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How cells reproduce?
Reproduction
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Produces a generation of individuals like parents
Cell division
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Bridges two generations
Each daughter cell receives
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A required number of DNA molecules
Some cytoplasm
Eukaryotes and Prokaryotes
Eukaryotic cells undergo mitosis and/or meiosis
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Separates duplicated chromosomes of parent cell into two daughter nuclei
Another mechanism divides cytoplasm
Prokaryotic cells divide by a different process
Mitosis and Meiosis
Mitosis
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Basis of growth, cell replacements, and tissue repair in multicelled species
Basis of asexual reproduction in many single-celled and multicelled species
Meiosis
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Basis of sexual reproduction
Precedes formation of gametes or sexual spores
Chromosome structure
Eukaryotic chromosome
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Association of DNA, histones, and other proteins
Proteins structurally organize the chromosome and affect access to its genes
Nucleosome
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Smallest unit of organization
Double-stranded DNA looped twice around a spool of histones
Introducing the cell cycle
Cell cycle
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Starts when a new cell forms
Runs through interphase
Ends when cell reproduces by nuclear and cytoplasmic division
Interphase
Most cellular activities occur in interphase
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G1: Cell grows in mass, doubles number of cytoplasmic components
S: DNA replication duplicates chromosomes
G2: Cell prepares for division
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Chromosome number
Sum of all chromosomes in cells of a given type
In human body cells, chromosome number is 46
Body cells are diploid (have two of each kind of chromosome)
Mitosis and chromosome number
Mitosis maintains parental chromosome number from one generation to the next
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Bipolar spindle divides sister chromatids
A closer look at mitosis
Mitosis
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A nuclear division mechanism that maintains the chromosome number
Mitosis proceeds in four stages:
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Prophase
Metaphase
Anaphase
Telophase
Prophase
Duplicated chromosomes become threadlike as they start to condense
Microtubules form a bipolar spindle
Nuclear envelope starts to break apart
Transition to Metaphase
Microtubules from one spindle pole harness one chromatid of each chromosome
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Microtubules from the opposite spindle pole harness its sister chromatid
Other microtubules extend from both poles and grow until they overlap at the spindle’s midpoint
Metaphase
All chromosomes become aligned midway between the two spindle poles
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Chromosomes in most condensed forms
Anaphase
Sister chromatids detach from each other
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Spindles move them toward opposite poles
Microtubules that overlap at spindle’s midpoint slide past each other, push poles farther apart
Motor proteins drive movements
Telophase
Two identical clusters (one chromosome of each type) reach opposite spindle poles
Nuclear envelope forms around each cluster
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Both new nuclei have the parental chromosome number
At the end!
Interphase
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Two daughter cells
Cytoplasmic Division Mechanisms
Mechanisms of cytoplasmic division differ in plant and animal cells
In animal cells
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A contractile ring of microfilaments (part of cell cortex) contracts and pulls the cell
surface inward until the cytoplasm is divided
Cytoplasmic Division in Plant Cells
In plant cells
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A band of microtubules and microfilaments forms around the nucleus before mitosis starts
Marks site where cell plate will form
Cell plate becomes a cross-wall that partitions the cytoplasm
Controls Over Cell Division
Products of checkpoint genes control cell cycle
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Kinases
Growth factors
When control is lost
Mutant checkpoint genes can cause tumors by disrupting normal controls
Cancers
Altered cells grow and divide abnormally
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Malignant cells may metastasize (break loose and colonize distant tissues)