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DNADNA

““The Secret of Life”The Secret of Life”

Basic Bases of DNABasic Bases of DNAFour Main Bases: (Nitrogenous Four Main Bases: (Nitrogenous

Bases)Bases)1)1) AdenineAdenine2)2) GuanineGuanine3)3) CytosineCytosine4)4) ThymineThymine5)5) Known as A, T, C, GKnown as A, T, C, G

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Purines and PyrimidinesPurines and Pyrimidines• Adenine and Guanine are purines. Adenine and Guanine are purines.

2 carbon rings2 carbon rings• Thymine and Cytosine are Thymine and Cytosine are

pyrimidines. 1 carbon ringpyrimidines. 1 carbon ring

NucleotideNucleotide• The nucleotide is the basic unit of a The nucleotide is the basic unit of a

DNA molecule.DNA molecule.• It has 3 parts:It has 3 parts:• 1) Nitrogen Base (A, T, C, G)1) Nitrogen Base (A, T, C, G)• 2) Phosphate Group2) Phosphate Group• 3) Sugar (Deoxyribose)3) Sugar (Deoxyribose)

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Double HelixDouble Helix• DNA is a double helix, meaning DNA is a double helix, meaning

there are 2 strands connected there are 2 strands connected together by together by hydrogen bonds!!hydrogen bonds!!

• It has a spiral shape like the border It has a spiral shape like the border on the left.on the left.

• The two strands are anti-parallel, The two strands are anti-parallel, meaning they run in opposite meaning they run in opposite directions.directions.

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Base Pair RulesBase Pair Rules• Adenine pairs with ThymineAdenine pairs with Thymine• A with TA with T• Guanine pairs with CytosineGuanine pairs with Cytosine• G with CG with C• Chargaff’s Rule!Chargaff’s Rule!

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DNA ReplicationDNA Replication• ““It has not escaped our notice that It has not escaped our notice that

the specific pairing we have the specific pairing we have postulated immediately suggests a postulated immediately suggests a possible copying mechanism for possible copying mechanism for the genetic material.”the genetic material.”

• J.D. WatsonJ.D. Watson• F. H. C. CrickF. H. C. Crick

What is DNA replication?What is DNA replication?• The process of producing two The process of producing two

identical strands/molecules of DNA.identical strands/molecules of DNA.• Each DNA molecule will have the Each DNA molecule will have the

same sequence.same sequence.

EnzymesEnzymes• There are There are many enzymesmany enzymes that that

help DNA replication work.help DNA replication work.• The important ones you need to The important ones you need to

know are:know are:

• 1. DNA polymerase: reads the 1. DNA polymerase: reads the original template strand and inserts original template strand and inserts the correct nucleotide using the the correct nucleotide using the base pair rules.base pair rules.

• Adenine with ThymineAdenine with Thymine• Guanine with CytosineGuanine with Cytosine

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• 2. Helicase: opens up the DNA 2. Helicase: opens up the DNA molecule. Unwinds the helix. molecule. Unwinds the helix.

• Simply creates the room to do Simply creates the room to do replication.replication.

• 3. Ligase: links copied DNA 3. Ligase: links copied DNA fragments together.fragments together.

Where?Where?• At replication forks! The point At replication forks! The point

where the double strand DNA is where the double strand DNA is splitting into 2 single strands.splitting into 2 single strands.

StepsSteps• 1. Helicase: unwinds helix1. Helicase: unwinds helix• 2. 2. DNA polymerase uses an DNA polymerase uses an

original template strand to create a original template strand to create a complementary strand going complementary strand going towards the replication forktowards the replication fork. . Always works in 5’ to 3’ direction.Always works in 5’ to 3’ direction.

• 3. The lagging strand (moving 3. The lagging strand (moving away from the repl. Fork) is made away from the repl. Fork) is made in several fragments.in several fragments.

• 4. Ligase: connects the fragments 4. Ligase: connects the fragments on the lagging strand.on the lagging strand.

Semi-conservativeSemi-conservative• 1. Original strand is a template to 1. Original strand is a template to

be copied. (like stencil)be copied. (like stencil)• 2. After replication there are two 2. After replication there are two

ds DNA molecules. ds DNA molecules. • 3. Each molecule will have an 3. Each molecule will have an

orginal parent strand and a newly orginal parent strand and a newly made strand.made strand.

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• DNA makes DNADNA makes DNA