Oct 25 Chapter 8 (continued)

Cell cycle

 

Cell cycle is an orderly sequence of events during cell division and cell growth.

·        Consists of two main phases.

- Interphase

- Mitotic phase

Each phase consist of sub-phases.

 

1) Interphase

- Chromosomes are diffused.

- Three sub-phases

i) G1 phase – cell growth, synthesis of cell components

ii) S phase – DNA synthesis

iii) G2 phase – preparation for cell division

 

2) Mitotic phase

- Chromosomes are condensed.

- Four sub-phases.

- Prophase

- Metaphase

- Anaphase

- Telophase and Cytokinesis

 

Can you identify different stages in Fig 8.11A?

 

A) Prophase

i) Coiling of chromatin – condensed chromosomes become visible.

ii) Sister chromatids are joined together at the centromere.

iii) Centrioles start to form mitotic spindles.

iv) Two centrioles move away from each other.

v) Nuclear envelop breaks down.

vi) Mitotic spindles make contact with chromosomes at their centromeres.

vii) With a help of spindle microtubules, chromosomes move towards the center of the cell.

 

B) Metaphase

- Chromosomes align at the center (methaphase plate) of the cells.

- Centrioles are at the two poles.

- One of sister chromatids is attached to the spindle that comes from one pole and the other sister chromatid is attached to the spindle from the other pole.

 

C) Anaphase

- Sister chromatids separate.  They are now called daughter chromosomes.

- The spindle microtubules that are attached to chromosomes shorten, pulling daughter chromosomes towards the two poles.

- The spindle microtubules that are not attached to chromosomes lengthen.

- The cell elongates.

 

D) Telophase and cytokinesis

- Chromosomes start uncoiling.

- Nuclear envelops are formed around the chromosomes.

- The mitotic spindle disappears.

- The cytoplasm divides (cytokinesis).

 

Cytokinesis in animal cells - Cleavage furrow pinches the cell in two.

Cytokinesis in plant cells - Cell plate divides two daughter cells.

 

3) Control of cell cycle -To divide or not to divide

- Decision to divide is influenced by environmental and developmental signals.

- Cells stop dividing when reaches a certain density (density-dependent inhibition). (Fig 8.8, p135)

- Most cells in mature adult do not normally divide.  Their cell cycles are arrested.

- Growth factors (protein signal molecules) can reinitiate the cell cycle.

 

- Cell cycle control system (Fig 8.9, 136; last year’s Nobel Prize in Medicine)

·        Proteins within the cell serve as “timer” to control the cell cycle.

·        Checkpoints determine whether to divide or not to divide.

·        Growth factors give a “go-ahead” signal at the checkpoints.