Dr. Siham Gritly 1

8- How cell dividedcell division 11

Meiosis; Reduction division

Dr. Siham Gritly

University of Bahri

Dr. Siham Gritly 2

Meiosis; Reduction division

That Occurs in Sexually Reproducing Organisms

• Part of the process of gamete formation,

• consisting of chromosome conjugation and two cell

divisions, in the course of which the diploid

chromosome number becomes reduced to the haploid.

• Diploid cells have two homologous copies of each

chromosome.

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• Meiosis is used only for the production of

sperm and eggs

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Meiosis has two unique features: synapsis and reduction division.

• 1- Synapsis

• * The process of pairing throughout the length

of the homologous chromosomes and

exchanging genetic fragments is called

synapsis.

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• 2-Reduction Division• Reduction division: The first cell division in

meiosis, the process by which germ cells are formed.

• In reduction division, the chromosome number is reduced from diploid (46 chromosomes) to haploid (23 chromosomes)

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Meiosis features

• -yields genetically unique haploid daughter

cells (cells with only one member of each

homologous chromosome pair).

• -involves 2 nuclear and cytoplasmic divisions

• -produces 4 haploid cells

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The Sexual Life Cycle

1. In the sexual life cycle, there is an alternation of diploid and haploid generations.

2. Normal diploid body cells are called somatic cells.

3. Somatic cells arise from the zygote and are all genetically identical.

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Germ-Line Tissues

1. Cells that produce gametes are called germ-line tissues.

2. Germ-line cells will undergo meiosis to produce haploid gametes.

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The stages of meiosis can be broken down into two main stages, Meiosis I and Meiosis II

• A. Meiosis I can be broken down into four substages:

• Prophase I,• Metaphase I, • Anaphase I • and Telophase I• Meiosis I serves to divide the two versions of

each chromosome;

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• B. Meiosis II can be broken down into four substages:

• Prophase II,• Metaphase II, • Anaphase II• and Telophase II• meiosis II separates the two replicas of each

chromosome.

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Meiosis I

• The first cell division in Meiosis is known as Meiosis I.

• The function of Meiosis I is very different from that of either Mitosis or Meiosis II.

• In both Mitosis 1 and Meiosis II sister

chromatids are separated during anaphase

to produce identical daughter cells.

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• In Meiosis I members of homologous chromosome pairs are separated.

• This results in the segregation of genes into the two gametes.

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stages of Meiosis I :

• 1-prophase I occurs as the chromosomes condense, homologues pair, and crossing over occurs;

• Prophase; Main event in prophase of M1 is

“crossing over”, also called

“recombination”.

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• In crossing over, homologous chromosomes pair up, and exchange segments by breaking and rejoining at identical locations.

• ( Crossing over allows for combinations of genes that may never have existed previously).

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Crossing over allows for combinations of genes that may never have existed previously

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• 2-metaphase I involves formation of the spindle apparatus and alignment of chromosome pairs along the center of the cell;

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• 3-anaphase I, The centromeres break and homologous chromosomes separate and pulled toward the poles (note that the sister chromatids are still attached)

• Cytokinesis begins

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• 4-telophase I, the chromosomes gather at each pole and prepare for the second division.

• cleavage furrow formed• Cytokinesis reaches completion, creating two

haploid daughter cells

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Meiosis II

• Meiosis II, which also has four stages, follows

after meiosis I,

• This result is the separation of the sister

chromatids to form four haploid gametes.

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4 stages of meiosis11

• * prophase 11, the chromosomes condense and the spindle forms.

• Centrioles form and move toward the poles• The nuclear membrane dissolves

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• *Metaphase 11; chromosomes line up singly on the cell equator.

• Microtubules grow from the centrioles and attach to the centromeres

• The sister chromatids line up along the cell equator

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• *Anaphase 11; centromeres divide and pulled to opposite poles.

• The centromeres break and sister chromatids separate

• Cytokinesis begins

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• *telophase 11; cytoplasm divided into 2 cells.• The chromosomes may decondense • Cytokinesis reaches completion, creating four

haploid daughter cell

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Cytokinesis

• the cytoplasmic division of a cell at the end of mitosis or meiosis, bringing about the separation into two daughter cells.

• The division of the cytoplasm and the plasma membrane following the division of the nucleus resulting into two cells, each having its own nucleus and cytoplasm surrounded by a plasma membrane.

• This process begins during the late stage of mitosis (Telophase) and meiosis (Telophase II).

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