dna and cell division
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DNA and Cell Division
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
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
Sexual Reproduction Genetic material from
two parents are combined to form a unique combination of genes
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
Gametes
Male and female gametes (sex cells) combine genetic material to form a fertilized egg (zygote) containing 46 chromosomes
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
Crossing Over
Independent Assortment
Errors in Meiosis
Polypoid gamete Contains extra
complete sets of chromosomes
Nondisjunction Replicated
chromosomes fail to separate
Prokaryotic Cell Division
Asexual Reproduction Daughter cells are
genetically identical to parent cell
Genetic Variability in Asexual Reproduction
Transformation Transduction Conjugation Mutations
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