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Nucleic Nucleic AcidsAcids

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DNA or Protein?DNA or Protein? Walter Sutton discovered Walter Sutton discovered

chromosomes chromosomes were made were made of of DNA and ProteinDNA and Protein

However, scientists were However, scientists were NOTNOT sure which one sure which one (protein or DNA) was (protein or DNA) was the the actual genetic material actual genetic material of the cellof the cell

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DNA!DNA! Frederick GriffithFrederick Griffith in in

1928 showed the 1928 showed the DNADNA was the cell’s was the cell’s genetic materialgenetic material

Watson & CrickWatson & Crick in in the 1950’s the 1950’s builtbuilt the the 11stst model of DNA model of DNA

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Structure of DNAStructure of DNA DNA is DNA is made of subunits made of subunits

called called nucleotidesnucleotides DNA nucleotides are DNA nucleotides are

composed of a composed of a phosphatephosphate, , deoxyribosedeoxyribose sugar, sugar, and and a a nitrogen-containing basenitrogen-containing base

The 4 bases in DNA are: The 4 bases in DNA are: adenine (A), adenine (A), thymine (T),thymine (T), guanine (G),guanine (G), and cytosine (C) and cytosine (C)

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DNA NucleotideDNA Nucleotide

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Base Pairing RuleBase Pairing Rule• Watson and Crick showed that Watson and Crick showed that DNA DNA

is a is a double helixdouble helix• AA (adenine) pairs (adenine) pairs with with TT (thymine) (thymine)• CC (cytosine) pairs (cytosine) pairs with with GG (guanine) (guanine)

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Nitrogen RingsNitrogen Rings• PurinesPurines have have doubledouble ringsrings of of

carbon-nitrogen (G, A)carbon-nitrogen (G, A)• PyrimidinesPyrimidines have have singlesingle

carbon-nitrogen carbon-nitrogen ringsrings (C, T) (C, T)• This is called This is called

complementary base pairingcomplementary base pairing because a because a purinepurine is always is always paired with a paired with a pyrimidinepyrimidine

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.5’ to 3’ Sugars5’ to 3’ Sugars

When the DNA When the DNA double helixdouble helix unwinds, it unwinds, it resembles a ladderresembles a ladder The The sidessides of the ladder are the of the ladder are the sugar-phosphate backbonessugar-phosphate backbones The The rungsrungs of the ladder are of the ladder are the the complementary paired complementary paired basesbasesThe two DNA strands are The two DNA strands are anti-anti-parallelparallel (they run in opposite (they run in opposite directions)directions)

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Anti-Parallel Strands of DNA

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DNA DNA ReplicatiReplicati

onon

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Steps in Steps in DNA DNA ReplicationReplication

Occurs when Occurs when chromosomes duplicatechromosomes duplicate (make copies)(make copies) An An exact copyexact copy ofof the the DNADNA is is produced produced with the aid of the enzyme with the aid of the enzyme DNA polymeraseDNA polymerase Hydrogen bondsHydrogen bonds between bases between bases breakbreak and enzymes “unzip” the and enzymes “unzip” the moleculemolecule Each Each old strandold strand of nucleotides of nucleotides servesserves as a as a templatetemplate for each new strand for each new strand New nucleotidesNew nucleotides move into move into complementary positions are joined complementary positions are joined by DNA polymeraseby DNA polymerase

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Two New, Two New, Identical Identical

DNA DNA Strands Strands Result Result from from

ReplicatiReplicati

onon

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Another View of Another View of ReplicationReplication

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RNARNA

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RNA Differs from DNARNA Differs from DNA1.1. RNARNA has a sugar has a sugar riboseribose

DNADNA has a sugar has a sugar deoxyribosedeoxyribose

2.2. RNARNA contains the base contains the base uracil uracil (U)(U)

DNADNA has has thymine (T)thymine (T)

3.3. RNARNA molecule is molecule is single-single-strandedstranded

DNADNA is is double-strandeddouble-stranded

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Structure of RNAStructure of RNA

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.

Three Types of RNAThree Types of RNA• Messenger RNAMessenger RNA (mRNA)(mRNA)

carriescarries genetic genetic infoinformation rmation toto the the ribosomesribosomes

• Ribosomal RNARibosomal RNA (rRNA)(rRNA),, along with protein, along with protein, makes makes upup the the ribosomesribosomes

• Transfer RNA (tRNA)Transfer RNA (tRNA) transferstransfers amino acids to the amino acids to the ribosomesribosomes where proteins where proteins are synthesizedare synthesized

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PROTEIN PROTEIN SYNTHESISSYNTHESIS

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Protein SynthesisProtein Synthesis The production (synthesis) of The production (synthesis) of

polypeptide chains polypeptide chains (proteins)(proteins) Two phases:Two phases:

Transcription Transcription & & TranslationTranslation mRNA must be mRNA must be processed before processed before

it leaves the nucleusit leaves the nucleus of of eukaryotic cellseukaryotic cells

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TranscriptionTranscription

TranslationTranslation

DNA

mRNA

Ribosome

Protein

Prokaryotic CellProkaryotic Cell

DNA DNA RNA RNA ProteinProtein

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DNA DNA RNA RNA ProteinProtein

Nuclearmembrane

TranscriptionTranscription

RNA ProcessingRNA Processing

TranslationTranslation

DNA

Pre-mRNA

mRNA

Ribosome

Protein

EukaryotEukaryotic Cellic Cell

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Pathway to Making a Pathway to Making a ProteinProtein

DNADNA

mRNAmRNA

tRNA (ribosomes)tRNA (ribosomes)

ProteinProtein

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Genes & ProteinsGenes & Proteins ProteinsProteins are made of are made of amino amino

acidsacids linked together by linked together by peptide bondspeptide bonds

2020 different amino acids different amino acids existexist Amino acids chains are called Amino acids chains are called

polypeptidespolypeptides Segment of DNA that codes Segment of DNA that codes

for the amino acid sequence for the amino acid sequence in a protein are called in a protein are called genesgenes

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Two Parts of Protein Two Parts of Protein SynthesisSynthesis

TranscriptionTranscription makes an RNAmakes an RNA molecule molecule complementary tocomplementary to a portion of a portion of DNADNA

TranslationTranslation occursoccurs whenwhen the the sequence of bases of sequence of bases of mRNAmRNA DIRECTSDIRECTS the the sequence of sequence of amino acidsamino acids in a polypeptide in a polypeptide

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Genetic CodeGenetic Code DNA contains a DNA contains a triplet codetriplet code Every three bases on DNAEvery three bases on DNA

standsstands for for ONE amino acidONE amino acid Each three-letter unitEach three-letter unit on on mRNAmRNA

is is called a called a codoncodon Most amino acids have more Most amino acids have more

than one codon!than one codon! There are There are 20 amino acids20 amino acids with with

a possible 64 different tripletsa possible 64 different triplets The code is nearly The code is nearly universal universal

among living organismsamong living organisms

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TranscriptioTranscriptionn

TranslatiTranslationon

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Overview of Overview of TranscriptionTranscription

During During transcriptiontranscription in the in the nucleus, a segment of DNA nucleus, a segment of DNA unwinds and unzips, and the unwinds and unzips, and the DNA DNA serves as aserves as a template for template for mRNA formationmRNA formation

RNA polymeraseRNA polymerase joins the RNA joins the RNA nucleotides so that the nucleotides so that the codons codons in mRNA are complementaryin mRNA are complementary to to the triplet code in DNAthe triplet code in DNA

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Steps in Steps in TranscriptionTranscription

The transfer of information in the The transfer of information in the nucleusnucleus from a from a DNADNA molecule to an molecule to an RNARNA molecule molecule

Only 1 Only 1 DNADNA strand serves as the strand serves as the templatetemplate

Starts at promoterStarts at promoter DNADNA (TATA box) (TATA box) Ends at terminatorEnds at terminator DNADNA (stop) (stop) When complete, When complete, pre-RNApre-RNA molecule is molecule is

releasedreleased

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TranscriptionTranscription

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What is the What is the enzymeenzyme

responsible for responsible for the production of the production of

the mRNA the mRNA molecule?molecule?

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RNA PolymeraseRNA Polymerase

EnzymeEnzyme found in the found in the nucleusnucleus

Separates Separates the two DNA the two DNA strands by strands by breaking the breaking the hydrogen bondshydrogen bonds between between the basesthe bases

Then moves along one of Then moves along one of the DNA strands and the DNA strands and links links RNA nucleotidesRNA nucleotides together together

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DNADNA

pre-mRNApre-mRNA

RNA PolymeraseRNA Polymerase

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Question:Question:

What would be the What would be the complementary RNA complementary RNA strand for the following strand for the following DNA sequence?DNA sequence?

DNA 5’-DNA 5’-GCGTATGGCGTATG-3’-3’

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Answer:Answer:

•DNA 5’-GCGTATG-3’DNA 5’-GCGTATG-3’•RNA 3’-CGCAUAC-5RNA 3’-CGCAUAC-5’’

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Messenger RNA Messenger RNA (mRNA)(mRNA)

• Carries the information for a Carries the information for a specific proteinspecific protein

• Made up of Made up of 500 to 1000500 to 1000 nucleotides longnucleotides long

• Sequence of 3 bases called Sequence of 3 bases called codoncodon• AUGAUG – methionine or – methionine or start start codoncodon• UAA, UAG, or UGAUAA, UAG, or UGA – – stopstop codons codons

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Messenger RNA Messenger RNA (mRNA)(mRNA)

methionine glycine serine isoleucine glycine alanine stopcodon

proteinprotein

A U G G G C U C C A U C G G C G C A U A AmRNAmRNA

startcodon

Primary structure of a proteinPrimary structure of a protein

aa1 aa2 aa3 aa4 aa5 aa6

peptide bonds

codon 2 codon 3 codon 4 codon 5 codon 6 codon 7codon 1

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RibosomesRibosomes

•Made of a large and small subunit

•Composed of rRNA (40%) and proteins (60%)

•Have two sites for tRNA attachment --- P and A

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RibosomesRibosomes

PSite

ASite

Largesubunit

Small subunit

mRNAmRNA

A U G C U A C U U C G

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Transfer RNA (tRNA)Transfer RNA (tRNA)

amino acidamino acidattachment siteattachment site

U A C

anticodonanticodon

methionine amino acidamino acid

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TranslationTranslation

• Synthesis of proteinsSynthesis of proteins in the in the cytoplasmcytoplasm

• Involves the following:Involves the following:

1.1. mRNA (codons)mRNA (codons)

2.2. tRNA (anticodons)tRNA (anticodons)

3.3. ribosomesribosomes

4.4. amino acidsamino acids

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TranslationTranslation• Three steps:Three steps:

1.1. initiationinitiation: start codon (AUG): start codon (AUG)

2.2. elongationelongation: amino acids : amino acids linkedlinked

3.3. terminationtermination: stop codon : stop codon (UAG, UAA, or UGA).(UAG, UAA, or UGA).

Let’s Make a Protein !Let’s Make a Protein !

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mRNA Codons Join the mRNA Codons Join the RibosomeRibosome

PSite

ASite

Largesubunit

Small subunit

mRNAmRNA

A U G C U A C U U C G

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InitiationInitiation

mRNAmRNA

A U G C U A C U U C G

2-tRNA

G

aa2

A U

A

1-tRNA

U A C

aa1

anticodon

hydrogenbonds codon

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mRNAmRNA

A U G C U A C U U C G

1-tRNA 2-tRNA

U A C G

aa1 aa2

A UA

anticodon

hydrogenbonds codon

peptide bond

3-tRNA

G A A

aa3

ElongationElongation

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mRNAmRNA

A U G C U A C U U C G

1-tRNA

2-tRNA

U A C

G

aa1

aa2

A UA

peptide bond

3-tRNA

G A A

aa3

Ribosomes move over one codon

(leaves)

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mRNAmRNA

A U G C U A C U U C G

2-tRNA

G

aa1

aa2

A UA

peptide bonds

3-tRNA

G A A

aa3

4-tRNA

G C U

aa4

A C U

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mRNAmRNA

A U G C U A C U U C G

2-tRNA

G

aa1aa2

A U

A

peptide bonds

3-tRNA

G A A

aa3

4-tRNA

G C U

aa4

A C U

(leaves)

Ribosomes move over one codon

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mRNAmRNA

G C U A C U U C G

aa1aa2

A

peptide bonds

3-tRNA

G A A

aa3

4-tRNA

G C U

aa4

A C U

U G A

5-tRNA

aa5

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mRNAmRNA

G C U A C U U C G

aa1aa2

A

peptide bonds

3-tRNA

G A A

aa3

4-tRNA

G C U

aa4

A C U

U G A

5-tRNA

aa5

Ribosomes move over one codon

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mRNAmRNA

A C A U G U

aa1

aa2

U

primaryprimarystructurestructureof a proteinof a protein

aa3

200-tRNA

aa4

U A G

aa5

C U

aa200

aa199

terminatorterminator or stopor stop codoncodon

TerminationTermination

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End Product –The End Product –The Protein!Protein!

• The end products of protein The end products of protein synthesis is a synthesis is a primary structureprimary structure of a proteinof a protein

• A A sequence of amino acidsequence of amino acid bonded together by peptide bonded together by peptide bondsbonds

aa1

aa2 aa3 aa4aa5

aa200

aa199