Cell Growth and Division Limits to Cell Growth

Cell Growth and Division

Limits to Cell Growth On average, the cells of an adult animal are no larger than those of a young animal, there are just more of them.

The larger a cell becomes, the more demands the cell places on its DNA. In addition, the cell has more trouble moving enough nutrients and wastes across the cell membrane.

As a cell increases in size, it usually does not make extra copies of DNA. If a cell were to grow without limit, an “information crisis” would occur. This means that if the cell is too large it can not send messages to all parts of the cell.

Another reason for cell size limitations is product transportation. You may recall that food, oxygen, and water enter the cell through its cell membrane. Waste products leave the cell in the same way.

The rate at which this exchange takes place depends on the surface area of the cell. However, the rate at which food and oxygen are used up and waste products are produced depends on the cell’s volume.

As a cell grows the volume increases much more rapidly than the surface area, causing the ratio of surface area to volume to decrease. If a cell gets too large, it would be more difficult to get sufficient amounts of oxygen and nutrients in and waste products out. These factors limit cell growth.

Division of the Cell Before a cell becomes too larger, a growing cell divides forming two “daughter” cells. The process by which a cell divides into two new daughter cells is called cell division.

Before cell division occurs, the cell replicates, or copies, all of its DNA. This replication of DNA solves the problem of information storage because each daughter cell gets one complete set of genetic information.

Cell cycle is the period from the beginning of one cell division to the beginning of the next cell division AND has two parts: 1. Growth and preparation (interphase) 2. Cell division which has two stages A. mitosis (nuclear division) B. cytokinesis (cytoplasm division)

Interphase occurs between divisions: A. Produces all materials for cell growth B. Prepares for cell division Chromosomes replicate C. Longest part of cell cycle

Prophase chromosomes in nucleus condense proteins begin to assemble into microtubules which form a fibrous structure called the mitotic spindle stretches between opposite poles during late prophase

nuclear envelope and nucleolus break up and disappear part of each chromosome’s centromere attaches to spindle fibers move sister chromatids toward center of cell

Metaphase chromosomes are pulled to center of cell chromosomes line up on imaginary line called metaphase plate

Anaphase centromeres divide spindle fibers pull one set of chromatids toward one pole and other set toward opposite pole

Telophase: reverse of prophase two daughter nuclei form nuclear envelopes form around each set of chromosomes uncoil to form loose mass of chromatin mitotic spindle disassembles

Cytokinesis: cytoplasm division which begins during telophase In animal cells the cell membrane pinches inward at center to form a cleavage furrow which divides the cell in the middle In plant cells membrane-bound fragments accumulate to form cell plate where cell wall forms