CHAPTER 16 The Molecular Basis of Inheritance

What is DNA? DNA stands for deoxyribonucleic acid. DNA is what makes our genes, and along with protein, makes our chromosomes. It encodes our hereditary information. It directs the development of our anatomical, physiological, and behavioral traits.

How did we find DNA? Thomas Hunt Morgan, in the early 1900s, studied fruit flies and showed that genes are on chromosomes.

How did we find DNA? In 1928, Frederick Griffith saw that when an organism gets some external bacteria, it changes, which is called transformation.

How did we find DNA? In 1944, Avery, McCarty, and MacLeod figured out that the bacteria had DNA in it, and that’s what caused transformation.

How did we find DNA? In 1952, Hershey and Chase showed that DNA is the genetic material of our cells.

How did we find DNA? In 1947, Erwin Chargaff figured out that DNA had four nitrogenous bases, A, T, C and G.

How did we find DNA? In the 1950s, Rosalind Franklin took an x-ray crystallography picture of DNA.

How did we find DNA? Watson and Crick used her picture to determine that DNA is a double helix.

The Structure of DNA DNA is a double helix. It is a polymer made of monomers called nucleotides. Each nucleotide is made of a nitrogenous base, a pentose sugar called deoxyribose, and a phosphate group. The backbone of DNA is called “sugar phosphate” and has bases attached to it like rungs of a ladder.

The Structure of DNA DNA is “right handed” and curves to the right. Hydrogen bonds hold the bases together The 5’ end has a phosphate group The 3’ end has an OH group Strands always line up with one 5’ strand face up attached to a 3’ strand

The Structure of DNA There are 4 different nitrogenous bases: Adenine (A) Thymine (T) Guanine (G) Cytosine (C)

The Structure of DNA Adenine always pairs with Thymine Cytosine always pairs with Guanine

Purines Purines are nitrogenous bases with 2 organic rings. G and A are purines

Pyrimidines Pyrimidines are nitrogenous bases with only 1 organic ring Cytosine and thymine

DNA Replication DNA replicates during the S phase of interphase, prior to cell division (mitosis). DNA replication is semi- conservative, meaning that new DNA strands are made of one new daughter strand attached to one old parent strand. Meselson and Stahl figured this out in late 1950s.

DNA Replication DNA replication begins at special sites called origins of replication, by opening up a replication bubble. At each end of the replication bubble there is a replication fork, a Y-shaped region where DNA is being replicated.

DNA Replication DNA polymerases are special enzymes that add complementary bases to the unzipped DNA.

DNA Replication DNA replication can ONLY go from 5’ to 3’ So replication is antiparallel, one strand elongates normally, called the leading strand. The other is going away from the replication fork, called the lagging strand.

DNA Replication As the bubble of replication grows, the lagging strand is made bit by bit in fragments, called Okazaki fragments. These are eventually joined by an enzyme called DNA ligase.

Proteins that help DNA replication A primer is an initial bit of nucleotide that helps the new base attach to the DNA strand. Primase is an enzyme that joins nucleotides together.

Proteins that help DNA replication Helicase is an enzyme that unwinds DNA at the replication fork. Topoisomerase is an enzyme that relieves the strain of untwisting the DNA Single strand binding proteins bind to unwound DNA strands and stabilize them until replication is done

Proofreading and Repairing DNA 1 in every 100,000 base pairs gets paired incorrectly Special enzymes repair mistakes A nuclease is an enzyme that cuts out bad DNA during nucleotide excision repair

Telomeres Telomeres are nucleotides at the end of our DNA strands Each time DNA replicates (as we age), telomeres get shorter Shorter or missing telomeres results in cancer and other effects from aging.