Intro sexual reproduction

Sexual Reproduction

The process where DNA from two individuals (via sperm and ova) combine to produce genetically unique offspring, allowing for variation and adaptability.

What are gametes and where are they formed?

Gametes (sperm and ova/oocytes) are reproductive cells formed in the gonads (testicles and ovaries) by meiosis, which reduces chromosome number by half.

Fertilization

The fusion of sperm and ovum that restores the full chromosome number, resulting in a zygote with genetic material from both parents.

Meiosis in Gametes

A special type of cell division that reduces chromosome number in gametes from diploid (2n) to haploid (n).

Oogenesis

The process of egg (oocyte) formation from stem cells called oogonia, surrounded by follicular cells in the ovary.

What is a primordial follicle?

A primordial follicle is an oocyte paused in meiosis I, surrounded by support cells, formed before birth and inactive until puberty.

How does oogenesis differ from spermatogenesis?

Oogenesis is finite and occurs before birth, producing a limited number of oocytes, while spermatogenesis is continuous throughout a male's life.

Oocyte Reserve

Females are born with 1–2 million oocytes, but only ~400,000 remain by puberty, and fewer than 500 will be ovulated in a lifetime.

• Most oocytes undergo degeneration (atresia), and only a few hundred are released during ovulation over a woman’s reproductive lifespan.

What role do FSH and LH play in the female reproductive cycle?

FSH (follicle-stimulating hormone) and LH (luteinizing hormone), released from the anterior pituitary starting at puberty, stimulate the growth and development of primordial follicles each month.

Graafian Follicle (Mature Follicle)

The Graafian follicle is the one follicle (out of several that begin to grow) that fully matures and gets released during ovulation.

What happens to primordial follicles each month after puberty?

Each month, several primordial follicles begin to develop due to FSH and LH, but typically only one becomes a mature (Graafian) follicle and is ovulated.

What is spermatogenesis, and where does it occur?

Spermatogenesis is the process of making sperm. It begins at puberty and takes place in the seminiferous tubules of the testes.

Spermatogonia

Spermatogonia are diploid (2n) stem cells that initiate spermatogenesis. Some enter meiosis to become primary spermatocytes.

What are the main stages of spermatogenesis?

1. Spermatogonia → primary spermatocytes

2. Primary spermatocytes → haploid secondary spermatocytes

3. Secondary spermatocytes → spermatids

4. Spermatids → mature sperm (via spermiogenesis)

Spermiogenesis

Spermiogenesis is the final step of spermatogenesis where spermatids mature into fully developed sperm cells.

Spermatogonia are diploid (2n), but after meiosis, the resulting sperm are called spermatozoa and are haploid (n), each carrying half the number of chromosomes. They will fuse with haploid egg to form diploid zygote.

How many chromosomes are in a human zygote, and how are they organized?

A human zygote has 46 chromosomes, organized into 23 pairs—22 pairs of autosomal chromosomes and 1 pair of sex chromosomes.

What determines the genetic sex of a human baby?

The sperm determines the genetic sex. If it contributes an X chromosome, the baby will be female (XX); if it contributes a Y, the baby will be male (XY).

Autosomal chromosomes

The 22 pairs of chromosomes (numbered 1–22) that are the same in both sexes and carry genes for most traits not related to sex determination.

Sex chromosomes

The 23rd chromosome pair that determines sex—females have two X chromosomes (XX), and males have one X and one Y (XY).

Why is the Y chromosome smaller than the X chromosome?

The Y chromosome has only about 80 genes, mainly related to male development, while the X chromosome has around 1090 genes, making the Y smaller.

What do the gonads look like right after fertilization in male and female embryos?

They look the same and are undifferentiated early on before developing into testes or ovaries.

Testis-Determining Factor (TDF)

A protein produced by the SRY gene on the Y chromosome that triggers the development of testes from undifferentiated gonads.

• SRY gene is located on Y chromsomes and codes for TDF, which causes gonads to develop into testes.

What happens in embryos if TDF is present?

Seminiferous tubules form, germ cells and Sertoli cells develop (around days 43-50), Leydig cells appear by day 65 to produce testosterone, and male reproductive organs begin developing around 8 weeks.

• if there is no TDF because its an XX The gonads develop into ovaries because there is no signal (TDF) to form testes.