Meiosis

Chapter 11

04/25/2016

Introduction to Meiosis: the Big Picture

Starts with 1 original cell

Homologous chromosomes cross over

Ends with 4 non-identical cellsEach has half the DNA of the original cell

Meiosis is Essential for Sexual Reproduction

meioticcell division

fertilization

diploidparentalcells

diploidfertilizedegg

haploidgametes

2n

2n

2n n

n

With Sexual Reproduction, Offspring Get Half of the DNA

from Each Parent

• 23 chromosomes from maternal (mom)

• 23 chromosome from paternal (dad)

Meiosis Allow For Genetic Recombination

• Genetic recombination (during formation of sperm and eggs) is why most siblings are not identical

– Exception: identical twins

Twins

http://www.embryology.ch/anglais/fplacenta/gemell01.html

Genetic Recombination by Crossing Over of Homologous Chromosomes

Figure 13.8a

Prophase I Metaphase I Anaphase I Telophase I andCytokinesis

Centrosome(with centriole pair)

Sisterchromatids

Chiasmata

Spindle

Homologouschromosomes

Fragmentsof nuclearenvelope

Duplicated homologouschromosomes (red and blue)pair and exchange segments;2n 6 in this example.

Centromere(with kinetochore)

Metaphaseplate

Microtubuleattached tokinetochore

Chromosomes line upby homologous pairs.

Sister chromatidsremain attached

Homologouschromosomesseparate

Each pair of homologous chromosomes separates.

Cleavagefurrow

Two haploid cells form; each chromosomestill consists of two sister chromatids.

Figure 13.8b

Prophase II Metaphase II Anaphase IITelophase II and

Cytokinesis

Sister chromatidsseparate

Haploid daughtercells forming

During another round of cell division, the sister chromatids finally separate;four haploid daughter cells result, containing unduplicated chromosomes.

Spermatogenesis Oogenesis

46

92

46 46

46

92

46

2323 23 23 23

Chromosome Non-Disjunction

• Chromosomes do not properly separate into daughter cells

• Can happen in mitosis; much more likely in meiosis

Chromosome Non-Disjunction

• Abnormal gamete can participate in fertilization with a normal gamete

• Offspring either…– Have one copy of a chromosome

• Monosomy

• Nearly always fatal

• Exception: Turner Syndrome (XO)

– Have 3 copies of a chromosome • Trisomy

• Usually fatal, some exceptions

Chromosome Non-Disjunction

• Trisomy 13 – Patau Syndrome

– 80% fatal in first year

• Trisomy 18 – Edward’s Syndrome

– 50% fatal in first year

• Trisomy 21 – Down Syndrome

– Survival to adulthood common

Is this karyotype normal?

Summary

Mitosis Meiosis

Where it occurs Body cells Reproductive organs

Why it occurs Cell repair, growth, asexual reproduction

Sexual reproduction

Number of cells produced 2 4

Number of divisions 1 2

Number of chromosomes in daughter cells

Same as mother cell(diploid)

Half that of mother cell (haploid)

Amount of variation in daughter cells None Lots

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The human life cycle

meiotic cell

division in

testes

meiotic cell

division in

ovaries

adults (2n)

egg (n)zygote (2n)

haploid (n)

diploid (2n)fusion of gametes

sperm (n)

embryo (2n)

baby (2n)

mitotic cell division,

differentiation, and growth

mitotic

cell division,

differentiation,

and growth

mitotic cell division,

differentiation,

and growth

Figure 9-19

Lifecycle of moss