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??

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.)

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

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

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

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

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

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

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

The Cell Cycle – A schematic overview

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

Schematic drawing of a chromosome

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

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

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

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

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

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

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

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

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

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

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

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)

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

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

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

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

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

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

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

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

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

Cell cycle - Regulation

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)

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

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

Molecular control of the cell cycle - MPF

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

For Wednesday: Ch. 13 Meiosis Sexual Life Cycles