Chapter 10: Cell Growth and Division

Why do cells divide – why can’t they just grow bigger? Surface area represents ability to take in/get rid of materials. Volume represents needs of the cell – food, oxygen, water, etc.

Larger cube 2 mm tall Surface area = 24 mm2 Volume = 8 mm3 Surface area to volume ratio = 3:1 Small cube 1 mm tall Surface area = 6 mm2 Volume = 1 mm3 Surface area to volume ratio = 6:1

As a cell grows, the volume of the inside grows faster than the cell membrane around the outside What happens when a cell gets too big? It could starve – can’t take in enough food to support needs It could poison itself – can’t get rid of waste fast enough So cells stay small for exchange of materials

Rather than grow bigger, cells divide and increase in number Cell division occurs when: Body growth Maintenance and repair Fighting infection Replacing worn/dead cells

Chromosome Structure Chromatin – made of DNA and protein Seen in a non-dividing cell Chromosomes – Chromatin that has coiled tightly Seen in a dividing cell Histones – proteins that help to coil the DNA and maintain the shape of chromosomes

After replication, there are 2 identical sister chromatids, held together by a centromere

Eukaryotic Cell Cycle A repeating sequence of cellular growth and division during the life of a cell. http://www.cellsalive.com/cell_cycle.htm

Most of the cell cycle is spent in interphase (time between cell divisions): G1 stage – cell growth and activity S stage – DNA replication occurs (S stands for synthesis of DNA) G2 stage – continues to grow and cell prepares for cell division

Amount of time spent in interphase varies – average for adult mammals is 20 hours Nerve cells and muscle cells exit the cell cycle  G0 phase Body never makes more of them

Following interphase is the M stage, including mitosis and cytokinesis 2 daughter cells are produced that are identical to the parent Occurs in body cells Diploid cells produce other diploid cells Takes about 4 hours http://www.cellsalive.com/mitosis.htm Animation of mitosis

Mitosis overview

Centriole Chromatin Nucleolus (in yellow) Nuclear membrane Spindle fibers Chromosome (replicated) Centromere Sister Chromatids (each half of replicated chromosomes) Daughter Chromosomes (once the replicated chromosome splits) Cell membrane Cell membrane pinches in

Early Prophase Chromatin coils to become chromosomes Centrioles move to opposite ends of the cell Nuclear membrane and nucleolus disappear

Late Prophase Spindle fibers form Chromosomes become attached to spindle fibers – centromere attaches to spindle fibers

Metaphase Chromosomes line up at equator (middle)

Anaphase Sister chromatids separate, becoming daughter chromosomes, and move toward opposite ends of cell

Telophase Spindle fibers disappear Nuclear membrane and nucleolus reappear Chromosomes uncoil and turn back into chromatin

Cytokinesis, or division of cytoplasm, accompanies mitosis. Division of the cytoplasm begins in anaphase, but is not completed until end of telophase. In animal cell, cell membrane is flexible and pinches in to divide cell. In plant cells, rigid cell wall does not bend, so a cell plate forms to divide the cell.

Mitosis in Plant Cells Same phases as in animal cells No centrioles Cell plate forms instead of cell membrane pinching in

Cytokinesis in animal cells

Cytokinesis in plant cells

Cell Division in Prokaryotes The process of asexual reproduction in prokaryotes is called binary fission. The two daughter cells are identical to the original parent cell, each with a single chromosome. Following DNA replication, the two resulting chromosomes separate as the cell elongates. Cell divides without cell structures seen in plants & animals

Animation of binary fission