Human Reproduction, Contraception, and Cell Division

Human Reproductive Anatomy

Male Reproductive System

• Scrotum: Protects the testes and maintains sperm below body temperature.
• Penis: Allows sperm to pass into the vagina; semen is ejaculated through it.
• Glans: The rounded structure at the end of the penis.
• Testis: Produces sperm and testosterone.
• Seminiferous tubules: Produce millions of sperm per day.
• Epididymis: Area where sperm mature and are stored.
• Seminal vesicle: Produces a nutrient-rich fluid that provides energy to the sperm.
• Vas deferens: Tube through which sperm are propelled from the epididymis to reach the ejaculatory duct.

Female Reproductive System

• Corpus luteum: Secretes female sex hormones, specifically progesterone and oestrogen.
• Follicle: Contains and nurtures the developing egg (ovum).
• Fallopian tubes: Transport ova from the ovaries toward the uterus and serve as the site of fertilisation.
• Cervix: Lower part of the uterus that allows sperm, menstrual blood, or a baby to pass.
• Vagina: Provides lubrication and stretches during sexual intercourse and childbirth.
• Labia minora and majora: External folds that protect the vagina.

The Menstrual Cycle

• Day 1: The lining of the uterus breaks down and menstrual bleeding starts.
• Day 4: The lining of the uterus builds up again and thickens into a layer enriched with blood cells; the pituitary gland produces regulatory hormones.
• Day 14: The mature egg is released and remains viable for around 3 days; the pituitary gland and the hypothalamus release luteinising hormone (LH), triggering ovulation.
• Day 28: The uterine lining stays thick, awaiting the arrival of a fertilised egg. If fertilisation does not occur, the lining breaks down again to begin a new cycle.

The Placenta

The placenta is an organ that facilitates a constant exchange of materials between the mother (supplying food and oxygen, O₂) and the foetus (removing waste materials). Blood vessels from the mother and the foetus lie in close proximity but do not touch. Foetal and maternal blood do not mix directly because the higher blood pressure in the mother could damage the delicate foetal vessels.

Methods of Birth Control

• Natural Methods: Involves avoiding intercourse during the woman's most fertile period (rhythm method) or withdrawing the penis before ejaculation.
• Chemical Methods: Includes the use of spermicidal creams or foams that kill sperm in the vagina (noting that some individuals may experience allergic reactions). Oral contraceptive pills contain synthetic female hormones that prevent the maturation and release of eggs, offering high efficacy when taken regularly.
• Mechanical Methods: Barrier methods such as condoms prevent sperm from entering the cervix, and diaphragms operate in a similar manner; these are often combined with spermicides to neutralise leakage. The intrauterine device (IUD or coil) interferes with sperm motility and prevents a fertilised egg from implanting in the uterine wall, providing high reliability.
• Surgical Methods: Tubal ligation in females involves cutting or sealing the fallopian tubes leading from the ovaries to the uterus to achieve permanent sterilisation. Vasectomy in males involves cutting the vas deferens connecting the testes to the prostate gland, permanently preventing the release of sperm; both procedures are difficult to reverse.

Sexually Transmitted Infections (STIs)

• Gonorrhoea: Characterised by anal irritation, painful urination, lower abdominal pain, abnormal vaginal discharge, or discharge from the tip of the penis; treated with appropriate antibiotics.
• Papilloma (HPV): Manifests as warts or benign tumours that appear on the skin and mucosal surfaces; managed via surgical removal or topical and systemic medications.

Modes of Reproduction

Asexual Reproduction

The process through which a single organism produces offspring that are genetically identical to the parent.

• Advantages: Only one parent is required; a large number of organisms can be produced in a short time; identical offspring thrive in the stable environment of their parents; the biological process is simpler.
• Disadvantages: Total lack of genetic variation, making the population vulnerable to environmental shifts and unsuitable for adaptive dispersal.

Sexual Reproduction

The process through which two organisms of different sexes combine genetic material to produce offspring genetically distinct from themselves.

• Advantages: Generates genetic variety among offspring, enabling evolutionary adaptation and natural selection.
• Disadvantages: Requires a second parent for fertilisation; the reproductive cycle is longer and biologically more complex.

The Cell Cycle and Cell Division

Phases of the Cell Cycle

• G₁ Phase (Gap 1): Intensive cellular synthesis takes place; the cell produces structural and functional proteins, leading to overall cell growth.
• S Phase (Synthesis): DNA replicates itself, and essential cytoplasmic substances accumulate.
• G₂ Phase (Gap 2): Cytoplasmic organelles replicate and organise in preparation for division.

Cytokinesis

Cytokinesis is the physical division of the cytoplasm following nuclear division.

• Animal Cells: Characterised by the formation of a cleavage furrow and the pinching inward of the cell membrane, cleaving the cell into two distinct daughter cells.
• Plant Cells: Involves the synthesis of a membrane-bound cell plate and subsequent cell wall across the equatorial plane, separating the cell into two daughter cells.

Mitosis

Mitosis is the process of nuclear division in which duplicated chromosomes separate to form two genetically identical daughter nuclei.

• Prophase: Chromatin condenses into visible chromosomes, the nuclear membrane disintegrates, the nucleolus disappears, and the mitotic spindle begins to assemble.
• Metaphase: Chromosomes attach to the mitotic spindle via their kinetochores and align along the metaphase plate at the cell equator.
• Anaphase: Kinetochore microtubules shorten, pulling sister chromatids toward opposite poles, while polar microtubules elongate to push the poles apart.
• Telophase: Polar microtubules continue to elongate, and a new nuclear membrane forms around each set of daughter chromosomes.

Meiosis

Meiosis is a specialised form of cell division that reduces chromosome numbers by half, producing genetically diverse gametes.

• Prophase I: Chromatin condenses tightly, and homologous chromosomes pair up and exchange genetic segments through crossing over.
• Metaphase I: The nuclear membrane disappears, the spindle apparatus forms, and homologous chromosome pairs align along the equator, attached to spindle fibres originating from opposite poles.
• Anaphase I: Spindle fibres shorten and pull homologous chromosome pairs apart toward opposite poles; each pole receives a distinct haploid set of duplicated chromosomes.
• Telophase I: The nuclear envelope reforms around each daughter nucleus, with each nucleus containing two sister chromatids joined at a common centromere.