UNIT IV - DNA & CELL DIVISION

1. UNIT IV - DNA & CELL DIVISION Big Campbell – Ch 12, 13, 16 Baby Campbell – Ch 8, 10.1-10.5

2. I. INTRODUCTION TO DNA • Genome • All of an organism’s DNA • Provides working instructions for cell through ______________________ • Must be copied prior to cell division • Chromosome • Single molecule of DNA wrapped in protein. Proteins maintain chromosome structure & control DNA activity • Somatic cells • Gametes • Chromatin • 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 • Watson & Crick • Realized DNA was a helix composed of 2 nucleotide strands • Franklin suggested backbone of DNA was composed of alternating sugar-phosphate molecules • Watson & Crick determined interior of DNA was made up of paired N-bases • 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 • Pyrimidines • Purines

6. II. A CLOSER LOOK AT DNA, cont • Structure of DNA • Double helix • Strand of nucleotides held together by covalent bonds • Nitrogen bases held together with hydrogen bonds • 2 nucleotide strands are antiparallel • Each strand has a 3’ end (terminus) and a 5’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 semiconservative model of replication • Meselson-Stahl Experiment

9. III. DNA REPLICATION, cont. • Chromatids • Two identical DNA molecules • Result of replication • Term is only used when identical DNAs are physically attached; described as onechromosome made up of two sister chromatids • Centromere – Site where sister chromatids are most closely attached

10. III. DNA REPLICATION, cont. • The Steps of Replication: • DNA helicase unwinds the DNA double helix • Replication begins at specific points on the DNA molecule known as origins of replication. The Y-shaped region where new strands of DNA are elongating are called replication forks

11. III. DNA REPLICATION, cont. • As DNA is “unzipped”, single-strand binding proteins hold the DNA open • A topoisomerase relieves tension creating by unwinding of DNA by making cuts, untwisting, & rejoining the nucleotide strand. • DNA polymerase can only add nucleotides to an already-existing strand so an RNA primer is synthesized to get replication going

12. III. DNA REPLICATION, cont. • DNA polymerases add complementary nucleotides to each side of the DNA molecule. DNA polymerase can only add nucleotides to the 3’ end of the growing strand, so the daughter DNA is synthesized 5’ – 3’; therefore, only one side of the DNA (3’ – 5’) molecule can be replicated as a continuous strand. Known as the leading strand.

13. III. DNA REPLICATION, cont. • Synthesis of lagging strand • 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 Okazaki fragments. • DNA ligase binds fragments together to form a continuous strand of nucleotides. • Proofreading & Repair • DNA Polymerase proofreads nucleotides as they are added

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

15. III. DNA REPLICATION, cont. • Telomeres • 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

16. IV. ASEXUAL REPRODUCTION • Cell Theory • Virchow • Cell Cycle • Single-celled • Organisms • Multicellular • Organisms

17. V. PROKARYOTIC CELL DIVISION • Known as binary fission • Asexual reproduction • Much shorter than euk cell cycle • Single chromosome replicates • Each copy begins moving to opposite ends of cell • Cell elongates • When bacterium is 2X original size, cell membrane pinches inward • Cell wall deposited • 2 identical cells produces

18. VI. EUKARYOTIC CELL CYCLE • Can be divided into:

19. VI. EUKARYOTIC CELL CYCLE, cont • Interphase • Portion of cell cycle in which cell is carrying out normal activities. • Approx 90% of normal cell cycle is spent in interphase. • DNA found in chromatin form • 3 sub-phases • G1 – Period of time following cell division in which cell is growing to normal size. Protein production, metabolism high. • S – DNA replication. Known as “point of no return”. Chromosome now consists of 2 sister chromatids. • G2 – Preparation for division; replication of centrioles in animal cells

20. VI. EUKARYOTIC CELL CYCLE, cont • Mitosis • Nuclear division • Requires all the cells energy, resources • Last step is cytokinesis – splitting of the cell

21. VI. EUKARYOTIC CELL CYCLE, cont

22. VI. EUKARYOTIC CELL CYCLE, cont

23. VI. EUKARYOTIC CELL CYCLE, cont • Cytokinesis in Animal & Plant Cells

24. VI. EUKARYOTIC CELL CYCLE, cont

25. VII. CONTROL OF THE CELL CYCLE, cont • Internal Signals • Three major checkpoints in cell cycle • Regulated by enzymes known as cyclin-dependent kinases or Cdks; activated when bound to proteins known as cyclins • Cdk concentration fairly constant; cyclin concentration is critical factor • Most important checkpoint is the G1 checkpoint. If there is no signal, cell exits cell cycle and switches to G0

26. VII. CONTROL OF THE CELL CYCLE • External Signals • Growth Factors • Proteins released by certain cells that stimulates other cells to divide. • Cells stop dividing when growth factor is depleted. • Examples include erythropoetin, interleukin • Density-dependent Inhibition • Results from crowded conditions • When one cell touches another, cell division stops • Anchorage Dependence • Most cells must be in contact with solid surface to divide

27. VII. CONTROL OF THE CELL CYCLE, cont • Cell Cycle Out of Control = CANCER • Cancer cells do not respond to normal cell cycle controls • Apoptosis – Programmed cell death • Uncontrolled growth • Deprive normal cells of nutrients • Named after type of cells affected • Carcinoma – originate in linings & coverings; for example, skin or lining of digestive tract • Sarcoma – originate in support tissues; for example, bone and muscle • Lymphoma/Leukemia – originate in blood-forming tissues; for example, bone marrow, spleen, lymph nodes

28. VII. CONTROL OF THE CELL CYCLE, cont • Tumor – Mass of abnormal cells • Benign – Mass remains at original site • Malignant – Mass spreads to other parts of the body • Metastasis – Separation of cancer cells from tumor; travel through circulatory system

29. VIII. MEIOSIS • Somatic Cells • Body cells • Human somatic cells contain 46 chromosomes, 23 from mom, 23 from dad • 2n or diploid • Matched pairs of chromosomes called homologous pairs. Each chromosome making up a homologous pair is known as a homologue. Both carry genes for same traits. The location of a gene on a chromosome is known as a locus. • Autosomes – Human somatic cells contain 44 autosomes • Sex chromosomes – 2 per somatic cell • XX = • XY =

30. VIII. MEIOSIS, cont • Gametes • Egg and sperm cells • Haploid or n • Contain 23 chromosomes • In fertilization, haploid (n) sperm fuses with haploid (n) egg → diploid (2n) zygote • Meiosis • Occurs in ovaries, testes • DNA replicated once, cell divides twice • Produces 4 cells with ½ the original chromosome number

31. VIII. MEIOSIS, cont

32. VIII. MEIOSIS, cont

33. VIII. MEIOSIS, cont • Nondisjunction – Failure of chromosomes to separate properly in meiosis

34. IX. MEIOSIS vs MITOSIS

35. X. GENETIC VARIATION

36. X. GENETIC VARIATION, cont • Crossing Over • Further increases genetic variability • Occurs during prophase I when tetrads are forming • Piece of one sister chromatid breaks off & exchanges places with piece of sister chromatid of homologue • Known as chiasma • Occurs very frequently