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Nucleic Acids: Cell Overview and Core Topics. Outline Cellular Overview Anatomy of the Nucleic Acids Building blocks Structure (DNA, RNA ) Looking at the Central Dogma DNA Replication RNA Transcription Protein Synthesis. DNA and RNA in the Cell. Cellular Overview. - PowerPoint PPT Presentation
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Nucleic Acids: Cell Overview and Core Topics
Outline
I.Cellular Overview
II.Anatomy of the Nucleic Acids1. Building blocks2. Structure (DNA, RNA)
III.Looking at the Central Dogma1. DNA Replication2. RNA Transcription3. Protein Synthesis
Cellular Overview
DNA and RNA in the Cell
Classes of Nucleic Acids: DNA
DNA is usually found in the nucleus
Small amounts are also found in:• mitochondria of eukaryotes• chloroplasts of plants
Packing of DNA:• 2-3 meters long• histones
genome = complete collection of hereditary information of an organism
Classes of Nucleic Acids: RNA
FOUR TYPES OF RNA
• mRNA - Messenger RNA
• tRNA - Transfer RNA
• rRNA - Ribosomal RNA
• snRNA - Small nuclear RNA
Anatomy of Nucleic Acids
THE BUILDING BLOCKS
Nucleic acids are linear polymers.
Each monomer consists of:
1. a sugar
2. a phosphate
3. a nitrogenous base
Nitrogenous Bases
Nitrogenous Bases
DNA (deoxyribonucleic acid):adenine (A) guanine (G)cytosine (C) thymine (T)
RNA (ribonucleic acid):adenine (A) guanine (G)cytosine (C) uracil (U)
Why ?
Properties of purines and pyrimidines:
1.keto – enol tautomerism2.strong UV absorbance
Pentoses of Nucleic Acids
This difference in structure affects secondary structure and stability.
Which is more stable?
Nucleosideslinkage of a base and a sugar.
Nucleotides- nucleoside + phosphate
- monomers of nucleic acids - NA are formed by 3’-to-5’ phosphodiester linkages
Shorthand notation:
- sequence is read from 5’ to 3’- corresponds to the N to C terminal of
proteins
Nucleic Acids: Structure
DNA
Primary Structure
• nucleotide sequences
DNA Double Helix
• Maurice Wilkins and Rosalind Franklin
• James Watson and Francis Crick Features:
• two helical polynucleotides coiled around an axis
• chains run in opposite directions• sugar-phosphate backbone on
the outside, bases on the inside
• bases nearly perpendicular to the axis
• repeats every 34 Å• 10 bases per turn of the helix• diameter of the helix is 20 Å
Secondary Structure
Double helix stabilized by hydrogen bonds.
Which is more stable?
Axial view of DNA
A and B forms are both right-handed double helix.
A-DNA has different characteristics from the more common B-DNA.
• left-handed• backbone phosphates zigzag
Z-DNA
Comparison Between A, B, and Z DNA: A-DNA: right-handed, short and broad, 11 bp per turn
B-DNA: right-handed, longer, thinner, 10 bp per turn
Z-DNA: left-handed, longest, thinnest, 12 bp per turn
Major and minor grooves are lined with sequence-specific H-bonding.
Consequences of double helical structure:
1. Facilitates accurate hereditary information transmission
2.Reversible melting• melting: dissociation of the double helix• melting temperature (Tm)• hypochromism• annealing
Supercoiling
relaxed DNA
supercoiled DNA
Tertiary Structure
Topoisomerase I – relaxation of supercoiled structures
Topoisomerase II – add negative supercoils to DNA
Structure of Single-stranded DNA
Stem Loop
Nucleic Acids: Structure
RNA
Secondary Structure
transfer RNA (tRNA) : Brings amino acids to
ribosomes during translation
Transfer RNA
Extensive H-bonding creates four double helical domains, three capped by loops, one by a stem
Only one tRNA structure (alone) is known
Many non-canonical base pairs found in tRNA
ribosomal RNA (rRNA) : Makes up the ribosomes, together with ribosomal proteins.
Ribosomes synthesize proteins
All ribosomes contain large and small subunits
rRNA molecules make up about 2/3 of ribosome
Secondary structure features seem to be conserved, whereas sequence is not
There must be common designs and functions that must be conserved
messenger RNA (mRNA) : Encodes amino acid sequence of a polypeptide
small nuclear RNA (snRNA) :With proteins, forms complexes that are used in RNA processing in eukaryotes. (Not found in prokaryotes.)
Central Dogma
DNA Replication, Transcription, and Translation
Central Dogma
DNA ReplicationCentral Dogma
DNA Replication – process of producing identical copies of original DNA
• strand separation followed by copying of each strand
• fixed by base-pairing rules
DNA replication is semi-conservative.
DNA replication is bidirectional.
involves two replication forks that move in opposite direction
DNA Replication
Begins at specific start sites• in E. coli, origin of replication, oriC locus• binding site for dnaA, initiation protein• rich in A-T
Overall: each of the two DNA duplexes contain one “old” and one “new” DNA strand (semi-conservative) and half of the new strand was formed by leading strand and the other half by lagging strand.
DNA replication requires unwinding of the DNA helix.
expose single-stranded templates
DNA gyrase – acts to overcome torsional stress imposed upon unwinding
helicases – catalyze unwinding of double helix- disrupts H-bonding of the two strands
SSB (single-stranded DNA-binding proteins) – binds to the unwound strands, preventing re-annealing
Primer
RNA primes the synthesis of DNA.
Primase synthesizes short RNA.
DNA replication is semidiscontinuous
DNA polymerase synthesizes the new DNA strand only in a 5’3’ direction. Dilemma: how is 5’ 3’ copied?
The leading strand copies continuously
The lagging strand copies in segments called Okazaki fragments (about 1000 nucleotides at a time) which will then be joined by DNA ligase
DNA Polymerase= enzymes that replicate DNA
All DNA Polymerases share the following:
1.Incoming base selected in the active site (base-complementarity)
2.Chain growth 5’ 3’ direction (antiparallel to template)3.Cannot initiate DNA synthesis de novo (requires primer)First DNA Polymerase discovered – E.coli DNA Polymerase I (by Arthur Kornberg and colleagues)
Roger D. Kornberg
2006 Nobel Prize in Chemistry
Arthur Kornberg
1959 Nobel Prize in Physiology and Medicine
http://www.nobelprize.org
DNA Polymerase
specificity dictated by H-bonding and shape complementarity between bases• binding of correct base is
favorable (more stable)• interaction of residues in
the enzyme to the minor groove of DNA
• close down around the incoming NTP
Mechanism of DNA linkage:
3’ 5’ exonuclease activity
- removes incorrect nucleotides from the 3’-end of the growing chain (proofreader and editor)- polymerase cannot elongate an improperly base-paired terminus
proofreading mechanisms• Klenow fragment – removes
mismatched nucleotides from the 3’’ end of DNA (exonuclease activity)
• detection of incorrect base- incorrect pairing with the template
(weak H-bonding)- unable to interact with the minor
groove (enzyme stalls)
5’ 3’ exonuclease activity
Exonuclease activity
- remove distorted segments lying in the path of the advancing polymerase
DNA Ligase = seals the nicks between Okazaki fragments
DNA ligase seals breaks in the double stranded DNA
DNA ligases use an energy source (ATP in eukaryotes and archaea, NAD+ in bacteria) to form a phosphodiester bond between the 3’ hydroxyl group at the end of one DNA chain and 5’-phosphate group at the end of the other.
DNA replication terminates at the Ter region.
• the oppositely moving replication forks meet here and replication is terminated
• contain core elements 5’-GTGTGTTGT
• binds termination protein (Tus protein)
Eukaryotic DNA Replication Like E. coli, but more complex
Human cell: 6 billion base pairs of DNA to copy
Multiple origins of replication: 1 per 3000-30000 base pairs
E.coli 1 chromosomeHuman 23E.coli circular chromosome; Human linear
Telomeres
The Ends of Linear DNA Possess TelomeresPresent because DNA is shortened after
each round of replicationContains hundreds of tandem repeats of a
hexanucleotide sequence (AGGGTT in humans)
Telomeres at the 3’ end is G rich and is slightly longer
May form large loops to protect chromosome ends
DNA Recombination =
recombinases Holliday junction –
crosslike structure
natural process of genetic rearrangement
Mutations
1. Substitution of base pair
a. transitionb. transversion
2. Deletion of base pair/s
3. Insertion/Addition of base pair/s
DNA replication error rate: 3 bp during copying of 6 billion bp
Macrolesions: Mutations involving changes in large portions of the genome
Agents of Mutations
1. Physical Agentsa) UV Lightb) Ionizing Radiation
2. Chemical AgentsSome chemical agents can be
classified further intoa) Alkylatingb) Intercalatingc) Deaminating
3. Viral
UV Light Causes Pyrimidine Dimerization
Replication and gene expression are blocked
Chemical mutagens
• 5-bromouracil and 2-aminopurine can be incorporated into DNA
Deaminating agentsEx: Nitrous acid (HNO2)Converts adenine to hypoxanthine, cytosine to uracil, and
guanine to xanthineCauses A-T to G-C transitions
Alkylating agents
Intercalating agents
AcridinesIntercalate in DNA, leading to insertion or
deletionThe reading frame during translation is changed
DNA Repair
Direct repairPhotolyase cleave pyrimidine dimers
Base excision repairE. coli enzyme AlkA removes modified bases
such as 3-methyladenine (glycosylase activity is present)
Nucleotide excision repairExcision of pyrimidine dimers (need different
enzymes for detection, excision, and repair synthesis)
Do we has a quiz?
QUIZ1. Draw the structure of any nitrogenous base of your
picking. (1 pt)
2. What is the difference between the glycosidic bond and the phosphodiester bond? (2 pts)
3. Give the reason why DNA utilizes the deoxyribose while RNA uses the ribose. (2 pts)
4. Enumerate all the enzymes and proteins involved in DNA replication and briefly state their importance/function. A short concise answer will suffice. (4 pts)
5. Give the partner strand of this piece of DNA:5-ACTCATGATTAGCAG-3 (1 pt)
RNA TranscriptionCentral Dogma
Process of Transcription has four stages:
1. Binding of RNA polymerase at promoter sites2. Initiation of polymerization3. Chain elongation4. Chain termination
Transcription (RNA Synthesis)
RNA PolymerasesTemplate (DNA)Activated precursors (NTP)Divalent metal ion (Mg2+ or Mn2+)
Mechanism is similar to DNA Synthesis
Reece R. Analysis of Genes and Genomes.2004. p47.
Limitations of RNAP II:
1. It can’t recognize its target promoter and gene. (BLIND)
2. It is unable to regulate mRNA production in response to developmental and environmental signals. (INSENSITIVE)
Start of Transcription
Promoter SitesWhere RNA Polymerase can indirectly bind
TATA box – a DNA sequence (5’—TATAA—3’) found in the promoter region of most eukaryotic genes.
Abeles F, et al. Biochemistry. 1992. p391.
Preinitiation Complex (PIC)
Transcription Factors (TF):
Hampsey M. Molecular Genetics of RNAP. Microbiology and Molecular Biology Reviews. 1998. p7.
TFIID binds to TATA; promotes TFIIB binding
TFIIA stabilizes TBP binding
TFIIB promotes TFIIF-pol II binding
TFIIF targets pol II to promoter
TFIIE stimulates TFIIH kinase and ATPase actiivities
TFII H helicase, ATPase, CTD kinase activities
Termination of Transcription
Terminator SequenceEncodes the
termination signalIn E. coli – base
paired hair pin (rich in GC) followed by UUU…
1. Intrinsic termination = termination sites
causes the RNAP to pause
causes the RNA strand to detach from the DNA template
Termination of Transcription
2. Rho termination = Rho protein, ρ
prokaryotes: transcription and translation happen in cytoplasm
eukaryotes: transcription (nucleus); translation (ribosome in cytoplasm)
In eukaryotes, mRNA is modified after transcriptionCapping, methylationPoly-(A) tailsplicing
capping: guanylyl residue
capping and methylation ensure stability of the mRNA template; resistance to exonuclease activity
Eukaryotic genes are split genes: coding regions (exons) and noncoding regions (introns)
Introns & Exons
IntronsIntervening
sequencesExons
Expressed sequences
Splicing
Spliceosome: multicomponent complex of small nuclear ribonucleoproteins (snRNPs)
splicing occurs in the spliceosome!
Reverse TranscriptionRNA-Directed DNA Polymerase
1964: Howard Temin notices that DNA synthesis inhibitors prevent infection of cells in culture by RNA tumor viruses. Temin predicts that DNA is an intermediate in RNA tumor virus replication
1970: Temin and David Baltimore (separately) discover the RNA-directed DNA polymerase - aka "reverse transcriptase"
Reverse Transcriptase
Primer required, but a strange one - a tRNA molecule that the virus captures from the host
RT transcribes the RNA template into a complementary DNA (cDNA) to form a DNA:RNA hybrid
All RNA tumor viruses contain a reverse transcriptase
RT II Three enzyme activities
RNA-directed DNA polymerase RNase H activity - degrades RNA in the DNA:RNA
hybrids DNA-directed DNA polymerase - which makes a
DNA duplex after RNase H activity destroys the viral genome
• HIV RT: very error-prone (1 bp /2000 to 4000 bp)
HIV therapy: AZT (or 3'-azido-2',3'- dideoxythymidine) specifically inhibits RT
Translation: Protein SynthesisCentral Dogma
TranslationStarring three types of RNA
1.mRNA
2.tRNA
3.rRNA
Properties of mRNA
1. In translation, mRNA is read in groups of bases called “codons”
2. One codon is made up of 3 nucleotides from 5’ to 3’ of mRNA
3. There are 64 possible codons
4. Each codon stands for a specific amino acid, corresponding to the genetic code
5. However, one amino acid has many possible codons. This property is termed degeneracy
6. 3 of the 64 codons are terminator codons, which signal the end of translation
Genetic Code
3 nucleotides (codon) encode an amino acid
The code is nonoverlappingThe code has no punctuation
Synonyms
Different codons, same amino acidMost differ by the last base
XYC & XYU XYG & XYA
Minimizes the deleterious effect of mutation
Encoded sequences. (a) Write the sequence of the mRNA molecule
synthesized from a DNA template strand having the sequence
(b) What amino acid sequence is encoded by the following base sequence of an mRNA molecule? Assume that the reading frame starts at the 5 end.
Practice
Answers
(a) 5’ -UAACGGUACGAU-3’ .(b) Leu-Pro-Ser-Asp-Trp-Met.
tRNA as Adaptor Molecules
Amino acid attachment site
Template recognition siteAnticodon
Recognizes codon in mRNA
tRNA as Adaptor Molecules
Mechanics of Protein Synthesis All protein synthesis involves three
phases: initiation, elongation, termination Initiation involves binding of mRNA and
initiator aminoacyl-tRNA to small subunit(30S), followed by binding of large subunit (50S) of the ribosome
Elongation: synthesis of all peptide bonds - with tRNAs bound to acceptor (A) and peptidyl (P) sites.
Termination occurs when "stop codon" reached
TranslationOccurs in the ribosomeProkaryote START
fMet (formylmethionine) bound to initiator tRNA
Recognizes AUG and sometimes GUG (but they also code for Met and Val respectively)
AUG (or GUG) only part of the initiation signal; preceded by a purine-rich sequence
Translation
Eukaryote START
AUG nearest the 5’ end is usually the start signal
Termination
Stop signals (UAA, UGA, UAG):• recognized by release factors (RFs)• hydrolysis of ester bond between polypeptide and
tRNA
Reference:
Garrett, R. and C. Grisham. Biochemistry. 3rd edition. 2005.
Berg, JM, Tymoczko, JL and L. Stryer. Biochemistry. 5th edition. 2002.