D3.1-A: Reproduction

Which of the following statements) include differences between male and female gametes in sexual reproduction?

i. Male gamete is smaller

ii. Male gamete has fewer food reserves

iii. Female gametes are motile

A. l only

B. I and II

C. I, II and III

D. III only

B

Which of the following is an example of negative feedback in the menstrual cycle?

A. Increasing FSH receptors in the follicles leading to increased ostradiol production

B. Oestradiol inhibiting secretion of FSH from the pituitary gland

C. Oestradiol stimulating the secretion of LH by the pituitary gland

D. Progesterone continuing to stimulate the build-up of the endometrium

B

Which of the following statements is the definition of fertilization?

A. Transfer of a male gamete to a female

B. Generation of gametes by meiosis

C. Fusion of a male and female gamete

D. Production of a haploid cell from a diploid cell

C

What does the release of gonadotropin-releasing hormone directly stimulate?

i. Puberty

ii. Release of an ovum from an ovary

iii. Increased LH and FSH

A. l only

B. l and II

C. I and III

D. I, II and III

C

Which of the following is a mechanism to prevent polyspermy?

A. Reduced meiotic division

B. The cortical reaction

C. Many sperm are produced but only one egg

D. Increased spermatogenesis

B

Which hormone is detected by a pregnancy test?

A. Insulin

B. Epinephrine

C. Thyroxin

D. Human chorionic gonadotropin

D

State the name of the hormones that:

a. trigger ovulation (1 mark)

b. stimulate the growth of the endometrium (1 mark)

c. maintain the endometrium and promote its vascularisation. (1 mark)

a. Luteinizing hormone (LH)

b. Oestradiol

c. Progesterone

Distinguish between asexual and sexual reproduction. (3 marks)

Explain how meiosis breaks up parental combinations of alleles, and how fusion of gametes produces new combinations. (4 marks)

• Random Assortment: Maternal and paternal chromosomes mix randomly to form haploid cells (gametes).

• Genetic Variation: Crossing over and random assortment produce gametes with unique allele sequences.

• Random Fertilization: Any male gamete can randomly fertilize any egg.

• Unique Offspring: The fusion of these unique gametes creates a brand-new organism with unique allele combinations.

Describe the use of hormones in IVF treatment. (5 marks)

• Down-regulation: Down-regulation stops pituitary and ovarian hormone secretion using a drug (such as a nasal spray) to shut down the menstrual cycle.

• Superovulation: High doses of FSH are injected to stimulate the development of multiple follicles.

• Maturation (hCG): When follicles reach 15–20 mm in diameter, an injection of hCG is given to start the maturation process.

• Egg Collection: After approximately 36 hours, the follicles are collected from the ovaries under a general anaesthetic.

• Fertilization & Implantation: The eggs are combined with sperm in sterile conditions and incubated, while progesterone is taken to maintain the endometrium for implantation.

Outline how meiosis and two other mechanisms promote genetic variability among individuals of the same species. (7 marks)

• Crossing over: Crossing over during prophase I produces recombinant chromosomes, which increases variability.

• Independent assortment: The random orientation and separation of chromosomes and chromatids during metaphase and anaphase are completely random processes of independent assortment.

• Allele combinations: In humans, this random orientation yields more than 8 million ($2^{23}$) possible allele combinations.

• Daughter cells: Because of these processes, daughter cells differ from parental cells.

• Sexual reproduction: Fertilization and mating processes in sexual reproduction also increase genetic variability.

• Mutations: Spontaneous mutations in gamete-producing cells can lead to new characteristics in the offspring.

a. Explain how the placenta is adapted to its function. [3]

b. List the substances exchanged between the fetus and the mother. [2]

c. State which processes are used to transfer substances to the fetal blood at the placenta. [2]

d. Suggest why is it essential that maternal and foetal blood circulations do not mix in the placenta. [2]

a.

• Large Surface Area: Features villi-like projections to maximize diffusion.

• Short Diffusion Pathway: The barrier is only a few cells thick.

• Maintains Concentration Gradient: A constant blood supply maintains the gradient between maternal and fetal blood

b.

• Respiratory Gases: Oxygen diffuses from maternal to fetal hemoglobin; carbon dioxide diffuses in the reverse direction.

• Nutrients & Protection: Water, glucose, ions, amino acids, and antibodies are transferred.

• Waste Products: Excretory products, including urea, leave the fetus

c.

• Diffusion: Respiratory gases and urea.

• Osmosis: Water.

• Facilitated Diffusion: Glucose.

• Active Transport: Ions and amino acids.

d.

• Different Blood Types: The mother and fetus may have incompatible blood types.

• Antibody Attack: If maternal blood enters the fetus, maternal antibodies will attack and destroy the fetal blood cells

Outline the role of mitosis and meiosis in gametogenesis. (6 marks)

• Multiplication and Growth: Mitosis multiplies cells in the ovaries and testes (multiplication phase), followed by a growth phase for the developing sex cells.

• Maturation: The maturation phase involves meiosis, which creates haploid gametes.

• Meiosis Stages: Meiosis I produces secondary spermatocytes and secondary oocytes, while meiosis II produces spermatids and ova.

• Spermatogenesis Output: Spermatogenesis results in the production of four sperm cells.

• Oogenesis Output: Oogenesis produces one ovum and three polar bodies.

• Fertilization Requirement in Oogenesis: During oogenesis, meiosis II begins but pauses and is only completed upon fertilization

Distinguish between spermatogenesis and oogenesis. [3]

Describe the hormonal control of pregnancy and childbirth. (7 marks)

• Role of hCG: hCG is initially secreted by the blastocyst and later by the placenta. It maintains the corpus luteum to secrete progesterone for the first 16 weeks of pregnancy.

• Maintenance of Pregnancy: After the corpus luteum breaks down, the placenta secretes oestradiol and progesterone, which maintain the uterine lining and continue the pregnancy.

• Trigger for Birth: Childbirth is triggered by a sharp decline in progesterone levels, which removes the inhibition preventing the uterine muscles from contracting.

• Function of Oxytocin: The posterior pituitary releases oxytocin, which stimulates rhythmic contractions of the uterus and relaxes pelvic girdle fibers to dilate the cervix.

• Contraction Pattern: Powerful, intermittent waves of contraction start at the top of the uterus and move down toward the cervix.

• Positive Feedback: Control of contractions occurs via a positive feedback loop, causing the rate and strength of contractions to progressively increase.

• Expulsion of Offspring: This escalating cycle of contractions continues until the offspring is expelled

Outline the role of chemical signalling in the control of pregnancy and childbirth. (7 marks)

• Chemical Signalling & Hormones: Chemical signalling involves releasing ligands to deliver signals within or to other cells, ensuring coordinated cellular activities. Hormones are extracellular signal molecules transported via the bloodstream for long-distance signalling.

• Feedback Regulation: Signalling pathways can be controlled by feedback. Positive feedback amplifies conditions away from equilibrium, whereas negative feedback returns conditions to initial states.

• Steroid Hormones: Steroid hormones (such as progesterone) are lipid-soluble and diffuse across the plasma membrane to bind to receptors located in the nucleus.

• Control of Pregnancy: Pregnancy is controlled by hCG, which maintains the corpus luteum as an endocrine gland.

• Progesterone Maintenance: The continuity of pregnancy is maintained by progesterone secretion, initially coming from the corpus luteum and later from the placenta.

• Trigger for Childbirth: Changes during childbirth are triggered by a decrease in progesterone levels.

• Role of Oxytocin: Oxytocin controls childbirth by binding to receptors on elastic fibres in the pelvic girdle to relax them and by stimulating uterine contractions