Cell Division • All cells are derived from pre-existing cells • New cells are produced for growth and to replace damaged or old cells • Differs in prokaryotes (bacteria) and eukaryotes (protists, fungi, plants, & animals)
Keeping Cells Identical The instructions for making cell parts are encoded in the DNA, so each new cell must get a complete set of the DNA molecules
DNA Replication Original DNA strand • DNA must be copied or replicated before cell division • Each new cell will then have an identical copy of the DNA Two new, identical DNA strands
Mitosis Notes • Cells divide to make more cells…this is called mitosis. • All organisms start as just one cell and continue to divide and make more. Humans have over 50,000,000,000,000 cells.
Mitosis • Mitosis is the division of the nucleus in eukaryotic cells. Mitosis has 6 steps. • In mitosis the DNA is copied so that each cell gets a copy. • The DNA is condensed into a chromosome. Humans have 46 chromosomes.
Chromosomes Notes • Each of the cells in your body have the same 46 chromosomes. You get 23 chromosomes from each parent. • Not all species have the same number of chromosomes. Corn has 20, flies have 10, chimpanzees have 48.
Chromosome Notes • Each chromosomes has a central region called a centromere that is an important attachment point for mitosis.
Chromosome Notes • The cell providing the DNA to be copied is the “parent cell,” the cells that are created are identical “daughter cell.”
Chromosome • A chromosome contains the DNA for the organism. • During mitosis the chromosome divides into two chromatids that are copied during mitosis. • These chromatids are “sister chromatids”
Karyotype • First 22 pairs are called autosomes • Last pair are the sex chromosomes • XX female or XY male
Boy or Girl? The Y Chromosome Decides Y - Chromosome X - Chromosome
Prokaryotic Chromosome • The DNA of prokaryotes (bacteria) is one, circular chromosome attached to the inside of the cell membrane
Cell Division in Prokaryotes Parent cell • Prokaryotes such as bacteria divide into 2 identical cells by the process of binary fission • Single chromosome makes a copy of itself • Cell wall forms between the chromosomes dividing the cell Chromosome doubles Cell splits 2 identical daughter cells
Types of Cell Reproduction • Asexual reproduction involves a single cell dividing to make 2 new, identical daughter cells • Mitosis & binary fission are examples of asexual reproduction • Sexual reproduction involves two cells (egg & sperm) joining to make a new cell (zygote) that is NOT identical to the original cells • Meiosis is an example
The cell cycle • Some cells divide constantly like skin cells and stomach cells. Some cells divide only every 10 years. Actively dividing cells go through the cell cycle. The cell cycle has four parts.
The Cell Cycle • G1- (GAP) Each chromosome has 1 chromatid. Cells spend most of their time here if they are not dividing. • S- (Synthesis) this is when DNA is copied. The chromosome goes from one chromatid to 2 chromatids. • G2- Is the period between S and mitosis. The cell prepares for Mitosis • M- is Mitosis
DNA Copied Cells prepare for Division Cells Mature Daughter Cells Cell Divides into Identical cells
Interphase • During Interphase chromosomes are copied (sister chromosome) but remain in loose structures called chromatin. • All the organelles are copied. • Interphase is not technically part of mitosis because mitosis is division of the nucleus and Interphase involves division of other organelles.
Prophase • Prophase means “the before stage.” Prophase is the start of mitosis. • Chromatin condenses into chromosomes. • The microtubules condense into centrisomes which move to opposite poles of the cell. • The nuclear membrane starts to dissolve. • Spindle fibers begin to form to help line the chromosomes up.
Metaphase • Chromosomes line up on the equator of the cell. Spindle fibers attach to the centromeres of the chromatids. • Everything is now aligned for the rest of division to occur.
Anaphase • Chromatids (or pairs of chromosomes) separate and move to opposite ends of the cell. • The spindle fibers contract and the chromosomes are pulled towards the centrioles.
Telophase • In telophase the cell divides. Two new nuclei begin to form. • The chromosomes are at the poles and the spindle fibers disintegrate. • Chromosomes begin to appear as chromatin (the threads not the rods). • This is the end of mitosis.
Cytokinesis • The organelles get divided evenly into each (daughter cell). Each cell has identical chromosomes in their own nucleus. • Plant and animal cells divide differently. Plants form a new cell wall that cuts the two cells apart. Animal cells form actin fibers around the equator of the cell and this pinches the cell in half. • Cytokinesis isn’t part of mitosis (doesn’t involve the nucleus). After Cytokinesis we go back to G1 and start over.