Cell Reproduction

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ChromosomesRemember that DNA is a long thin molecule that stores genetic information. The DNA in a human cell is estimated to consist of 3 billion pairs of nucleotides.

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To visualize the size of 3 billion pairs of nucleotides, imagine increasing a cell nucleus to the size of a basketball. Then imagine taking the DNA out of the basketball-sized nucleus and stretching it into a straight line. That line of DNA would stretch for 64 km!

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Chromosome structure

During cell division, DNA coiled into chromosomesDNA wraps around proteins called histonesHelp maintain shape of chromosome

Nonhistones - involved in controlling activity of specific regions of DNA

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Notice each chromosome made of two identical halvesEach half is called a chromatidConstricted area in center is centromere

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Between cell divisions, DNA not so tightly coiled into chromosomesParts of DNA uncoil so they can be read and information can be used to direct activities of cellLess tightly coiled DNA is called chromatin

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Prokaryote and Eukaryote

Prokaryotic DNA much simpler than eukaryotic DNA

Single loopOne or two chromosomes

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Chromosome numbers

Each species has characteristic number of chromosomes

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Sex chromosomes and autosomes

Human and animal chromosomes categorized as:Sex chromosomes - determine genderX and Y

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Autosomes - All other chromosomes

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Every cell has two copies of each autosomeHomologous chromosomesHave same genes

Karyotype - photograph of chromosomes

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Diploid and Haploid Cells

Diploid - cells having two sets of chromosomesHave both homologous chromosomesAll cells but sperm and eggCalled 2n

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Haploid - only one set of chromosomesSperm and egg cells1n

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Cell Division

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Approximately 2 trillion cells – – are produced by an adult human body every day. about 25 million cells per secondAll cells come from the division of preexisting cells Cell division (cell reproduction) is the process by which cells produce offspring cells

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Cell division differs in prokaryotes and eukaryotes. But cell reproduction in both produces the same result – two cells from one.

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Cell Division in Prokaryotes

Binary fission - division of prokaryotic cell into two offspring cells1. Chromosome copies itself, resulting in two identical chromosomes

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2. Cell grows until it is twice the original size3. Cell wall forms in middle, splits the two cells

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Cell Division in Eukaryotes

Both cytoplasm and nucleus divideTwo typesMitosis - new cells with genetic material identical to parent cellAsexual reproduction – production of offspring from one parentReproduction of unicellular organismsAddition of cells to tissue or organ in muticellular

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Meiosis - reduces chromosome number by halfEach new cell genetically different than parentthis is sexual reproduction - results in genetic variationFormation of gametes – egg and sperm

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The Cell CycleCell cycle - repeating set of events that make up life of cell

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Interphase - time between divisions

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Cell division has two phasesMitosis – division of nucleusCytokinesis – division of cytoplasm

Interphase3 stages1. G1 phase - offspring grow to mature size2. S phase - DNA is copied3. G2 phase - cell prepares to divideCells can exit cell cycle and enter Go phaseCells do not copy DNA, do not divide

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Mitosis

Division of nucleus4 phasesProphaseMetaphaseAnaphase 32

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Prophase

DNA coils into chromosomesNucleus starts to disappearCentrosomes (2 dark spots) appearIn animal cells, they contain centrioles, cylindrical 34

Centrosomes move to opposite endsSpindle fibers made of microtubules radiate from centrosomesCalled mitotic spindleMade of kinetochore fibers and polar fibers

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spindle fibers attach to a structure on each chromatid called a kinetochorekinetochores are part of the centromere

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MetaphaseKinetochore fibers move chromosomes to center of cellOnce in center, chromosomes are held in place by kinetochore fibers

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where the chromosomes line up in the middle is called the metaphase

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AnaphaseChromatids separate at centromereMove, centromere first, to opposite ends

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TelophaseAfter chromosomes reach opposite ends, spindle fibers disassemble Chromosomes uncoil slightlyNuclear envelope forms around each set of chromosomes

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Cytokinesis

Division of cytoplasm

Different in animal cells and plant cells

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Animal cells - cleavage furrow forms

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Plant cells - cell plate forms

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Cytokinesis in Animal Cells

Cell membrane pinches inward between the two dividing cellsCleavage furrow eventually splits the two cellsAction of microfilaments

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Cytokinesis in Plant Cells

Vesicles formed by Golgi apparatus fuse at midline of dividing cellForms cell plate

Cell plate separates two cells

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Control of Cell Division

Cell spends most time in interphaseWhat triggers cell to leave interphase and start dividing?

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In eukaryotes proteins regulate progress of cell division at certain checkpointsCertain feedback signals from cell can make proteins start next phase of cell cycleThree main checkpoints

1. Cell growth (G1) checkpoint

Proteins control whether the cell will divideIf cell healthy and is big enough proteins will initiate S phaseCell copies DNA 52

If conditions not favorable, cell cycle will stopWill also stop if cell needs rest periodCertain cells enter G0 at this point

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2. DNA synthesis (G2) checkpoint

DNA repair enzymes check results of DNA replicationIf passed, proteins signal cell to start molecular processes that allow cell to enter mitosis

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3. Mitosis checkpoint

If cell passes, proteins signal cell to exit mitosisCell enters G1 phase again

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When control is lost: cancer

Proteins that regulate cell division are coded for by genesIf mutation happens in genes, proteins may not function properly

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Cell growth and division may be disruptedDisruption can lead to cancer – uncontrolled growth of cells

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Cancer cells do not respond normally to control mechanismsCan overproduce growth-promoting moleculesIncreased cell divisionMay interfere with control proteins that slow or stop cell cycle

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Meiosis is a process of nuclear division that reduces the number of chromosomes in new cells to half the number in the original number

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The halving of the chromosome number counteracts a fusion of cells later in the life cycle of the organism

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For example, in humans, meiosis produces haploid reproductive cells called gametes. Human gametes are sperm cells and egg cells, each of which contains 23 (1n) chromosomes

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The fusion of a sperm and an egg results in a zygote that contains 46 (2n) chromosomes

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Formation of haploid cells

In animals meiosis produces gametes – haploid reproductive cellsSperm and egg cells each have 23 (1n) chromosomesFusion of sperm and egg results in zygote that contains 46 (2n) chromosomes

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Cells preparing for meiosis undergo G1, S, and G2 of interphaseDuring interphase cell grows to mature size and copies DNA

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Cells start with duplicate set of chromosomes (just like mitosis)Cells in meiosis go through TWO divisions

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Meiosis ISame stages as mitosisProphase IMetaphase IAnaphase ITelophase I

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Prophase ISignificant different than prophase of mitosis

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Homologous chromosomes line up next to each other - synapsis

Twist around each other, which may exchange portions of genetic material this results in crossing overcrossing over results in Genetic recombination - provide genetic variety in species

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Metaphase ITetrads (pairs of homologous chromosomes) line up at centerLining up is RANDOMSpindle fibers attach to each of the pair

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Anaphase IEach homologous chromosome (made of two chromatids) moves to opposite endsSeparation is RANDOM independent assortmentRandom separation of maternal and paternal chromosomesGenetic variation 73

Telophase IChromosomes reach opposite endsCytokinesis beginsEach new cell is haploid (only half the number of chromosomes)

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BUT each has two copies of chromosomeDNA copied before during S phase

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Meiosis IIDNA NOT copied before Prophase II

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Formation of Gametes

In animals, meiosis produces gametesEgg and sperm

Only happens in reproductive organsOvaries and testes

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SpermatogenesisDiploid reproductive cell divides to make 4 haploid cells called spermatidsEach spermatid develops into a mature sperm cell

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OogenesisDiploid reproductive cell divides to produce 1 mature egg cell (ovum) and 3 polar bodies (waste)Because mature egg cell needs most of the cytoplasmPolar bodies degenerate

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Asexual Reproduction

Production of offspring from 1 parentDoes not involve meiosis or union of gametesUnicellular organisms - by binary fission or mitosisOffspring genetically identical to parent

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Sexual reproduction

Production of offspring through meiosis and union of sperm and eggOffspring genetically different than parents

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Evolutionary advantage: enables species to adapt quickly to new conditionsDisease hits - some individuals may be resistant, survive to reproduce

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