CELLS: THE LIVING UNITS

1. CHAPTER # 3(d) CELLS: THE LIVING UNITS

2. Cell Cycle • Defines changes from formation of the cell until it reproduces • Includes: • Interphase • Cell division (mitotic phase)

3. Interphase • Period from cell formation to cell division • Nuclear material called chromatin • Four subphases: • G1 (gap 1)—vigorous growth and metabolism • G0—gap phase in cells that permanently cease dividing • S (synthetic)—DNA replication • G2 (gap 2)—preparation for division

4. G1 checkpoint (restriction point) S Growth and DNA synthesis G2 Growth and final preparations for division G1 Growth M G2 checkpoint Figure 3.31

5. Centrosomes (each has 2 centrioles) Interphase Plasma membrane Nucleolus Chromatin Nuclear envelope Interphase Figure 3.33

6. DNA Replication • DNA helices begin unwinding from the nucleosomes • Helicase untwists the double helix and exposes complementary chains • The Y-shaped site of replication is the replication fork • Each nucleotide strand serves as a template for building a new complementary strand

7. DNA Replication • DNA polymerase only works in one direction • Continuous leading strand is synthesized • Discontinuous lagging strand is synthesized in segments • DNA ligase splices together short segments of discontinuous strand

8. DNA Replication • End result: two DNA molecules formed from the original • This process is called semiconservative replication

9. Old strand acts as a template for synthesis of new strand DNA polymerase Free nucleotides Chromosome Leading strand Two new strands (leading and lagging) synthesized in opposite directions Lagging strand Old DNA Helicase unwinds the double helix and exposes the bases Replication fork Adenine Thymine Cytosine DNA polymerase Old (template) strand Guanine Figure 3.32

10. DNA Replication PLAY Animation: DNA Replication

11. Cell Division • Mitotic (M) phase of the cell cycle • Essential for body growth and tissue repair • Does not occur in most mature cells of nervous tissue, skeletal muscle, and cardiac muscle

12. Cell Division • Includes two distinct events: • Mitosis—four stages of nuclear division: • Prophase • Metaphase • Anaphase • Telophase • Cytokinesis—division of cytoplasm by cleavage furrow

13. G1 checkpoint (restriction point) S Growth and DNA synthesis G2 Growth and final preparations for division G1 Growth M G2 checkpoint Figure 3.31

14. Cell Division PLAY Animation: Mitosis

15. Prophase • Chromosomes become visible, each with two chromatids joined at a centromere • Centrosomes separate and migrate toward opposite poles • Mitotic spindles and asters form

16. Prophase • Nuclear envelope fragments • Kinetochore microtubules attach to kinetochore of centromeres and draw them toward the equator of the cell • Polar microtubules assist in forcing the poles apart

17. Early mitotic spindle Early Prophase Aster Chromosome consisting of two sister chromatids Centromere Early Prophase Figure 3.33

18. Polar microtubule Spindle pole Fragments of nuclear envelope Late Prophase Kinetochore Kinetochore microtubule Late Prophase Figure 3.33

19. Metaphase • Centromeres of chromosomes are aligned at the equator • This plane midway between the poles is called the metaphase plate

20. Metaphase Spindle Metaphase plate Metaphase Figure 3.33

21. Anaphase • Shortest phase • Centromeres of chromosomes split simultaneously—each chromatid now becomes a chromosome • Chromosomes (V shaped) are pulled toward poles by motor proteins of kinetochores • Polar microtubules continue forcing the poles apart

22. Anaphase Daughter chromosomes Anaphase Figure 3.33

23. Telophase • Begins when chromosome movement stops • The two sets of chromosomes uncoil to form chromatin • New nuclear membrane forms around each chromatin mass • Nucleoli reappear • Spindle disappears

24. Cytokinesis • Begins during late anaphase • Ring of actin microfilaments contracts to form a cleavage furrow • Two daughter cells are pinched apart, each containing a nucleus identical to the original

25. Nuclear envelope forming Nucleolus forming Contractile ring at cleavage furrow Telophase and Cytokinesis Telophase Figure 3.33

26. Control of Cell Division • “Go” signals: • Critical volume of cell when area of membrane is inadequate for exchange • Chemicals (e.g., growth factors, hormones, cyclins, and cyclin-dependent kinases (Cdks))

27. Control of Cell Division • “Stop” signals: • Contact inhibition • Growth-inhibiting factors produced by repressor genes

28. Protein Synthesis • DNA is the master blueprint for protein synthesis • Gene: Segment of DNA with blueprint for one polypeptide • Triplets of nucleotide bases form genetic library • Each triplet specifies coding for an amino acid PLAY Animation: DNA and RNA

29. Nuclear envelope DNA Transcription RNA Processing Pre-mRNA mRNA Nuclear pores Ribosome Translation Polypeptide Figure 3.34

30. Roles of the Three Main Types of RNA • Messenger RNA (mRNA) • Carries instructions for building a polypeptide, from gene in DNA to ribosomes in cytoplasm

31. Roles of the Three Main Types of RNA • Ribosomal RNA (rRNA) • A structural component of ribosomes that, along with tRNA, helps translate message from mRNA

32. Roles of the Three Main Types of RNA • Transfer RNAs (tRNAs) • Bind to amino acids and pair with bases of codons of mRNA at ribosome to begin process of protein synthesis