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Cell division is at the heart of the reproduction of cells and organisms
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Organisms can reproduce sexually or asexually
8.1 Like begets like, more or less
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Some organisms make exact copies of themselves, asexual reproduction
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Other organisms make similar copies of themselves in a more complex process, sexual
reproduction
8.2 Cells arise only from preexisting cells
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All cells come from cells
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Cellular reproduction is called cell division
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Cell division allows an embryo to develop into an adult
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It also ensures the continuity of life from one generation to the next
8.3 Prokaryotes reproduce by binary fission
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Prokaryotic cells divide asexually
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These cells possess a single chromosome, containing genes
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The chromosome is replicated
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The cell then divides into two cells, a process called binary fission
8.4 The large, complex chromosomes of eukaryotes duplicate with each cell division
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A eukaryotic cell has many more genes than a prokaryotic cell
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The genes are grouped into multiple chromosomes, found in the nucleus
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The chromosomes of this plant cell are stained dark purple
Chromosomes contain a very long DNA molecule with thousands of genes
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Individual chromosomes are only visible during cell division
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They are packaged as chromatin
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Before a cell starts dividing, the chromosomes are duplicated
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This process produces sister chromatids
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When the cell divides, the sister chromatids separate
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Two daughter cells are produced
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Each has a complete and identical set of chromosomes
8.5 The cell cycle multiplies cells
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The cell cycle consists of two major phases:
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Interphase, where chromosomes duplicate and cell parts are made
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The mitotic phase, when cell division occurs
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8.6 Cell division is a continuum of dynamic changes
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Eukaryotic cell division consists of two stages:
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Mitosis
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Cytokinesis
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In mitosis, the duplicated chromosomes are distributed into two daughter nuclei
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After the chromosomes coil up, a mitotic spindle moves them to the middle of the cell
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The sister chromatids then separate and move to opposite poles of the cell
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The process of cytokinesis divides the cell into two genetically identical cells
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8.7 Cytokinesis differs for plant and animal cells
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In animals, cytokinesis occurs by cleavage
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This process pinches the cell apart
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In plants, a membranous cell plate splits the cell in two
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8.8 Anchorage, cell density, and chemical growth factors affect cell division
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Most animal cells divide only when stimulated, and others not at all
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In laboratory cultures, most normal cells divide only when attached to a surface
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They are anchorage dependent
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Cells continue dividing until they touch one another. This is called density-dependent
inhibition
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Growth factors are proteins secreted by cells that stimulate other cells to divide
8.9 Growth factors signal the cell cycle control system
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Proteins within the cell control the cell cycle
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Signals affecting critical checkpoints determine whether the cell will go through a
complete cycle and divide
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The binding of growth factors to specific receptors on the plasma membrane is usually
necessary for cell division
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8.10 Connection: Growing out of control, cancer cells produce malignant tumors
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Cancer cells have abnormal cell cycles
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They divide excessively and can form abnormal masses called tumors
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Radiation and chemotherapy are effective as cancer treatments because they interfere with cell
division
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Malignant tumors can invade other tissues and may kill the organism
8.11 Review of the functions of mitosis: Growth, cell replacement, and asexual reproduction
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When the cell cycle operates normally, mitotic cell division functions in:
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Growth (in an onion root)
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Cell replacement (in skin)
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Asexual reproduction (in a hydra)
8.12 Chromosomes are matched in homologous pairs
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Somatic cells of each species contain a specific number of chromosomes
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Human cells have 46, making up 23 pairs of homologous chromosomes
8.13 Gametes have a single set of chromosomes
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Cells with two sets of chromosomes are said to be diploid
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Gametes are haploid, with only one set of chromosomes
At fertilization, a sperm fuses with an egg, forming a diploid zygote
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Repeated mitotic divisions lead to the development of a mature adult
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The adult makes haploid gametes by meiosis
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All of these processes make up the sexual life cycle of organisms
8.14 Meiosis reduces the chromosome number from diploid to haploid
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Meiosis, like mitosis, is preceded by chromosome duplication
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However, in meiosis the cell divides twice to form four daughter cells
In the first division, meiosis I, homologous chromosomes are paired
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While they are paired, they cross over and exchange genetic information
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The homologous pairs are then separated, and two daughter cells are produced
Meiosis II is essentially the same as mitosis
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The sister chromatids of each chromosome separate
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The result is four haploid daughter cells
8.15 Review: A comparison of mitosis and meiosis
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For both processes, chromosomes replicate only once, during interphase
8.16 Independent orientation of chromosomes in meiosis and random fertilization lead to varied
offspring
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Each chromosome of a homologous pair comes from a different parent
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Each chromosome thus differs at many points from the other member of the pairThe large
number of possible arrangements of chromosome pairs at metaphase I of meiosis leads to many
different combinations of chromosomes in gametes
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Random fertilization also increases variation in offspring
8.17 Homologous chromosomes carry different versions of genes
The differences between homologous chromosomes are based on the fact that they can carry different
versions of a gene at corresponding loci
8.18 Crossing over further increases genetic variability
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Crossing over is the exchange of corresponding segments between two homologous
chromosomes
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Genetic recombination results from crossing over during prophase I of meiosis
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This increases variation further
How crossing over leads to genetic recombination
8.19 A karyotype is a photographic inventory of an individual’s chromosomes
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To study human chromosomes microscopically, researchers stain and display them as a
karyotype
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A karyotype usually shows 22 pairs of autosomes and one pair of sex chromosomes
8.20 Connection: An extra copy of chromosome 21 causes Down syndrome
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This karyotype shows three number 21 chromosomes
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An extra copy of chromosome 21 causes Down syndrome
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The chance of having a Down syndrome child goes up with maternal age
8.21 Accidents during meiosis can alter chromosome number
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Abnormal chromosome count is a result of nondisjunction
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Either homologous pairs fail to separate during meiosis I
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Or sister chromatids fail to separate during meiosis II
Fertilization after nondisjunction in the mother results in a zygote with an extra chromosome
8.22 Connection: Abnormal numbers of sex chromosomes do not usually affect survival
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Nondisjunction can also produce gametes with extra or missing sex chromosomes
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Unusual numbers of sex chromosomes upset the genetic balance less than an unusual
number of autosomes
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A man with Klinefelter syndrome has an extra X chromosome
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A woman with Turner syndrome lacks an X chromosome
8.23 Connection: Alterations of chromosome structure can cause birth defects and cancer
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Chromosome breakage can lead to rearrangements that can produce genetic disorders or
cancer
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Four types of rearrangement are deletion, duplication, inversion, and translocation
Chromosomal changes in a somatic cell can cause cancer
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A chromosomal translocation in the bone marrow is associated with chronic
myelogenous leukemia