DNA & DNA Replication

History DNA DNA Comprised of genes In non-dividing cell nucleus as chromatin Protein/DNA complex Chromosomes form during cell division Duplicate to yield a full set in daughter cell

From Chapter 3 Nucleic acids are polymers Nucleic acids are polymers Monomers are called nucleotides Monomers are called nucleotides Nucleotides = base + sugar + phosphate Nucleotides = base + sugar + phosphate Base = Base = adenine, guanine adenine, guanine thymine, cytosine, uracil thymine, cytosine, uracil Sugar = deoxyribose or ribose Sugar = deoxyribose or ribose Phosphate, a single phosphate in DNA Phosphate, a single phosphate in DNA Sugar is linked to the phosphate Sugar is linked to the phosphate

DNA is a Double Helix Nucleotides Nucleotides A, G, T, C A, G, T, C Sugar and phosphate form the backbone Sugar and phosphate form the backbone Bases lie between the backbone Bases lie between the backbone Held together by H-bonds between the bases Held together by H-bonds between the bases A-T – 2 H bonds A-T – 2 H bonds G-C – 3 H bonds G-C – 3 H bonds

H - Bonds Base-pairing rules Base-pairing rules A  T only (A  U if DNA-RNA hybrid) G  C only

Double Helix

Nucleotides as Language We must start to think of the nucleotides – A, G, C and T as part of a special language – the language of genes that we will see translated to the language of amino acids in proteins We must start to think of the nucleotides – A, G, C and T as part of a special language – the language of genes that we will see translated to the language of amino acids in proteins

Genes as Information Transfer A gene is the sequence of nucleotides within a portion of DNA that codes for a peptide or a functional RNA A gene is the sequence of nucleotides within a portion of DNA that codes for a peptide or a functional RNA Sum of all genes = genome Sum of all genes = genome

DNA Replication Semiconservative Semiconservative Daughter DNA is a double helix with 1 parent strand and 1 new strand Daughter DNA is a double helix with 1 parent strand and 1 new strand Found that 1 strand serves as the template for new strand Found that 1 strand serves as the template for new strand ReplicationFork Parental DNA Molecule 3’ 5’ 3’ 5’

DNA Template Each strand of the parent DNA is used as a template to make the new daughter strand Each strand of the parent DNA is used as a template to make the new daughter strand DNA replication makes 2 new complete double helices each with 1 old and 1 new strand DNA replication makes 2 new complete double helices each with 1 old and 1 new strand

How is DNA Synthesized? DNA is synthesized by DNA is synthesized by Simple addition of nucleotides along one strand (1/2 the double helix) Simple addition of nucleotides along one strand (1/2 the double helix)

Mistakes during Replication Base pairing rules must be maintained Base pairing rules must be maintained Mistake = genome mutation, may have consequence on daughter cells Mistake = genome mutation, may have consequence on daughter cells Only correct pairings fit Only correct pairings fit If wrong nucleotide is included If wrong nucleotide is included Special enzyme has “proofreading” and can remove incorrect nucleotide Special enzyme has “proofreading” and can remove incorrect nucleotide Another enzyme then adds correct base Another enzyme then adds correct base

Proofreading

Other Necessary Proteins Helicase opens the double helix and helps it uncoil Helicase opens the double helix and helps it uncoil Specialized binding proteins keep strands separated Specialized binding proteins keep strands separated

DNA Repair For the rare mutations occurring during replication that isn’t caught by DNA proofreading enzyme For the rare mutations occurring during replication that isn’t caught by DNA proofreading enzyme If no repair If no repair In germ (sex) cells  inherited diseases In germ (sex) cells  inherited diseases In somatic (regular) cells  cancer In somatic (regular) cells  cancer

Effect of Mutation

Repair Mechanisms Different enzymes correct different mistakes Different enzymes correct different mistakes Other enzymes make the proper strand piece Other enzymes make the proper strand piece Still another enzyme will join new the new piece in its proper spot on the strand Still another enzyme will join new the new piece in its proper spot on the strand