1. Part II: Genetics – The basis of heredity
FACT: Related individuals resemble each other more than randomly chosen individuals do
Within populations
Among populations within species
Within species among species
Etc.
…WHY??

2. Resemblance among relatives…”heredity”
Information is passed from parent to offspring
Information is passed largely intact
That information encodes the phenotype
Phenotype is any property of an organism that can be attributed to that organism (height, weight, eye color, obnoxiousness, etc.)

3. Outline of today’s lecture – (Ch. 12)
Review of (some) Eukaryotic cell structures
Schematic overview of mitosis
The mitotic spindle
Cytokinesis
Cell cycle control

4. Review of Eukaryotic cell structures
Nucleus
Surrounded by membrane
Contains the genetic material (DNA)
During interphase, DNA uncoiled, complexed with proteins, called “chromatin”
In mitosis, condenses into chromosomes

5. Review of Eukaryotic cell structures
Nucleolus
Point of synthesis of ribosomal RNA
Ribosomal subunits assembled
Nucleolus

6. Review of Eukaryotic cell structures
Centrosomes
Region near nucleus that is the cellular “microtubule organizing center”
Associated with formation of spindle fibers
Form “spindle poles” during mitosis

7. Review of Eukaryotic cell structures
Cytoskeleton
Provides mechanical support to the cell
Dynamic (disassemble and reassemble)
Constructed of microtubules, microfilaments, intermediate filaments

8. Microtubules: Structure and Function
Composed of polymers of dimers of α-tubulin, β-tubulin
Lengthen by adding dimers, shorten by losing dimers
“Molecular motors” can move along microtubules

9. Mitosis and the Cell Cycle
All organisms must grow and reproduce
During organismal growth, all cells must grow and divide
Genetic information must be faithfully passed between parent cell and daughter cells
MITOSIS is the process by which genetic information is passed from parent to daughter cells

10. The Cell Cycle – A schematic overview

11. Some notation Chromosome Centromere Spindle pole (centrosome)
Spindle fiber
Nuclear envelope

12. Schematic drawing of a chromosome

13. G1 (Interphase)
Each chromosome is a single, unreplicated double strand of DNA
One chromosome from each parent (Male, Female) forms a Homologous Pair (= “homologs”)
Chromosomes in nucleus, surrounded by nuclear membrane
Single centrosome F M

14. S-phase (DNA synthesis)
After DNA replication, TWO “sister chromatids” are present for each homolog
Each sister chromatid is the SAME double-stranded DNA molecule
Attached by proteins, tightly at centromere and more loosely throughout their length F F M M

15. G2 (Interphase) Duplication of centrosomes
Cell receives signal to enter mitosis F F M M

16. M1 (Early Prophase) Chromosomes condense
Mitotic spindle forms from centrosome
Centrosomes begin to migrate to poles of cell
Nucleoli disappear F M

17. M2 (Mid-prophase) Chromosomes fully condensed
Centrosomes complete migration to the poles
Nuclear envelope begins to degrade
Spindle fibers enter nuclear area from the pole
Kinetochores form at centromeres F M

18. M3 (Prometaphase) Nuclear envelope completely degraded
Kinetochores form at centromeres
Some spindle fibers attach at kinetochores
Sister chromatids attached to opposite poles F M

19. M4 (Metaphase) Spindle fibers attached to centromere at kinetochore
All sister chromatids attached to opposite poles
Chromosomes migrate to center plane of cell, “metaphase plate” F M

20. M5 (Early Anaphase) Protein bond between sister chromatids degrades
Sister chromatids separate, begin migration toward opposite poles
Poles move farther apart as non-kinetochore spindle fibers lengthen

21. M5 (Late Anaphase)
Chromosomes (no longer “chromatids”) have reached poles

22. M6 (Telophase)
Non-kinetochore spindle fibers continue to elongate cell
Nuclear envelopes begin to form at poles
Chromosomes de-condense back into chromatin
Nucleoli re-form, cytokinesis begins

23. The mitotic spindle: Structure and Function
Made of microtubules, associated proteins
Cytoskeleton partially disassembles to provide materials
Assembly starts in centrosomes
Centrosomes “pushed” away from each other as microtubules grow

24. The mitotic spindle: Structure and Function
Some spindle fibers grow and attach to the kinetochore while sister chromatids are still attached
“Equilibrium” reached when sister chromatids are midway between the poles (metaphase plate)

25. Microtubules: Structure and Function
Composed of polymers of dimers of α-tubulin, β-tubulin
Lengthen by adding dimers, shorten by losing dimers
“Molecular motors” can move along microtubules

26. The mitotic spindle: Structure and Function
Hypothesized mechanism for chromosome movement

27. The mitotic spindle: Structure and Function
Experimental Test of the hypothesized mechanism for chromosome movement
What is an obvious alternative hypothesis?

28. Cytokinesis – Animal cells
Cleavage furrow formed by contraction of a ring of microfilaments
Associated with actin and myosin (motor protein system)
Forms along metaphase plate

29. Cytokinesis – Plant cells
Plants have cell walls
Vesicles (derived from Golgi) move along microtubules to the middle of the cell
Vesicles coalesce to form a “cell plate”; membrane derived from vesicles
Cell plate membrane fuses with outer cell membrane, forms daughter cells

30. Cell Division in Prokaryotes - Binary fission
Prokaryotes preceded Eukaryotes by billions of years
Bacterial chromosome is circular DNA molecule
Origin of replication

31. Cell Division in Prokaryotes - Binary fission
As DNA replication proceeds, one copy of the origin migrates to each end of the cell

32. Cell Division in Prokaryotes - Binary fission
Cell membrane grows inward
New cell wall forms
Two daughter cells result
Migration of origin(s) is similar to migration of centromeres

33. Cell cycle - Regulation
Timing and rate of cell division are critical
Chemical signals regulate “checkpoints” in the cell cycle
RESULTS of certain chemical processes are “checked”
If results not appropriate, cell division does not proceed
Evidence from cell fusion experiments

34. Cell cycle - Regulation

35. Cell cycle - Regulation
G1 checkpoint: Cell will not enter into S phase unless appropriate signal is received
Many cells in adult mammals are in G0 and thus do not divide (e.g., nerve)
If not, enters non-dividing “G0” phase
Variety of external chemical cues to trigger entry into G1 and G2 phase (or not)

36. Cell cycle - Regulation
Additional Regulatory Checkpoints
Cell will not proceed through S-phase unless DNA synthesis completed
M-checkpoint: Cell will not enter anaphase until chromosomes lined up on metaphase plate

37. Cell cycle - Regulation
Cell cycle controlled by two types of proteins, kinases and cyclins
Kinases active when attached to cyclins
E.g., “Maturation-promoting factor” MPF

38. Molecular control of the cell cycle - MPF

39. Cell cycle regulation – internal and external controls
M-checkpoint: if kinetochores not attached to spindle microtubules, sister chromatids remain attached
G1-checkpoint: growth factors / receptors
E.g., platelet-derived gf (PDGF), healing wounds
Density-dependent inhibition (nutrient concentration below a certain level)
Anchorage-dependence

40. For Wednesday: Ch. 13
Meiosis
Sexual Life Cycles