MEIOSIS (making sperm and egg cells

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MEIOSIS(making sperm and egg cells)
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DNA Passes from Parent to Offspring

• Two types of reproduction (making more of you)
• Asexual reproduction ? a single parent reproduces
  by itself
• Parent and offspring (child) are genetically
  identical
• e.g., bacteria, many plants and fungi

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DNA Passes from Parent to Offspring

• Sexual reproduction ? 2 cells (different parents)
  unite to produce the first cell of the new
  offspring
• parents and offspring are genetically unique

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Review of Chromosomes

• Chromosome ? condensed DNA, containing all genes
• Normal cells diploid (2n) ? 2 copies of each
  chromosome (good idea! backup copy!)
• In diploid cells, 1 copy came from each parent
  (people 1 from mom, 1 from dad)
• Offspring have the same amount of DNA as their
  parentsso how can parents only pass on HALF
  their genetic material?

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What is meiosis?

• Meiosis ? special cell division for sexual
  reproduction
• Produces haploid (1n) cells ? 1 copy of each
  chromosome
• Haploid cells gametes (sperm/eggs)
• Human gametes
• In ?, meiosis produces (in testes) 4 sperm cells
• In ?, meiosis produces (in ovaries) 1 egg cell
  and 3 polar bodies (that later disintegrate)

Mitosis cells divide once Meiosis cells divide
twice
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Why do meiosis?
Why do ovary and testes cells go through meiosis?
Meiosis is reduction division. It reduces the
chromosome number in the gametes by one half.
From diploid (2n) to haploid (n).
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Chromosome numbers
In humans, 2n 46 A normal body cell has 46
chromosomes ? 23 came from mom, 23 from dad each
set of 23 from mom is similar to the set of 23
from dad In humans, n 23 A normal gamete has
half the normal chromosome number (n) ? just one
set
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Chromosome number questions

1. If a horse retina cell has 36 chromosomes, how
   many chromosomes does a horse sperm cell have?
2. If n 4 for fruit flies, how many chromosomes
   does a wing cell have?
3. If the 2n number of a pine tree is 86, how many
   chromosomes would be found in a pine ovum?
4. If n 16 for goldfish, how many chromosomes are
   in a fin cell?

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Homologous Pairs

• Homologous pairs ? Pairs of chromosomes with the
  same sequence of genes
• One is from mom, one from dad
• Genes are in the same order, but
  not identical!
• e.g., blood type

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Phases of Meiosis

• Remember mitosis? (I)PMAT
• In meiosis, each phase occurs twice
• Meiosis I Prophase I, Metaphase I, Anaphase I,
  Telophase I, Cytokinesis I
• Meiosis II Prophase II, Metaphase II, Anaphase
  II, and Telophase II, Cytokinesis II

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Dont freak out I changed the order of the next
slides
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Meiosis I

• In Prophase I, homologous chromosomes pair up in
  a tetrad
• same gene sequences, same banding pattern, same
  position of centromere
• Crossing-over takes place

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A Sketch of Chromosomes Crossing Over (Prophase I)

• During crossing-over, homologous chromosomes
  exchange pieces of DNA
• Allows for a reshuffling of genetic material

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Meiosis I

• Meiosis I results in 2 haploid daughter cells

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Meiosis II
Meiosis

• Each of the haploid daughter cells from meiosis I
  divide again to make 4 haploid cells.

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Comparing Mitosis and Meiosis

• The processes of mitosis and meiosis are very
  similar, but with some important differences

• Meiosis
• Includes 2 cycles of cell division (meiosis I and
  meiosis II)
• Occurs only in sex cells (gametes)
• Chromosomes cross over in Prophase I
• Results in four genetically different haploid
  cells
• Metaphase I 2 duplicated chromosomes per spindle
  fiber

• Mitosis
• Includes only 1 cycle of cell division
• Occurs in body cells (somatic cells)
• No crossing over of chromosomes
• Results in two genetically identical diploid
  cells
• Metaphase 1 duplicated chromosome per spindle
  fiber

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Now, lets look at our flipbooks and compare!!
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Interphase

• DNA replication occurs
• DNA is in chromatin form (unwound, like spaghetti)

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Prophase I

• DNA condenses to form chromosomes
• Homologous chromosomes pair up to form tetrads
• Crossing over takes place

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Metaphase I

• Homologous pairs line up along center of cell
• 1 homologous chromosome and its duplicate are
  attached to each spindle fiber

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Anaphase I

• Homologous pairs separate, move to opposite ends
  of the cell

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Telophase I

• Nuclear membrane reappears
• Spindle fibers disappear

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Cytokinesis

• Cytoplasm of the two cells separate
• Meiosis I results in 2 haploid cells that are
  genetically different

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Prophase II

• The 2 haploid cells from Meiosis I will divide
  again

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Metaphase II

• Chromosomes (set of sister chromatids) line up
  along the center of the cell
• 1 sister chromatid attaches to each spindle fiber

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Anaphase II

• Centromeres split, sister chromatids are pulled
  apart

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Telophase II and Cytokinesis

• Cytoplasm divides, resulting in 4 genetically
  different haploid cells