DNA & DNA ReplicationDNA & DNA Replication

HistoryHistoryDNADNA

Comprised of genesComprised of genes In non-dividing cell nucleus In non-dividing cell nucleus

as chromatinas chromatinProtein/DNA complexProtein/DNA complex

Chromosomes form during Chromosomes form during cell divisioncell divisionDuplicate to yield a full set in Duplicate to yield a full set in

daughter celldaughter cell

From Chapter 3From Chapter 3Nucleic acids are polymersNucleic acids are polymers

Monomers are called nucleotidesMonomers are called nucleotidesNucleotides = base + sugar + phosphateNucleotides = base + sugar + phosphate

Base = Base = adenine, guanineadenine, guanine thymine, cytosine, uracilthymine, cytosine, uracil

Sugar = deoxyribose or riboseSugar = deoxyribose or ribosePhosphate, a single phosphate in DNAPhosphate, a single phosphate in DNA

Sugar is linked to the phosphateSugar is linked to the phosphate

DNA is a Double HelixDNA is a Double Helix NucleotidesNucleotides

A, G, T, CA, G, T, C

Sugar and phosphate Sugar and phosphate form the backboneform the backbone

Bases lie between the Bases lie between the backbonebackbone

Held together by Held together by H-bonds between the H-bonds between the basesbases A-T – 2 H bondsA-T – 2 H bonds G-C – 3 H bondsG-C – 3 H bonds

H - BondsH - Bonds Base-pairing rulesBase-pairing rulesAAT only (AT only (AU U

if DNA-RNA if DNA-RNA hybrid)hybrid)

GGC onlyC only

Double HelixDouble Helix

Nucleotides as LanguageNucleotides as Language

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

Genes as Information TransferGenes as Information Transfer

A A genegene is the sequence of nucleotides is the sequence of nucleotides within a portion of DNA that codes for a within a portion of DNA that codes for a peptide or a functional RNApeptide or a functional RNA

Sum of all genes = Sum of all genes = genomegenome

DNA ReplicationDNA Replication

SemiconservativeSemiconservative Daughter DNA is a double helix with 1 parent Daughter DNA is a double helix with 1 parent

strand and 1 new strandstrand and 1 new strand Found that 1 strand serves as the templateFound that 1 strand serves as the template for for

new strandnew strand

ReplicationReplicationForkFork

Parental DNA MoleculeParental DNA Molecule

3’

5’

3’

5’

DNA TemplateDNA Template

Each strand of the parent DNA is used as a Each strand of the parent DNA is used as a templatetemplate to make the new daughter strand to make the new daughter strand

DNA replication makes 2 new complete double DNA replication makes 2 new complete double helices each with 1 old and 1 new strandhelices each with 1 old and 1 new strand

How is DNA Synthesized?How is DNA Synthesized?

DNA is synthesized byDNA is synthesized bySimple addition of nucleotides Simple addition of nucleotides

along one strand (1/2 the double along one strand (1/2 the double helix)helix)

Mistakes during ReplicationMistakes during Replication

Base pairing rules must be maintainedBase pairing rules must be maintainedMistake = genome mutation, may have Mistake = genome mutation, may have

consequence on daughter cellsconsequence on daughter cellsOnly correct pairings fit Only correct pairings fit If wrong nucleotide is includedIf wrong nucleotide is included

Special enzyme has “Special enzyme has “proofreading”proofreading” and and can remove incorrect nucleotidecan remove incorrect nucleotide

Another enzyme then adds correct baseAnother enzyme then adds correct base

ProofreadingProofreading

Other Necessary ProteinsOther Necessary Proteins

HelicaseHelicase opens the double helix opens the double helix and helps it uncoiland helps it uncoil

Specialized binding proteinsSpecialized binding proteins keep strands separated keep strands separated

DNA RepairDNA RepairFor the rare mutations occurring during For the rare mutations occurring during

replication that isn’t caught by DNA replication that isn’t caught by DNA proofreading enzymeproofreading enzyme

If no repairIf no repair In germ (sex) cells In germ (sex) cells inherited diseases inherited diseases In somatic (regular) cells In somatic (regular) cells cancer cancer

Effect of MutationEffect of Mutation

Repair Repair MechanismsMechanisms

Different enzymes Different enzymes correct different correct different mistakesmistakes

Other enzymes make Other enzymes make the proper strand piecethe proper strand piece

Still another enzyme Still another enzyme will join new the new will join new the new piece in its proper spot piece in its proper spot on the strandon the strand