Biology - Mitosis and Cell Reproduction - Quick Guides | Biology

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Study GuideBiology–Mitosis and Cell Reproduction1. The Cell Cycle: How Cells Grow and DivideLiving cells are constantly growing, working, and reproducing. Most cells in the body repeat thispattern over and over in a process called thecell cycle. With a few exceptions—such as red bloodcells, which do not have a nucleus—almost all cells go through this cycle.The cell cycle hastwo main stages:1.Interphase–when the cell grows and carries out its normal functions2.Mitosis–when the cell divides to form two new cells1.1 Interphase: The Cell’s Busy LifeInterphase is the longest part of the cell cycle. During this time, the cell is not resting—it is very active.Interphase is divided intothree phases: G1, S, and G2.1.1.1 G1 Phase: Growth and Normal WorkTheG1 phasehappens right after mitosis. During this phase, the cell grows and produces theproteins and enzymes it needs to do its specific job.For example:•Apancreas cellproduces insulin•Amuscle cellcontracts to allow movement•Asalivary gland cellproduces digestive enzymesAt this stage, each chromosome is made ofone DNA moleculewrapped around histone proteins.•Normal human body cells have46 chromosomes•Sex cells have23 chromosomes•Red blood cells haveno chromosomesbecause they lack a nucleus

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Study Guide1.1.2 S Phase: DNA Is CopiedTheS (synthesis) phaseis when DNA replication occurs. During this phase:•Each chromosome is copied exactly•Every DNA molecule is duplicatedBy the end of the S phase, the cell containstwice the amount of DNA.In human cells, this means there are92 chromosomespresent, although they are still arranged inpairs.1.1.3 G2 Phase: Preparing for DivisionIn theG2 phase, the cell gets ready for mitosis. Special proteins assemble to form thespindle, astructure that helps move chromosomes during cell division.•The spindle is made ofmicrotubules•It is built for each mitosis and taken apart afterward•The amino acids are reused by the cellOnce preparation is complete, the cell enters mitosis.1.2 Mitosis: Dividing the NucleusThe wordmitosiscomes from a Latin term meaning “threads,” because chromosomes look like thinthreads when they become visible under a microscope.During mitosis:•Chromosomes become visible•They line up in the center of the cell•They separate evenly into two new cellsAlthough mitosis is continuous, scientists divide it intofour stagesto make it easier to study:prophase, metaphase, anaphase, and telophase.

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Study Guide1.2.1 Prophase: Chromosomes AppearMitosis begins withprophase. During this stage:•Chromatin condenses into visible chromosomes•Each chromosome hastwo identical copies, calledsister chromatids•The chromatids are joined at a region called thecentromereAs prophase continues:•Thenucleoli disappear•Thenuclear envelope breaks down•Spindlefibersbegin to formIn animal cells:•Centriolesmove to opposite ends of the cell•They form star-shaped structures calledasters•Centrioles and asters are usuallyabsent in plant and fungal cellsSpindle fibersattach to the chromosomes at thekinetochore, a protein structure located on thecentromere. Eventually, all chromosomes move toward the middle of the cell, known as theequatorial plate.1.2.2 Metaphase: Chromosomes Line UpDuringmetaphase, all chromosome pairs line up neatly along the equatorial plate (also called themetaphase plate).In a human cell:•92 chromosomes(46 pairs) are aligned•Each pair is still connected at the centromere•Spindle fibers remain attached at the kinetochoreThis careful alignment ensures each new cell will receive the correct number of chromosomes.

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