copyright © 2003 pearson education, inc. publishing as benjamin cummings all cells come from cells...
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Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• All cells come from cells
• Cellular reproduction is called cell division
– Cell division allows an embryo to develop into an adult
– It also ensures the continuity of life from one generation to the next
8.2 Cells arise only from preexisting cells
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Figure 8.3A
• Binary fission of a prokaryotic cell
Prokaryoticchromosome
Plasmamembrane
Cell wall
Duplication of chromosomeand separation of copies
Continued growth of the cell and movement of copies
Division intotwo cells
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• The cell cycle consists of two major phases:
– Interphase, where chromosomes duplicate
and cell parts are made
– The mitotic phase, when cell division occurs
8.5 The cell cycle multiplies cells
Figure 8.5
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• Before a cell starts dividing, the chromosomes are duplicated
– This process produces sister chromatids
Centromere
Sister chromatids
Figure 8.4B
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• Chromosomes contain a very long DNA molecule with thousands of genes
– Individual chromosomes are only visibleduring cell division
– They are packaged as chromatin
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• When the cell divides, the sister chromatids separate – Two daughter
cells are produced
– Each has a complete and identical set of chromosomes
Centromere Sister chromatids
Figure 8.4C
Chromosomeduplication
Chromosomedistribution
todaughter
cells
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• Cell Cycle:
• Interphase: G1, G0, S, G2
• Mitotic Phase: Mitosis / Cytokinesis
(Mitosis : PMAT)
8.6 Cell division is a continuum of dynamic changes
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Interphase
• G1 – cell grows / developes
• G0 – cell does what it normally supposed to do, some cells stay in this phase forever, ex nerve
• S – DNA Replication
• G2 – organelles double, enzymes for cell division made
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INTERPHASE PROPHASE
Centrosomes(with centriole pairs)
Chromatin
Nucleolus Nuclearenvelope
Plasmamembrane
Early mitoticspindle
Centrosome
CentrosomeChromosome,consisting of twosister chromatids
Fragmentsof nuclearenvelope
Kinetochore
Spindlemicrotubules
Figure 8.6
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METAPHASE TELOPHASE AND CYTOKINESIS
Metaphaseplate
Spindle Daughterchromosomes
Cleavagefurrow
Nucleolusforming
Nuclearenvelopeforming
ANAPHASE
Figure 8.6 (continued)
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• In animals, cytokinesis occurs by cleavage
– This process pinches the cell apart
8.7 Cytokinesis differs for plant and animal cells
Figure 8.7A
Cleavagefurrow
Cleavagefurrow
Contracting ring ofmicrofilaments
Daughter cells
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• In plants, a membranous cell plate splits the cell in two
Vesicles containingcell wall material
Cell plateforming
Figure 8.7BCell plate Daughter
cells
Wall ofparent cell
Daughternucleus
Cell wall New cell wall
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• Cancer cells have abnormal cell cycles
– They divide excessively and can form abnormal masses called tumors
• Radiation and chemotherapy are effective as cancer treatments because they interfere with cell division
8.10 Connection: Growing out of control, cancer cells produce malignant tumors
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• Homologous pairs – chromosomes from each parent that have same genes but not necessarily same alleles
– Human cells have 46, making up 23 pairs of homologous chromosomes
MEIOSIS AND CROSSING OVER
8.12 Chromosomes are matched in homologous pairs
Chromosomes
Centromere
Sister chromatids Figure 8.12
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• Diploid – somatic cells, 2 sets of chromosomes
• Haploid – gamete cells, 1 set chromosomes
8.13 Gametes have a single set of chromosomes
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• At fertilization, a sperm fuses with an egg, forming a diploid zygote
– Repeated mitotic divisions lead to the development of a mature adult
– The adult makes haploid gametes by meiosis
– All of these processes make up the sexual life cycle of organisms
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• The human life cycle
Figure 8.13
MEIOSIS FERTILIZATION
Haploid gametes (n = 23)
Egg cell
Sperm cell
Diploidzygote
(2n = 46)Multicellular
diploid adults (2n = 46)
Mitosis anddevelopment
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• Meiosis, like mitosis, is preceded by chromosome duplication
– However, in meiosis the cell divides twice to form four daughter cells
8.14 Meiosis reduces the chromosome number from diploid to haploid
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• Meiosis I
- Homologous pairs separate
- During Prophase I, tetrads can cross over to swap genetic info
- End with 2 Haploid cells with sister chromatids
• Meiosis II
- sister chromatids separate
- end with 4 Haploid cells, no sister chromatids
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Figure 8.14, part 1
MEIOSIS I: Homologous chromosomes separate
INTERPHASE PROPHASE I METAPHASE I ANAPHASE I
Centrosomes(withcentriolepairs)
Nuclearenvelope
Chromatin
Sites of crossing over
Spindle
Sisterchromatids
Tetrad
Microtubules attached tokinetochore
Metaphaseplate
Centromere(with kinetochore)
Sister chromatidsremain attached
Homologouschromosomes separate
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Figure 8.14, part 2
MEIOSIS II: Sister chromatids separate
TELOPHASE IAND CYTOKINESIS PROPHASE II METAPHASE II ANAPHASE II
Cleavagefurrow
Sister chromatidsseparate
TELOPHASE IIAND CYTOKINESIS
Haploiddaughter cellsforming
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Figure 8.18A
TetradChaisma
Centromere
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• How crossing over leads to genetic recombination
Figure 8.18B
Tetrad(homologous pair ofchromosomes in synapsis)
Breakage of homologous chromatids
Joining of homologous chromatids
Chiasma
Separation of homologouschromosomes at anaphase I
Separation of chromatids atanaphase II and completion of meiosis
Parental type of chromosome
Recombinant chromosome
Recombinant chromosome
Parental type of chromosome
Gametes of four genetic types
1
2
3
4
Coat-colorgenes
Eye-colorgenes
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• For both processes, chromosomes replicate only once, during interphase
8.15 Review: A comparison of mitosis and meiosis
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Figure 8.15
MITOSIS MEIOSIS
PARENT CELL(before chromosome replication)
Site ofcrossing over
MEIOSIS I
PROPHASE ITetrad formedby synapsis of homologous chromosomes
PROPHASE
Duplicatedchromosome(two sister chromatids)
METAPHASE
Chromosomereplication
Chromosomereplication
2n = 4
ANAPHASETELOPHASE
Chromosomes align at the metaphase plate
Tetradsalign at themetaphase plate
METAPHASE I
ANAPHASE ITELOPHASE I
Sister chromatidsseparate duringanaphase
Homologouschromosomesseparateduringanaphase I;sisterchromatids remain together
No further chromosomal replication; sister chromatids separate during anaphase II
2n 2n
Daughter cellsof mitosis
Daughter cells of meiosis II
MEIOSIS II
Daughtercells of
meiosis I
Haploidn = 2
n n n n
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• GENETIC VARIATION CAN RESULT FROM:
- crossing over
- homologous pairs rearranging
- random fertilization
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Figure 8.16
POSSIBILITY 1 POSSIBILITY 2
Two equally probable
arrangements of chromosomes at
metaphase I
Metaphase II
Gametes
Combination 1 Combination 2 Combination 3 Combination 4
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• Preparation of a karyotype
Figure 8.19
Blood culture
1
Centrifuge
Packed redAnd white blood cells
Fluid
2
Hypotonic solution
3
Fixative
WhiteBloodcells
Stain
4 5
Centromere
Sisterchromatids
Pair of homologouschromosomes
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• This karyotype shows three number 21 chromosomes
• An extra copy of chromosome 21 causes Down syndrome
8.20 Connection: An extra copy of chromosome 21 causes Down syndrome
Figure 8.20A, B
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• Abnormal chromosome count is a result of nondisjunction
– Either homologous pairs fail to separate during meiosis I
8.21 Accidents during meiosis can alter chromosome number
Figure 8.21A
Nondisjunctionin meiosis I
Normalmeiosis II
Gametes
n + 1 n + 1 n – 1 n – 1
Number of chromosomes
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– Or sister chromatids fail to separate during meiosis II
Figure 8.21B
Normalmeiosis I
Nondisjunctionin meiosis II
Gametes
n + 1 n – 1 n n
Number of chromosomes
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• Fertilization after nondisjunction in the mother results in a zygote with an extra chromosome
Figure 8.21C
Eggcell
Spermcell
n + 1
n (normal)
Zygote2n + 1
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• Nondisjunction can also produce gametes with extra or missing sex chromosomes
– Unusual numbers of sex chromosomes upset the genetic balance less than an unusual number of autosomes
8.22 Connection: Abnormal numbers of sex chromosomes do not usually affect survival
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SPERMATOGENESIS
– Spermatogenesis: the formation of sperm cells
•Diploid cells made continuously in seminiferous tubules of testes
•Differentiated primary spermatocytes
•Haploid secondary spermatocytes
•Haploid sperm
LE 27-04a
Testis
Scrotum
Diploid cell
Differentiation andonset of Meiosis
Primary spermatocyte
Secondary spermatocyte
Meiosis
(in prophase of Meiosis
(haploid; double chromatids)
(haploid; single chromatids)Developing sperm cells
Differentiation
Sperm cells
Epididymis
Penis
Seminiferous tubule
Cross section ofseminiferoustubule
Center ofseminiferous tubule
Testis
(haploid)n n n n
nnnn
n n
2n
2n
completedMeiosis
LE 27-04b
Diploid cell
Primary oocyte
(arrested in prophaseof Meiosis )
Secondary oocyte
In embryo
Differentiation andonset of Meiosis
Present at birth
Completion of Meiosis and onset of Meiosis
Firstpolar body
2n
2n
n n(arrested at meta-phase of Meiosis ;
released from ovary)
Entry of sperm triggerscompletion of Meiosis
Secondpolar body
nn
Ovum
(haploid)
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Difference between Oogenesis and Spermatogenesis
• Oogenesis
- starts at birth / primary oocyte arrested in Prophase I
- puberty – release secondary oocyte once a month – STOPS after eggs run out
- complete Meiosis II if fertilized
- 1 egg, polar bodies (unequal divisions)
• Spermatogenesis
- starts at puberty
- division continuous
- 4 haploid cells