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– Interphase,where chromosomes duplicate and cell parts are made

– The mitotic phase, when nuclear division occurs

The life cycle of a cellCell cycle consists of 2 major phases

Figure 8.5


• During interphase a cell performs all of its regular functions and gets ready to divide

• Metabolic activity is very high

Most of the life of a cell is spent in InterphaseCell does most of its’ growth during interphase

Figure 8.5


• Untwisting and replication of DNA

Figure 10.4B


• Before a cell starts dividing, the chromosomes are duplicated

– This process produces sister chromatids

– EM of human chromosome that has duplicated

Centromere

Sister chromatids

Figure 8.4B


Structure of Chromosomes

– Homologous chromosomes are identical pairs of chromosomes.

– One inherited from mother and one from father

– made up of sister chromatids joined at thecentromere.

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• This phase spans the time from the completion of DNA synthesis to the onset of cell division

• Following DNA replication, the cell spends about 2-5 hours making proteins prior to entering the M phase

G2 Phase

Figure 8.5


INTERPHASE PROPHASECentrosomes(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

– The first sign of cleavage is the appearance of a cleavage furrow

Cytokinesis differs for plant and animal cells

Figure 8.7A

Cleavagefurrow

Cleavagefurrow

Contracting ring ofmicrofilaments

Daughter cells


– As the daughter chormosomes move to opposite poles

– The cytoplasm constricts along the plane of the metaphase plate

The process of cytokinesis divides the cell into two genetically identical cells

Cytokinesis differs for plant and animal cells

Figure 8.7A

Cleavagefurrow

Cleavagefurrow

Contracting ring ofmicrofilaments

Daughter cells


• When the cell divides, the sister chromatidsseparate

– Two daughter cells are produced

– Each has a complete and identical set of chromosomes

Centromere Sister chromatids

Figure 8.4C

Chromosomeduplication

Chromosomedistribution

todaughter

cells


• The human life cycle

• Meiosis is a special form of cell division that produces gametes

Figure 8.13

MEIOSIS FERTILIZATION

Haploid gametes (n = 23)

Egg cell haploid

Sperm cell haploid

Diploidzygote

(2n = 46)Multicellular

diploid adults(2n = 46)

Mitosis anddevelopment


• There is a special mechanism to produce gametes

• Each gamete has a single set of chromosomes

• 22 autosomesand a single sex chromosomeFigure 8.13

MEIOSIS FERTILIZATION

Haploid gametes (n = 23)

Egg cell haploid

Sperm cell haploid

Diploidzygote

(2n = 46)Multicellular

diploid adults(2n = 46)

Mitosis anddevelopment


• Haploid gametes keeps the chromosome number from doubling in each succeeding generation

• Haploid gametes are produced by a special sort of cell division called meiosis

• Which occurs only in reproductive organs, ovaries and testes

• Purpose of meiosis is to produce sperm and egg

Gametes have a single set of chromosomes


• Meiosis involves 2 cell divisions

• Meiosis produces 4 cells from 1 parental cell

• Each of the 4 daughter cells has 23 individual chromosomes rather than 23 pairs of chromosomes

• Meiosis reduces the chromosome number from diploid to haploid

• Meiosis, like mitosis, is preceded by chromosome duplication

– However, in meiosis the cell divides twice to form four daughter cells

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 ISister chromatids

separate duringanaphase

Homologouschromosomesseparateduringanaphase I;sisterchromatidsremain together

No further chromosomal replication; sister chromatidsseparate 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


Figure 8.14, part 1

MEIOSIS I: Homologous chromosomes separate

INTERPHASE PROPHASE I METAPHASE I ANAPHASE I

Centrosomes(withcentriolepairs)

Nuclearenvelope

Chromatin

Sites of crossing overSpindle

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 8.16

POSSIBILITY 1 POSSIBILITY 2

Two equally probable

arrangements of chromosomes at

metaphase I

Metaphase II

Gametes

Combination 1 Combination 2 Combination 3 Combination 4


• Each synapsis is made up of 2 pairs of sister chromatids

• This matched set of 4 chromatids is called a tetrad

MEIOSIS AND CROSSING OVER

Chromosomes are matched in homologous pairs

Chromosomes

Centromere

Sister chromatids Figure 8.12


• Crossing over is the exchange of corresponding segments between two non-sister chromatids of homologous chromosomes

• Genetic recombination results from crossing over during prophase I of meiosis

– This increases variation further

Crossing over further increases genetic variability


• How crossing over leads to genetic recombination

• Nonsisterchromatidsbreak in two at the same spot

• The 2 broken chromatids join together in a new way 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



Gametes of four genetic types

1

2

3

4

Coat-colorgenes

Eye-colorgenes


• A segment of one chromatid has changed places with the equivalent segment of its nonsisterhomologue

• If there were no crossing over meiosis could only produce 2 types of gametes

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





Gametes of four genetic types

1

2

3

4

Coat-colorgenes

Eye-colorgenes

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