Chapter 12: Molecular Genetics What You’ll Learn How DNA was discovered to be the genetic material & know its structure DNA replication Protein synthesis Gene regulation & mutations

Section 12.1: DNA: The Genetic Material Section Objectives: Summarize the experiments leading to the discovery of DNA as the gentic material Analyze the structure of DNA Describe the basic structure of the eukaryotic chromosome

What is DNA? Although the environment influences how an organism develops, the genetic information that is held in the molecules of DNA ultimately determines an organism’s traits. DNA achieves its control by determining the structure of proteins. Within the structure of DNA is the information for life—the complete instructions for manufacturing all the proteins for an organism.

 Bodies are made up of cells  All cells run on a set of instructions spelled out in DNA Bodies  Cells  DNA

The “Transforming Principle” 1928 Frederick Griffith – Streptococcus pneumonia bacteria was working to find cure for pneumonia – harmless live bacteria (“rough”) mixed with heat-killed pathogenic bacteria (“smooth”) causes fatal disease in mice – a substance passed from dead bacteria to live bacteria to change their phenotype “Transforming Principle”

The “Transforming Principle” Transformation = change in phenotype something in heat-killed bacteria could still transmit disease-causing properties live pathogenic strain of bacteria live non-pathogenic strain of bacteria mice diemice live heat-killed pathogenic bacteria mix heat-killed pathogenic & non-pathogenic bacteria mice livemice die A.B. C. D.

DNA is the “Transforming Principle” Avery, McCarty & MacLeod – purified both DNA & proteins separately from Streptococcus pneumonia bacteria which will transform non-pathogenic bacteria? – injected protein into bacteria no effect – injected DNA into bacteria transformed harmless bacteria into virulent bacteria 1944 What ’ s the conclusion? mice die

Confirmation of DNA Hershey & Chase – classic “blender” experiment – worked with bacteriophage viruses that infect bacteria – grew phage viruses in 2 media, radioactively labeled with either 35 S in their proteins 32 P in their DNA – infected bacteria with labeled phages 1952 | 1969 Hershey Why use S ulfur vs. P hosphorus?

Protein coat labeled with 35 S DNA labeled with 32 P bacteriophages infect bacterial cells T2 bacteriophages are labeled with radioactive isotopes S vs. P bacterial cells are agitated to remove viral protein coats 35 S radioactivity found in the medium 32 P radioactivity found in the bacterial cells Which radioactive marker is found inside the cell? Which molecule carries viral genetic info? Hershey & Chase

Blender experiment Radioactive phage & bacteria in blender – 35 S phage radioactive proteins stayed in supernatant therefore viral protein did NOT enter bacteria – 32 P phage radioactive DNA stayed in pellet therefore viral DNA did enter bacteria – Confirmed DNA is “transforming factor” Taaa-Daaa!

Hershey & Chase Alfred HersheyMartha Chase 1952 | 1969 Hershey

The structure of nucleotides DNA is a polymer made of repeating subunits called nucleotides.(the monomer) Nucleotides have three parts: a simple sugar, a phosphate group, and a nitrogenous base. Sugar (deoxyribose) Nitrogenous base Phosphate group

The structure of nucleotides in DNA there are four possible nucleotides, each containing one of these four bases. The phosphate groups and deoxyribose molecules form the backbone of the chain, and the nitrogenous bases stick out like the teeth of a zipper. Nucleotide Sugar-phosphate backbone Phosphate group Sugar (deoxyribose) DNA nucleotide Nitrogenous base (A, G, C, or T) Thymine (T)

Chargaff DNA composition: “Chargaff’s rules” – varies from species to species – all 4 bases not in equal quantity – bases present in characteristic ratio humans: A = 30.9% T = 29.4% G = 19.9% C = 19.8% 1947 That ’ s interesting! What do you notice? Rules A = T C = G

Paired bases DNA structure – double helix 2 sides like a ladder Bases match together ( – A pairs with T A : T – C pairs with G C : G

Structure of DNA James Watson and Francis Crick worked out the three-dimensional structure of DNA, based on work by Rosalind Franklin and Maurice Wilkens 1953 | 1962 Wilkins

Rosalind Franklin ( )

DNA is a double-stranded helix Watson and Crick also proposed that DNA is shaped like a long zipper that is twisted into a coil like a spring. Because DNA is composed of two strands twisted together, its shape is called double helix.

The structure of DNA Ribbon model Hydrogen bond Partial chemical structure Computer model Base pair

Anti-parallel strands – DNA molecule has “direction” – complementary strand runs in opposite direction

The importance of nucleotide sequences The sequence of nucleotides forms the unique genetic information of an organism. The closer the relationship is between two organisms, the more similar their DNA nucleotide sequences will be. Scientists use nucleotide sequences to determine evolutionary relationships among organisms, to determine whether two people are related, and to identify bodies of crime victims. Chromosome

Organizing & packaging DNA nucleus cell DNA nucleus cell 4 chromosomes in this organism DNA in chromosomes in everyday “working” cell DNA in chromosomes in cell getting ready to divide DNA has been “wound up”

DNA Packing DNA double helix (2-nm diamete r Histones “Beads on a string” Nucleosome (10-nm diameter) Tight helical fiber (30-nm diameter ) Supercoil (200-nm diameter ) Metaphase chromosome 700 nm

Nucleosomes “Beads on a string” – 1 st level of DNA packing – histone proteins 8 protein molecules positively charged amino acids bind tightly to negatively charged DNA 8 histone molecules