MEIOSISMEIOSIS

Two types of reproduction

• Asexual: – only one parent cell needed– Parent cell divides using mitosis to

produce new, exactly identical offspring

• Sexual reproduction:– Two parent cells join together– Parent cells known as sex cells or

reproductive cells– Different from ordinary body cells in

that they have only HALF the usual amount of DNA (chromosomes)

– WHY?

How are these sex cells How are these sex cells made?made?

• Through a process called MEIOSIS

• Diploid (2n) Diploid (2n) haploid (n)haploid (n)(paired chromosomes) (single chromosome for each

characteristic)

• MeiosisMeiosis is necessary for sexual sexual reproduction.reproduction.

• Two divisionsTwo divisions (meiosis I meiosis I and meiosis meiosis IIII).

MeiosisMeiosis• Sex cellsSex cells have halfhalf the # of

chromosomeschromosomes.

• SimilarSimilar to mitosismitosis with some chromosomal differences.

• Below is model of mitosis.

So how do we get half the So how do we get half the number of chromosomes?number of chromosomes?

• Chromosomes are copied ONCE (replication), but the nucleus divides TWICE.

• Two stages: Meiosis I and Meiosis II• Each stage still has prophase,

metaphase, anaphase and telophase, where the same things occur as in mitosis

Remember…

• Prophase: Prepare: Chromosomes form, nucleus disappears

• Metaphase: chromosomes line up in MIDDLE

• Anaphase: Chromosomes move APART

• Telophase and Cytokinesis : Cell TEARS in two

This is a model of Meiosis

InterphaseInterphase

• DNA is Replicated (copied).

Prophase IProphase I

Metaphase IMetaphase I

Anaphase IAnaphase I

Note that the sister chromatids are still attached at the centromere (different from mitosis)

Telophase ITelophase I

Cytokinesis ICytokinesis I

Meiosis 1 Review

Meiosis IIMeiosis II

• No interphase II No interphase II (no more DNA replicationDNA replication)

Prophase IIProphase II• same as prophaseprophase in mitosismitosis

Metaphase IIMetaphase II• same as metaphasemetaphase in mitosismitosis

Anaphase IIAnaphase II

• same as anaphaseanaphase in mitosismitosis• sister chromatids separatesister chromatids separate

Telophase IITelophase II

• Same as telophasetelophase in mitosismitosis.

• Nuclei form.

• Afterwards, CytokinesisAfterwards, Cytokinesis occurs.

• Remember: Four haploid daughter Remember: Four haploid daughter cells are cells are produced.produced.

Telophase IITelophase II

Cytokinesis Cytokinesis IIII

Meiosis 2 Review

Meiosis

Question:Question:

• A cell containing 20 chromosomes20 chromosomes (diploid)(diploid) at the beginning of meiosis would, at its completion, produce cells containing how many chromosomeschromosomes?

• 10 Chromosomes (haploid)

FertilizationFertilization

• The fusion of a sperm sperm (gamete or “S” Cell) and eggegg (gamete or “S” Cell) to form a zygotezygote.

• A zygote is a fertilized egg

n=23egg

sperm n=23

2n=46zygote

human

human

human

Were you Paying AttentionWere you Paying Attention1.1. In asexual reproduction the genes of In asexual reproduction the genes of

the offspring are _____________ those of the offspring are _____________ those of the parent.the parent.

2.2. In what stage of Meiosis are the In what stage of Meiosis are the homologous pairs pulled apart? homologous pairs pulled apart? ____________________________________

3.3. What is another name for reproductive What is another name for reproductive cells or sex cells? _________________cells or sex cells? _________________

4. How many gametes are produced at the . How many gametes are produced at the end of Meiosis from one parent cell?end of Meiosis from one parent cell?

______________________

5. A sperm cell from a frog has 14 5. A sperm cell from a frog has 14 chromosomes how many chromosomes chromosomes how many chromosomes will a fertilized frog egg have? _________will a fertilized frog egg have? _________

6. Why is Meiosis important to sexual 6. Why is Meiosis important to sexual reproduction?reproduction?________________________________________________________________________________________________________________________________________________________________________________