The Cell CycleThe Cell Cycle

Types of Cell DivisionTypes of Cell Division

Mitosis Mitosis – provides cells for growth, cell replacement, and provides cells for growth, cell replacement, and

asexual reproductionasexual reproduction– maintains chromosome numbermaintains chromosome number

Meiosis Meiosis – produces cells for sexual reproductionproduces cells for sexual reproduction– reduces chromosome number by halfreduces chromosome number by half

Chromosome ReplicationChromosome Replication

All the DNA is duplicated.All the DNA is duplicated. Duplicated chromosomes separate as cell Duplicated chromosomes separate as cell

divides.divides.

Asexual Reproduction in Asexual Reproduction in BacteriaBacteria

Bacteria reproduce asexually by binary Bacteria reproduce asexually by binary fission. fission.

A bacterium possesses a single, circular A bacterium possesses a single, circular chromosome which is replicated; and then chromosome which is replicated; and then the replicates (chromosomes) are distributed the replicates (chromosomes) are distributed to two new cells formed by division of the to two new cells formed by division of the original cell.original cell.

The Eukaryotic Cell CycleThe Eukaryotic Cell Cycle InterphaseInterphase

– GG11 growth phase; synthesis of organelles growth phase; synthesis of organelles

– S synthesis of DNA (replication)S synthesis of DNA (replication)

– GG2 2 growth; synthesis of proteins essential to cell divisiongrowth; synthesis of proteins essential to cell division

MitosisMitosis– prophaseprophase– metaphasemetaphase– anaphaseanaphase– telophasetelophase

Important TermsImportant Terms

chromatin - the form taken by chromosomes when chromatin - the form taken by chromosomes when a eukaryotic cell is not dividinga eukaryotic cell is not dividing

monad chromosome - unreplicated chromosome monad chromosome - unreplicated chromosome (single chromosome)(single chromosome)

dyad chromosome - replicated chromosome dyad chromosome - replicated chromosome (double chromosome)(double chromosome)

chromatid - half of a dyad chromosomechromatid - half of a dyad chromosome centromere - structure by which chromatids of a centromere - structure by which chromatids of a

dyad chromosome are held togetherdyad chromosome are held together

Mitosis*Mitosis*

1. Prophase: dyad (replicated) chromosomes 1. Prophase: dyad (replicated) chromosomes resemble long threads (chromatin); replicated resemble long threads (chromatin); replicated centrioles move apart toward opposite poles; centrioles move apart toward opposite poles; (animal cell); nucleolus disappears; dyad (animal cell); nucleolus disappears; dyad chromosomes condense (coil tightly); nuclear chromosomes condense (coil tightly); nuclear membrane disintegrates; spindle fibers appear membrane disintegrates; spindle fibers appear between centrioles; dyad chromosomes between centrioles; dyad chromosomes migrate toward the equator of the cellmigrate toward the equator of the cell

Mitosis*Mitosis*

2. Metaphase: dyad chromosomes line up 2. Metaphase: dyad chromosomes line up along the equator of the cellalong the equator of the cell

Mitosis*Mitosis*

3. Anaphase: dyad chromosomes separate 3. Anaphase: dyad chromosomes separate forming monad chromosomes; forming monad chromosomes; monad chromosomes move toward opposite monad chromosomes move toward opposite polespoles

Mitosis*Mitosis*

4. Telophase: monad chromosomes cluster 4. Telophase: monad chromosomes cluster at poles; cytokinesis occurs gradually; at poles; cytokinesis occurs gradually; nuclear membrane forms; nucleolus nuclear membrane forms; nucleolus reappears; chromosomes uncoil; cytokinesis reappears; chromosomes uncoil; cytokinesis is completed; two daughters cells are is completed; two daughters cells are formedformed

Daughter Cells of MitosisDaughter Cells of Mitosis

have the same number of chromosomes as have the same number of chromosomes as each other and as the original cell from each other and as the original cell from which they were formedwhich they were formed

e.g. Mitosis in a cell which possesses 4 e.g. Mitosis in a cell which possesses 4 chromosomes will result in the formation of chromosomes will result in the formation of two daughter cells each having 4 two daughter cells each having 4 chromosomes.chromosomes.

Can you identify the stages Can you identify the stages (phases) of mitosis in both plant (phases) of mitosis in both plant

cells and animal cells?cells and animal cells?

Can you cite differences between Can you cite differences between plant cell mitosis and animal cell plant cell mitosis and animal cell

mitosis?mitosis?

Types of Cell DivisionTypes of Cell Division

Mitosis Mitosis – provides cells for growth, cell replacement, and provides cells for growth, cell replacement, and

asexual reproductionasexual reproduction– maintains chromosome numbermaintains chromosome number

Meiosis Meiosis – produces cells for sexual reproductionproduces cells for sexual reproduction– reduces chromosome number by halfreduces chromosome number by half

MeiosisMeiosis

preceded by interphase which includes preceded by interphase which includes chromosome replicationchromosome replication

two meiotic divisionstwo meiotic divisions four daughter cells producedfour daughter cells produced daughter cells contain half the number of daughter cells contain half the number of

chromosomes as the original cellchromosomes as the original cell

Important terms in the study of Important terms in the study of meiosismeiosis

monad chromosome - unreplicated chromosomemonad chromosome - unreplicated chromosome dyad chromosome - chromosome composed of dyad chromosome - chromosome composed of

two chromatids (replicates)two chromatids (replicates) homologous - similar in size and shapehomologous - similar in size and shape synapsis - pairing of homologous chromosomessynapsis - pairing of homologous chromosomes crossover - exchange of segments between crossover - exchange of segments between

chromatids of homologous chromosomes chromatids of homologous chromosomes

Chromosomes are replicated Chromosomes are replicated during S-phase of interphase. As during S-phase of interphase. As a result, monad chromosomes a result, monad chromosomes

become dyad chromosomes.become dyad chromosomes.

Meiosis I(Figure 6.6, p. 106)

Prophase I: homologous chromosomes of diploid cell synapse; homologous pairs migrate toward equator of cell; crossover may occur during synapsis

Metaphase I: dyad pairs of homologous chromosomes align at the equator of the cell

Anaphase I: homologous chromosomes move apart toward opposite poles

Telophase 1: chromosomes cluster at poles of cell; cytokinesis occurs forming two haploid daughter cells each containing one member of each homologous pair thus reducing chromosome by half

Meiosis II (resembles mitosis)(Figure 6.6, p. 106)

Prophase II: dyad chromosomes move toward the equator of each cell (2)

Metaphase II: dyad chromosomes align at equator of cell Anaphase II: dyad chromosomes separate forming monad

chromosomes (chromatids separate); monads migrate toward opposite poles of each cell

Telophase II: cytokinesis occurs at right angles to the first plane of division in each cell (2); four daughter cells are formed each having half the number of chromosomes as the original cell

Gamete formation - meiosisGamete formation - meiosis

meiosis in males - spermatogenesismeiosis in males - spermatogenesis meiosis in females - oogenesismeiosis in females - oogenesis

SpermatogenesisSpermatogenesis

4 mature sperm cells produced from 4 mature sperm cells produced from meiosis in a single spermatocytemeiosis in a single spermatocyte

OogenesisOogenesis

one mature ovum (egg cell) and 3 polar bodies formed one mature ovum (egg cell) and 3 polar bodies formed from meiosis in a single oocytefrom meiosis in a single oocyte

human female embryo - oocytes formed by mitosis and human female embryo - oocytes formed by mitosis and then complete prophase I of meiosis by time of birththen complete prophase I of meiosis by time of birth

puberty - one oocyte in alternating ovaries resumes puberty - one oocyte in alternating ovaries resumes meiosis under the influence of hormones; at ovulation meiosis under the influence of hormones; at ovulation the oocyte is at metaphase IIthe oocyte is at metaphase II

oogenesis will be completed in the event of oogenesis will be completed in the event of fertilizationfertilization

Comparison of Mitosis and MeiosisComparison of Mitosis and Meiosis(Table 6.2; Figure 6.8)(Table 6.2; Figure 6.8)

MitosisMitosis– synapsis does not occursynapsis does not occur

– one division occursone division occurs

– two daughter cells are formedtwo daughter cells are formed

– chromosome number is maintainedchromosome number is maintained MeiosisMeiosis

– synapsis occurs during Prophase Isynapsis occurs during Prophase I

– two divisions occurtwo divisions occur

– four daughter cells are formedfour daughter cells are formed

– chromosome number is reduced by halfchromosome number is reduced by half

NondisjunctionNondisjunction(Essay 6.1, p. 109)(Essay 6.1, p. 109)

failure of chromosomes to separate during failure of chromosomes to separate during anaphaseanaphase

results: daughter cells contain abnormal results: daughter cells contain abnormal number of chromosomesnumber of chromosomes

Chromosome anomaliesChromosome anomalies

Down syndromeDown syndrome Turner syndromeTurner syndrome Klinefelter syndromeKlinefelter syndrome

Down syndromeDown syndrome

Cause: trisomy 21Cause: trisomy 21 Effects: mental retardation, weakened Effects: mental retardation, weakened

cardiovascular system, shortened limbs, cardiovascular system, shortened limbs, widely-spaced eyes, receding forehead widely-spaced eyes, receding forehead and chin, protruding tongue and chin, protruding tongue

Turner syndromeTurner syndrome

Cause: one X chromosome in female (XO)Cause: one X chromosome in female (XO) Effects: underdeveloped ovaries; failure of Effects: underdeveloped ovaries; failure of

secondary sex characteristics to appear; secondary sex characteristics to appear; sterility; webbing of neck; weakened sterility; webbing of neck; weakened pancreas, liver, and heart in 40%; short pancreas, liver, and heart in 40%; short staturestature

Klinefelter syndromeKlinefelter syndrome

Cause: extra X chromosome in males Cause: extra X chromosome in males (XXY)(XXY)

Effects: underdeveloped testes, sterility, Effects: underdeveloped testes, sterility, failure of secondary sex characteristics to failure of secondary sex characteristics to appear, taller than average appear, taller than average

Detection of chromosome Detection of chromosome anomalies in a fetus:anomalies in a fetus:

amniocentesis amniocentesis chorionic villi samplingchorionic villi sampling

Questions for next class meeting:Questions for next class meeting:

During which weeks of a pregnancy, may During which weeks of a pregnancy, may the following procedures be conducted:the following procedures be conducted:– amniocentesisamniocentesis

– chorionic villi sampling chorionic villi sampling