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All Cells Come From Cells • Most of the cells on the outer layer of your skin are dead • Underneath the dead cells are living cells providing life functions. • The dead cells need to be replaced – the regeneration of cells from other cells is called mitosis • Mitosis is necessary to both replace damaged cells and to allow growth of an organism Reproduction • During mitosis, DNA is duplicated and the cell splits. The resulting 2 cells are genetically identical – this is called asexual reproduction – no genetic material (DNA) has been mixed • Sexual reproduction involves the combining of genetic material (DNA) – the resulting cells or organisms are not genetically identical to each other Chromosome structure • Our genetic material consists of chromatin, a combination of DNA and protein molecules. • When cells divide, the chromatin fibers condense, becoming visible under a microscope as the structures called chromosomes. • Human body cells each have 46 chromosomes. Each chromosome may contain many hundreds of genes. The chromosome duplicates before mitosis begins The centromere holds the two sister chromatids When the two sister chromatids separate they are genetically identical Cell Cycle – all the processes that result in new cells from old cells • Interphase – Cells are functioning –– This is when the cell is functioning – fulfilling life functions – Cells are MOST OFTEN in interphase • Prophase – DNA duplicates and uncoils as the cell prepares for Mitosis • Metaphase – chromosomes line up in the center of the cell – this is part of the organizing part of mitosis • Anaphase – The chromosomes are divided and moved to opposite sides of the cell • Telophase/Cytokinesis – the cell now splits into 2 identical cells Prophase – DNA becomes visible – nuclear envelope breaks up Structures of mitosis • During Prophase the DNA uncoils and becomes visible • Spindle fibers help to direct the chromosomes as they line up during metaphase • These spindles continue to direct movement during anaphase as the chromosomes are divided and pulled apart Telophase and cytokinesis • In animals, during telophase an indentation appears and the two cells separate • In plants, a cell plate forms that separates the two cells • Cytokinesis is the completion of the creation of the 2 cells Regulation of mitosis • Hormones and enzymes control the processes involved in the cell cycle • When new cells are needed, the cell is large enough and DNA is duplicated mitosis can proceed. Cancer • G1 and G2 are “checkpoints” where the cells pause before continuing with mitosis. • Cancer results when the cells do not pause here, and just continue to grow. • A tumor of growing cells is the result Meiosis – reproduction of sex cells • Sperm and egg are the sex cells that we call gametes • Meiosis produces 4 gametes that contain ½ of the total chromosomes of the parent organism • DNA is also combined, restoring the full DNA count • Cells containing the entire chromosome count are considered to be diploid – all of our body cells are diploid • Sex cells, containing only ½ the chromosome count are described as haploid • When gametes combine during fertilization a diploid zygote is formed Fertilization restores full chromosome count •During fertilization, sperm and egg combine DNA This combining of DNA is Sexual Reproduction Meisosis • The stages of meiosis are very similar to mitosis • However, there are 2 cycles • The first cycle is the same as mitosis – creating 2 cells – with a combination of DNA derived from the male parent and female parent • These chromosomes are described as homologous – They code for the same genes, but different versions of the gene Prophase II • In the second cycle, during prophase II the DNA is NOT duplicated • During prophase II of meiosis, some genetic material from the chromosomes switches from one homologous chromosome to another • Crossing over is genetic recombination and creates greater genetic diversity