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Chapter 42

Chapter 42 Textbook notes

Biological Sciences
Course Code
Andrew Mason

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How do animals reproduce without sex?
x Sexual reproduction is universal in almost all organisms
x Some can reproduce sexually and asexually and some can only reproduce
x Offspring that are reproduced asexually are identical to each other and parents
o Can convert resources to reproduce
o But does not have the genetic diversity that comes with sexual
reproduction which is a disadvantage in changing environments
x Animals that reproduce asexually are usually invertebrates
o Are sessile
o Live in constant environment where reproducing asexually would be
x Three ways animals reproduce asexually is by
1. budding,
2. regeneration
3. parthenogenesis
Budding and regeneration produce new individuals by mitosis
x Budding²new individuals form as outgrowths or buds from older animals
o Bud formed by mitosis
o Bud is identical to parent
o Can grow to be as big as parent before being independent
x Regeneration
o Usually thought as replacing damage tissue or lost limbs
o But some organisms can regenerate to a new individual
o If you cut a sea star into individual pieces then each piece will become an
individual organism
x Parthenogenesis
o The development of unfertilized eggs
o Occurs in arthropods and some fish, amphibians and reptiles
o Some species parthenogenesis determines sex²honey bees, males come
from unfertilized eggs, females come from fertilized eggs

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o Parthenogenesis sometimes requires sexual behaviour²like in whiptail
All are female
Some will act like male to stimulate another female to release eggs
Whether they act like a male or female depends on hormonal
states²when estragon levels are high it acts female, when estragon
levels go down and progesterone levels go up, then it acts male
42.2 How Do Animals Reproduce Sexually
x Large portion of time and energy budgets of sexually reproducing animals goes
into mating
o Mating exposes them to predators and usually takes a lot of energy, also
keeps them from doing other things like feeding and taking care of
existing offspring.
x Sexual reproduction requires 2 haploid cells to come together to make a diploid
o Haploid cells are also called gametes
o Gametes are formed by gametogenesis
x Sexual reproduction has 3 parts
o Gametogenesis (making gametes)
o Mating (getting gametes together)
o Fertilization (fusing gametes together)
Gametogenesis produces eggs and sperm
x Gametogenesis happens in the gonads²testes in males and ovaries in females
x Gametes formed in males are sperm and eggs in female
x Sperm moving using a flagellum and eggs are non-motile
x Gametes form from germ cell²different from other cells in the body which are
somatic cells
x Germ cells are secluded in the body until gonads start to form
x Then germ cells go to the gonads and by mitosis become spermatagonia and
oogonia which are diploid
x They multiply by mitosis and become primary spermocytes and primary oocytes
x Then through meiosis the chromosomes reduce to haploids²these become sperm
and ova
x Mitosis in embryo comes before male germ cells become spermatogonia
x Primary spermocytes divide by meiosis to form secondary spermocytes
x Secondary spermocytes become 4 haploid spermatids
x Spermatids are connected by cytoplasmic bridges
o Bridge happens because of asymmetry in sperm cells²one get X and the
other gets Y, X chromosomes have more genes than Y. some genes on the
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