Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings All cells come from cells...

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


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


• 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


Karyotype


• Before a cell starts dividing, the chromosomes are duplicated

– This process produces sister chromatids

Centromere

Sister chromatids

Figure 8.4B


• Chromosomes contain a very long DNA molecule with thousands of genes

– Individual chromosomes are only visibleduring cell division

– They are packaged as chromatin


• 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


• Cell Cycle:

• Interphase: G1, G0, S, G2

• Mitotic Phase: Mitosis / Cytokinesis

(Mitosis : PMAT)

8.6 Cell division is a continuum of dynamic changes


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


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


METAPHASE TELOPHASE AND CYTOKINESIS

Metaphaseplate

Spindle Daughterchromosomes

Cleavagefurrow

Nucleolusforming

Nuclearenvelopeforming

ANAPHASE

Figure 8.6 (continued)


• 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


• 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


• 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


• 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


• Diploid – somatic cells, 2 sets of chromosomes

• Haploid – gamete cells, 1 set chromosomes

8.13 Gametes have a single set of chromosomes


• 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


• 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


• 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


• 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


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


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


Figure 8.18A

TetradChaisma

Centromere


• 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


• For both processes, chromosomes replicate only once, during interphase

8.15 Review: A comparison of mitosis and meiosis


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


• GENETIC VARIATION CAN RESULT FROM:

- crossing over

- homologous pairs rearranging

- random fertilization


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


• 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


• 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


• 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


– 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


• 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


• 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


Table 8.22


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)


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

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings All cells come from cells...

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