you reload the tRNA once information technology gives its AA away? Loading requires enzymes and energy -- we'll look at it carefully next time. For now we'll just assume each tRNA is loaded with its respective amino acid,

C. How does the new peptide chain grow?  Run across handout 13B or Sadava fig. 14.xvi (12.12) or Becker fig. 22-10.

For a video of the class sit-in meet video (windows media file) by Peter Sloane at http://www.columbia.edu/cu/biology/courses/c2005/lectures/translation.wmv

1. Chain adds to newest AA. When each peptide bond is made, the growing chain is transferred (from the tRNA that previously held it) to the next amino acid (still attached to its tRNA), non the other way around, for logistical reasons. The newest amino acid is non added to the free end of the chain. Instead, the chain is added to the newest amino acid. (The current system allows the translation mechanism to slide down the mRNA reading two adjacent codons at a time. The other fashion doesn't.)

       Catalyst for formation of peptide bonds is chosen peptidyl transferase considering the growing peptide concatenation is transferred as described above. This catalyst is part of the ribosome.

ii. Peptide chain grows amino → carboxyl. This follows because the amino acids are held downwardly (attached to tRNA) by their COOH ends. And so if concatenation must add together to free end of next AA, must add to amino finish of next AA. (Note for those who accept had organic: From the point of view of mechanism, the electrons go the other way; the electrons of the amino assail the carboxyl.)

3. Energy for peptide synthesis. The energy derived from splitting the tRNA~AA (really the tRNA~concatenation) bond drives peptide bond synthesis. In other words, the AA-tRNA connection is a high energy bond. How it is formed at the expense of ATP will be discussed next time. (Additional energy is required to bind the AA~tRNA and move the ribosome downwards the mRNA, but we will ignore the free energy details of those steps, as well as the proteins needed to promote them.)

4.  Stops. The peptide chain stops growing when the translation machine comes to a stop codon. There are no tRNA'southward for the stop codons, so at that place is no mode that the chain tin proceed growing if a end codon comes next. Come across Sadava fig. fourteen.17 (12.thirteen) or Becker fig. 22-11.

To review protein synthesis so far, and the role of tRNA, try problem 7-21.

D. How exercise ribosomes fit in?

one. Part. You demand something to concur tRNA (two loaded ones at a time) onto mRNA while amino acids are being hooked up and you lot need to provide necessary enzymes for making peptide bond etc. (How many weak bonds hold a tRNA and mRNA together?)

2. Ribosome contains both RNA and poly peptide. Holding of tRNA etc. is done by a structure that contains both RNA(s) and poly peptide(s). Anything made of both is called an RNP = ribonucleoprotein or ribonucleoprotein particle. This detail RNP construction = ribosome; RNA within it is chosen ribosomal RNA or rRNA.  Be careful not to confuse ribosomal RNA (rRNA) & ribosomes.

For pictures of ribosome structure come across Sadava fig. 14.14 (12.10) and/or Becker figs. 22-1 & 22-two & table 22-ane. )Molecular details of construction next fourth dimension.

3.  Important Structural Features (Come across Becker, fig. 22-2 or Sadava fig. 14.14 (12.x) Run into handout 13-A.

    a. i site or groove for mRNA.

    b. two sites for loaded tRNA (hybridized to mRNA) per ribosome -- These are chosen A and P; more than details below. These sites bind both mRNA and (loaded) tRNA.

    c. Ane site for unloaded tRNA This site binds binds empty, used tRNA before it is bumped off the ribosome. (It's called Due east for exit site). This site is sometimes omitted in diagrams of elongation. (The T site shown in the seventh ed. of Purves probably does not exist and should be ignored.) The E site binds tRNA but not mRNA.

    d. All ribosomes are the same. Which protein is fabricated does not depend on the ribosome.

iv. How Ribosomes Move (See Becker fig. 22-vii & 22-10 or Sadava fig. 14.16 (12.12)

    a. Directions : Ribosome moves down mRNA 5' to 3' (or mRNA slides through ribosome) as peptide is made amino to carboxyl. Both peptides and nucleic acids are both fabricated/read as written, left to right.
    How mRNA is made and how it is translated happen to be in the same direction, only transcription and translation are two separate processes (which are commonly coupled in prokaryotes but not eukaryotes).

    b. A & P sites. The ii binding sites for loaded tRNA are different -- 1 called A binds amino acyl tRNA & 1 called P binds peptidyl tRNA.

    c . Translocation -- Movement of mRNA (& tRNA's) relative to the Ribosome.

    (one). Differences between the A & P sites permit unidirectional movement. Before peptide bond is formed, AA-tRNA is in A site and peptidyl-tRNA is in P site. Equally presently equally peptide bond is formed, tRNA in A site becomes a peptidyl-tRNA, and tRNA in P site becomes unloaded or empty tRNA, Since "wrong" types of  tRNA are now in A & P sites, ribosome no longer fits properly and moves over one codon, shifting peptidyl-tRNA to P site, empty tRNA to E site and leaving A site empty to agree side by side AA-tRNA. When the side by side AA-tRNA arrives, the empty or unloaded tRNA is then released to exist reloaded and used again.

    (2). Which function actually moves? Ribosome or mRNA?

mRNA & ribosome: Motion one codon relative to each other. On handout 13B, in steps 5 & 6, information technology looks like the ribosome moves 1 codon toward the 3' cease of the message. Probably, the ribosome stays in stock-still position and the mRNA advances one codon through the ribosome in the five' direction, as shown in pace two → iii. (In other words, if drawn correctly, the mRNA moves to left instead of the ribosome moving to the right.)

Messenger RNA & tRNA: These do not move relative to each other but are pulled together.

Annotation that the effect is the same whether the ribosome or the mRNA (& attached tRNAs) motility -- the ribosome and mRNA are shifted i codon relative to each other and all the tRNA'due south shift down one site. Either style you wait at information technology, the overall result is:

• The empty tRNA moves into the East site,
• The peptidyl tRNA moves into the P site, and
• The A site becomes empty, fix for the next AA-tRNA.

    (3). Poly peptide Synthesis uses upward a lot of Free energy . Movement and bounden tRNA both require free energy which nosotros are ignoring. You probably need at to the lowest degree 5 P's split from ATP (or GTP) per AA added if you lot count all the steps involved, not just growth of peptide chain. So making proteins is a very expensive procedure, and making unnecessary proteins is very wasteful. Every bit a result, there has been potent selection for efficient regulation of poly peptide synthesis; how regulation works will exist explained next time. (For interest of GTP in translation meet Becker figs. 22-8 & 22-10.)

To review how the A & P sites fit in, try trouble 7-12, office C.

5. How Ribosomes attach to mRNA

    a. Attachment. When not in use, ribosomes come apart into subunits. The cell contains a pool of subunits. When translation starts, i small subunit and one large subunit clench onto the mRNA to form a ribosome and begin translation. When translation ends, the two subunits come apart, fall off the mRNA, and render to the pool -- ready to exist used again.

    b.  Polysomes -- More than one ribosome can read a single message at one time.
The first ribosome attaches near the 5' finish of the mRNA. And so the ribosome moves (run into note below) down the mRNA toward the 3' finish, making protein. Once the ribosome has moved far enough down, a second ribosome tin attach behind it (on the 5' side) and follow the commencement ribosome down the message. Every bit each ribosome moves toward the iii' end, making protein, another ribosome attaches subsequently it until the entire mRNA is covered with ribosomes. The mRNA remains covered with ribosomes; although some ribosomes finish and fall off the three' end, others continually attach at the 5' terminate. The mRNA covered with multiple ribosomes is called a polyribosome or polysome for short. Sadava fig. 14.18 (12.14).

Note: This description assumes that the ribosomes move downwardly the mRNA, 5' to 3'. The effect is the same if yous assume the ribosomes stay put while the mRNA moves through the ribosomes, 5' terminate first. (Which is more likely.) One time enough mRNA has slid through the first ribosome,  a second ribosome can adhere to the space on the 5' cease and the mRNA can thread through that 1 adjacent, and so on.

To review polysomes, endeavor problem seven-sixteen, part B. E. Peptidyl Transferase is a Ribozyme

            Peptidyl transferase is part of the ribosome. The catalytic activeness is a holding of the rRNA in the big subunit, not a protein, so this is non really an enzyme (goad made of protein) but a ribozyme (goad fabricated of RNA). It is presumed that it is a relic of the "RNA earth" that existed before Dna and protein took over many of the early on functions of RNA (which has both catalytic and informational properties). Peptidyl transferase is not the only ribozyme -- other catalytic RNA's are known.
For more details come across  http://world wide web.sciencemag.org/cgi/content/full/289/5481/878 You tin can reach this site from any Columbia computer; I don't know if you can become information technology from a personal computer if y'all are not a subscriber to Science Online. Note that this site has detailed "hypernotes" which list many sites useful to molecular biologists. If you find any of these useful, please tell Dr. Grand. so she can tell other students. (The site possibly slow to load, merely the link works.)

Next fourth dimension: Whatever details of the above we don't get to, plus some more of import details to wrap upwards  translation. Then (one) what happens when macromolecular synthesis makes mistakes, and (2) how is protein synthesis regulated in prokaryotes?

� Copyright 2010 Deborah Mowshowitz and Lawrence Chasin Department of Biological Sciences Columbia University New York, NY

In What Direction Does Rna Polymerase Read a Template Strand

Source: http://www.columbia.edu/cu/biology/courses/c2005/lectures/lec13_10.html

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