2.4 gametogenesis in mammals

importance of centrioles from sperm cells after fertilisation. Egg cells don’t have them. (3)

• sperm cells are source of centrioles for zygote

• so spindle fibres can be synthesised

• so zygote can divide by mitosis to develop into embryo

how fertilisation produces zygote (4)

• head of sperm come into contact with membrane of oocyte- triggers acrosome reaction

• completes meiosis of oocyte

• cortical reaction occurs (prevention of polyspermy)

• fusion of fusion of 2 nuclei from sperm and egg cell to form diploid zygote

how can: low sperm count, structural defects of sperm, chromosomal mutations, absence of an acrosome and mutations of mitochondrial DNA—> lead to infertility. (6)

• low sperm count: not enough enzymes to digest through outer layer of egg cell to fertilise

• structural defects: no mid piece= no mitochondria to provide energy

• chromosomal mutations: lack of/too much genetic material

• absence of acrosome: no enzymes to digest through zona pellucida and follicle cells

• mutations of mitochondrial DNA: less energy to swim

when oocytes produced(1)

before birth

difference between spermatic and mature spermatozoon(1)

• spermatids have no acrosome/flagellum

what’s the advantage of competition between sperm (2)

• the fastest/healthiest sperm will fertilise egg

• pass on the advantageous alleles to offspring

what’s the importance of DNA replication during development of zygote into blastocyst(3)

• zygote undergoes mitosis several times to form blastocyst

• makes identical copies of DNA molecules

• so all cells are diploid (46)

• so when mitochondria divides they’ll have a copy of DNA

why mutations in nuclear DNA can be inherited from either the mother or the father whereas mutations in mitochondrial DNA are only inherited from the mother (2)

• both gametes contain DNA

• only secondary oocyte contains mitochondria and not in the sperms head (only head fuses with membrane and the mitochondria is found in the mid piece)