The Molecular Basis of Inheritance

 Used bacteriophages (viruses that infect bacteria)  Only made up of DNA and protein  Used phosphorus to “tag” the DNA in one culture of bacteriophages and sulfur to “tag” the protein in a second culture  Their results showed that only DNA entered bacteria infected by the virus

 Used previous data from scientists (Rosalind Franklin) to build their model of DNA as we know it, the double helix  Also used X-ray crystallography  Purified samples of DNA are bombarded with X- rays, which are then diffracted depending on the pattern of the subject in question  X-ray crystallography of DNA showed a helix

 1) DNA is a double helix, which can be described as a twisted ladder with the sides made up of sugar-phosphate components and the rungs made up of pairs of nitrogenous bases  2) The nitrogenous bases of DNA are adenine (A), thymine (T), guanine (G) and cytosine (C) ▪ A pairs with T, C pairs with G

 The two sides of the double helix are antiparallel and run in different directions  One runs 5’ to 3’ (right side up)  The other runs 3’ to 5’ (upside down)  A double hydrogen bond binds A to T  A triple hydrogen bond binds C to G

 DNA replication is semiconservative  at the end of replication, each of the daughter DNA molecules has one old strand, derived from the parent strand of DNA

 Begins at origins of replication, where 2 strands of DNA separate to form replication bubbles, which speed up the process  Helicases are enzymes that untwist the DNA double helix  A group of enzymes called DNA polymerases catalyze the elongation of new DNA at the replication fork

 DNA polymerase adds nucleotides to the growing chain one by one, working in a 5’ to 3’ direction, matching A with T and G with C  DNA replication occurs continuously along the 5’ to 3’ strand, which is called the leading strand (towards the fork)  The strand that runs 3’ to 5’ is copied in segments and called the lagging strand (away from the fork)

 The lagging strand is synthesized in separate pieces called Okazaki fragments, which are then sealed together by DNA ligase

 Specificity of base pairings  Mismatch repair, in which special repair enzymes fix incorrectly paired nucleotides  Nucleotide excision repair, in which incorrectly placed nucleotides are excised or removed by enzymes called nucleases, and the gap left over is filled in correctly

 Each time DNA replicates, some nucleotides from the ends of the chromosomes are lost  To protect against the loss of genes, eukaryotes have special nonsense nucleotide sequences at the ends of chromosomes that repeat thousands of times called telomeres

 Every time DNA replicates, telomeres get shorter (serves as a clock that counts cell divisions)

 A bacterial chromosome is one double- stranded, circular DNA molecule associated with a small amount of protein  Eukaryotic chromosomes are linear DNA molecules associated with large amounts of protein  Eukaryotic cells contain chromatin, packed DNA and proteins

 1) Nucleosome  DNA wrapped around proteins called histones  This complex resembles beads on a string and is termed a nucleosome, the basic unit of DNA packaging  2) 30nm Fiber  Strings of nucleosomes fold to form a 30nm fiber

 3) Looped Domains of 300nm  Further folding of the 30nm fiber  4) Metaphase chromosome  Folding of looped domains