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A&P I - DNA, RNA & PROTEIN SYNTHESIS

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Readings for DNA and Protein

Synthesis Chap. 3: pages

93; 96; 100-107 Skim text -

concentrate on lecture notes

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I. Importance and structure of DNA: Deoxyribose Nucleic Acid

A. Historical Review 1. 1900s - Morgan’s studies with fruit flies

showed that genes were located on chromosomes, and chromosomes consisted of protein and DNA

2. 1952 - Hershey-Chase demonstrated that DNA (not protein) was the genetic material of a viral phage

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B. Structure of DNA

1. Nucleotide monomers: Phosphate Pentose sugar(C5)

(Deoxyribose sugar) Organic Nitrogen

group: cytosine, adenine, guanine, thymine

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B. Structure of DNA (continued)

2. Polynucleotide chain with linkage via phosphates to next sugar, with nitrogen base away from backbone of Phos-Sugar-Phos-Sugar

3. Dehydration synthesis

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•4. 1954 - classical one page paper in Nature by Watson & Crick using Franklin’s and Wilkins’ data

A double helix - 2 polynucleotide strands

Sugar-phosphate chains of each strand are like the side ropes of a rope ladder

Pairs of nitrogen bases, one from each strand, form the rungs or steps

The ladder forms a twist every 10 bases (all from x-ray studies!)

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•5. Later confirmation that:

# of adenine equal to # of thymine

# of guanine equal to # of cytosine

This dictates combinations of N-bases that form steps/rungs

Does not restrict the sequence of bases along each DNA strand

10Fig. 3.32

Copyright © 2010 Pearson Education, Inc.

Figure 3.32 Replication of DNA.

AdenineThymineCytosineGuanine Old (template) strand

Two new strands (leading and lagging)synthesized in opposite directions

DNA polymerase

DNA polymerase

Laggingstrand

Leading strand

Free nucleotides

Old strand acts as atemplate for synthesisof new strandChromosome

Helicase unwindsthe double helix andexposes the bases

Old DNA

Replicationfork

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C. Replication / Duplication of DNA

1. Due to complimentary base pairing – one strand of DNA polynucleotide determines the sequence of the other polynucleotide strand

2. Therefore, each strand of double stranded DNA acts as a template

3. The double helix first unwinds – controlled by enzymes and uses new nucleotides that are free in the nucleus to copy a complimentary strand off the original DNA strand

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Models of DNA Replication

View A&P Animation - Structure & Replication

View 16-07 L4 from BIO text View animation 16-10 (3 and 2) View 16-11

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4. Information storage in DNA

The 4 nitrogen bases are the “alphabet” or code for all the traits an organism possesses

Different genes or traits vary in the sequence and length of the bases

ATTTCGGAC vs..... ATTTAC Every three bases = one amino acid in

a protein/peptide

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II. Ribonucleic Acid (RNA)

A. Structure of RNA 1. Nucleotide monomer

Phosphate Pentose sugar = ribose (extra oxygen) Nitrogen base = A / G / C plus U = uracil

instead of thymine Single stranded - possibly 3 types (messenger/transfer/ribosomal RNA)

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B. Synthesis of RNA - Transcription

1. DNA acts as template, but only one strand of DNA utilized at a given time

2. This exposed strand is controlled by specific enzymes that pair the DNA nucleotides with free RNA nucleotides, which are also present in the nucleus

3. These RNA nucleotides form a single stranded RNA nucleic acid

4. DNA = ATTCGCAT 5. RNA = UAAGCGUA 6. Short segments of DNA transcribed at a time, with

start and stop messages

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Figure 3.35 Overview of stages of transcription.

RNA polymerase

RNA polymerase

RNApolymerase

DNA

Coding strand

Template strandPromoterregion

Terminationsignal

mRNA

mRNA

Template strand

mRNA transcript

Completed mRNA transcript

Rewinding of DNA

Coding strand of DNA

DNA-RNA hybrid region

The DNA-RNA hybrid: At any given moment, 16–18 base pairs ofDNA are unwound and the most recently made RNA is still bound to DNA. This small region is called the DNA-RNA hybrid.

Templatestrand

Unwindingof DNA

RNA nucleotides

Direction oftranscription

1 Initiation: With the help of transcription factors, RNA polymerase binds to the promoter, pries apart the two DNA strands, and initiates mRNA synthesis at the start point on the template strand.

3 Termination: mRNA synthesis ends when the termination signal is reached. RNA polymerase and the completed mRNA transcript are released.

2 Elongation: As the RNA polymerase moves along the template strand, elongating the mRNA transcript one base at a time, it unwinds the DNA double helix before it and rewinds the double helix behind it.

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View 17-02 View 17-06 movie View 17-06 photos

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C. Three types of RNA 1. m-RNA: messenger RNA

Transcribed from a specific segment of DNA which represents a specific gene or genetic unit

2. t-RNA: transfer RNA Transcribed from different segments of DNA and

their function is to find a specific amino acid in cytoplasm and bring it to the mRNA

3. r-RNA: ribosomal RNA Transcribed at the nucleolus -with proteins

function as the site of protein synthesis (in cytoplasm)

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Three types of RNA

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III. Protein synthesis = Translation

A. Ribosomes = sites of protein synthsis 1. 30 to 40% protein 2. 60 to 70% RNA (rRNA) 3. Assembled in nucleus and exported via nuclear pores 4. Antibiotics can paralyze bacterial ribosomes, but not eukaryotic

ribosomes 5. 2 ribosomal subunits - a large and a small 6. There are three sites on the ribosome that are involved in protein

synthesis

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A. Ribosomes (continued) - bring mRNA together with amino acid bearing tRNAs

8. Three ribosomal sites P site - (peptidyl-tRNA) holds the tRNA

carrying the growing peptide chain, after several amino acids have been added

A site - (aminoaccyl-tRNA) holds the next single amino acid to be added to the chain

E site - (exit site) site where discharged tRNA minus amino acids leave ribosome

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C. Protein Synthesis

1. One mRNA can bind to several ribosomes termed a polyribosome

17-17 movie and stills 17-18 17-19 17-20 17-21

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Fig. 3.37 modified

25Copyright © 2010 Pearson Education, Inc.

Figure 3.36 The genetic code.

SECOND BASE

UUG

UUA

UUC

UUUPhe

Leu

CUG

CUA

CUC

CUU

Leu

AUA

AUC

AUU

Ile

GUG

GUA

GUC

GUU

Val

UCG

UCA

UCC

UCU

Ser

CCG

CCA

CCC

CCU

Pro

ACG

ACA

ACC

ACU

Thr

GCG

GCA

GCC

GCU

Ala

UAC

UAUTyr

CAG

CAA

CAC

CAUHis

Gln

AAG

AAA

AAC

AAUAsn

Lys

GAG

GAA

GAC

GAUAsp

Glu

UGC

UGUCys

Trp

CGG

CGA

CGC

CGU

Arg

AGG

AGA

AGC

AGUSer

Arg

GGG

GGA

GGC

GGU

Gly

UAA Stop UGA Stop

AUG Met orStart

UAG Stop UGG

U C A G

G

A

C

U

G

A

C

U

G

A

C

U

G

A

C

U

U

C

A

G

26Fig. 3.37Copyright © 2010 Pearson Education, Inc.

Figure 3.37 The basic steps of translation.

1

2

3

4

Leu

Leu

Energized by ATP, the correct aminoacid is attached to each species oftRNA by aminoacyl-tRNA synthetaseenzyme.

Amino acid

tRNA

Aminoacyl-tRNAsynthetase

G A A

tRNA “head”bearinganticodon

Psite A

siteE

site

Ile

Pro

A AU U UC C C

CG G

G

Largeribosomalsubunit

Smallribosomalsubunit

Direction ofribosome advancePortion of mRNA

already translated

Codon15

Codon16

Codon17

Nucleus

mRNA

Released mRNA

Nuclearmembrane

Nuclear pore

RNA polymerase

Templatestrand ofDNA

After mRNA synthesis in thenucleus, mRNA leaves the nucleusand attaches to a ribosome.

Translation begins as incomingaminoacyl-tRNA recognizes thecomplementary codon calling forit at the A site on the ribosome. Ithydrogen-bonds to the codon viaits anticodon.

As the ribosome moves alongthe mRNA, and each codon isread in sequence, a new aminoacid is added to the growingprotein chain and the tRNA inthe A site is translocated to theP site.

Once its amino acid is releasedfrom the P site, tRNA is ratchetedto the E site and then released toreenter the cytoplasmic pool,ready to be recharged with a newamino acid. The polypeptide isreleased when the stop codon isread.

GA A

U

UA

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Fig. 3.38 modified

28Copyright © 2010 Pearson Education, Inc.

The protein is enclosed within aprotein (coatomer)-coated transportvesicle. The transport vesicles maketheir way to the Golgi apparatus,where further processing of theproteins occurs (see Figure 3.19).

In this example, the completedprotein is released from the ribosomeand folds into its 3-D conformation,a process aided by molecular chaperones.

The signal sequence is clipped off by anenzyme. As protein synthesis continues, sugargroups may be added to the protein.

Once attached to the ER, the SRP is releasedand the growing polypeptide snakes through theER membrane pore into the cisterna.

The mRNA-ribosome complex isdirected to the rough ER by the SRP.There the SRP binds to a receptor site.

1 2

3

4

5

Ribosome

ER signalsequence

Signalrecognitionparticle(SRP)

Receptor site

mRNA

Growingpolypeptide

Signalsequenceremoved

Sugargroup

Releasedprotein

Transport vesiclepinching off

Coatomer-coatedtransport vesicle

Rough ER cisterna

Cytoplasm

Figure 3.39 Presence of an ER signal sequence in a newly forming protein causes the signal recognition particle (SRP)to direct the mRNA-ribosome complex to the rough ER.

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Figure 3.40 Information transfer from DNA to RNA to polypeptide.

DNAmolecule

Gene 1

Gene 2

Gene 4

DNA: DNA basesequence (triplets) ofthe gene codes forsynthesis of a particularpolypeptide chain

T A

1

C G T A G C G A T T T C C C T G C G A A A A C T

2 3 4 5 6 7 8 9

Codons

Triplets

tRNA

A U

1

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

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

U G A

2 3 4 5 6 7 8 9

Met Pro Ser Leu Lys Gly Arg Phe

Start

Stop;detach

G

Anticodon

mRNA: Base sequence(codons) of thetranscribed mRNA

tRNA : Consecutivebase sequences oftRNA anticodons ableto recognize the mRNA codons calling for the amino acids theytransport

Polypeptide: Aminoacid sequence of the polypeptide chain