CELL GROWTH & DIVISION Chapter 10 Biology 392

Smaller is Better The bigger the cell becomes… the more demands the cell places on its DNA. the more trouble moving enough nutrients and wastes across the cell membranes

DNA Overload DNA  contains the information that controls a cell’s functions – Eukaryotes – DNA is found in the nucleus – Prokaryotes – DNA is found in the cytoplasm As cell increases in size, DNA does not have enough information to maintain it and service the needs of the cell

Exchanging Materials Rate at which exchange of wastes, nutrients, gases takes place depends on the surface of the cell  Surface area = length x width x # of sides Rate at which nutrients and gases are used up and wastes are made depends on volume of the cell  Volume = length x width x height

Surface to Volume Ratio  Volume increases much faster than surface area when a cell is growing – Exchange of materials cannot keep up with the demand Smaller cells have a larger surface area to volume ratio – This means there is more surface area for materials to fit into

SA to Volume Ratios Surface Area Volume SA to Vol ratio

Eukaryotic Cell Division In order to stay the same size, cells must divide – Splits in 2 creating “daughter cells” – EXACT copies of each other  CLONES Before dividing, the cell must duplicate everything (DNA & organelles) so that everything is the same

Prokaryotic Cell Division Prokaryotes are simpler (no organelles) – After DNA is copied, cell splits in two – Each new cell contains a copy of the DNA - called binary fission – Example of asexual reproduction

Prokaryotic Cell Division

Cell Division in Eukaryotes Occurs in 2 main stages: Division of cell nucleus Division of cytoplasm

Chromosomes Structure that carry genetic information – Made up of DNA and protein (histones) DNA coils very tightly  degree of coiling leads to whether DNA is visible or invisible Each eukaryote has a characteristic number of chromosomes in the nucleus of each cell. Examples: – Humans- 46 – Dog- 78 – Housefly- 12 – Potato plant- 48 – Cat- 38 – Aspergillus (fungus)- 8 – Drosophila (fly) - 8

What is a chromosome?

Chromosomes While the cell is “resting”, you cannot see the chromosomes – It is unwound  Called chromatin While the cell is dividing, you can see the chromosomes – Wound up  Made of two sister chromatids connected by a centromere

The Cell Cycle Occurs in eukaryotic cells  It is the period of time from the beginning of one cell division to the beginning of the next Cell doubles its contents so that it is ready to divide into two completely independent cells

The Cell Cycle Consists of 4 phases – G1  intense growth and activity; new proteins and organelles are produced – S phase  copying of chromosomes (DNA) – G2  intense growth and activity; new proteins and organelles are produced Shortest of the three phases – M phase – mitosis  the division of the cell nucleus and cytokinesis, division of the cytoplasm

G 2 phase S phase G 1 phase The Cell Cycle

Cell Cycle Rates  Cells do not all move through the cell cycle at the same rate Cells in a developing embryo replicate rapidly- 3 minutes Average time of cell cycle- 20 hours Lining of esophagus- 2-3 days Lining of small intestine- 1-2 days Lining of large intestine- 6 days Red blood cells-120 days White blood cells- 10 hrs-decades

Cell Cycle Rates Once conditions are right, cells will leave G1 and go through S phase, G2, and M phase Some cells stop going through the cell cycle. They are permanently in the G1 stage (normal cell function). This stage is referred to as G0. – Cardiac muscle – Nerve cells (most) – Red blood cells

MITOSIS

M phase Cell division – Process in which the cell divides into two independent cells  daughter cells – This process is called mitosis in eukaryotes and binary fission in prokaryotes

Mitosis Time of cell division Divided into four main phases – Prophase – Metaphase – Anaphase – Telophase

Prophase Longest of the four phases Appearance of chromosomes – Creation of sister chromatids – Attached at the centromere Nucleus undergoes several changes  Nuclear membrane begins to breakdown

Prophase Spindle begins to form  Series of microtubules that span the cell  Purpose is to pull chromatids apart Chromosomes attach to the spindle at the centromere In animals, spindle is created by the centriole In plants, spindle is created between the cell wall  Plants do not have centrioles

Metaphase Shortest phase of mitosis  Chromosomes line up in the middle of the cell Centriole Spindle Centriole

Anaphase Centromeres split  Sister chromatids are pulled to the opposite ends of the cell Individual chromosomes

Telophase  Spindle breaks down  Nuclear membrane forms around the chromosomes again  Chromosomes unwind to chromatin again

Cytokinesis  Process of the cytoplasm dividing In animals  the cell pinches in at the middle to create two new cells In plants  a cell plate is created in the middle of the cell