ch25-nucleic acid and protein synthesis

7/29/2019 Ch25-Nucleic Acid and Protein Synthesis

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Chapter 25Nucleic Acids and Protein Synthesis


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Chapter 25 2

IntroductionDeoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are the

molecules that carry genetic information in the cell

DNA is the molecular archive for protein synthesis RNA molecules transcribe and translate the information from DNA so it can be

used to direct protein synthesis

DNA is comprised of two polymer strands held together byhydrogen bonds

Its overall structure is that of a twisted ladder

The sides of the ladder are alternating sugar and phosphate units

The rungs of the ladder are hydrogen-bonded pairs of heterocyclic amine bases


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DNA polymers are very long molecules DNA is supercoiled and bundled into 23 chromosomes for packaging in the cell

nucleus

The sequence of heterocyclic amine bases in DNA encodes thegenetic information required to synthesize proteins

Only four different bases are used for the code in DNA

A section of DNA that encodes for a specific protein is called a gene

The set of all genetic information coded by the DNA in an organism is its genome

The set of all proteins encoded in the genome of an organism and expressed atany given time is its proteome

The sequence of the human genome is providing valuableinformation related to human health

Example: A schematic map of genes on chromosome 19 that are related todisease


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Nucleotides and NucleosidesMild degradation of nucleic acids yields monomer units called

nucleotides

Further hydrolysis of a nucleotide yields: A heterocyclic amine base

D-ribose (from RNA) or 2-deoxy-D-ribose (from DNA); both are C5monosaccharides

A phosphate ion

The heterocylic base is bonded by a bN-glycosidic linkage to C1of the monosaccharide

Examples: A general structure of an RNA nucleotide (a) and adenylic acid (b)

A nucleoside is a nucleotide without the phosphate group A nucleoside of DNA contains 2-deoxy-D-ribose and one of the following four

bases


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A nucleoside of RNA contain the sugar D-ribose and one of thefour bases adenine, guanine, cytosine or uracil


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Nucleosides that can be obtained from DNA


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Nucleosides that can be obtained from RNA


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Nucleotides can be named in several ways Adenylic acid is usually called AMP (adenosine monophosphate)

It can also be called adenosine 5-monophosphate or 5-adenylic acid

Adenosine triphosphate (ATP) is an important energy storagemolecule

The molecule 3,5-cyclic adenylic acid (cyclic AMP) is animportant regulator of hormone activity

This molecule is biosynthesized from ATP by the enzyme adenylate cyclase


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Laboratory Synthesis of Nucleosides andNucleotides

Silyl-Hilbert-Johnson Nucleosidation An N-benzoyl protected base reacts with a benzoyl protected sugar in thepresence of tin chloride and BSA (a trimethylsilylating agent)

The trimethylsilyl protecting groups are removed with aqueous acid in the 2ndstep

The benzoyl groups can be removed with base


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Unnatural nucleotide derivatives can be synthesized fromnucleosides bearing a substitutable group on the heterocyclic ring


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Dibenzyl phosphochloridate is a phosphorylating agent forconverting nucleosides to nucleotides

The 5-OH is phosphorylated selectively if the 2- and 3-OH groups are protected


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Deoxyribonucleic Acid: DNA Primary Structure

The monomer units of nucleic acids are nucleotides

Nucleotides are connected by phosphate ester linkagesThe backbone of nucleic acids consists of alternating phosphate

and sugar units

Heterocyclic bases are bonded to the backbone at each sugar unit

The base sequence contains the encoded genetic information

The base sequence is always specified from the 5 end of the

nucleic acid


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Secondary StructureThe secondary structure of DNA was proposed by Watson and

Crick in 1953

E. Chargaff noted that in DNA the percentage of pyrimidine baseswas approximately equal to the percentage of purine bases

Also the mole percentage of adenine Is nearly equal to that of thymine

The mole percentage of guanine is nearly equal to cytosine

Chargaff also noted that the ratio of A and T versus G and C variesby species but the ratio is the same for different tissues in the

same organism


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X-ray crystallographic data showed the bond lengths and anglesof purine and pyrimidine bases

X-ray data also showed DNA had a long repeat distance (34 )

Based on this data, Watson and Crick proposed the double helixmodel of DNA (next slide) Two nucleic acid chains are held together by hydrogen bonding between the

bases on opposite strands

The double chain is wound into a helix

Each turn in the helix is 34 long and involves 10 successive nucleotide pairs

Each base pair must involve a purine and a pyrimidine to achieve the properdistance between the sugar-phosphate backbones

Base pairing can occur only between thymine and adenine, or cytosine andguanine; no other pairing has the optimum pattern of hydrogen bonding or wouldallow the distance between sugar-phosphate backbones to be regular


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Specific pairing of bases means the two chains of DNA arecomplementary

Knowing the sequence of one chain allows one to also know the sequence of theother


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Replication of DNA (see next slide)The DNA strand begins to unwind just prior to cell division

Complementary strands are formed along each chain (each chain

acts as a template for a new chain)Two new DNA molecules result; one strand goes to each daughter

cell


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RNA and Protein SynthesisThe central dogma of molecular genetics

A gene is the portion of a DNA molecule which codes for oneprotein

Proteins have many critical functions, e.g., catalysis, structure, motion, cellsignaling, the immune response, etc.

DNA resides in the nucleus and protein synthesis occurs in thecytoplasm Transcription of DNA into messenger RNA (mRNA) occurs in the nucleus

mRNA moves to the cytoplasm and the translation into proteins occurs using twoother forms of RNA: ribosomal RNA (rRNA) and transfer RNA (tRNA)


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Transcription: Synthesis of Messenger RNA (mRNA)In the nucleus a DNA molecule partially unwinds to expose a

portion corresponding to at least one gene

Ribonucleotides with complementary bases assemble along theDNA strand

Base-pairing is the same in RNA, except that in RNA uracil replaces thymine

Ribonucleotides are joined into a chain of mRNA by the enzymeRNA polymerase


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An intron (intervening sequence) is a segment of DNA which istranscribed into mRNA but not actually used when a protein isexpressed

An exon (expressed sequence) in the part of the DNA gene whichis expressed

Each gene usually contains a number of introns and exons Introns are excised from mRNA after transcription


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Ribosomes - rRNAProtein synthesis is catalyzed in the cytoplasm by ribosomes

A ribosome consists of approximately two thirds RNA and one third protein

A ribosome is a ribozyme ( an reaction catalyst made of ribonucleic acid)A ribosome has 2 large subunits

The 30S subunit binds the mRNA that codes for the protein to be translated

The 50S subunit catalyzes formation of the amide bond in protein synthesis

Transfer of an amino acid to the growing peptide chain is aided byacid-base catalysis involving an adenine in the 50S subunits

See Figure 25.14, page 1238


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Transfer RNA (tRNA)Transfer RNAs (tRNAs), specific to each amino acid, transport

amino acids to complimentary binding sites on the mRNA bound

to the ribosome More than one tRNA codes for each amino acid

tRNA is comprised of a relatively small number of nucleotideswhose chain is folded into a structure with several loops

One arm of the tRNA always terminates in the sequence cytosine-cytosine-adenine, and it is here the amino acid is attached

On another arm is a sequence of three bases called the anticodon, which binds

with the complementary codon on mRNAThe mRNA genetic code is shown on the next page

The structure of a tRNA molecule is shown in Figure 25.15, page1240


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The Genetic CodeThe genetic code is based on three-base sequences in mRNA

Each three-base sequence corresponds to a particular amino acid The fact that three bases are used to code for each amino acid provides

redundancy in the overall code and in the start and stop signals

N-formyl methionine (fMet) is the first amino acid incorporated into bacterialprotein and appears to be the start signal

fMet is removed from the protein chain before its synthesis is complete


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TranslationTranslation is peptide synthesis by a ribosome using the code

from an mRNA

The following is an example (see figure on next page): An mRNA binds to a ribosome

A tRNA with the anticodon for fMet associates with the fMet codon on the mRNA

A tRNA with anticodon UUU brings a lysine residue to the AAA mRNA codon

The 50S ribosome catalyzes amide bond formation between the fMET and lysine

The ribosome moves down the mRNA chain to the next codon (GUA)

A tRNA with the anticodon CAU brings a valine residue

The ribosome catalyzes amide bond formation between Lys and Val The ribosome moves along the mRNA chain and the process continues, e.g., with

the tRNA for phenylalanine binding to the ribosome

A stop signal is reached and the ribosome separates from the mRNA

At this point the polypeptide also separates from the ribosome

The polypeptide begins to acquire its secondary and tertiarystructure as it is being synthesized

Several ribosomes can be translating the same mRNA moleculesimultaneously

Protein molecules are synthesized only when they are needed Regulator molecules determine when and if a particular protein will be expressed

i.e. synthesized


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Determining the Base Sequence of DNAThe Chain-Terminating (Dideoxynucleotide) Method

DNA molecules are replicated in such a way that a family of partial copies isgenerated; each DNA copy differs in length by only one base

Random chain-termination is done by poisoning a replication reaction with a lowconcentration of 23-dideoxynucleotides, which are incapable of chain elongationat their 3 position

The 23-dideoxynucleotides are labeled with covalently attached coloredfluorescent dye molecules, with each color representing a base type

The partial copies are separated according to length by capillary electrophoresis

The terminal base on each strand is detected by the color of laser-inducedfluorescence as each DNA molecule passes the detector

A four-color chromatogram is generated (see Figure 25.17, page 1246)

Automation of high-throughput dideoxy sequencing madepossible completion of the Human Genome Project by the 50thanniversary of Watson and Cricks elucidation of the structure of

DNA in 2003


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Laboratory Synthesis of DNASolid-phase methods for laboratory synthesis of DNA are similar

to those used for laboratory synthesis of proteins

The solid phase is often controlled-pore glass (CPG) Protecting/blocking reagents are needed (e.g., the dimethoxytrityl and b-

cyanoethyl groups)

A coupling reagent (1,2,3,4-tetrazole) is used to join the protected nucleotides


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The Polymerase Chain Reaction (PCR)PCR is an extraordinarily simple and effective method for

exponentially multiplying (amplifying) the number of copies of a

DNA molecule. PCR beginning with a single molecule can lead to 100 billion copies in an

afternoon

The Nobel Prize was awarded to K. Mullis in 1993 for invention of PCR

PCR requires: A sample of the DNA to be copied

The enzyme DNA polymerase

A short primer sequence complimentary to the template DNA A supply of A, C, G, and T nucleotide triphosphate monomers

A simple device for thermal cycling during the reaction sequence

The PCR process is summarized on the next 2 slides


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ch25-nucleic acid and protein synthesis

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