Meiosis: Making haploid reproductive cells

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MeiosisMaking haploid reproductive cells
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Goal of Meiosis

• The purpose is to make haploid cells so that when
  2 haploid cells combine you have a new diploid
  offspring that has an original combination of
  genes.
• Haploid cell Haploid cell Diploid cell
  (2n) (1n) (1n) that turns into a
  muticellular offspring

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Meiosis Overview

• Makes 4 haploid cells
• Takes place in sex cells (gametes) such as egg
  sperm cells
• Location Testes (male) and ovaries (female)
• The Goal
• To make 4 haploid daughter cells for reproduction
  that are genetically different from the parent
  cells.
• Each haploid daughter cell will contain a mixture
  of the parents chromosomes, so that the 4
  daughter cells should be different.

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Meiosis

• Involves 2 cell divisions
• The cell will go through interphase (G1, S,
  G2)but then will divide twice ? resulting in 4
  haploid cells
• There are 2 parts in Meiosis
• 1. Meiosis 1 and Meiosis 2

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Meiosis I and Meiosis II

• Both go through each of the phases of mitosis
  (PMAT)
• After each one cytokinesis occurs
• The goal is to reduce chromosome number to
  haploid so that 2 cells can combine to make a
  diploid organism

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Meiosis 1 Prophase 1

• DNA coils into chromatids
• Spindle fibers appear
• Nucleus/nucleolus disappear
• Homologous chromosomes pair up into tetrads of 4
  chromatids, called synapsis
• Crossing over may occur causing genetic
  recombination

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What is crossing over?

• When a homologous pair bumps into each other
  during prophase I and they exchange portions one
  chromatid ? leads to genetic recombination

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Metaphase I Anaphase I

• Metaphase I
• Tetrads line up randomly in the center. Spindle
  fibers attach to each end of the homologous pair

• Anaphase I
• Homologous pairs split randomly, called
  independent assortment

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Independent Assortment

• The random lining up of homologous pairs during
  metaphase I. This allows for different gametes to
  be possible and also leads toward genetic
  recombination

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More independent assortment
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What is genetic recombination?

• when the combination of genes in an offspring
  are different from the gene combination found in
  the parents
• Occurs in 2 ways in meiosis
• 1. Crossing over where homologous pairs exchange
  portions of one chromatid
• 2. Independent assortment where the homologous
  pairs line up randomly or without influences like
  which parent they came from or whether they are
  dominant or recessive.

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Telophase I and Cytokinesis I

• Chromosomes reach poles
• Cytokinesis occurs forming 2 haploid cells with 2
  copies of their chromosomes
• Nuclear membrane may or may not reform before
  meiosis II

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Meiosis II (2nd cell division, similar to
mitosis steps, PMAT)

• Prophase II Spindle fibers form
• Metaphase II Chromosomes move to center (along
  equator)
• Anaphase II Chromatids separate toward the poles
• Telophase II Nuclear membrane reforms. Spindle
  fibers disassemble.
• Cytokinesis occurs forming 4 haploid cells with 1
  copy of each chromosome.

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How does Meiosis differ in males vs. females?

• Since meiosis is producing sex cells (sperm
  eggs) it takes place in the testes (males) the
  ovaries (female)
• In males
• Spermatogenesis - the formation of 4 spermatids
  (haploid cells) which will turn into spermatozoa
  (sperm cells).
• In females
• Oogenesis - the formation of 1 egg cell (ova)
  with more cytoplasm than the other 3 cells. The
  remaining 3 cells are called polar bodies and
  will disintegrate

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• The phases of Meiosis

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Diagrams
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Let us compareMitosis VS. Meiosis

• Mitosis makes IDENTCIAL cells to the parent
• Mitosis make DIPLOID cells
• Mitosis occurs in BODY cells
• Mitosis makes 2 DAUGHTER CELLS

• Meiosis make DIFFERENT cells from the parent
• Meiosis makes HAPLOID cells
• Meiosis occurs in REPRODUCTIVE cells
• Meiosis makes 4 DAUGHTER CELLS

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Animations
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