DNA: Structure and Replication
DNA DNA. DNA is often called the blueprint of life. DNA contains the instructions for making proteins within the cell. Nucleic acid
Why do we study DNA? We study DNA for many reasons, e.g., its central importance to all life on Earth, medical benefits such as cures for diseases, better food crops. Solves crimes
Chromosomes and DNA Our genes are on our chromosomes. Chromosomes are made up of a chemical called DNA.
The Shape of the Molecule DNA is a very long polymer – made of nucelotides The basic shape is like a twisted ladder This is called a double helix.
The Shape of the Molecule Sides - alternating phosphates and sugars Center – made of nitrogen bases
Nucleotides CC C O Phosphate O C C O -P O O O One deoxyribose sugar together with its phosphate and base make a nucleotide. Nitrogenous base Deoxyribose
Nucleotides CC C O Phosphate O C C O -P O O O Nitrogenous base Deoxyribose
Four nitrogenous bases 1. Cytosine C 2. Thymine T 3. Adenine A 4. Guanine G DNA has four different bases:
Chargraff’s Rule: Adenine and Thymine always join together A T Cytosine and Guanine always join together C G
Three Representations of DNA
DNA Replication
Process of copying DNA – done by a series of enzymes Process of copying DNA – done by a series of enzymes Essential in all life forms Essential in all life forms Must occur before the cell divides – ensures each cell will have identical DNA Must occur before the cell divides – ensures each cell will have identical DNA Takes place in the nucleus Takes place in the nucleus
Steps of DNA Replication 1. Helicase enzyme breaks the H-bonds between bases 2. DNA polymerase adds complementary bases 3. Process continues until entire DNA is copied
Steps of DNA Replication Enzymes proofread and correct these mistakes 4. Enzymes proofread and correct these mistakes 5. The new error rate for DNA that has been proofread is 1 in 1 billion base pairing errors
Steps of DNA Replication
T A C C G A T G G C #1#2 Enzyme Helicase T A C C G #1 A T G G C A T G G C #2 T A C C G New DNA Polymerase DNA Polymerase
Semiconservative Model of Replication Idea presented by Watson & Crick Idea presented by Watson & Crick The The two strands of the parental molecule separate, and each acts as a template for a new complementary strand New DNA consists of 1 PARENTAL (original) and 1 NEW strand of DNA Parental DNA DNA Template New DNA
Proofreading New DNA DNA polymerase initially makes about 1 in 10,000 base pairing errors DNA polymerase initially makes about 1 in 10,000 base pairing errors Enzymes proofread and correct these mistakes Enzymes proofread and correct these mistakes The new error rate for DNA that has been proofread is 1 in 1 billion base pairing errors The new error rate for DNA that has been proofread is 1 in 1 billion base pairing errors
Steps of DNA Replication 1. Helicase enzyme breaks the H-bonds between bases and “unzips” the DNA 2. DNA polymerase adds complementary nucelotides to the exposed bases 3. Process continues until entire DNA is copied
Steps of DNA Replication Enzymes proofread and correct these mistakes 4. Enzymes proofread and correct these mistakes 5. The new error rate for DNA that has been proofread is 1 in 1 billion base pairing errors
DNA Molecule S P AT S P P P SS SS P P G C C G Nucleotide Hydrogen bonds