Cell Division

Ch 10

Sections 10-2 & 10-3

10-2 Cell Division

In Eukaryotes (Cells with a nucleus) cellular division occurs in 2 stages:

1. Mitosis= 1st stage where the cell’s nuclues divides

2. Cytokinesis= 2nd stage where the cytoplasm divides

Chromosomes

Chromosomes: Found in eukaryotic cells Pass genetic information from one generation

to the next Made up of DNA The cells of every organism have a specific #

of chromosomes EX: Fruit flies= 8 chromosomes, Humans= 46 chromosomes, carrot cells= 18 chromosomes

Each chromosome is composed of two chromatids

Chromatids

Chromatids: 2 identical strands that make up 1 chromosome During cell division, sister chromatids separate

from each other One chromatid then goes to each of the

2 new cells

The Cell Cycle

The Cell cycle= A series of events that cells go through as the grow and divide Cells grow Prepare for division Divide to form 2 daughter cells Each daughter cell then begins the cycle

again

Events of the Cell Cycle

Events of the cell cycle can be divided into two major stages:1. Interphase= Can be quite long. Divided into 3

parts G1= Cells grow and increase their size S= Chromosomes replicate, DNA is synthesized G2= Produces organelles required for cell

division

2. Cell Division= AKA Mitosis Takes place quickly

Mitosis

Mitosis= Process of cellular division. Divided into 4 phases:

1. Prophase

2. Metaphase

3. Anaphase

4. Telophase

Prophase

Prophase= 1st and longest phase of mitosis (50-60%) of total time required to complete mitosis Chromosomes become visible Centrioles (2 tiny structures located in the

cytoplasm) separate and take opposite sides of nucleus

Condensed chromosomes attach to the spindle Chromosomes coil more tightly Nuclear envelope breaks down & nucleolus

disappears

Metaphase

Metaphase= 2nd phase of mitosis. Lasts only a few minutes Chromosomes line up in center of the cell Microtubules connect centromere of each

chromosome to the 2 poles of the spindle

Anaphase

Anaphase= 3rd phase of mitosis Sister chromatids split & separate becoming

individual chromosomes Chromosomes move into two groups near

poles of the spindle Anaphase ends when chromosomes stop

moving

Telophase

Telophase= 4th and final phase of mitosis Chromosomes begin to disappear into tangle

of dense material Nuclear envelope reforms around each cluster

of chromosomes Spindles break apart Nucleolus becomes visible in each daughter

nucleus Mitosis is complete**CELL DIVISION HOWEVER IS NOT

COMPLETE**

Cytokinesis

Cytokinesis= Division of the cytoplasm itself and is the final phase of cellular division Usually occurs during the same time as

telophase In animal cells= cytoplasm is drawn inward

and pinched into two equal parts In plant cells= Cell plate forms causing a cell

wall to form between the 2 divided nuclei

10-3 Regulating the Cell Cycle

Controls on Cell Division: Cell growth can be turned on and off depending on the situation EX: if cells growing on a Petri dish are

removed, the cells surrounding that now empty space will start dividing, filling in the area where the cells were removed

Cell Cycle Regulators

Cell Cycle Regulators= Help maintain homeostasis (relatively stable environment) in the body.

Cyclins= A family of closely related proteins that regulate the timing of the cell cycle in eukaryotic cells

Internal Regulators= Proteins that respond to events inside cell Allow the cell cycle to proceed only when certain

processes have happened inside the cell. EX: Several proteins make sure cell doesn’t start

mitosis until all chromosomes have been replicated.

Cell Cycle Regulators Con’t…

External Regulators= Proteins that respond to events outside the cell. Direct cells to speed up or slow down the cell

cycle. EX: Growth regulators are important during

embryonic development & wound healing EX: molecules on neighboring cells can cause

cell cycles to speed up or slow down keeping tissues of the body from disrupting each other.

Uncontrolled Cell Growth

The consequences of uncontrolled cell growth in multicellular organisms can be very severe. Cancer cells DO NOT respond to the signals

that regulate the growth of most cells They divide uncontrollably forming tumors Some fail to respond to internal regulator proteins Some fail to respond to external regulator

proteins Causes of cancer can be: smoking, radiation

exposure, and even some viral infections