Meiosis - Oogenesis and Spermatogenesis

How is diploid different to haploid?

Diploid (2n)	Haploid (n)
2 sets of chromosomes Chromosomes in homologous pairs One set from each parent All somatic cells are diploid Humans 2n = 46 chromosomes	1 set of chromosomes Chromosomes not found in pairs Gametes are the only haploid cells Humans n = 23 chromosomes

Why is meiosis essential?

Meiosis: Specialised cell division that produces halpoid gametes

• Each gamete (sperm and egg) must be haploid to ensure the zygote is diploid

• Differs between sexes

What processes in oogenesis occur before birth during fetal development?

• Germ cells (stem cells) migrate to the developing ovaries

• They divide via mitosis to produce many diploid oogonia

• Oogonia develop into primary oocytes (with 2 sets of chromosomes)

• Each primary oocyte begins meiosis but arrests in Prophase i

• Primary oocytes are enclosed by follicle cells, forming primary follicles

How does the follicle enable oogenesis?

Follicle: The functional, fluid-filled sac in the ovary that houses, nourishes, and protects a developing primary oocyte

• Ensures the primary oocyte matures for eventual release during ovulation (as a secondary oocyte)

How is oogenesis resumed during puberty?

• Puberty triggers hormone release which causes a primary oocyte to resume meiosis I during each menstrual cycle

• Usually one oocyte completes meiosis I during a cycle

When is Meiosis I completed and what does it involve the production of?

• Completed right before ovulation

• The primary oocyte divides unequally via cytokinesis, producing:

1. A large haploid secondary oocyte containing almost all the cytoplasm and nutrients

2. A small polar body containing excess chromosomes and very little cytoplasm which usually disingegrates

Why is the division of cytoplasm unequal during the completion of Meiosis I in oogenesis?

• The future egg requires a large amount of cytoplasm, nutrients and organelles to support early embryonic development if fertilisation occurs

• Unequal division ensures the secondary oocyte keeps this to create a single viable egg whilst shedding unneeded genetic material into small, disposable polar bodies

What immediately follows the completion of Meiosis I in oogensis and what does it involve?

• Meiosis II follows Meiosis I (formation of secondary oocyte)

• The secondary oocyte immediately begins Meiosis II but arrests in metaphase II

• Approx 14 days before the next period, ovulation occurs

• The arrested secondary oocyte in metaphase II ruptures from the follicle

• Released into the ovary to enter the fallopian tube

Why do arrests occur in particular stages of oogenesis?

Primary oocytes: Arrest in prophase I before birth to preserve a supply of immature eggs for future reproductive years

Secondary oocytes: Arrest in metaphase II after ovulation and only complete meiosis II if fertilisation by a sperm occurs

• Arrests ensure that usually only one egg matures per menstrual cycle and that meiosis is only completed when fertilisation is possible

What occurs from puberty in spermatogenesis before Meiosis I occurs?

• Germ cells (spermatogonoia) within the seminiferous tubules of the testes divide via mitosis to maintain a supply of cells for sperm production

• Some spermatogonia develop into primary spermatocytes

• Sertoli cells provide nutrients for this development

What occurs after the production of primary spermatocytes? What does this produce?

• Each primary spermatocyte undergoes meiosis I

• Produces 2 haploid secondary spermatocytes

• Cytokinesis is equal, both cells recieve similar amounts of cytoplasm

How does one primary spermatocyte become 4 haploid spermatids following Meiosis I?

• Through Meiosis II

• Each secondary spermatocyte undergoes this to produce 4 haploid spermatids

How do spermatids become mature sperm?

• Spermatids are transported from the seminiferous tubules to the epididmyis

• They mature and differentiate into mature sperm

• Involves development of a tail from the cytoplasm to allow movement (motile)

How do you define oogenesis and spermatogenesis?

Oogenesis: Process by which female gametes (ova/egg cells) are produced by meiosis in the ovaries

Spermatogenesis: Process by which male gametes (sperm cells) are produced by meiosis in the testes

How does oogenesis different to spermatogenesis?

	Oogenesis	Spermatogenesis
Location?	Ovaries	Testes (seminiferous tubules)
Beginning?	Begins before birth	Begins at puberty
Ending?	Ends at menopause	Continues throughout life
Amount able to be produced?	Finite number of primary oocytes to produce a finite amount of ovum	Sperm produced continuously
Final product?	Produces 1 ovum + polar bodies	Produces 4 sperm
Size and complexity?	Large ovum contains abundant cytoplasm, organelles and nutrients	Small sperm with few organelles (mainly mitochondria to swim)
Motility?	Non-motile egg	Motile sperm
Arrests?	Arrests in prophase I & metaphase II	No prolonged arrests
How many released?	~1 egg released per cycle	Millions of sperm produced daily
Time to develop?	—	~74 days for one sperm to develop