Why do cells divide? INFORMATION Exchanging Materials Living cells store critical information in DNA. As a cell grows, that information is used to build the molecules needed for cell growth. As size increases, the demands on that information grow as well. If a cell were to grow without limit, an “information crisis” would occur. Exchanging Materials Food, oxygen, and water enter a cell through the cell membrane. Waste products leave in the same way. The rate at which this exchange takes place depends on the surface area of a cell. The rate at which food and oxygen are used up and waste products are produced depends on the cell’s volume.

About Cell Division About 2 trillion cells- about 25 million cells per second- are produced by an adult human body every day. All cells come from the division of preexisting cells. Cell division or reproduction is the process by which cells produce offspring cells. Cell division differs in prokaryotes and eukaryotes. Cell reproduction in both prok and euk produce the same result – two cells from one!!

So what happens? Before a cell grows too large, it divides into two new “daughter” cells in a process called cell division. Before cell division, the cell copies all of its DNA. It then divides into two “daughter” cells. Each daughter cell receives a complete set of DNA. Cell division reduces cell volume. It also results in an increased ratio of surface area to volume, for each daughter cell.

How does it relate to reproduction? The production of genetically identical offspring from a single parent is known as asexual reproduction. Offspring produced by sexual reproduction inherit some of their genetic information from each parent.

Asexual Reproduction Asexual reproduction is reproduction that involves a single parent producing an offspring. The offspring produced are, in most cases, genetically identical to the single cell that produced them. Asexual reproduction is a simple, efficient, and effective way for an organism to produce a large number of offspring. Cells reproduce by binary fission

Sexual Reproduction In sexual reproduction, offspring are produced by the fusion of two sex cells – one from each of two parents. These fuse into a single cell before the offspring can grow. The offspring produced inherit some genetic information from both parents. Most animals and plants, and many single-celled organisms, reproduce sexually.

Cell Division Binary Fission, Mitosis & Meiosis

Binary Fission Most cells reproduce through some sort of Cell Division Prokaryotic cells divide through a simple form of division called Binary Fission 3 step process Single “naked” strand splits and forms a duplicate of itself. The two copies move to opposite sides of the cell Cell “pinches” into two new and identical cells called "daughter cells". (Cell wall then forms if applicable)

Cell Division -Eukaryotes Two main types of division: Mitosis- results in new cells with genetic material that is identical to the genetic material of original cell During: growth, development, repair, asexual reproduction (production of offspring from one parent) 2. Meiosis- occurs during the formation of gametes (reproductive cells, 1n), reduces the chromosome number by ½ in new cells, often combine to make 2n cells

Mitosis Eukaryotes divide by a more complicated system called Mitosis This is because: They have a nucleus which must be broken up and then reformed They have their DNA “packaged” in the form of Chromosomes Chromosomes are composed of Chromatin Made of DNA Strands & Proteins Also contain Nucleosomes containing Histones - Proteins the DNA is wrapped around, DNA/Protein complex is Chromatin They usually have more than 1 chromosome (Humans have 23 pairs) They have numerous organelles to equally share

Chromatin / Chromosomes

The Cell Cycle Most of the cell's life is spent doing its regular function. Cells divide along a rough time frame called its Cell Cycle. The Cell cycle consists of the folowing steps: G1 (Gap 1) Phase - Cell performs its normal function (cells which do not divide stay in this stage for their entire life span) S (Synthesis) Phase - Here the cell actively duplicates its DNA in preparation for division G2 (Gap 2) Phase - Amount of cytoplasm (including organelles) increases in preparation for division. Mitosis - Actual division occurs

Interphase Cell Replicates its DNA/Chromosomes in preparation of upcoming division Animal Cell Plant cell

Prophase 1.Chromosomes Shorten and become visible. 2. Centrioles move to opposite sides of the cell 3. Nuclear envelope disappears 4. Spindle Fibers begin to form Animal Cell Plant Cell

Metaphase Chromosomes line up along center of cell called the Metaphase Plate Chromosomes attach to spindle fibers Spindle fibers are now clearly visible Animal Cell Plant Cell

Anaphase Centromeres break up separating chromosome copies Chromosomes are pulled apart to opposite sides of cell Spindle fibers begin to break down Animal Cell Plant Cell

Telophase (Cytokinesis) Nuclear envelope forms around both sets of chromosomes DNA uncoils Spindle fibers completely disappear Cytokinesis happens with most (but not all) cells Cytoplasm & organelles move (mostly equally) to either side of the cell.Cell Membrane “pinches” to form 2 separate cells Animal Cell Plant Cell

Animal Cytokinesis Cytokinesis differs significantly between Animal & Plant cells. With animals, the membranes pinch together to form a Cleavage Furrow, which eventually fuses to form two daughter cells

Plant Cytokinesis With Plants, a cell wall must be formed between the 2 daughter cells. Vesicles containing Cellulose form and fuse between the two daughter cells, eventually forming a complete cell wall.

Overview of Mitosis

Meiosis Similar in many ways to mitosis Several differences Involves 2 cell divisions Results in 4 cells with 1/2 the normal genetic information

Vocabulary Diploid (2N) - Normal amount of genetic material Haploid (N) - 1/2 the genetic material. Meiosis results in the formation of haploid cells. In Humans, these are the Ova (egg) and sperm. Ova are produced in the ovaries in females Process is called oogenesis Sperm are produced in the testes of males. Process is called spermatogenesis

Meiosis Phases Meiosis occurs in 2 phases; Meiosis I, & Meiosis II. Prior to division, amount of DNA doubles

Crossing Over During metaphase 1 homologous chromosomes line-up along the metaphase plate Areas of homologous chromosomes connect at areas called chiasmata

Crossing over contd. Crossing Over of genes occurs now Segments of homologous chromosomes break and reform at similar locations. Results in new genetic combinations of offspring. This is the main advantage of sexual reproduction

Chromosome reduction During anaphase 1, each homologous chromosome is pulled to opposite sides of the cell. Unlike mitosis, THE CENTROMERES DO NOT BREAK.

Meiosis I continued Nuclei may or may not reform following division. Cytokinesis may or may not occur

Meiosis II DNA does not double Chromosomes randomly line-up along metaphase plate like regular mitosis. During anaphase 2, CENTROMERES BREAK and each chromosome is pulled to opposite sides of the cell. Nuclei reform and Cytokinesis usually occurs (although it is often unequal).

Overview of Meiosis

Comparison of Mitosis & Meiosis