DNA: Deoxyribonucleic Acid Biology

Structure 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 bases Adenine = A Guanine = G Cytosine = C Thymine = T

Nucleotide 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 Structure Pyrimidines Bases that have only one ring of carbon and nitrogen atoms Cytosine and thymine

The Origin of the Double Helix Rosalind Franklin X-ray photographs of DNA crystals Research partner Maurice Wilkins

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 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 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

Complementary Base Pairing DNA nucleotides pair in specific combinations Base-pairing rules Cytosine & Guanine C - G Adenine & Thymine A - T

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 Base pairs are connected to one another by hydrogen bonds

Replication of DNA Replication The process of copying DNA in the cell Replication The process of copying DNA in the cell

Replication of DNA-Step 1 The separation of the two nucleotide strands The point at which the two chains separate Replication fork

Replication 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 2 DNA polymerase binds to the separated chains of DNA Runs along the separated strands

Replication of DNA-Step 2 New chains of DNA are assembled using nucleotides in the surrounding medium that are complementary to the existing DNA chains

Replication of DNA-Step 3 Once the DNA is copied the cell is ready to undergo cell division

Accuracy 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 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