Fertilisation and the first cleavages · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Sperm-entry barriers and early cleavage

Science · Organisms & Life Processes · ages 19-20
Name ______________________   Date ____________

  1. What produces the brief fast block to further sperm fusion in a sea urchin egg?

    • A change in the egg membrane voltage
    • Release of DNA from the egg nucleus
    • Division of the egg into smaller cells

  2. Which statement describes normal fertilisation in birds?

    • Only one sperm enters; any additional sperm entry necessarily prevents normal bird development.
    • Several sperm enter, and all of their nuclei combine with the egg nucleus during fertilisation.
    • Several sperm enter, but only one sperm nucleus combines with the egg nucleus.

  3. Which process forms the longer-lasting slow block in a sea urchin egg?

    • A voltage change with no change to the envelope
    • Cortical-granule release that changes the envelope
    • Repeated mitoses that split the envelope into cells

  4. Which statement compares sea urchin and mammalian sperm-entry barriers accurately?

    • Both use the same electrical block as their only lasting barrier.
    • Mammals lack an established sea-urchin-like electrical fast block.
    • Sea urchins lack an electrical fast block and rely only on egg division.

  5. One zygote undergoes three complete rounds of equal cleavage divisions in a no-growth model. What fraction of its starting volume is in each final cell?

    • It is one half of the starting volume
    • It is one quarter of the starting volume
    • It is one eighth of the starting volume

  6. Which comparison correctly relates yolk to cleavage in a sea urchin and a chicken?

    • Both eggs divide completely; yolk changes only the number of sperm that enter.
    • Sea urchin: whole egg divides; chicken: a disc above the yolk divides.
    • Sea urchin: only a disc divides; chicken: the whole yolk-rich egg divides.

  7. Sam says the rapid S/M model applies to every animal embryo at every stage, and that DNA is not copied when there is little overall growth. Which correction addresses both claims?

    • S phase copies DNA; mammalian cleavage is generally slower, and cycle structure varies.
    • S phase copies DNA; rapid S/M cycles describe every animal embryo at every developmental stage.
    • S phase is skipped; rapid S/M cycles occur only in mammalian embryos throughout all cleavage stages.

  8. If cortical-granule release is prevented in a sea urchin egg, a normal fast electrical block alone can still provide lasting protection against extra sperm.

    Circle one:   True   False

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Answer key

For grown-ups. Fold this page away before handing over the rest.

Sperm-entry barriers and early cleavage W1-mt_9BXQsdP8fI-s1

  1. A change in the egg membrane voltage · Sperm-egg fusion rapidly depolarises the egg membrane. This voltage change briefly blocks additional sperm fusion.
  2. Several sperm enter, but only one sperm nucleus combines with the egg nucleus. · Bird eggs normally admit several sperm. Only one sperm nucleus combines with the egg nucleus, so normal bird fertilisation is an exception to the one-sperm-entry rule.
  3. Cortical-granule release that changes the envelope · A rise in calcium triggers cortical-granule release. The released contents modify and lift the outer envelope to form the slow-block barrier.
  4. Mammals lack an established sea-urchin-like electrical fast block. · Mammals have no established equivalent of the sea urchin electrical fast block. Changes to the egg coat and membrane help limit additional sperm.
  5. It is one eighth of the starting volume · Three complete rounds produce 2 × 2 × 2 = 8 cells. Equal division of the unchanged starting volume gives one eighth to each cell.
  6. Sea urchin: whole egg divides; chicken: a disc above the yolk divides. · Sea urchin eggs have little yolk and undergo whole-egg, holoblastic cleavage. Chicken cleavage is meroblastic, confined to a disc above the yolk.
  7. S phase copies DNA; mammalian cleavage is generally slower, and cycle structure varies. · S phase still copies DNA before mitosis. Rapid S/M cycles describe many early non-mammalian embryos, not all embryos at every stage. Mammalian cleavage is generally slower, and cycle structure varies with species and stage.
  8. False · The electrical fast block is brief. Preventing cortical-granule release prevents the usual slow-block envelope change, so the fast block alone does not provide lasting protection.
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