CELL DIVISION

Cells divide. This makes cells small. Why do cells divide? Why must cells be small?

PURPOSES OF CELL DIVISION 1. Growth- increase in size of the organism (occurs by dividing cells, not by increasing the size of the individual cells) 2. Repair – needed because of worn out or injured cells (your skin cells are replaced every 28 days; your stomach every 7) 3. Reproduction (2 types) Asexual – one parent. Offspring identical to parent - mitosis Sexual – combination of genetic material from two parents - meiosis

CELLS ARE SMALL “Bigger is Not Better” Surface area to volume ratio must remain high for the cell Cells need to stay small so they can effectively transport materials: To take in sufficient nutrients and oxygen to maintain life To remove waste To move molecules across the cell efficiently This is the reason why cells do not grow bigger even if the organism they are part of does

High Surface Area to Volume Ratio????? The size of the nucleus with the DNA is not increasing but cell size is increasing. DNA has a difficult time regulating and meeting the needs of the larger cell. Surface area = 6 sides x length x width 6 x 1 x 1 = 6 Volume = L x W x H = 1 Ratio SA:V = 6:1 --------> = 6 Surface area = 6 sides x length x width = 6 x 4 x 4 = 96 Volume = L x W x H = 64 Ratio SA:V = 96:64= 3:2 -------> = 1.5 6 is higher than 1.5!

Reproduction Asexual Reproduction – passes on chromosomes through mitosis to make a clone (like binary fission in bacteria) Sexual reproduction – chromosomes are separated in meiosis (more later…) and then combine to make a new organism (like egg & sperm fusing to make a zygote…more later…)

Vocabulary Chromosome – structure found in the nucleus of eukaryotic cells that contains the genetic material; made of chromatin Chromatin – strands of DNA found in the nucleus; makes up chromosomes when folded and thickened around proteins

Nuclesome – “DNA Packing” Strand of DNA called chromatin is tightly coiled and foiled around a protein called histone DNA and histone form a bead-like structure called a nucleosome. These are packed with one another to form a thick chromosome (only this is visible under a light microscope)

Vocabulary Chromatids—one of the two strands of a chromosome that become visible during mitosis or meiosis Centromere—the region of the chromosome that holds the two sister chromatids together

Vocabulary Mitosis – division of the nucleus. It is an asexual process since cells produced by mitosis are genetically identical to the parent cell Cytokinesis - division of the cytoplasm

Prokaryotic Cell Division Prokaryotes reproduce by binary fission - dividing in half after the DNA is replicated. Eukaryotic cells have to form nuclei so it is more complicated process called mitosis.

The Cell Cycle The cell cycle is a continuous process that occurs in SOMATIC CELLS (body cells ex. skin). It is an ordered set of events of cell growth and division resulting in two daughter cells (….. then the process starts again). 2 main parts: Growth & preparation - the “getting ready stage” Interphase (cell spends 90% of the its life here) G1 S G2 II. Cell division Mitosis – division of the nucleus Cytokinesis – division of the cytoplasm; follows after mitosis

6 Stages of Mitosis

Mitosis Continued

Cytokinesis

Plant cell plate Animal cell cleavage

6 Stages of Mitosis Under a Microscope

I. Growth & Preparation (Must occur before mitosis) STAGES OF CELL CYCLE I. Growth & Preparation (Must occur before mitosis) INTERPHASE- getting ready stage (happens before mitosis; can often see nucleolus and DNA with threadlike chromatin)

INTERPHASE 90% of the time, the cell is in this phase Grows Performs the job that is unique to that type of cell

INTERPHASE a) GROWTH STAGE 1 – G1 Decides whether or not the cell will divide Cell makes structural proteins and enzymes to perform the functions carried out by that particular cell A pancreas cell will produce and secrete insulin Salivary gland will produce and secrete enzymes in the mouth to aid in digestion

INTERPHASE b) S Synthesis (DNA Replication) Each of the chromosomes is copied

INTERPHASE c) GROWTH PHASE 2 – G2 DNA replication is checked by DNA repair enzymes Cell prepares for mitosis Proteins organize themselves to form a series of fibers called the spindles Spindles are involved in chromosome movement during mitosis Spindle fibers composed of microtubules

INTERPHASE IN AN ANIMAL CELL INTERPHASE IN A PLANT CELL

II. Cell Division - MITOSIS Continuous process 4 main parts prophase, metaphase, anaphase, telophase P-MAT (pray more at the ….church)

PROPHASE ANAPHASE METAPHASE TELOPHASE

MITOSIS PROPHASE Condensing (thickening/fattening) of 2 chromatids to form chromosome hinged by a centromere Coil up Become visible Centrioles begin to move to opposite ends of the cell Nuclear envelope begins to dissolve

MITOSIS METAPHASE Spindle fibers align the chromosomes along the middle of the cell nucleus. This line is referred to as the metaphase plate. - Each chromosome is connected to a spindle fiber at its centromere This organization helps to ensure that in the next phase, when the chromosomes are separated, each new nucleus will receive one copy of each chromosome

MITOSIS ANAPHASE Centromere that joins the sister chromatids splits. This allows the sister chromatids to separate and move apart from one another as individual chromosomes Each chromosome is attached to a spindle which moves it toward one pole Results in equal separation and distribution of chromosomes

MITOSIS TELOPHASE Chromosomes arrive at opposite ends of cell and lose their distinct shape New nuclear envelope begins to reform around each cluster of chromosomes of the daughter nuclei Nucleolus becomes visible again The chromosomes uncoil and are no longer visible under the light microscope. The spindle fibers break apart Cytokinesis begins at the end of this stage This phase just reverses many of the processes of prophase

Tissue sample showing cells in multiple phases of mitosis

CYTOKINESIS Process in which the cytoplasm divides and two separate cells form. In animals, it begins with the formation of a cleavage furrow Microfilaments contract during cleavage and assist the division of the cell into two daughter cells “It is like a string being pulled around a cube of jello”

CYTOKINESIS In plant cells, cleavage does not occur New cell wall is formed in the center of the cell by vesicles from the golgi apparatus As the vesicles join, they form a double membrane called the cell plate Forms in middle and moves outward Separates the daughter cells

Cytokinesis

MITOSIS—Summary Animation: http://www.hybridmedicalanimation.com/work/animation/the-stages-of-mitosis/ PROPHASE- P is for Phat (fat), chromosomes condense/fatten and become visible METAPHASE- M is for middle as the chromosomes line up in the middle of cell

ANAPHASE- A is for Apart or Away because the chromatids pull apart and move away from center TELOPHASE- T is for Two new nuclear envelopes forming around the cluster of chromosomes Cleavage furrow Cell plate

Not all cells reproduce… Some leave the cell cycle here and do not undergo cell division Red Blood Cells – which “kick out” their nucleus to make room for the hemoglobin and therefore can’t divide Brain and spinal cord cells – rarely if ever divide

Other cells can’t stop dividing… Uncontrolled cell growth is known as cancer. Cancer is a disease of the cell cycle.

Regulating Cell Cycle 3 major checkpoints: G1, G2, and M phases G1 seems to be the most important checkpoint. Cells will either continue through it and complete the cycle or be changed into a G0 phase, which is a non-dividing phase.

Cell Cycle Regulators Scientific Evidence: In the early 1980’s a group of scientists discovered that cells in mitosis contained a group of proteins that when injected into a non-dividing cell, would stimulate the cell to begin dividing .

Cell Cycle Regulators These scientists also discovered that the amount of these proteins in the cell would increase and decrease with the cell cycle. These proteins were called cyclins . They seemed to regulate the timing and progression timing and progression of eukaryotic cells through the entire cell cycle

Cell Cycle Regulators Protein regulators will give either the “green light” to start cell division, the “yellow light” to delay or the “red light” to stop cell division. There are Two Types of Cell Cycle Regulators: Internal Regulators (the “checkpoints”) These are cyclin proteins that respond to events occurring inside the cell They will only let the cell cycle proceed if certain processes have been completed inside the cell

Examples of Internal Regulators: Regulatory proteins will not let the cell enter mitosis until all DNA has been replicated Regulatory proteins will prevent the cell from entering anaphase until all the chromosomes are attached to the spindle fiber (mitotic spindle) Other regulatory proteins trigger the exit from mitosis and the beginning of the G1 phase of interphase

2. External Regulators these are proteins (growth factors) that respond to events outside the cell They will direct the cell to speed up or slow down the cell cycle Examples of External Regulators: Proteins found on the surface of neighboring cells will cause cells to slow down their growth or stop the cell cycles to prevent excessive growth Are important in wound healing, bone healing and growth of newly formed embryo

Cancer Terms: Benign Tumor- a cluster of cells that stays at the original site; normally not serious and can be successfully removed. Malignant Tumor- invasive tumors that impair the function of the tissue or organ it is within. Metastasis -cancer cells break away from the tumor and travel to form tumors in other locations, usually serious.

Cancer

Cancer Oncogenes: are genes that cause cells to divide leading to cancer Repressor genes: genes that stop mitosis. Usually people that end up with cancer have both an oncogene and a problem in the repressors.