Section 3.4: The Cell Cycle Hiba Reda, Ali Osman, Aya Nassar, Tasneem Ahmed 2nd Hour

The Cell Cycle The cell cycle is: the sequence of growth and division of a cell (from the time it forms until it divides) A new cell will grow for a period of time and then it divides into two new cells, called daughter cells, which then continue the cycle The phases, or parts, of the cycle include: interphase, mitosis, cytoplasmic division (or cytokinesis), and differentiation Checkpoints: groups of special proteins that interact at certain points of the cell cycle to keep the cycle under control Restriction Checkpoints determine whether or not a cell is fit enough to continue growing and dividing, or if the cell will move into a nondividing stage as a specialized cell (ex: nerve cells), or if the cell will die

The Cell Cycle Cont. Disruption of the cell cycle can affect health: If cell division is infrequent, wounds will not heal If cell division is too frequent, a cancer grows Not all cells normally divide continually: some cells divide 40-60 times (in labs), others divide the maximum number of times (cells that line the small intestine, and others don’t divide at all (nerve cells) The time taken to complete the cell cycle can vary greatly as well. Example: The cell cycle of a fly embryo cell takes just eight minutes while a human liver cell cycle can last longer than a year Cells have a built-in “clock” in the form of chromosome tips called telomeres so that a cell “knows when to stop dividing Telomeres shorten after each cell division and when they shorten to a certain length, the cell no longer divides

Interphase A cell must grow and duplicate most of its components before it divides so that two daughter cells can form and this period of preparedness is called interphase During interphase, a cell acquires nutrients, uses them to produce new living material, and maintains routine “housekeeping” functions Cell duplicates membranes, ribosomes, lysosomes, mitochondria, and most importantly, genetic material. In the end, the two new cells will have a complete set of genetic instruction (DNA) Broken down into 3 phases or parts: G1 Phase: the cell begins to grow to reach the mature size S (synthesis) Phase: DNA begins to replicate itself G2 Phase: cell prepares for cell division, in this case, mitosis

Cell Division A cell can divide in two ways: Meiosis: part of gametogenesis- forms sex cells→ in female=formation of egg cell, in male=formation of sperm cells After the egg is fertilized by the sperm, it should have the normal number of of 46 chromosomes. Therefore, both the egg and the sperm must first halve their normal chromosome number to 23 chromosomes The halving of the chromosomes in meiosis is accomplished by reduction division 2. Mitosis: more common form of cell division which increases cell number and consists of 2 processes First part= Mitosis which is the division of the nucleus Second part= Cytokinesis which is the division of the cytoplasm All cells can divide by mitosis except sex cells and red blood cells

Cell Division Cont. Division of the nucleus is important since each daughter cell should receive identical and precise genetic information or else the cell will not survive DNA replicates during interphase and DNA is divided evenly to each daughter cell in mitosis Mitosis is broken down into phases The four stages of mitosis are: Prophase Metaphase Anaphase Telophase

Prophase Chromosomes condense into tightly-coiled rods and become visible Chromosome has two identical parts called chromatids which are attached at the centromere One centriole is on one side of the cell and the other is at the opposite end of the cell The nuclear envelope and nucleolus disperse and disappear Microtubules begin to grow from centrioles Spindle fibers form between the centrioles to equally divide the chromatids and connect to the centromere

Metaphase The chromosomes line up along the middle (midline) of the cell Spindle fibers from the centrioles attach to the centromeres of each chromosome so that each sister chromatid can go to opposite sides of the cell

Anaphase Centromeres are pulled apart Sister chromatids separate and slowly move (centromere first) towards the opposite sides of the cell After they are separated, they are considered individual chromosomes

Telophase Chromosomes reach opposite ends of the cell Chromosomes unwind into threadlike and less tightly coiled chromatin form Nuclear envelope forms around each newly formed cell and nucleolus forms in each cell Spindle fibers disappear

Cytokinesis It is the division of the cytoplasm that forms two daughter cells Begins during anaphase and the cell membrane will begin to pinch midway between cells=cleavage furrow Cleavage furrow pinches the cell into two cells with the help of microfilaments This is complete at the end of telophase during mitosis Result: two daughter cells with identical genetic material

Cell Differentiation Cell differentiation: a cell that becomes specialized to perform a specific function like a liver cell, a blood cell, or a neuron There are more than 260 general types of cells in the human body Differentiation is why cells vary greatly from other cells like their size and shape Stem Cell: a cell that is able to divide repeatedly without specializing Progenitor Cell: descendants of stem cells that can differentiate to form one or more kinds of cells, but cannot divide and reproduce indefinitely Self-Renewal: stem cell process of giving rise to at least one more stem cell by dividing Occurs through gene expression: specific combinations of genes that are turned on or off in a cell compared to another→ controls how a cell functions

Cell Death A cell that does not divide or specialize can die Apoptosis- a type of cell death that is a normal part of development Occurs after a sunburn and apoptosis peels away the damaged skin cells. If apoptosis does not occur, the damaged cells can become cancerous

Cell Death Continued A.) Cell in the process of apoptosis- Rounds up and swells The nuclear membrane breaks down Chromatin condenses Enzymes cut the chromosomes into many pieces of DNA The cell shatters into membrane enclosed fragments and scavenger cell engulfs and destroys them

Use What You Know A= G1 Phase B= S Phase C= G2 Phase E= Cytokinesis D= Mitosis E= Cytokinesis F= Prophase G= Metaphase H= Anaphase I= Telophase

Use What You Know Centrioles Chromatin Spindle Fibers Interphase Prophase Metaphase Anaphase Telophase Cytokinesis

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