More on DNA Chromosomes and Replication

Chromosomes A single molecule of DNA Eukaryotes usually have many linear chromosomes Prokaryotes usually have one circular chromosome Prokaryotes also have plasmids

Eukaryotic Chromosomes Are highly condensed and coiled many times to fit a lot of DNA into a tiny nucleus The length of all the DNA in your body would stretch to the sun and back 70 times

Histone Proteins Helix only the first level DNA is wrapped tightly around histone proteins Control access to DNA

Fibers When genes are less active they get coiled more tightly into fibers Changes in gene expression are caused by coiling or uncoiling of chromosome

Fibers Loop and Coil Scaffold proteins coil and condense DNA further During replication extra scaffold proteins condense the DNA into chromosomes

Chromatin v. Chromosome

DNA Replication Watson and Crick noticed the huge benefit of double strands Each strand can serve as a template for making the other

Semiconservative Model Each strand serves as a template for the creation of a new strand of DNA 2 DNA molecules are created, each containing 1 strand of the original DNA

Semiconservative Replication

How Do We Know? Meselson-Stahl experiment

DNA Replication is Remarkably Fast and Accurate! Humans have 46 chromosomes, and thus 46 DNA molecules About 6 billion base pairs DNA replication takes just a few hours, even in humans Only 1 error per 1 billion nucleotides

The Beginning Topoisomerase unwinds the helix Helicase separates the strands Single stranded binding proteins (SSBs) keep the strands separated This forms a replication fork

Topoisomerase, Helicase and Single-Strand Binding Protein

DNA Polymerases Each nucleotide is added one by one by DNA polymerases Nucleoside Triphosphates are added Nucleoside loses 2 phosphate groups, providing energy to synthesize new strand

The Leading Strand Synthesis always occurs in the 5' to 3' direction (the 5' end of the new strand is synthesized first) (attaches to the 3’ end of the template) One strand can grow continuously as the fork opens in front of it

The Lagging Strand Built discontinuously in the opposite direction of replication Built in fragments, called Okazaki fragments DNA ligase connects fragments

Priming DNA DNA polymerases can only add nucleotides to an existing strand A RNA primer, first binds to the template strand with the help of primase (aka RNA Polymerase) Eventually replaced by DNA molecules

Primer continued... The leading strand requires only one primer For the lagging strand, each fragment requires a new primer

DNA Polymerase – An Amazing Enzyme DNA pol proofreads each nucleotide that it adds against the template If an error is made, the enzyme deletes the nucleotide and continues synthesizing DNA Other proteins do the same thing DNA is also repaired after damage, such as exposure to X-rays Over 100 DNA repair enzymes! Extremely, extremely important

DNA Repair/ Excision Repair Nucleases cut out (incise) the incorrect nucleotide DNA polymerase adds the correct nucleotide Ligase connects the new nucleotide to the strand

Topoisomerase Helicase Single Stranded Binding Proteins

5’ 3’ DNA Ligase Lagging Strand Okazaki Fragments Leading Strand DNA Polymerase Primase