Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

BIOLOGY · Stage 9 · Chapter 11: Human Genetics and Development

Biology 11.2 Fertilisation: new life begins — Nuclei Fusion and Zygote Formation — Alhussan International Academy - Biology (Stages 7-9) Revision Notes & Topical Questions

5 min read 5 parts Lesson 2 of 9

📚 Alhussan International Academy - Biology (Stages 7-9) Hub / Topical Revision
🩺
100% Curriculum Specification Match Lesson Doctor Verified
The only platform with 100% curriculum specification match and verified interactive question banks.
0%read
Lesson summary

11.2 Fertilisation: new life begins — Nuclei Fusion and Zygote Formation — Alhussan International Academy — Biology (Stages 7–9) notes: Describe how fertilisation occurs, how the zygote becomes an embryo that implants in the uterus, and how twins and IVF arise.Fertilisation = the…

  • Sperm cells
  • Cell division
  • Implantation
  • Development
  • Identical twins
  • IVF (in vitro fertilisation)

Section 1 of 5What fertilisation is

  • Fertilisation = the fusion of the nucleus of a sperm with the nucleus of an egg to form a single cell called a zygote.
    • Each nucleus has 23 chromosomes, so the zygote has the normal 46, with half from each parent.
    • The zygote has a new combination of DNA, so the offspring differs genetically from either parent.
  • Egg cells
    • they are large, contain a food store, cannot move themselves and are formed and stored in the ovaries before birth.
    • A few mature and one is released each month once a girl is a teenager.
  • Sperm cells
    • they are very small and made in the testes, and boys make millions every day once they are teenagers.

Section 2 of 5Stages of fertilisation

  1. During intercourse, semen is released into the vagina near the cervix.
  2. Millions of sperm swim through the cervix and uterus into the oviducts using their tails, and many die on the way.
  3. If an egg has been released recently, sperm surround it and enzymes in the acrosomes digest a path through the jelly coat.
  4. One sperm head enters the egg and the egg forms a protective layer at once, so no other sperm can enter.
  5. The sperm nucleus fuses with the egg nucleus, forming the zygote.
  • Timing
    • an egg survives about 24 hours after ovulation and sperm live inside the female body for several days,
    • so intercourse around ovulation gives the highest chance of pregnancy.

Section 3 of 5From zygote to embryo

  • Cell division
    • the zygote divides by mitosis as it travels down the oviduct to the uterus, taking about 3 to 4 days.
    • First two cells form, then four, then eight, until a ball of cells forms.
  • Embryo
    • the ball of cells is called the embryo.
  • Implantation
    • about 6 to 7 days after fertilisation the embryo sinks into the soft lining of the uterus.
  • Development
    • the cells keep dividing and begin to specialise into tissues and organs.
  • Fetus
    • from about eight weeks, when the main organs have formed, the developing baby is called a fetus.

Section 4 of 5The importance of genes

  • Control
    • once the embryo begins to grow, the inherited genes determine its characteristics.
    • They control skin and eye colour, how the heart forms and the length of the arms and legs.
  • Sex
    • the chromosomes also determine whether the individual is a boy or a girl.

Section 5 of 5Twins and fertility treatment

1Identical twins2 points
  • they form when the embryo splits in two early on.
  • They have the same genes and are always the same sex.
2Non-identical twins2 points
  • they form when two eggs are fertilised by two sperm.
  • They are as different as other brothers and sisters.
3IVF (in vitro fertilisation)3 points
  • eggs are collected from the mother and fertilised with sperm in a laboratory dish.
  • A few embryos are then placed in the uterus.
  • It helps couples who cannot conceive naturally.

Last updated: