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Meiosis. . Two Types of Diploid Cells In Sexual Organisms. Somatic. GERM. Body Cells Diploid Divide by mitosis to make exact copies In every tissue except reproductive tissue. “Germinating” (reproductive) cells Diploid

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two types of diploid cells in sexual organisms

Two Types of Diploid Cells In Sexual Organisms

Somatic

GERM

  • Body Cells
  • Diploid
  • Divide by mitosis to make exact copies
  • In every tissue except reproductive tissue
  • “Germinating” (reproductive) cells
  • Diploid
  • (Most) Divide by meiosis to make gametes (sex cells) (sperm & ova)
  • In testes & ovaries
what is an n
What is an “n”?

Haploid (n) is the number

of chromosomes in a gamete.

Diploid (2n) – number of chromosomes in a

somatic cell. You have one copy from

the mother, and one copy from the

Father.

slide6

In Meiosis: ONEdiploid, germ cell divides to become FOUR

(4) daughter cells

  • (*circle*) … diploid / haploid
  • (*circle*) … somatic / gamete
slide7

In order to go from diploid to haploid, Meiosis Involves Two Divisions

Meiosis I

Meiosis II

  • Splits homologous pairs of chromosomes
  • Splits sister chromatidsapart
time to name the phases
Time To Name The Phases
  • PROPHASE I
  • METAPHASE I
  • ANAPHASE I
  • TELOPHASE I
  • PROPHASE II
  • METAPHASE II
  • ANAPHASE II
  • TELOPHASE II
prophase i

PROPHASE I
  • Nuclear envelope disintegrates
  • Centrioles migrate, produce spindle fibers
  • Homologous chromosomes pair up
  • Homologous pairsCROSS OVER!
prophase i1

PROPHASE I

CROSSING OVER

Occurs when homologous chromosomes exchange segments of genes

metaphase i

METAPHASE I
  • Homologous pairs of chromosomes line up along metaphase plate
anaphase i

ANAPHASE I
  • Homologous pairs pulled apart to opposite poles
telophase i cytokinesis

TELOPHASE I & CYTOKINESIS
  • Chromosomes (no longer homologous pairs!) prepare for meiosis II
  • Cytoplasm begins to split into first 2 (of 4) daughter cells
telophase i cytokinesis1

TELOPHASE I & CYTOKINESIS

NOTE:

At the end of meiosis I, daughter cells are already Haploid!

(because homologous pairs were separated)

2 haploid daughter cells then go through meiosis ii

2 haploid daughter cells then go through MEIOSIS II

In Meiosis II, 2 haploid cells will separate sister chromatids to make 4 gametes!

prophase ii

PROPHASE II
  • Centrioles migrate, produce spindle fibers

Just like in mitosis!

metaphase ii

METAPHASE II
  • Chromosomes line up along metaphase plate

Just like in mitosis!

anaphase ii

ANAPHASE II
  • Sister chromatids pulled apart to opposite poles

Just like in mitosis!

telophase ii cytokinesis

TELOPHASE II & CYTOKINESIS
  • Nuclear envelopes reform around 4 daughter cells
  • Cytoplasm splits to separate the 4 daughter cells
  • Each of the 4 daughter cells is a new gamete withgenetic information different from either parent!
meiosis i

Meiosis II

Meiosis I

Check – Point: At what point do the cells become haploid?

slide26

200 to 400 million sperm per ejaculate!

But what happens to the other 3 eggs?

The largest ovum absorbs the others!

Only one wins per cycle, usually

finally fertilization
Finally, Fertilization
  • Male and female gametes unite
  • Fusion of two haploid nuclei produces diploid nucleus
  • Egg + Sperm = Zygote(new life in one diploid cell)
meiosis and sexual reproduction guarantee variation and diversity1
Meiosis and Sexual Reproduction Guarantee Variation and Diversity!

In Three Ways

  • Independent Assortment
    • Homologous Chromosomes separate randomly
  • Crossing Over
    • Homologous pairs exchange gene segments
  • Random Fertilization
    • Which sperm meets which egg? (game of chance)