fertilisation

OVUM:

nucleus - Contains only one copy of each chromosome (haploid)

follicle cells - Secrete chemicals that triggers the acrosome reaction

cytoplasm - Very large so fertilised cell can divide immediately (mitosis without interphase) - ribosomes, mitochondria

lipid droplets - source of energy for future growth and division

zona pellucida - surrounds plasma membrane - Hardens once sperm has entered ova, stops further cells entering. releases chemicals that trigger the release of enzymes in the acrosome

lysosomes/ cortical granules - activated on fertilisation to release chemicals Cause the zona pellucida to harden once a sperm’s nucleus has entered the ova.

SPERM:

nucleus - Contains only one copy of each chromosome (haploid)

head - Detachable

middle - Contains lots of mitochondria, which make ATP (fructose supplied in surrounding semen)

tail (undulipodia) - Made from motor proteins, which use ATP to propel the sperm forwards

acrosome - An specialised lysosome on the top of the sperm’s head. The acrosome swells and bursts when the sperm embeds in the zona pellucida. The enzymes in the acrosome digest the follicle cells and the zona pellucida and allow the cell membranes to fuse.

cytoplasm - Very little cytoplasm, which means cells are small and therefore can be released in large numbers.

Fertilisation in Mammals

• During sexual intercourse, semen is ejaculated high up into the vagina of the female, near the cervix

• The sperm cells then follow a chemical trail released by the egg cell and travel up through the cervix to reach the uterus (the womb)

• The sperm cells then travel into the oviduct containing the egg cell

• If a sperm cell meets the egg cell in the oviduct, fertilisation can occur

  • This is most likely to occur 1-2 days after the female has ovulated (i.e. released an egg cell from one of her ovaries into an oviduct)

• Fertilisation is the fusion of the nuclei from a male gamete (sperm cell) and a female gamete (egg cell)

  • During fertilisation, the head of a sperm cell releases enzymes that digest a path through the protective outer layer of the egg cell (the zona pellucida), allowing the sperm to pass through the egg cell membrane

    • This process is known as the acrosome reaction

  • Once this occurs, the egg cell immediately releases the contents of vesicles known as cortical granules into the space between the egg cell membrane and the zona pellucida

  • The chemicals contained within the cortical granules cause the zona pellucida to rapidly thicken and harden, preventing any more sperm cells from entering, ensuring only one sperm cell can fertilise the egg cell

    • This process is known as the cortical reaction

• The nucleus of the sperm cell then enters the egg and fuses with the nucleus of the egg cell

• When the male and female gamete nuclei fuse, they become a zygote (fertilised egg cell)

• This zygote contains the full 46 chromosomes (23 pairs of chromosomes), half of which came from the father and half from the mother

• The zygote divides by mitosis to form two new cells, which then continue to divide like this until an embryo is formed after a few days

• Cell division continues and eventually many of the new cells produced become specialised to perform particular functions and form all the body tissues of the offspring

The process of fertilisation in mammals

describe and explain how, in mammals, events following the acrosome reaction prevent more than one sperm fertilising an egg

• after the acrosome swells, the sperm cell can fuse with the cell surface. the sperm cell releases the male nucleus into the ovum. cortical granules in the cytoplasm then moves to the cell surface membrane . it fuses with the cell surface membrane which triggers the release of enzymes (by exocytosis) which then causes the zona pellucida to harden. this prevents more sperm from entering the ovum.