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Lecture 05 - DNAStructureBased on Chapter 2 - DNA: The Genetic Material

1. The Search for the Genetic Material

2. Griffith’s Transformation Experiment

3. Avery’s Transformation Experiment

Download and play the Avery Experiment annimation above by clicking on “play”.

Avery Experiment

4. Hershey and Chase’s Bacteriophage Experiment

Download and play the Avery Experiment annimation above by clicking on “play”.

Hershey Chase Experiment

5. RNA as Viral Genetic Material

• All known cellular organisms and many viruses have DNA as their genetic material. Some viruses, however, use RNA instead.

• Examples of RNA viruses include:– Bacteriophages such as MS2 and Qb.– Animal viruses such as poliovirus and human

immunodeficiency virus (HIV).– Plant viruses such as tobacco mosaic virus (TMV)

and barley yellow dwarf virus.

6a. The Composition and Structure of DNA and RNA

6b. The Composition and Structure of DNA and RNA

6c. The Composition and Structure of DNA and RNA

6d. The Composition and Structure of DNA and RNA

6e. The Composition and Structure of DNA and RNA

7a. The DNA Double Helix

7b. The DNA Double Helix

Chargaff’s Rules –

From inspection of data such as that shown in Table 2.2. above Erwin Chargaff concluded that:

• the amount of purine always equals the amount of pyrimidine

• the amount of G equals C, and the amount of A equals T

7c. The DNA Double Helix

7d. The DNA Double Helix

7e. The DNA Double Helix

8. RNA Structure

Similarities and differences between DNA and RNA structure:

• RNA structure is very similar to that of DNA.• It is a polymer of ribonucleotides (the sugar is ribose

rather than deoxyribose).• Three of its bases are the same (A, G, and C) while it

contains U rather than T.• Functional RNA in a cell is single-stranded, but internal

base pairing can produce secondary structure in the molecule.

• Some viruses use either dsRNA or ssRNA for their genomes. Double-stranded RNA is structurally very similar to dsDNA.

9. The Organization of DNA in Chromosomes

• Cellular DNA is organized into chromosomes. • A genome is the chromosome or set of

chromosomes that contains all the DNA of an organism.

• In prokaryotes the genome is usually a single circular chromosome.

• In eukaryotes, the genome is one complete

haploid set of nuclear chromosomes. Mitochondrial and sometimes chloroplast DNA are also present.

10a. Prokaryotic Chromosomes

10b. Prokaryotic Chromosomes

DNA Supercoiling

10c. Prokaryotic Chromosomes

11. Eukaryotic Chromosomes

12a. The Structure of Chromatin

12b. The Structure of Chromatin

12c. The Structure of Chromatin

13. Euchromatin and Heterochromatin

• The cell cycle affects DNA packing, with DNA condensing for mitosis and meiosis and then decondensing during interphase, being most dispersed at S phase.

• Staining of chromatin reveals two forms:– Euchromatin condenses and decondenses with the

cell cycle. Euchromatin accounts for most of the genome in active cells.

– Heterochromatin remains condensed throughout the cell cycle. There are two types based on activity:

• Constitutive heterochromatin• Facultative heterochromatin varies between cell

types or developmental stages

14. Unique- and Repetitive-Sequence DNA

• Sequences vary widely in how often they occur within a genome. The categories are:– Unique-sequence DNA, present in one or a few copies.– Moderately repetitive DNA, present in a few to 105

copies.– Highly repetitive DNA, present in about 105–107 copies.

• Prokaryotes - mostly unique-sequence DNA• Eukaryotes have a mix of unique and repetitive sequences.

– Unique-sequence DNA includes most of the genes that encode proteins.

– Human DNA contains about 65% unique sequences.– Repetitive-sequence DNA includes the moderately and

highly repeated sequences. They may be dispersed throughout the genome or clustered in tandem repeats.