MITOSIS

The Cell Cycle

Outcomes1. Explain the events of the cell cycle ● Interphase ● Mitosis ● Prophase ● Metaphase ● Anaphase ● Telophase

● Cytokinesis 2. Use a simulation to demonstrate the behaviour of

chromosomes during mitosis 3. Observe and identify stages of the cell cycle from

onion root tip cells, and determine the duration of each stage.

Cell Cycle● Recall: cell divisions after

fertilization - continues to increase in size, maintains grown individual

● Division of body cells (somatic cells)

● Cell cycle: phases, no pauses in between

● There is a maximum cell size ! because of surface area:volume (impact on diffusion)

● Is the reason why large organisms are made of many tiny cells

● Cells divide in order to create more cells, either to increase organism’s size or replace old/damaged cells

Cell Cycle Phases

● Division Phase: components of cytoplasm and nucleus of parent cell divided - gives rise to TWO identical daughter cells - Mitosis: duplication of chromosomes, each daughter cell has same number of chromosomes as parent cell - Cytokinesis: divides cytoplasm and organelles equally

● Interphase: stage between divisions - G1: rapid growth - S: synthesis of chromosomes (duplication)- G2: growth, preparation for division

Interphase● Majority of time spent in interphase

- includes G1, S, G2 ● Cells not actively dividing

- grow and undergo metabolic processes ● Chromosomes uncondensed (chromatin) ● G1: period of rapid growth,

chromosomes unduplicated ● S phase: prepare for division, duplication

of chromosomes ● G2: growth, completes prep for division

Stages of Mitosis● Recall: mitosis is the process in which a parent cell

divides into two identical daughter cells with the same number of chromosomes as the parent.

● 4 stages of Mitosis - Prophase- Metaphase- Anaphase- Telophase

● Cytokinesis: occurs after Telophase- division of cytoplasm

I / P M A T

Prophase● Early Prophase: chromosomes start to

condense- become shorter, thicker and visible

● Centrioles move to opposite poles - site of attachment of protein-based SPINDLE FIBRES- “guide wires” for attachment and movement of chromosomes - form spindle apparatus - most plant cells lacking centrioles

● Centromere: joins the two chromatids, anchors chromosomes to spindle fibres

● Late Prophase: nuclear membrane dissolves

Early Prophase Late Prophase

Metaphase● Chromosomes composed of

sister chromatids move toward centre of cell- line up on equatorial plate “like train tracks”

● Attached to spindle fibres at centromere

● Most visible at this stage- dark, thick filamentous structures

● Nuclear membrane completely dissolved

Anaphase● Centromeres divide ● Sister chromatids move to

opposite poles - now considered chromosomes (unduplicated)

● Same number and type of chromosomes at each pole (if mitosis occurs correctly)

Telophase● Last phase of mitosis ● Chromosomes reach

opposite poles, begin to lengthen (forming chromatin)

● Spindle fibres dissolve ● Nuclear membrane begins

to form around each mass of chromatin

Cytokinesis● Chromosomes already at each

pole, nuclear membrane forming

● Cytoplasm begins to divide ● Animal cells: furrow develops,

pinches off the cell into two parts (TWO daughter cells) - marks end of cell division

● Plant cells: cell plate forms between two chromatin masses - develops into new cell wall

Mitosis and Cytokinesis cell division on contrast microscope

Whitefish Prophase

Whitefish Metaphase

Whitefish Anaphase

Whitefish Telophase

REVIEW

Cell Biological Clock

● Cells have a finite number of divisions they can undergo ● Internal memory for number of divisions already undergone

ex) heart cells: frozen after 10 divisions, underwent another 40 when thawed- always total of 50 divisions no matter how long frozen

● Not all cells have same ability to mitotically divide ● WHY???

- AGE: stop dividing (due to length of telomeres)- SPECIALIZATION: more specialized, reproduce less

● 2 types of cells divide endlessly - spermatagonia- cancerous cells (spend most of life cycle in cell division not interphase)