DNA What is the major compon onent nt of all cells? s? PROTEIN - - PDF document

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DNA

What is the major compon

• nent

nt of all cells? s? PROTEIN INS Why would protein synthesis be important?

• cellular structures
• enzymes
• cell membrane structures
• organelles
• direct all other cellular activities

What substance directs protein synthesis?

DNA

DNA molecule responsible for all cell activities and contains the genetic code Genetic tic Code method cells use to store the program that is passed from one generation to another DISCOVERY OF THE GENETIC CODE

1928: Frederick Griffith

(studied how bacteria cause pneumonia)

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Griffith Experiment 1

• 1. Grew 2 strains of bacteria on

plates

• smooth colonies- caused

disease (viru rule lent)

• rough edge colonies-did not

cause disease (aviru rule lent)

• 2. Injected into mice

Results:

• smooth colonies: died
• rought colonies: lived

Conclusion: bacteria didn’t produce a toxin to kill mice

Griffith Experiment 2

• 1. Injected mice with heat

killed virulent strain 2. Injected mice with non -virulent strain + heat killed virulent strain Results:

• heat killed: lived
• mixed strains: mice

developed pneumonia Conclusion: heat killed virulent strain passed disease causing abilities to non virulent strain

After Experiment

Cultured bacteria from dead mice and they grew virulent strain. Griffith hypothesized that a factor was transferred from heat killed cells to live cells .

TRANS NSFOR FORMA MATIO ION

1944: Avery (et al)

• 1. Repeated Griffith’s experiment with same results.
• result: transformation occurred
• 2. Did a second experiment using enzymes that would

destroy RNA.

• result: transformation occurred
• 3. Did third experiment using enzymes that would destroy DNA.
• result: no transformation

CONCLUSION DNA was transforming factor

1952: Hershey / Chase

• studied how viruses (bacteriophage) affect bacteria.

Bacterio

riophage ge Virus composed of DNA core and protein coat

How Bacteriophages Work

• 1. bacteriophage attaches to surface of bacteria and

injects DNA

• 2. bacteria makes phage DNA
• 3. bacterial cell bursts
• 4. sends out new phages to infect more bacteria

animation

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Hershey Chase Experiment

• 1. They labeled virus protein

coat with radioactive sulfur

• 2. They labeled virus DNA

with radioactive phosphorous Result lt

• bserved that bacteria had

phosphorous *** virus injected bacterial cells with its phosphorous labeled DNA*** Conclu clusion DNA carried genetic code since bacteria made new DNA.

DISCOV OVERY RY OF STRUCTU TURE OF DNA Early 1950’s: Rosalind Franklin (English)

x ray eviden dence: X pattern showed that fibers of DNA twisted and molecules are spaced at regular intevals on length fiber.

Maurice Wilkins: x ray diffraction, worked with Franklin

1950’s Watson (American) & Crick (English)

**double helix model** won Nobel prize in 1962

DNA

• double strand of nucleotides
• may have 1000’s of nucleotides in 1 strand

(very long molecule)

• bases join up in specific (complementary) pairs:
• complementary pairs (base pairing rules)

1 purin ine bonds with 1 pyrimid idin ine on one rung of the ladder connected by a weak H bond

C - G A – T

Order r of nucle leotid ides not import rtant, proper r comple lementary ry bases must be paire red.

Same time period:

Chargaff (American biochemist)

Chargaff’s Rule:

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STRUCT CTURE URE OF DNA

Composed of:

• A. Phosphate
• B. Deoxyribose sugar (5 C)
• C. 4 Nitrogenous bases
• Purines

Adenine A Guanine G

• Pyrimidines

Thymine T Cytosine C D bases attached to sugar

• E. bases attached to each
• ther by weak H bond

Nucleotide Structure

Purines

Pyrimidines Sugar Base Phosphate

REPLIC ICATION ION OF DNA

Process of duplication of DNA

• Before cell can divide a new copy of DNA

must be made for the new cell

• Semicon
• nser

ervative ive replication ion: each strand acts as a template (pattern) for new strand to be made End Result lt:

• ne old strand, one new daughter strand

DNA REPLICATION

Models of DNA Replication

Steps of Replication

• 1. Enzyme DNA helica

icase attaches to DNA molecule and unwinds 2 strands at various points on the strand (breaks H bonds so strand unwinds)

• replica

licatio ion forks ks: two areas on either end of the DNA where double helix separates

• forms replica

icatio ion bubble le: “bubble” under electron microscope

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• 2. Enzyme DNA polymera

rase moves along each of DNA strand and adds complementary bases of nucleotides floating freely in nucleus

• A. DNA polymerase begins

synthesis at RNA primer segment

• enzyme RNA Prima

imase lays down this section on DNA strand

• RNA primer segment

signals beginning of replication

DNA Directionality

• directionality: DNA

polymerase reads the template in the 3’ to 5’ direction Daughter DNA strand (since it is complementary) must be synthesized in the 5’ to 3’ direction Strands are antiparallel.

But if there exist no DNA polymerases capable of polymerizing DNA in the 3' to 5' direction, how could this be?

Discontinuous synthesis

• synthesis only occurs when a

large amount of single strand DNA is present

• daughter DNA is then synthesized

in 5’ to 3’ direction

• leading and lagging strands:
• lead

ading strand – continuously synthesized DNA strand

• lagging strand - delayed,

fragmented, daughter DNA

• Okazaki

aki fragments- discontinuous fragmented DNA segments

• D. DNA ligase stitches

together Okazaki fragments into a single, unfragmented daughter molecule

• E. enzyme chops off

RNA primer and replaces it with DNA

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• 3. DNA polymerase catalyzes

formation of H bonds between nucleotides of template and newly arriving nucleotides which will form daughter DNA 4. Once all DNA is copied, daughter DNA detaches

animation

End Replication Problem

• On one end, RNA primer cannot be replaced with

DNA because it is a 5’ (DNA polymerase can only read from 3’ to 5’)

• Causes daughter DNA’s to be shorter with each

replication (cell division) 3’__________________________________ 5’ 5’-------------------------------------------------- 3’ 5’__________________________________ 3 3’-------------------------------------------------5’

Solution to End Replication Problem

telomere res: regions of repeated non coding sequences at end of chromosomes (protective sacrificial ends)

• become shorter with repeated cell divisions
• once telomeres are gone, coding sections of chrom.

are lost and cell does not have enough DNA to function ***telomere theory of aging***

• telomera

rase: special enzyme that contains an RNA template molecule so that telomeres can be added back on to DNA (rebuilds telomeres) ** found in: Cancer cells - immortal in culture Stem cells ** not found in most differentiated cells

Speed of Replication

• Multiple replication forks- replication occurs simultaneously on many

points of the DNA molecule

• Would take 16 days to replicate 1 strand from one end to the other on

a fruit fly DNA without multiple forks

• Actually takes ~ 3 minutes / 6000 sites replicate at one time
• Human chromosome replicated in about 8 hours with multiple

replication forks working together

Accuracy and Repair

• Cell has proofreading functions
• DNA polymerase can remove damaged nucleotides

and replace with new ones for accurate replication

• RNA does not have this ability- reason RNA viruses

mutate so much

• DNA damaged by heat, radiation, chemicals and
• ther factors

Importance of DNA

1. Controls formation of all substances in the cell by the genetic code 2. Directs the synthesis of specific strands of m RNA to make proteins

RNA (Ribonucleic acid)

Another nucleic acid takes orders from DNA Used in protein synthesis