Cell Growth Why are cells so small? DNA “Overload” Exchanging Materials

DNA Overload DNA “Overload” When cells are small the DNA of the cell can meet all of the needs of the cell. Analogy: Small library in a big town cannot meet the needs of all the people.

Exchange of Materials Exchanging Materials Food, oxygen, water, and wastes must move across the cell membrane. The rate of movement depends on the surface area of the cell. The rate of use depends on the volume of the cell. As cells grow the volume grows much faster than the surface area. The surface area to volume ratio decreases as the cell grows. Analogy: A single highway in a growing city becomes congested.

Cell Size Surface Area (length x width x 6) Volume (length x width x height) Ratio of Surface Area to Volume Ratio of Surface Area to Volume in Cells Go to Section:

Cell Division Cell division overcomes the problems of DNA overload and exchange of materials. Two new “Daughter Cells” are formed. During cell division the following happens: 1. Cells replicate the DNA so each daughter cell has its own identical copy of genetic instructions overcoming the DNA “overload” problem. 2. The cell divides, creating two smaller cells, increasing the surface to volume ratio to overcome material exchange problems.

Chromosomes Chromosomes are made of DNA and carry the genetic instructions for the cell. Every organism has a specific number of chromosomes (fruit flies=8, humans=46). Chromosomes are only visible during cell division when they condense. Prior to cell division, each chromosome is replicated and the identical copies (sister chromatids) are joined together by a centromere.

DNA double helix DNA and histones Chromatin Supercoiled DNA

Anatomy of a chromosome

DNA Replication When the cell divides, the DNA must be copied. Each strand of the original DNA serves as a template for the new strands of DNA. Base pairing allows for each strand of DNA to make the other complimentary half. A series of enzymes are used to replicate the DNA The DNA molecule separates into two strands. An enzyme called DNA helicase “unzips” and unwinds the DNA by breaking the H-bonds between the bases. Two new complementary strands are formed following the rules of base pairing. DNA polymerase attaches nucleotides. A—T and C—G. As it goes, DNA polymerase “proofreads” its work to make sure it doesn’t make mistakes.

The Cell Cycle The cell cycle consists of two main parts: 1. Interphase: this is the period between cell divisions. (the majority of the cells life is spent in interphase) 2. Cell Division: this is when the cell actually divides. Interphase is divided into three parts: 1. G 1 Phase: During this phase cells grow and make new proteins and organelles. 2. S Phase: Chromosomes replicate during this period. If S Phase starts the cell will finish cell division. 3. G 2 Phase: Usually the shortest phase, in which the cell produces everything necessary for cell division prior to cell division (M Phase).

Cell Division Cell division is divided into two main parts. 1. Cytokinesis: Division of the cytoplasm. 2. Mitosis: division of the nucleus.

M phase G 2 phase S phase G 1 phase Figure 10–4 The Cell Cycle Go to Section:

Mitosis Mitosis is divided into four phases: 1. Prophase 2. Metaphase 3. Anaphase 4. Telophase Mitosis is followed by cytokinesis which is the dividing of the cell into two daughter cells.

Centrioles Chromatin Interphase Nuclear envelope Cytokinesis Nuclear envelope reforming Telophase Anaphase Individual chromosomes Metaphase Centriole Spindle Centriole Chromosomes (paired chromatids) Prophase Centromere Spindle forming Figure 10–5 Mitosis and Cytokinesis Go to Section: Interphase is when the cell grows and replicates the DNA.

Centrioles Chromatin Interphase Nuclear envelope Cytokinesis Nuclear envelope reforming Telophase Anaphase Individual chromosomes Metaphase Centriole Spindle Centriole Chromosomes (paired chromatids) Prophase Centromere Spindle forming Go to Section: Prophase: 1.The nuclear envelope disintegrates. 2.Chromatin condense into chromosomes. 3.Centrioles separate. 4.A spindle begins to form.

Centrioles Chromatin Interphase Nuclear envelope Cytokinesis Nuclear envelope reforming Telophase Anaphase Individual chromosomes Metaphase Centriole Spindle Centriole Chromosomes (paired chromatids) Prophase Centromere Spindle forming Go to Section: Metaphase 1.Chromosomes line up across the center of the cell. 2.Each chromosome is connected to a spindle fiber at the centromere.

Centrioles Chromatin Interphase Nuclear envelope Cytokinesis Nuclear envelope reforming Telophase Anaphase Individual chromosomes Metaphase Centriole Spindle Centriole Chromosomes (paired chromatids) Prophase Centromere Spindle forming Go to Section: Anaphase 1.The sister chromatids separate into individual chromosomes and move apart.

Centrioles Chromatin Interphase Nuclear envelope Cytokinesis Nuclear envelope reforming Telophase Anaphase Individual chromosomes Metaphase Centriole Spindle Centriole Chromosomes (paired chromatids) Prophase Centromere Spindle forming Go to Section: Telophase 1.The chromosomes gather at opposite ends of the cell. 2.The chromosomes lose their distinct shape. 3.Two new nuclear envelopes start to form.

Centrioles Chromatin Interphase Nuclear envelope Cytokinesis Nuclear envelope reforming Telophase Anaphase Individual chromosomes Metaphase Centriole Spindle Centriole Chromosomes (paired chromatids) Prophase Centromere Spindle forming Go to Section: Cytokinesis 1.The cytoplasm pinches in half. 2.Each daughter cell has an identical set of duplicate chromosomes. 3.In plant cells a cell plate forms between the two cells.