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DNA and Cell Division
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Packaging of DNA
DNA segments are coiled around histones forming a complex called a nucleosome
Non-dividing cells DNA is loosely coiled forming network
called chromatin Cells preparing to divide
DNA becomes tightly coiled forming distinct structures called chromosomes
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Chromosomes Human somatic cells contain 46
chromosomes that exist as 23 pairs of homologous chromosomes
Homologous chromosomes: same size and shape
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Homologous Chromosomes
Code for the same traits Cells contain two
genes for every trait
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Cell Cycle
Period of time from cell formation to cell division
Phases: Interphase• Period of cell
growth Mitotic phase• Cell division
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Mitosis
One cell divides into 2 daughter cells with identical copies of DNA
Functions of mitosis embryonic development tissue growth replacement of dead cells repair of injured tissues
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Phases of Mitosis
Prophase Metaphase Anaphase Telophase
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Prophase Chromatin condenses into
chromosomes Spindle fibers grow from centrioles
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Metaphase
Chromosomes line up along equator of cell Spindle fibers attach to chromosomes
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Anaphase
Spindle fibers separate sister chromatids Daughter chromosomes move towards opposite poles
of cells
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Telophase New nuclear envelopes assemble around each set
of daughter chromosomes Chromosomes uncoil into chromatin
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Cytokinesis
Division of cytoplasm into 2 cells Myosin pulls on microfilaments of actin in the
membrane cytoskeleton (animal cells) creates crease around cell equator called
“cleavage furrow” Cell pinches in two
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Sexual Reproduction Genetic material from
two parents are combined to form a unique combination of genes
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Meiosis
Unique type of cell division that reduces chromosome number by half
Produces gametes or sex cells Spermatogenesis sperm cell production Oogenesis egg cell production
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Gametes
Male and female gametes (sex cells) combine genetic material to form a fertilized egg (zygote) containing 46 chromosomes
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Formation of Gametes
Gametes contain a single copy of 23 chromosomes (haploid)
Requires two cell divisions Meiosis• Meiosis I separates homologous chromosome pairs into
two haploid cells
• Meiosis II separates duplicated sister chromatids into four haploid cells
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Crossing Over
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Independent Assortment
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Errors in Meiosis
Polypoid gamete Contains extra
complete sets of chromosomes
Nondisjunction Replicated
chromosomes fail to separate
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Prokaryotic Cell Division
Asexual Reproduction Daughter cells are
genetically identical to parent cell
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Genetic Variability in Asexual Reproduction
Transformation Transduction Conjugation Mutations