MEIOSISHonors Biology

Spring 2013

Warm-Up With a neighbor, discuss the following:

What is meiosis? How does it differ from mitosis?

Essential QuestionsWhat is meiosis and why is it necessary?

What are the stages of meiosis?

How does meiosis compare to mitosis?

Haploid vs. DiploidDiploid cells

1. Body cells2. Contains homologous chromosomes3. 2N

B. Haploid cells1. Sex cells2. contains one member of each pair3. 1N

Meiosis Meiosis– process which reduces by half the

number of chromosomes in a diploid cell

• meiosis occurs in sex organs of individual

Overview of MeiosisA. Meiosis - formation of sex cells

1. Meiosis is cell division that results in a gamete containing half the number of chromosomes of its parent.2. Meiosis includes 2 steps that each closely resemble corresponding steps in mitosis.

Phases of Meiosismeiosis I•Prophase I•Metaphase I•Anaphase I•Telophase I &?•Cytokinesis PMAT I

Meiosis II•Prophase II•Metaphase II•Anaphase II•Telophase II &?• Cytokinesis PMAT II

Meiosis 1Meiosis I - reduction division

1. Interphase I2. Prophase I3. Metaphase I4. Anaphase I5. Telophase I

Meiosis 1: Prophase 1–chromatin condenses to form chromosomes–homologous chromosomes attach to form tetrads

Meiosis 1: Prophase 1– when tetrads

form, crossing over can occur

– crossing over is the

exchange of part of

homologous chromatids

Meiosis 1: Metaphase 1

–tetrads line up on equator–spindle fibers attach to centromeres

Meiosis 1: Anaphase 1– homologous

chromosomes separate– spindle fibers

pull them to opposite

poles

Meiosis 1: Telophase and Cytokinesis

• telophase I– nuclear

membrane reforms

• cytokinesis– cytoplasm

splits

Meiosis 1: Results• results– two haploid (n)

daughter cells = ½ the

number of chromosomes as

parent cell

Meiosis IIMeiosis II - separation of chromatids

1. Interphase II2. Prophase II3. Metaphase II4. Anaphase II5. Telophase II

–chromosomes already formed

Meiosis II: Prophase II

–chromosomes line up along equator

Meiosis II: Metaphase II

–sister chromatids separate–move to opposite ends

Meiosis II: Anaphase II

•telophase II–nuclear envelope reforms

•cytokinesis–cytoplasm splits

Meiosis II: Telophase and Cytokinesis

Meiosis II: Results– 4 haploid (n) daughter cells– all genetically different

Ploidy

parent cell?•dipliod (2n)after meiosis I?•haploid (n) (monopliod)after meiosis II?•hapliod (n)

Purpose of Meiosis: Diversity

Meiosis and sexual reproduction significantly contribute to genetic variation among offspring.

Purpose of Meiosis: Gametes

• role of meiosis is to produce gametes

• meiosis in males– spermatogenesis– produces 4 sperm cells

• meiosis in females– oogenesis– produces 1 ovum (egg cell) and 3

polar bodies

Where Does Meiosis Occur?

Mitosis vs. Meiosis where does is occur?• meiosis – sex organs; mitosis – somatic cells number of cell divisions?• meiosis – 2; mitosis – 1 number of cells produced?• meiosis – 4; mitosis – 2 ploidy of daughter cells?• meiosis – hapliod (n); mitosis – dipliod (2n) # of chromosomes compared to parent cell?• meiosis – ½ # chromosomes; mitosis – same

Mitosis vs. Meiosis