More On Cell DivisionMore On Cell Division

What if you got paid…What if you got paid…

Just 1 sen per week.Just 1 sen per week. Every week your pay doubles.Every week your pay doubles. How much would you be paid the 4How much would you be paid the 4thth week? week? 1 sen, 2 sen, 4 sen, 8 sen…1 sen, 2 sen, 4 sen, 8 sen… How much would you be paid the 10How much would you be paid the 10thth week? week? 1, 2, 4, 8, 16, 32, 64, RM 1.28, 2.56, 5.121, 2, 4, 8, 16, 32, 64, RM 1.28, 2.56, 5.12 How much would you be paid the 16How much would you be paid the 16thth week? week? RM10.24, 20.48, 40.86, 81.72, 163.44, 326.88RM10.24, 20.48, 40.86, 81.72, 163.44, 326.88

Geometric ProgressionGeometric Progression

FertilizationFertilization

Zygote

Gametes Sperm Ovum

In sexual reproduction, reproductive In sexual reproduction, reproductive cells combine—cells combine—

female — ovum, male — spermfemale — ovum, male — sperm

What would happen if the What would happen if the reproductive cells each had the same reproductive cells each had the same number of chromosomes as the number of chromosomes as the parent’s body cells?parent’s body cells?

This Leads To A QuestionThis Leads To A QuestionWhat happens so that living things What happens so that living things avoid having too many avoid having too many chromosomes from sexual chromosomes from sexual reproduction?reproduction?

As we begin…As we begin…

Let’s review the stages of cell Let’s review the stages of cell division as we already know them…division as we already know them…

InterphaseInterphase ProphaseProphase MetaphaseMetaphase AnaphaseAnaphase TelophaseTelophase These are the stages of…These are the stages of…

Mitosis

Video 1

Video 2

A B C D E F

AB CD E F

A B C D E F

A BCDE F

What happens so that living things avoid What happens so that living things avoid having too many chromosomes from sexual having too many chromosomes from sexual

reproduction?reproduction?

?

Somatic Cell2n=23

pairs=diploid

kary

What happens so that living things avoid What happens so that living things avoid having too many chromosomes from sexual having too many chromosomes from sexual

reproduction?reproduction?

Mitosis ?

Somatic Cell2n=23

pairs=diploid

Normal Somatic Cells

2n=23 pairs=diploid

A Different Kind of Cell A Different Kind of Cell DivisionDivision

Meiosis ?

Somatic Cell2n=23 pairs of chromosomes

diploid

Reproductive Cellsn=23 chromosomeshaploid (gametes)

FertilizationFertilization

Zygote (diploid)

Gametes Sperm Ovum(haploid) (haploid)

A Different Kind of Cell A Different Kind of Cell DivisionDivisionMeiosisMeiosis

2 CyclesMeiosis I•Interphase I•Prophase I•Metaphase I•Anaphase I•Telophase I

Meiosis II

•Prophase II•Metaphase II•Anaphase II•Telophase II

Meiosis I

InterphaseInterphase

Mother cell

Meiosis IIMeiosis II

Prophase I:Tetrad

formation

Prophase I:Tetrad

formation Metaphase I Metaphase I

Prophase I:Condensing

Chromosomes

Prophase I:Condensing

Chromosomes

Anaphase I Anaphase I

Telophase ITelophase I

Note Centromeres

Note bivalentsof homologousChromosomes

Telophase ITelophase I

Meiosis IIMeiosis II

Metaphase II Metaphase II

Anaphase II Anaphase II

Telophase II Telophase II

Prophase IIProphase II

Note that before Prophase IIThere is no replication of DNA

Prophase I:

Tetrad formation/

crossing over

Prophase I:

Tetrad formation/

crossing over

Anaphase I Anaphase I

Metaphase I Metaphase I

Telophase ITelophase IBecause of crossing over, every gamete receives a unique set of genetic information.

Crossing OverCrossing OverSite of crossoveris called chiasma

What is each diagram What is each diagram showing?showing?

What is shown?What is shown?

Mitosis--InterphaseMitosis--Interphase

Meiotic InterphaseMeiotic InterphaseGrowth 1 (G1) phaseGrowth 1 (G1) phase:: Immediately follows cytokinesis. Immediately follows cytokinesis. Very active period,Very active period, Synthesis of proteinsSynthesis of proteins each of the 46 human chromosomes consists of a each of the 46 human chromosomes consists of a

single (very long) molecule of DNA. single (very long) molecule of DNA. At this point cells are At this point cells are 46,2N46,2N, identical to somatic , identical to somatic

cells. cells. Synthesis (S) phaseSynthesis (S) phase:: genetic material is replicated: genetic material is replicated: each of its chromosomes duplicates (each of its chromosomes duplicates (46,2N46,2N). ). cell is still diploid, however, because it still contains cell is still diploid, however, because it still contains

the same number of centromeres. the same number of centromeres. chromatids are in the chromatin form because chromatids are in the chromatin form because

spiralisation and condensation into denser spiralisation and condensation into denser chromosomes have not taken place yet. chromosomes have not taken place yet.

Growth 2 (G2) phaseGrowth 2 (G2) phase:: G2 phase is absent in Meiosis G2 phase is absent in Meiosis

How could you explain…How could you explain…

Remember our question:Remember our question:

What happens so that living things avoid What happens so that living things avoid having too many chromosomes from sexual having too many chromosomes from sexual reproduction?reproduction?

Write a short explanation as you would for Write a short explanation as you would for someone who does NOT know about someone who does NOT know about meiosis. meiosis.

Avoid science vocabulary for now. Avoid science vocabulary for now.

Use 5-10 sentences—try to be brief.Use 5-10 sentences—try to be brief.