Molecular Biology of the Gene

DNA Lesson Part 1

Dr. Wilson MuseSchoolcraft College

10.1 Experiments showed that DNA is the genetic material

– Frederick Griffith discovered that a “transforming factor” could be transferred into a bacterial cell

– Disease-causing bacteria were killed by heat– Harmless bacteria were incubated with heat-killed

bacteria– Some harmless cells were converted to disease-

causing bacteria, a process called transformation – The disease-causing characteristic was inherited

by descendants of the transformed cells

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10.1 Experiments showed that DNA is the genetic material

– Alfred Hershey and Martha Chase used bacteriophages to show that DNA is the genetic material

– Bacteriophages are viruses that infect bacterial cells– Phages were labeled with radioactive sulfur to detect

proteins or radioactive phosphorus to detect DNA– Bacteria were infected with either type of labeled

phage to determine which substance was injected into cells and which remained outside

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10.1 Experiments showed that DNA is the genetic material

– The sulfur-labeled protein stayed with the phages outside the bacterial cell, while the phosphorus-labeled DNA was detected inside cells

– Cells with phosphorus-labeled DNA produced new bacteriophages with radioactivity in DNA but not in protein

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Animation: Hershey-Chase Experiment

Animation: Phage T2 Reproductive Cycle

Head

Tail fiber

DNA

Tail

Head

Tail fiber

DNA

Tail

Batch 1Radioactiveprotein

Bacterium

Radioactiveprotein

DNA

Phage

Pellet

RadioactiveDNA

Batch 2RadioactiveDNA

Emptyprotein shell

PhageDNA

Centrifuge

Radioactivityin liquid

Measure theradioactivity inthe pellet andthe liquid.

4Centrifuge the mixtureso bacteria form apellet at the bottom ofthe test tube.

3Agitate in a blender toseparate phagesoutside the bacteriafrom the cells andtheir contents.

2Mix radioactivelylabeled phages withbacteria. The phagesinfect the bacterial cells.

1

Pellet

CentrifugeRadioactivityin pellet

Batch 1Radioactiveprotein

Bacterium

Radioactiveprotein

DNA

Phage

RadioactiveDNA

Batch 2RadioactiveDNA

Agitate in a blender toseparate phagesoutside the bacteriafrom the cells andtheir contents.

2Mix radioactivelylabeled phages withbacteria. The phagesinfect the bacterial cells.

1

Empty protein shell

Phage DNA

Pellet

Emptyprotein shell

PhageDNA

Centrifuge

Radioactivityin liquid

Measure theradioactivity inthe pellet andthe liquid.

Centrifuge the mixtureso bacteria form apellet at the bottom ofthe test tube.

Pellet

CentrifugeRadioactivityin pellet

43

Phage attachesto bacterial cell.

Phage injects DNA. Phage DNA directs hostcell to make more phageDNA and protein parts.New phages assemble.

Cell lyses andreleases new phages.

DNADNA• DNADNA is often called

the blueprint of life..• In simple terms,

DNA contains the instructions for making proteins within the cell.

NucleotidesNucleotides

C C

C

OPhosphate

O

CC

O -P OO

O

O -P OO

O

O -P OO

O One deoxyribose together with its phosphate and base make a

nucleotide.

Nitrogenous base

Deoxyribose

10.2 DNA and RNA are polymers of nucleotides

– The monomer unit of DNA and RNA is the nucleotide, containing

– Nitrogenous base– 5-carbon sugar– Phosphate group

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– DNA and RNA are polymers called polynucleotides

– A sugar-phosphate backbone is formed by covalent bonding between the phosphate of one nucleotide and the sugar of the next nucleotide

– Nitrogenous bases extend from the sugar-phosphate backbone

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Animation: DNA and RNA Structure

Sugar-phosphate backbone

DNA nucleotide

Phosphate group

Nitrogenous baseSugar

DNA polynucleotide

DNA nucleotide

Sugar(deoxyribose)

Thymine (T)

Nitrogenous base(A, G, C, or T)

Phosphategroup

Sugar(deoxyribose)

Thymine (T)

Nitrogenous base(A, G, C, or T)

Phosphategroup

Pyrimidines

Guanine (G)Adenine (A)Cytosine (C)Thymine (T)

Purines

Sugar(ribose)

Uracil (U)

Nitrogenous base(A, G, C, or U)

Phosphategroup

Hydrogen Bonds, Hydrogen Bonds, cont.

• When making hydrogen bonds, cytosine always pairs up with guanine,

• And adenine always pairs up with thymine.

• (Adenine and thymine are shown here.)

C

C

CC

N

N

N

N

N

C

C

C

C

C

N

N

OO

OO

C

C

C

C

C

N

N

OO

N

C

C

C

C

N N

OO

NN

N C

Hydrogen BondsHydrogen Bonds

• The bases attract each other because of hydrogen bonds.

• Hydrogen bonds are weak but there are millions and millions of them in a single molecule of DNA.

• (The bonds between cytosine and guanine are shown here.)

Twist

Hydrogen bond

Basepair

Partial chemical structure Computer modelRibbon model

Basepair

Ribbon model

Hydrogen bond

Partial chemical structure

10.4 DNA replication depends on specific base pairing

– DNA replication follows a semiconservative model

– The two DNA strands separate– Each strand is used as a pattern to produce a

complementary strand, using specific base pairing– Each new DNA helix has one old strand with one

new strand

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Animation: DNA Replication Overview

Two Stranded DNATwo Stranded DNA• Remember, DNA

has two strands that fit together something like a zipper.

• The teeth are the nitrogenous bases but why do they stick together?

DNA by the numbersDNA by the numbers• Each cell has about 2 m

of DNA.• The average human has

75 trillion cells.• The average human has

enough DNA to go from the earth to the sun more than 400 times.

• DNA has a diameter of only 0.000000002 m.

The earth is 150 billion mor 93 million miles from the sun.

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DNA REPLICATION

Makin’ copies ........

10.4 DNA replication depends on specific base pairing

– DNA replication follows a semiconservative model

– The two DNA strands separate– Each strand is used as a pattern to produce a

complementary strand, using specific base pairing– Each new DNA helix has one old strand with one

new strand

Copyright © 2009 Pearson Education, Inc.

Animation: DNA Replication Overview

Parentalmoleculeof DNA

Parentalmoleculeof DNA

Nucleotides

Both parentalstrands serveas templates

Parentalmoleculeof DNA

Nucleotides

Both parentalstrands serveas templates

Two identicaldaughter

molecules of DNA

– DNA replication begins at the origins of replication

– DNA unwinds at the origin to produce a “bubble”

– Replication proceeds in both directions from the origin

– Replication ends when products from the bubbles merge with each other

– DNA replication occurs in the 5’ 3’ direction

– Replication is continuous on the 3’ 5’ template

– Replication is discontinuous on the 5’ 3’ template, forming short segments

10.5 DNA replication proceeds in two directions at many sites simultaneously

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Animation: Leading Strand

10.5 DNA replication proceeds in two directions at many sites

simultaneously– Proteins involved in DNA replication– DNA polymerase adds nucleotides to a growing

chain– DNA ligase joins small fragments into a

continuous chain

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Animation: Lagging Strand

Animation: DNA Replication Review

Animation: Origins of Replication

Origin of replication Parental strand

Daughter strand

Bubble

Two daughter DNA molecules

3 end 5 end

3 end5 end

3

5

2

4

13

5

2

41

P

P

P

P

P

P

P

P

Parental DNA

35

DNA polymerasemolecule

DNA ligase

35

Overall direction of replication

Daughter strandsynthesizedcontinuously

35

35

Daughter strandsynthesizedin pieces

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THE FLOW OF GENETIC INFORMATION FROM DNA

TO RNA TO PROTEIN

10.6 The DNA genotype is expressed as proteins, which

provide the molecular basis for phenotypic traits

– A gene is a sequence of DNA that directs the synthesis of a specific protein

– DNA is transcribed into RNA– RNA is translated into protein

– The presence and action of proteins determine the phenotype of an organism

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10.6 The DNA genotype is expressed as proteins, which

provide the molecular basis for phenotypic traits– Demonstrating the connections between genes

and proteins – The one gene–one enzyme hypothesis was based

on studies of inherited metabolic diseases– The one gene–one protein hypothesis expands the

relationship to proteins other than enzymes– The one gene–one polypeptide hypothesis

recognizes that some proteins are composed of multiple polypeptides

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Cytoplasm

Nucleus

DNA

Cytoplasm

Nucleus

DNA

Transcription

RNA

Cytoplasm

Nucleus

DNA

Transcription

RNA

Translation

Protein