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 (by dividing cells, not by increasing the size of the 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 Surface area to volume ratio must remain high for the cell: To take in sufficient nutrients and oxygen to maintain life To remove waste To move molecules across the cell efficiently When cells in an organism divide, growth and repair result.

High Surface Area to Volume Ratio????? Surface area = 6 x height x width = 6 Volume = l x w x h = 1 Ratio = 6:1 = 6 Surface area = 6 x height x width = 150 Volume = l x w x h = 125 Ratio = 150:125 or is higher than 1.2!

Asexual reproduction – passes on chromosomes through mitosis to make a clone (like binary fission in bacteria…more later…) 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 condensed around proteins

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 – Process of nuclear division; karyokinesis Cytokinesis - Process of division of the cytoplasm

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, which then start the process again. 2 main parts: I.Growth & preparation Interphase (90% of the cell’s life) –G 1 –S –G 2 II. Cell division Mitosis – division of the nucleus Cytokinesis – division of the cytoplasm; usually follows mitosis, but sometimes doesn’t occur –What would this result in?

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

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

INTERPHASE GROWTH 1 STAGE – G 1 –Decides whether or not the cell will divide –Makes its structural proteins and enzymes to perform its functions A pancreas cell will produce and secrete insulin Salivary gland will produce and secrete enzymes in the mouth to aid in digestion –Each chromosome is a single molecule of DNA and associated proteins

INTERPHASE S Synthesis (DNA Replication) –Each of the chromosomes is copied (in humans this makes 92 chromatids held by 46 centromeres)

INTERPHASE GROWTH 2 PHASE – G 2 –DNA replication is checked by DNA repair enzymes –Cell prepares for mitosis –Proteins organize themselves to form a series of fibers called the spindles 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

PROPHASE METAPHASE ANAPHASE TELOPHASE

MITOSIS PROPHASE –Condensing of 2 chromatids to form chromosome hinged by a centromere Coil up Become visible –Centrioles begin to migrate to opposite sides of the cell –Nuclear envelope dis-assembles

MITOSIS METAPHASE –Spindle fibers align the chromosomes along the middle of the cell nucleus. This line is referred to as the metaphase plate. –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 –Chromatids move apart from one another –Each chromosome is attached to a spindle which moves it toward one pole –Results in equal separation and distribution of chromosomes

MITOSIS TELOPHASE –Chromatids arrive at opposite poles of cell –New membranes form around the daughter nuclei. –The chromosomes disperse (uncoil) and are no longer visible under the light microscope. –The spindle fibers continue to dis-assemble –Cytokinesis may also begin during this stage. –This phase 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 (actin fibers) contract during cleavage and assist the division of the cell into two daughter cells –Think of a string being pulled tight around a cube of jello (gelatin)

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

Animal cell cleavage Plant cell plate

MITOSIS—Summary Animation: of-mitosis/ of-mitosis/ PROPHASE- chromosomes evident, nuclear membrane disappearing (P for Phat (fat), chromosomes condense/fatten and become visible) METAPHASE- sister chromatids lined up in the middle/equator (M for middle, chromosomes lined up in the middle of cell)

ANAPHASE- sister chromatids pulled apart (A for Apart or Away because the chromatids pull apart and move away from center) TELOPHASE- chromosomes are at ends of cell, cells prepare to separate (T for Two new nuclear envelopes are forming) 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; called G 0 (pronounced G naught)

Uncontrolled cell growth is known as cancer. Read how this can occur on pp & in your text. Other cells can’t stop dividing…

Websites Cell Mitosis Lab Practice le/activity_description.html le/activity_description.html Mitosis pictures & Video ml#mitosis Cell Cycle Interactive Game ellcycle.html