Cell Reproduction

I. Cell Growth Why do cells divide rather than grow into 1 giant cell? A.DNA “Overload” 1. A cell’s nucleus contains DNA which has all the information needed for a cell to function

2. As a cell increases in size, it doesn’t make copies of the cell. 3. There is not enough DNA to support a large cell.

B. Exchanging materials such as oxygen and water through diffusion becomes harder as the cell gets larger.

C. Surface area- to- volume ratio: 1. As a cell’s size increases, it’s volume increases much faster than it’s surface area

2. If a cell became too big, it’s surface area would be too small to allow certain things to enter and leave the cell.

D. Before it becomes too large, a cell will divide to form 2 “daughter” cells in a process called cell division. -Cell division is for growth, repair, and maintenance.

II. The Cell Cycle The cell cycle: sequence of growth and division of a cell, consisting of 4 main phases: A. Gap 1 (or G1) phase: cell grows, new proteins are made

B. Synthesis (or S) phase: DNA is copied C. Gap 2 (or G2) phase: cell gets ready for M phase D. M phase: when mitosis, or division of the cell nucleus, occurs

G1, S, and G2 are also called INTERPHAS E cell cycle

III. Mitosis Mitosis- divided into 4 phases A. Prophase 1. Stringy chromatin coil into visible chromosomes

2. Nuclear membrane and nucleolus disappear 3. Centrioles move to opposite ends and a spindle fibers forms between them

B. Metaphase 1. Chromosomes attach to the spindle fibers by their centromeres

2. The spindle fibers push and pull the chromosomes to the middle, or equator, of the cell

C. Anaphase 1. Centromeres split 2. Chromosomes split apart and move to opposite ends of the cell

D. Telophase 1.Chromosomes unwind 2. Nuclear membrane forms around each new nucleus

3. Nucleolus reappears 4. Plasma membrane begins to separate 2 new nuclei

E. Cytokinesis: division of the cytoplasm 1. In animal cells, the plasma membrane pinches inward along the equator (cleavage furrow) until the cell is pinched into 2 new cells.

2. In plant cells, a cell plate forms at the equator, forming 2 new cells.

F. Results of Mitosis: 1.Production of 2 new cells with chromosomes identical to the original parent cell. G. Enzymes control the cell cycle. Failure to respond to the signals that regulate growth can result in cancer, or the uncontrolled dividing of cells. Mitosis

IV. Meiosis *Mitosis produces 2 exact copies of “body” cells A.In meiosis, sex cells are produced B.Body cells are diploid which means they have 2 of each chromosome, or a pair of each chromosomes. {1 from mom, 1 from dad}

The number of chromosomes in a cell depends on what kind of organism it is. - Ex. Humans have 46 chromosomes, or 23 pairs.

1. Homologous chromosomes: chromosomes containing same information –One from mother & 1 from father; represented as 2n

2. Haploid cells contain only 1 chromosome of a homologous pair. They are called sex cells or gametes (sperm and egg). a. Represented as n. Humans: n= 23 Fruit flies: n= 4

*Gametes (haploid sex cells) fuse during sexual reproduction to form a diploid zygote. *Meiosis is the process of forming haploid cells from diploid cells. It has 2 main parts:

C. Meiosis I: 1. Chromosomes have replicated during interphase 2. Matching (or homologous chromosomes) move together, forming a tetrad. One chromosome is from mom and one is from dad. A tetrad is composed of 2 chromosomes each made up by 2 chromatids so has a total of 4 chromatids.

3. Nucleoli and nuclear membrane disappears. 4. Spindle fibers forms. 5. Tetrads line up at equator. 6. The homologous chromosomes separate and move to opposite ends.

7. Cell divides into 2 daughter cells- each still contains a double-stranded chromosome. 8. Divided into: Prophase I Metaphase I Anaphase I Telophase I

D. Meiosis II 1. Chromosomes line up at equator. 2. Centromere splits, and sister chromatids separate.

3. Each cell separates into 2 daughter cells, so now there are 4 cells, each with a single- stranded chromosome. 4. Divided into Pro II, Meta II, Ana II, Telo II Meiosis

E. During spermatogenesis, 4 haploid sperm are produced. During oogenesis, 1 haploid egg is produced and 3 polar bodies are produced. F. Meiosis is sometimes called reduction division and is important because it provides for genetic variation.