DNA:Deoxyribonucleic

Acid

Biology

Structure of DNAStructure of DNA

• DNA nucleotide has 3 parts:• Sugar molecule

• Deoxyribose

• Phosphate group• Nitrogen-

containing base

• DNA nucleotide has 3 parts:• Sugar molecule

• Deoxyribose

• Phosphate group• Nitrogen-

containing base

Nitrogen-containing basesNitrogen-containing bases

• Adenine = A• Guanine = G• Cytosine = C• Thymine = T

• Adenine = A• Guanine = G• Cytosine = C• Thymine = T

Nucleotide StructureNucleotide Structure

• Purines• Bases that

have two rings of carbon and nitrogen atoms

• Adenine and guanine

• Purines• Bases that

have two rings of carbon and nitrogen atoms

• Adenine and guanine

Nucleotide StructureNucleotide Structure

• Pyrimidines• Bases that

have only one ring of carbon and nitrogen atoms

• Cytosine and thymine

• Pyrimidines• Bases that

have only one ring of carbon and nitrogen atoms

• Cytosine and thymine

The Origin of the Double Helix

The Origin of the Double Helix

• Rosalind Franklin• X-ray photographs

of DNA crystals • Research partner

Maurice Wilkins

• Rosalind Franklin• X-ray photographs

of DNA crystals • Research partner

Maurice Wilkins

The Origin of the Double Helix

The Origin of the Double Helix

• Watson & Crick• Used

Franklin’s stolen research and photographs to determine the structure of DNA

• 1953

• Watson & Crick• Used

Franklin’s stolen research and photographs to determine the structure of DNA

• 1953

The Double HelixThe Double Helix

• DNA is composed of two nucleotide chains that wrap around each other to form a double spiral, like a spiral staircase• Double helix

• DNA is composed of two nucleotide chains that wrap around each other to form a double spiral, like a spiral staircase• Double helix

The Double HelixThe Double Helix

• The “backbone” of the double helix• Alternating sugar

and phosphate molecules

• Nitrogen-containing bases attach to the sugar molecules in the “backbone” and occupy the space in the center

• The “backbone” of the double helix• Alternating sugar

and phosphate molecules

• Nitrogen-containing bases attach to the sugar molecules in the “backbone” and occupy the space in the center

Complementary Base Pairing

Complementary Base Pairing

• DNA nucleotides pair in specific combinations• Base-pairing rules

• Cytosine & Guanine

• C - G

• Adenine & Thymine

• A - T

• DNA nucleotides pair in specific combinations• Base-pairing rules

• Cytosine & Guanine

• C - G

• Adenine & Thymine

• A - T

Complementary Base Pairing

Complementary Base Pairing

• The complementary base pairs are a combination• Purine with a

pyrimidine• Two rings paired

with a single ring

• The complementary base pairs are a combination• Purine with a

pyrimidine• Two rings paired

with a single ring

Complementary Base Pairing

Complementary Base Pairing

• Base pairs are connected to one another by hydrogen bonds

• Base pairs are connected to one another by hydrogen bonds

Replication of DNAReplication of DNA

• Replication• The process of

copying DNA in the cell

• Replication• The process of

copying DNA in the cell

Replication of DNA-Step 1Replication of DNA-Step 1

• The separation of the two nucleotide strands

• The point at which the two chains separate• Replication

fork

• The separation of the two nucleotide strands

• The point at which the two chains separate• Replication

fork

Replication of DNA-Step 1Replication of DNA-Step 1• The

nucleotide chains are separated by enzymes • Helicases

• Run down the strand and break the bonds holding the complemen-tary bases together

• The nucleotide chains are separated by enzymes • Helicases

• Run down the strand and break the bonds holding the complemen-tary bases together

Replication of DNA-Step 2Replication of DNA-Step 2

• DNA polymerase binds to the separated chains of DNA• Runs along

the separated strands

• DNA polymerase binds to the separated chains of DNA• Runs along

the separated strands

Replication of DNA-Step 2Replication of DNA-Step 2

• New chains of DNA are assembled using nucleotides in the surrounding medium that are complementary to the existing DNA chains

• New chains of DNA are assembled using nucleotides in the surrounding medium that are complementary to the existing DNA chains

Replication of DNA-Step 3Replication of DNA-Step 3

• Once the DNA is copied the cell is ready to undergo cell division

• Once the DNA is copied the cell is ready to undergo cell division

Accuracy and RepairAccuracy and Repair• Replication is

very accurate• One error in

every 10,000 pairs of nucleotides

• Mutations do occur• A mistake that

results in a change in the nucleotide sequence

• Replication is very accurate

• One error in every 10,000 pairs of nucleotides

• Mutations do occur• A mistake that

results in a change in the nucleotide sequence

Accuracy and RepairAccuracy and Repair• The number of

errors and mutations are reduced by proofreading enzymes• They detect errors

and replace the wrong nucleotide with the correct nucleotide

• Doesn’t always work• Skin cancer

• The number of errors and mutations are reduced by proofreading enzymes• They detect errors

and replace the wrong nucleotide with the correct nucleotide

• Doesn’t always work• Skin cancer