1. Molecular Genetics Eukaryotic and Prokaryotic Chromosomes

2. Eukaryotic Chromosomes They are linear and organized in pairs They are replicated at many points along their length simultaneously Their number varies greatly from one organism to another, but humans have 46 chromosomes; 22 pairs plus an X and either a second X or a Y Before replication, they are organized as chromatin

3. Chromatin DNA is wrapped around collections of proteins called histones Histones carry a positive charge and DNA carries a negative charge, so they are held together by electrostatic attraction 9 histones with DNA coiled twice around the 8 histone core is called a nucleosome

4. Chromatid Once replication is complete, the chromatin coils and condenses into the familiar chromosome (2 chromatid) that begins mitosis

5. Prokaryotic Chromosomes Prokaryotes are single celled organisms that lack membrane bound organelles, including a nucleus DNA in prokaryotes is in the form of a single, double stranded loop, as opposed to the linear eukaryotic chromosomes These “naked” loops of DNA attach to the inner membrane of the prokaryote There are also small, free floating loops of DNA called plasmids

6. Prokaryotic Replication – Binary Fission They replicate in one, continuous sweep of polymerase enzymes moving in opposite directions The bacteria grows to twice its normal size, the DNA loops separate and then the bacteria divides