The document discusses the structure and function of nucleic acids and their components. It covers the following key points: 1. Nucleic acids are made of nucleotides, which consist of a nucleoside (a nitrogenous base linked to a 5-carbon sugar) and one or more phosphate groups. The bases are either pyrimidines or purines. 2. DNA contains the bases adenine, guanine, cytosine, and thymine, while RNA contains adenine, guanine, cytosine, and uracil instead of thymine. 3. Nucleotides join together via phosphodiester bonds between the 3' carbon of one sugar and the 5' carbon of the next,

2. The nucleic acid bases are of two types – pyrimidines and purines. Three pyrimidine bases (single-ring aromatic compounds) – cytosine, thymine, and uracil – commonly occur. Cytosine is found both in RNA and in DNA. Uracil occurs only in RNA. In DNA, thymine is substituted for uracil. The common purine bases are adenine and guanine, both of which are found in RNA and in DNA.
A nucleoside is a compound that consists of a base and a sugar covalently linked together. When the sugar is β-D-ribose, the resulting compound is a ribonucleoside; when the sugar is β-D- deoxyribose, the resulting compound is a deoxyribonucleoside.

3. When phosphoric acid is esterified to one of the hydroxyl groups of the sugar portion of a nucleoside, a nucleotide is formed. A nucleotide is named for the parent nucleoside, with the suffix “monophosphate” added. The 5’ nucleotides are most commonly encountered in nature. If additional phosphate groups form anhydride linkages to the first phosphate, the corresponding nucleoside diphosphates and triphosphates are formed.

5. The polymerization of nucleotides gives rise to nucleic acids. The linkage between monomers in nucleic acids involves formation of two ester bonds by phosphoric acid. The hydroxyl groups to which the phosphoric acid is esterified are those bonded to the 3’ and 5’ carbons on adjacent residues. The resulting repeated linkage is a 3’,5’-phosphodiester bond. The nucleotide residues of nucleic acids are numbered from the 5’ end, which normally carries a phosphate group, to the 3’end, which normally has a free hydroxyl group.

6. Double helix
In the double helix, the two chains are coiled around a common axis called the axis of symmetry. The chains are paired in an antiparallel manner, that is, the 5’-end of one strand is paired with the 3’-end of the other strand. In the DNA helix, the hydrophilic deoxyribose- phosphate backbone of each chain is on the outside of the molecule, whereas the hydrophobic bases are stacked inside.

7. Watson and Crick model of DNA
As originally presented by Watson and Crick, DNA is composed of two strands, wound around each other in a right-handed, helical structure with the base pairs in the middle and the deoxyribosylphosphate chains on the outside. The orientation of the DNA strands is anti-parallel (i.e. the strands run in opposite directions). The nucleotide bases on each strand interact with the nucleotide bases on the other strand to form base pairs.
Watson-Crick base pairing of nucleotides in DNA.

8. The base pairs are planar and are oriented nearly perpendicular to the axis of the helix. Each base pair is formed by hydrogen bonding between a purine and a pyrimidine. Guanine forms three hydrogen bonds with cytosine, and adenine forms two with thymine. Because of the specificity of this interaction between purines and pyrimidines on the opposite strands, the opposing strands of DNA are said to have complementary structures.

11. General classes of RNA
RNA
Size and length
Percent of total cellular RNA
Function
rRNA
28s, 18s, 5.8s, 5s
(26s, 16s, 5s)8
80
interact to form ribosomes
tRNA
65-110 nt
15
adapter
mRNA
0.5-6 kb
5
direct synthesis of cellular proteins
- ribosomal RNA (rRNA) from prokaryotes consists of three different sizes of RNA, while rRNA from eukaryotes - four different sizes of RNA. These RNAs interact with each other, and with proteins, to form a ribosome that provides the basic machinery on which protein synthesis takes place;
- transfer RNAs (tRNAs) consist of one size class of RNA that are 65-110 nucleotides in length; they function as adapter molecules that translate the information stored in the mRNA nucleotide sequence to the amino acid sequence of proteins;
- messenger RNAs (mRNAs) represent the most heterogeneous class of RNAs found in cells, ranging in size from 500 nt to>6 kb; they are carriers of genetic-information, defining the sequence of all proteins in the cell

12. The roles of different kinds of RNA
RNA type
Size
Function
Transfer RNA
Small
Transports amino acids to site of protein synthesis
Ribosomal RNA
Several-kinds – variable in size
Combines with proteins to form ribosomes, the site of protein synthesis
Messenger RNA
Variable
Directs amino acid sequence of proteins
Small nuclear RNA
Small
Processes initial mRNA to its mature form in eukaryotes
Small interfering RNA
Small
Affects gene expression; used by scientists to knock out a gene being studied
Micro RNA
Small
Affects gene expression; important in growth and development

13. Transfer RNA
The smallest of the three important kinds of RNA is tRNA. Different types of tRNA molecules can be found in every living cell because at least on tRNA bonds specifically to each of the amino acids that commonly occur in proteins. Frequently there are several tRNA molecules for each amino acid.
The molecule can be drawn as a cloverleaf structure, which can be considered the secondary structure of tRNA because it shows the hydrogen bonding between certain bases. During protein synthesis, both tRNA and mRNA are bound to the ribosome in a definite spatial arrangement that ultimately ensures the correct order of the amino acids in the growing polypeptide chain.

14. A schematic drawing of a proposed secondary structure for 16S rRNA.
Ribosomal RNA
In contrast with tRNA, rRNA molecules tend to be quite large, and only a few types of rRNA exist in a cell. The RNA portion of a ribosome accounts for 60%-65% of the total weight.
An E.coli ribosome typically has a sedimentation coefficient of 70S. When an intact 70S bacterial ribosome dissociates, it produces a light 30S subunit and a heavy 50S subunit. The 30S subunit contains a 16S rRNA and 21 different proteins. The 50S subunit contains a 5S rRNA, a 23S rRNA, and 34 different proteins.

15. The structure of a typical prokaryotic ribosome. The individual components can be mixed, producing functional subunits.

16. Messenger RNA
The least abundant of the main types of RNA is mRNA. In most cells, it constitutes no more than 5%-10% of the total cellular RNA. The sequences of bases in mRNA specify the order of the amino acids in proteins. In rapidly growing cells, many different proteins are needed within a short time interval. Consequently, it is logical that mRNA is formed when it is needed, directs the synthesis of proteins, and then is degraded so that the nucleotides can be recycled. Both tRNA and rRNA can be recycled intact for many rounds of protein synthesis.
The sequence of mRNA bases that directs the synthesis of a protein reflects the sequence of DNA bases in the gene that codes for that protein. Messenger RNA molecules are heterogeneous in size, as are the proteins whose sequences they specify.

17. Replication
DNA replication yields two DNA molecules identical to the original one, ensuring transmission of genetic information to daughter cells with exceptional fidelity.
Transcription
The sequence of bases in DNA is recorded as a sequence of complementary bases in a single- stranded mRNA molecule.
Translation
Three-base codons on the mRNA corresponding to specific amino acids direct the sequence of building a protein.
Six kinds of RNA – transfer RNA (tRNA), ribosomal RNA (rRNA), messenger RNA (mRNA), small nuclear RNA (snRNA), micro RNA (miRNA), and small interfering RNA (siRNA) – play an important role in the life process of cells.

18. Replication fork - each set of replication machinery together with DNA that it is replicating
Leading strand - no problems with newly synthesized DNA laid down in a 5’ to 3’ direction
Lagging strand - replicates in a series of short segments, every time the DNA strands have been peeled apart by 250 nucleotides a polymerase/primase complex initiates DNA synthesis running back toward the replication origin in a 5’ to 3’ direction
Okazaki fragments - small fragments enabling replication in proper direction

19. Replication fork - each set of replication machinery together with DNA that it is replicating
Leading strand - no problems with newly synthesized DNA laid down in a 5’ to 3’ direction
Lagging strand - replicates in a series of short segments, every time the DNA strands have been peeled apart by 250 nucleotides a polymerase/primase complex initiates DNA synthesis running back toward the replication origin in a 5’ to 3’ direction
Okazaki fragments - small fragments enabling replication in proper direction

20. Doubling of Information Before a cell divides to become two, its DNA must be doubled so that each daughter cell will receive a perfect copy. This means the strands of DNA must first be separated, then complementary nucleotides must be linked along each of the separated strands.

22. The “Unzipper”
(helicase)
The “Builders”
(polymerases)
The “Eraser” (repair nuclease)
The “Untwister”
(topoisomerase)
The “Straighteners”
(single-strand DNA-
binding proteins)
The “Stitcher”
(ligase)
The “Initiator”
initiator protein)
How Enzymes Copy DNA
A Cast of Ingenious Characters
The sequence at the left oversimplifies. DNA doesn't copy itself any more than a recipe bakes a cake. DNA passively stores information. The team of proteins shown above does the actual copying, or replication. And they do it with an accuracy of only one mistake in every hundred thousand or so nucleotides!

23. DNA Replication — The Details
1. The initiator finds the place to begin copying and guides the unzipper to the correct position.
2. The unzipper separates the DNA strands by breaking the weak bonds between the nucleotides.
3. Then the builders arrive to assemble a new DNA strand along each of the exposed strands.
4. They build by joining individual nucleotides to their matching complements on the old strand.

24. How Enzymes Copy DNA (continued)
DNA Replication — The Details
5. Free-floating nucleotides bring their own energy. Remember ATP? There's also GTP, CTP, and TTP.
6. As each new nucleotide is added to the growing chain, its phosphate bond energy goes into making the new bond.
7. The upper builder follows behind the unzipper, but the lower strand runs the opposite way.
8. Yet the lower builder must build in the same chemical direction. She solves this by making a loop...

25. 9. ...and building along the bottom half of it.
10. When she finishes a length, she lets go of the completed end...
11. ...grabs a new loop, and continues linking nucleotides along a new stretch.
12. So, while the top new strand is built continuously, the bottom new strand is assembled in short lengths...

26. 13. ...which are then spliced together by the stitcher. This reaction requires energy, supplied by ATP.
14. The straighteners keep the single DNA strands from getting tangled.
15. And the untwister unwinds the double helix in advance of the unzipper.
DNA Replication –The Details

27. 16. The initiator, the unzipper, the builders, the stitcher, the untwister, and the straighteners work together in tight coordination, making near-perfect copies at the rate of fifty nucleotides per second!

28. Despite the elaborate proofreading system employed during DNA synthesis, errors – including incorrect base-pairing or insertion of one to a few extra nucleotides – can occur. In addition, DNA is constantly being subjected to environmental insults that cause the alteration or removal of nucleotide bases. The damaging agents can be either chemicals, for example, nitrous acid, or radiation, for example, ultraviolet light, and high-energy ionizing radiation, which can cause double-strand breaks. Bases are also altered or lost spontaneously from mammalian DNA at a rate of many thousands per cell per day. If the damage is not repaired, a permanent change (mutation) is introduced that can result in any of a number of deleterious effects, including loss of control over the proliferation of the mutated cell, leading to cancer.
DNA REPAIR

29. Cells are remarkably efficient at repairing damage done to their DNA. Most of the repair systems involve recognition of the damage (lesion) on the DNA, removal or excision of the damage, replacement or filling the gap left by excision using the sister strand as a template for DNA synthesis, and ligation. These repair systems thus perform excision repair, with the removal of one to tens of nucleotides.
- Methyl-directed mismatch repair
- Repair of damage caused by ultraviolet (UV) light
- Correction of base alterations (base excision repair)
- Repair of double-strand breaks

30. 1860 - chromosomes (chromos - color, soma - body)
Every somatic cell contains normally two copies of each chromosome
The number of unique chromosomes (N) in such a cell is known as its haploid number.
The total number of chromosomes (2N) is its diploid number
Number of chromosomes (2N) in some eukariotes
Organism
Chromosomes
Humans
46
Dog
78
Rat
42
Turkey
82
Frog
26
Fruit fly
8
Hermit crab
~254
Garden pea
14
Potato
48
Yeast
34

31. ORGANIZATION OF EUKARYOTIC DNA
A typical human cell contains 46 chromosomes, whose total DNA is approximately 1m long! Such a large amount of genetic material can be effectively packaged into a volume the size of a cell nucleus so that it can be efficiently replicated and its genetic information expressed. To do so requires the interaction of DNA with a large number of proteins, each of which performs a specific function in the ordered packaging of these long molecules of DNA. Eukaryotic DNA is associated with tightly bound basic proteins, called histones. These serve to order the DNA into fundamental structural units, called nucleosomes, that resemble beads on a string. Nucleosomes are further arranged into increasingly more complex structures that organize and condense the long DNA molecules into chromosomes that can be segregated during cell division.

32. E.coli RNA polymerase (holoenzyme) – ~450 kDa protein α2ββ σ. After initiation a subunit dissociate from the core enzyme, α2ββ which carries out the actual polymerization process.
Several function of holoenzyme:
1. template binding
2. RNA chain initiation
3. chain elongation
4. chain termination
The synthesis of a RNA by a polymerase always requires a template.
The copying of DNA by an RNA polymerase to make RNA is called transcription (two DNA strands are complementary, but not identical - different protein-coding potentials).
The nucleotide at the terminal 5’ end of a growing RNA strand is chemically distinct from the nucleotides within the strand in that it retains all three phosphate groups. When an additional nucleotide is added to the 3’ end of the growing strand, only the a phosphate is retained; the β and γ are lost.
RNA polymerase

33. TRANSCRIPTION
The process of transcription can be divided into three phases: initiation, elongation, and termination. A transcription unit extends from the promoter to the termination region, and the initial product of transcription by RNA polymerase is termed the primary transcript. Initiation: Transcription begins with the binding of the RNA polymerase holoenzyme to a region of the DNA known as the promoter, which is not transcribed. The prokaryotic promoter contains characteristic consensus sequences.

34. Elongation: Once the promoter region has been recognized and bound by the holoenzyme, local unwinding of the DNA helix continues, mediated by the polymerase. RNA polymerase begins to synthesize a transcript of the DNA sequence. The elongation phase is said to begin when the transcript (typically starting with a purine) exceeds ten nucleotides in length. The core enzyme is able to leave (“clear”) the promoter and move along the template strand in a processive manner. During transcription, a short DNA- RNA hybrid helix is formed. Like DNA polymerase, RNA polymerase uses nucleoside triphosphates as substrates and releases pyrophosphate each time a nucleoside monophosphate is added to the growing chain.
Termination: The elongation of the single-stranded RNA chain continues until a termination signal is reached. Termination can be intrinsic (spontaneous) or dependent upon the participation of a protein known as the ρ (rho) factor.

35. Three stages in transcription. During initiation of transcription, RNA polymerase forms a transcription bubble and begins polymerization of ribonucleotides (rNTPs) at the start site, which is located within the promoter region. Once a DNA region has been transcribed, the separated strands reassociate into a double helix, displacing the nascent RNA except at its 3’ end 5’ and of the RNA strand exits the RNA polymerase through a channel in the enzyme. Termination occurs when the polymerase encounters a specific termination sequence (stop site). See the text for details.

38. PROTEIN SYNTHESIS
Genetic information, stored in the chromosomes and transmitted to daughter cells through DNA replication, is expressed through transcription to RNA and translation into proteins (polypeptide chains). The pathway of protein synthesis is called translation because the “language” of the nucleotide sequence on the mRNA is translated into the “language” of an amino acid sequence. The process of translation requires a genetic code, through which the information contained in the nucleic acid sequence is expressed to produce a specific sequence of amino acids. Any alteration in the nucleic acid sequence may result in an incorrect amino acid being inserted into the polypeptide chain, potentially causing disease or even death of the organism.
Overview

40. The genetic code
Codons Codons are presented in the mRNA language of adenine (A), guanine (G), cytosine (C), and uracil (U). Their nucleotide sequences are always written from the 5'-end to the 3'-end. The four nucleotide bases are used to produce the three-base codons. There are, therefore, 64 different combinations of bases, taken three at a time. 1. How to translate a codon: This table (or “dictionary”) can be used to translate any codon and, thus, to determine which amino acids are coded for by an mRNA sequence. Sixty-one of the 64 codons code for the 20 common amino acids. 2. Termination (“stop” or “nonsense”) codons: Three of the codons, UAG, UGA, and UAA, do not code for amino acids, but rather are termination codons. When one of these codons appears in an mRNA sequence, synthesis of the polypeptide coded for by that mRNA stops.

41. Usually, only one reading frame (#3) will produce a functional protein since the other two reading frames will include several Stop codons
READING FRAMES
Reading frame 1
5' 3'
UUA UGA GCG CUA AAU
Leu Stop Ala Leu Asn
Reading frame 2
U UAU GAG CGC UAA AU
Tyr Glu Arg Stop
Reading frame 3
UU AUG AGC GCU AAA U
Met Ser Ala Lys

42. 1. Specificity: The genetic code is specific, that is, a particular codon always codes for the same amino acid. 2. Universality: The genetic code is virtually universal, that is, its specificity has been conserved from very early stages of evolution, with only slight differences in the manner in which the code is translated. 3. Degeneracy: The genetic code is degenerate. Although each codon corresponds to a single amino acid, a given amino acid may have more than one triplet coding for it. Only Met and Trp have just one coding triplet. 4. Nonoverlapping and commaless: The genetic code is nonover-lapping and commaless, that is, the code is read from a fixed starting point as a continuous sequence of bases, taken three at a time. For example, AGCUGGAUACAU is read as AGC/UGG/AUA/CAU without any “punctuation” between the codons.
Characteristics of the genetic code

43. A large number of components are required for the synthesis of a protein. These include all the amino acids that are found in the finished product, the mRNA to be translated, transfer RNA (tRNA) for each of the amino acids, functional ribosomes, energy sources, and enzymes, as well as protein factors needed for initiation, elongation, and termination steps of polypeptide chain synthesis.
COMPONENTS REQUIRED FOR TRANSLATION

44. - Amino acids
All the amino acids that eventually appear in the finished protein must be present at the time of protein synthesis. If one amino acid is missing, translation stops at the codon specifying that amino acid.
- Transfer RNA
At least one specific type of tRNA is required for each amino acid. In humans, there are at least 50 species of tRNA. Because there are only 20 different amino acids commonly carried by tRNA, some amino acids have more than one specific tRNA molecule. This is particularly true of those amino acids that are coded for by several codons.

45. 1. Amino acid attachment site: Each tRNA molecule has an attachment site for a specific (cognate) amino acid at its 3'- end. The carboxyl group of the amino acid is in an ester linkage with the 3'-hydroxyl of the ribose portion of the adenosine (A) nucleotide in the – CCA sequence at the 3'-end of the tRNA.
2. Anticodon: Each tRNA molecule also contains a three-base nucleotide sequence – the anticodon – that pairs with a specific codon on the mRNA. This codon specifies the insertion into the growing peptide chain of the amino acid carried by that tRNA.

46. Is required for attachment of amino acids to their corresponding tRNAs. Each member of this family recognizes a specific amino acid and all the tRNAs that correspond to that amino acid.
Aminoacyl-tRNA synthetases catalyze a two-step reaction that results in the covalent attachment of the carboxyl group of an amino acid to the 3'-end of its corresponding tRNA.
Aminoacyl-tRNA synthetases

47. The extreme specificity of the synthetase in recognizing both the amino acid and its cognate tRNA contributes to the high fidelity of translation of the genetic message. In addition, the synthetases have a “proofreading” or “editing” activity that can remove amino acids from the enzyme or the tRNA molecule.

49. Ribosomes are large complexes of protein and ribosomal RNA. They consist of two subunits – one large and one small – whose relative sizes are given in terms of their sedimentation coefficients, or S (Svedberg) values.
1. Ribosomal RNA
2. Ribosomal proteins
3. A, P, and E sites on the ribosome: Together, they cover three neighboring codons. During translation, the A site binds an incoming aminoacyl-tRNA as directed by the codon currently occupying this site. This codon specifies the next amino acid to be added to the growing peptide chain. The P-site codon carries the chain of amino acids that has already been synthesized. The E site is occupied by the empty tRNA as it is about to exit the ribosome.
4. In eukaryotic cells, the ribosomes are either “free” in the cytosol or are in close association with the endoplasmic reticulum.

50. STEPS IN PROTEIN SYNTHESIS
Initiation of protein synthesis involves the assembly of the components of the translation system before peptide bond formation occurs. These components include the two ribosomal subunits, the mRNA to be translated, the aminoacyl-tRNA specified by the first codon in the message, GTP, and initiation factors that facilitate the assembly of this initiation complex.
The initiating AUG is recognized by a special initiator tRNA. Recognition is facilitated by IF-2-GTP in prokaryotes and eIF-2-GTP in eukaryotes. In bacteria and in mitochondria, the initiator tRNA carries an N-formylated methionine. After methionine is attached to the initiator tRNA, the formyl group is added by the enzyme transformylase, which uses N10-formyl tetrahydrofolate. In eukaryotes, the initiator tRNA carries a methionine that is not formylated.
Initiation

51. Elongation of the polypeptide chain involves the addition of amino acids to the carboxyl end of the growing chain. During elongation, the ribosome moves from the 5'-end to the 3'-end of the mRNA that is being translated.
Elongation

52. The formation of the peptide bond is catalyzed by peptidyltransferase, an activity intrinsic to the 23S rRNA found in the large ribosomal subunit. After the peptide bond has been formed, what was attached to the tRNA at the P site is now linked to the amino acid on the tRNA at the A site. Translocation causes movement of the uncharged tRNA from the P to the E site (before being released), and movement of the peptidyl-tRNA from the A to the P site. The process is repeated until a termination codon is encountered.

57. Termination occurs when one of the three termination codons moves into the A site. The newly synthesized polypeptide may undergo further modification and the ribosomal subunits, mRNA, tRNA, and protein factors can be recycled and used to synthesize another polypeptide.
Termination
Translation begins at the 5'-end of the mRNA, with the ribosome proceeding along the RNA molecule. Because of the length of most mRNAs, more than one ribosome at a time can translate a message. Such a complex of one mRNA and a number of ribosomes is called a polysome or polyribosome.
Polysomes

58. Many polypeptide chains are covalently modified, either while they are still attached to the ribosome (cotranslational) or after their synthesis has been completed (posttranslational). These modifications may include removal of part of translated sequence, or the covalent addition of one more chemical groups required for protein activity.
Trimming
Many proteins destined for secretion from the cell are initially made as large, precursor molecules that are not functionally active. Portions of the protein chain must be removed by specialized endoproteases, resulting in the release of an active molecule. Zymogens are inactive precursors of secreted enzymes.
CO- AND POSTTRANSLATIONAL MODIFICATION OF POLYPEPTIDE CHAINS

59. 1. Phosphorylation: Phosphorylation occurs on the hydroxyl groups of serine, threonine, or, less frequently, tyrosine residues in a protein. The phosphorylation may increase or decrease the functional activity of the protein.
2. Glycosylation: Many of the proteins that are destined to become part of a plasma membrane or to be secreted from the cell, have carbohydrate chains attached to the amide nitrogen of asparagine or the hydroxyl groups of serine, threonine, or hydroxylysine. Glycosylation is also used to target proteins to the matrix of lysosomes.

60. Covalent attachment
- Hydroxylation: Proline and lysine residues of the α chains of collagen are extensively hydroxylated by vitamin C-dependent hydroxylases in the endoplasmic reticulum.
- Other covalent modifications: For example, additional carboxyl groups can be added to glutamate residues by vitamin K-dependent carboxylation. The resulting γ- carboxyglutamate (Gla) residues are essential for the activity of several of the blood-clotting proteins. Biotin is covalently bound to the ε-amino groups of lysine residues of biotin-dependent enzymes that catalyze carboxylation reactions, such as pyruvate carboxylase. Attachment of lipids, such as farnesyl groups, can help anchor proteins to membranes. Histone proteins can be reversibly acetylated.

61. Protein processing by proteolysis and self-splicing, (a) Three successive enzyme-catalyzed cleavages produce insulin from preproinsulin. The first cleavage occurs immediately after synthesis of preproinsulin, a single chain of 108 amino acids. This cleavage removes the 25-aa signal sequence from the amino end of the molecule. The remaining 84 amino acids constitute proinsuiin, a molecule in which all the correct disulfide bridges are present While the hormone is being packaged for secretion, the 33-residue C segment is removed via two proteolytic cleavages, yielding the A and B chains of insulin.

62. - Protein folding
Folding can be spontaneous (as a result of the primary structure), or facilitated by proteins known as “chaperones”.
- Protein degradation
Proteins that are defective, for example, misfolded, or destined for rapid turnover are often marked for destruction by ubiquitination – the attachment of chains of a small, highly conserved protein, called ubiquitin. Proteins marked in this way are rapidly degraded by a cellular component known as the proteasome, which is a macromolecular, ATP- dependent, proteolytic system located in the cytosol.

63. Action of chaperones during translation
Chaperones bind to the amino (N) terminus of the growing polypeptide chain, stabilizing it in an unfolded configuration until synthesis of the polypeptide is completed. The completed protein is then released from the ribosome and is able to fold into its correct three-dimensional conformation.

64. Action of chaperones during protein transport
A partially unfolded polypeptide is transported from the cytosol to a mitochondrion. Mitochondrial chaperones facilitate transport and subsequent folding of the polypeptide chain within the organelle.

67. 1. RNA polymerase I, located in the nucleoli (dense granular bodies with ribosomal genes in nucleus). 2. RNA polymerase II, occurs in nucleoplasm, synthesized mRNA precursors. 3. RNA polymerase III, occurs in nucleoplasm, synthesizes the precursors of 5S ribosomal RNA, tRNA, and variety of other small nuclear and cytosolic RNAs 4. Mitochondrial RNA polymerase
Amanita phalloides (death cap)
Tight 1:1 complex with RNA polymerase II (K=10-8M) and RNA polymerase III (K=10-6M) block of elongation step. Despite the amatoxins’ high toxicity (5-6 mg, which occur in ~40 g of fresh mushrooms, are sufficient to kill a human adult), they act slowly (liver dysfunction ~7th day). This, in part, reflects the slow turnover rate of eukaryotic mRNA and proteins.

68. Antibiotics are bacterially or fungally produces substances that inhibit the growth of other organisms. DNA replication - novobicin, transcription - rifamycin, bacterial cell wall synthesis - penicillin, translation - majority.
SOME RIBOSOMAL INHIBITORS
Inhibitor
Action
Chloramphenicol
Inhibits peptidyl transferase on the prokaryotic large subunit
Cycloheximide
Inhibits peptidyl transferase on the eukaryotic large subunit
Erythromycin
Inhibits translocation by the prokaryotic large subunit
Fusidic acid
Inhibits elongation in prokaryotes by binding to EF-G-GDP in a way that prvents its dissociation from the large subunit
Puromycin
An aminoacyl-tRNA analog that causes premature chain termination in all cells
Streptomycin
Causes mRNA misreading and inhibits chain initiation in prokaryotes
Tetracycline
Inhibits the binding of aminoacyl-tRNAs to the prokaryotic small subunit
Diphteria toxin
Catalytically inactivates eEF-2 by ADP-ribosylation
Ricin/abrin
Poisonous plant proteins that catalytically inactivate the eukaryotic large subunit

69. A selection on of antibiotics that act as translational inhibitors