Understanding DNA Structure and Cell Division: Key Concepts and Processes

1. UNIT IV - DNA & CELL DIVISION

2. I. INTRODUCTION TO DNA • ________________ – All of an organism’s DNA; must be copied prior to cell division • __________________– Single molecule of DNA wrapped in protein. Proteins maintain chromosome structure & control DNA activity • Somatic cells - ___________ _________________________ • Gametes - ___________ _________________________ • ________________ – Term used to describe fine strands of uncoiled DNA

3. II. A CLOSER LOOK AT DNA • Discovery of DNA • Early 1900s – Scientists determined genes determined inherited characteristics. Also realized chromosomes were composed of DNA & protein. • Griffith (1928) – Studied 2 strains of bacteria. Determined that pathogenicity could be transferred when living non-pathogens were exposed to remains of dead pathogens. • Avery (1944) – Identified “transforming substance” as DNA • Hershey & Chase (1952) – Used radioactively-viruses that infect bacteria - known as bacteriophages. Virus is made up of DNA & protein – Hershey & Chase proved it was the DNA component that was injected into host cell and used to make new virus particles. • Rosalind Franklin (late 1950s) – Produced x-ray crystallography image of DNA; “borrowed” by Watson & Crick

4. II. A CLOSER LOOK AT DNA, cont • _______________ & _______________ • Realized DNA was a __________ composed of _____ nucleotide strands • Franklin suggested backbone of DNA was composed of alternating ______________________________ molecules • Watson & Crick determined interior of DNA was made up of paired ___________________ • Eventually deduced bases always paired a specific way • Chargaff • Chemically proved the same base-pairing rules that Watson & Crick proved structurally

5. II. A CLOSER LOOK AT DNA, cont • Monomers of DNA • Nucleotides • Composed of _____________,_____________, & _________ • _______________ • Single-ring N-base • Ex: Cytosine & Thymine • _______________ • Double-ring structures • Ex: Adenine & Guanine

6. II. A CLOSER LOOK AT DNA, cont • Structure of DNA • Double helix • Strand of nucleotides held together by ________________ bonds • Nitrogen bases held together with ________________ bonds • Pairs • _____________________ • _____________________ • 2 nucleotide strands are ____________________ • Each strand has a ______ end (terminus) and a _______ end; named for carbon on deoxyribose

7. II. A CLOSER LOOK AT DNA, cont

8. III. DNA REPLICATION • DNA Replication • Prior to cell division, DNA must be replicated • Known as _______________________model of replication • Meselson-Stahl Experiment

9. III. DNA REPLICATION, cont. • ________________- Two identical DNA molecules; copied prior to cell division • ________________ – Site where sister chromatids are most closely attached

10. III. DNA REPLICATION, cont. • Steps Involved in Replication • ___________________________unwinds the DNA double helix • Replication begins at specific points on the DNA molecule known as ____________________________________________. The Y-shaped region where new strands of DNA are elongating are called ________________________________

11. III. DNA REPLICATION, cont. • As DNA is “unzipped”, _______________________ binding proteins hold the DNA open • A______________________________untwists the nucleotide strand. • DNA polymerase can only add nucleotides to an already-existing strand so an ___________________ is synthesized to get replication going

12. III. DNA REPLICATION, cont. • ______________________________ add complementary nucleotides to each side of the DNA molecule. • Can only add nucleotides to the _______ end of the growing strand, so the daughter DNA is synthesized _________________ • So…..only one side of the DNA (3’ – 5’) molecule can be replicated as a continuous strand which is known as the _______________________________.

13. III. DNA REPLICATION, cont. • Synthesis of ____________________ • To synthesize the other new strand of DNA, DNA polymerase must work away from the replication fork. Leads to synthesis of short pieces of DNA known as ________________ fragments. • ____________________binds fragments together to form a continuous strand of nucleotides. • Proofreading & Repair • DNA Polymerase proofread nucleotides as they are added

14. III. DNA REPLICATION, cont.

15. III. DNA REPLICATION, cont. An Overview of Replication

16. III. DNA REPLICATION, cont. • ___________________ • 5’ ends of daughter strands cannot be completed because DNA polymerase can only add nucleotides to the 3’ end • Results in shorter and shorter DNA molecules with jagged ends • To protect genetic integrity, ends of chromosomes do not contain genes – instead there are nucleotide sequences known as telomeres • Contain nucleotide repeat sequences • Telomeres shorten each time cell divides - limits the number of times a cell can divide; thought to protect organism from cancer • Telomerase – Enzyme produced by stem cells, cancer cells that restores telomere length