CHAPTER 10 Molecular Biology of the Gene Modules 10.1 – 10.5

Saboteurs Inside Our Cells The invasion and damage of cells by the herpesvirus can be compared to the actions of a saboteur intent on taking over a factory The herpesvirus hijacks the host cell’s molecules and organelles to produce new copies of the virus

Viruses provided some of the earliest evidence that genes are made of DNA Molecular biology studies how DNA serves as the molecular basis of heredity

10.1 Experiments showed that DNA is the genetic material THE STRUCTURE OF THE GENETIC MATERIAL 10.1 Experiments showed that DNA is the genetic material The Hershey-Chase experiment showed that certain viruses reprogram host cells to produce more viruses by injecting their DNA Head DNA Tail Tail fiber Figure 10.1A

The Hershey-Chase Experiment 1 Mix radioactively labeled phages with bacteria. The phages infect the bacterial cells. 2 Agitate in a blender to separate phages outside the bacteria from the cells and their contents. 3 Centrifuge the mixture so bacteria form a pellet at the bottom of the test tube. 4 Measure the radioactivity in the pellet and liquid. Radioactive protein Empty protein shell Radioactivity in liquid Phage Bacterium Phage DNA DNA Batch 1 Radioactive protein Centrifuge Pellet Radioactive DNA Batch 2 Radioactive DNA Centrifuge Radioactivity in pellet Figure 10.1B Pellet

Phage reproductive cycle Phage attaches to bacterial cell. Phage injects DNA. Phage DNA directs host cell to make more phage DNA and protein parts. New phages assemble. Cell lyses and releases new phages. Figure 10.1C

10.2 DNA and RNA are polymers of nucleotides DNA is a nucleic acid, made of long chains of nucleotides Phosphate group Nitrogenous base Nitrogenous base (A, G, C, or T) Sugar Phosphate group Nucleotide Thymine (T) Sugar (deoxyribose) DNA nucleotide Polynucleotide Sugar-phosphate backbone Figure 10.2A

DNA has four kinds of bases, A, T, C, and G Thymine (T) Cytosine (C) Adenine (A) Guanine (G) Pyrimidines Purines Figure 10.2B

Nitrogenous base (A, G, C, or U) RNA is also a nucleic acid RNA has a slightly different sugar RNA has U instead of T Nitrogenous base (A, G, C, or U) Phosphate group Uracil (U) Sugar (ribose) Figure 10.2C, D

10.3 DNA is a double-stranded helix James Watson and Francis Crick worked out the three-dimensional structure of DNA, based on work by Rosalind Franklin Figure 10.3A, B

The structure of DNA consists of two polynucleotide strands wrapped around each other in a double helix 1 chocolate coat, Blind (PRA) Figure 10.3C Twist

Hydrogen bonds between bases hold the strands together Each base pairs with a complementary partner A pairs with T G pairs with C

Partial chemical structure Three representations of DNA Hydrogen bond Ribbon model Partial chemical structure Computer model Figure 10.3D

10.4 DNA replication depends on specific base pairing In DNA replication, the strands separate Enzymes use each strand as a template to assemble the new strands A A Nucleotides Parental molecule of DNA Both parental strands serve as templates Two identical daughter molecules of DNA Figure 10.4A

Untwisting and replication of DNA Figure 10.4B

10.5 DNA replication: A closer look DNA replication begins at specific sites Parental strand Origin of replication Daughter strand Bubble Two daughter DNA molecules Figure 10.5A

Each strand of the double helix is oriented in the opposite direction 5 end 3 end P P P P P P P P 3 end 5 end Figure 10.5B

Overall direction of replication 3 DNA polymerase molecule How DNA daughter strands are synthesized 5 end 5 Daughter strand synthesized continuously Parental DNA 5 3 Daughter strand synthesized in pieces 3 P 5 The daughter strands are identical to the parent molecule 5 P 3 DNA ligase Overall direction of replication Figure 10.5C