the role of reproduction (part 7)

8/2/2019 The Role of Reproduction (Part 7)

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The Role of Reproduction (Part 7)

Challenges to Successful Fertilization Environmental

Sperm egg recognition

o Sperm contacts the egg via an elongated acrosomal process Sperm egg fusion

o Egg membrane rises around sperm head

o Microvilli elongate around entering


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Mammalian Ovum Fertilization The human egg is protected by the cumulus and the zona pellucida


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Fertilization: More than Just Fusion of the Gametes Primordial follicles: the egg is arrested in the prophase of meiosis 1

Hormonal stimulation reinitiates meiosis

Meiosis 1 completes at the time of ovulation

Meiosis 2 does not complete until the time of fertilization

Anisogamy


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In anisogamous species, the extra chromosome sets are extruded as

polar bodies small cells with little cytoplas

Anisogamy: Stages of Meiosis

Double Fertilization Angiosperms


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External Fertilization Plants

o Air environment

o Water environment

Animals

o Aqueous environment

Works well for sessile and not so well for mobile life forms

Internal Fertilization Sperm can only move through liquid

Sperm released into the air will dry out and die

All terrestrial animals and plants use internal union of gametes

Evolution of Internal Fertilization Earliest plants and animals evolved in aquatic environments

Internal gamete union in plants parallels the exploitation of

terrestrial environments


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Possible stages

in evolution of carpel (a) and stamen (b) from leaf-like structures


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Amphibians: First Animals to Move to Land Must move back to, and use water, for external fertilization

Use behavioural adaptations to ensure reproductive success

Reptiles: First Fully Terrestrial Animals Even those that have returned to the water still use the land for

reproductive purposes

Must use internal fertilization shelled egg formed

Eggs must be penetrated by sperm prior to release

Birds and Mammals Birds: as with reptiles, the shelled egg requires internal fertilization

Terrestrial animals have evolved an incredible diversity of

behavioural and anatomical adaptations to get male gametes

into the female reproductive tract


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Where does the embryo develop? Oviparous


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o Lay eggs in the environment and the embryo develop outside the

mothers body

Viviparous

o Retain the embryo within the mothers body during its early

development stages

the role of reproduction (part 7)

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