HSC Biology Comprehensive Study Guide: Heredity, Genetic Change, and Disease
Reproductive Mechanisms and the Continuity of Species
Reproduction is the fundamental process ensuring the continuity of a species through the transmission of genetic information from one generation to the next. Organisms utilize two primary methods: asexual and sexual reproduction. Asexual reproduction involves a single parent and produces offspring that are genetically identical clones. This process occurs via mitosis and is highly efficient in stable environments where rapid population increase is advantageous and adaptation is less critical. Conversely, sexual reproduction involve two parents where specialized gametes fuse to form a zygote. This method allows for significant genetic variation through the processes of meiosis and fertilization. Although it is a slower process, sexual reproduction increases the adaptability and long-term survival of a species in fluctuating or changing environments.
Animals exhibit two main methods of fertilization to facilitate sexual reproduction: external and internal. External fertilization occurs primarily in aquatic environments where large numbers of both male and female gametes are released into the water simultaneously. There is a low chance of fertilization success because gametes are dispersed across a large, open area. The resulting zygotes are unprotected and vulnerable to environmental factors like temperature, predation, infection, and rapid dispersal. Consequently, species using this method breed more frequently to compensate for low success rates. Internal fertilization is more suited to terrestrial environments as sperm is deposited directly inside the female reproductive tract, a confined space that increases the success rate of fertilization. This method requires fewer eggs but many sperm. The zygote develops in a protected environment inside the female body where temperature is controlled and the risks of predation or infection are minimized. Breeding occurs less frequently, often seasonally, due to the high efficacy of this method.
Asexual and Sexual Reproduction in Plants and Other Microorganisms
Plants and microorganisms employ diverse asexual strategies to ensure survival. Binary fission is common in prokaryotes like bacteria and unicellular protists such as amoeba or algae; it involves DNA replication followed by cytokinesis to form two identical cells. Spore formation is used by non-flowering plants and microbes like mushrooms; spores are produced via mitosis or meiosis and dispersed by the wind to grow when conditions are favorable. Mushrooms specifically contain sporangia known as basidia. Budding involves an outgrowth or bud developing via mitosis on a parent cell, such as in yeast. The nucleus divides equally but the cytoplasm divides unequally before the bud detaches or remains to form a colony. Vegetative propagation allows new plants to grow from nodes on roots or stems, such as runners on strawberries or bulbs on onions. Regeneration is a process where organisms grow back parts that have been removed or lost.
Sexual reproduction in plants involves the production of genetically different offspring through fertilization. Pollination is the transfer of male gametes, contained in pollen produced by the anther, to the female stigma. Self-pollination occurs when pollen fertilizes the same plant, leading to less genetic variation, whereas cross-pollination involves a different plant of the same species. The male organ, the stamen, consists of the anther and filament. The female organ, the pistil or carpel, includes the stigma, style, and ovary. The process begins with meiosis in the anther producing haploid pollen grains. Once pollen lands on a compatible stigma, it germinates and a pollen tube grows down the style. Two haploid nuclei travel down: one becomes the tube nucleus and the other divides into two sperm nuclei. Double fertilization occurs when one sperm nucleus fuses with the ovum to form a diploid zygote, and the second fuses with two polar nuclei to form a triploid endosperm nucleus. Seeds are then dispersed via wind, insects, or animals to prevent overcrowding and reduce competition.
Fungi reproduce asexually through budding or spore formation. Unicellular fungi like yeast use budding, where a bulge forms, the nucleus divides via mitosis, and a clone detaches. Multicellular fungi release thousands of lightweight, airborne spores produced via mitosis. Bacteria reproduce exclusively via binary fission, involving the replication of a single circular DNA molecule, cell elongation, cytokinesis (septum formation), and division into two identical cells. Protists like Amoeba and Paramecium also use binary fission through mitosis and cytokinesis, or budding where the bud may remain attached to form a colony.
Fertilization, Implantation, and Hormonal Control in Mammals
Mammalian reproduction is a complex process involving fertilization, usually occurring in the upper third of the fallopian tube. After ovulation, sperm travels through the tract to fuse with the egg, forming a diploid zygote. The zygote develops a strong outer membrane to prevent polyspermy before traveling to the uterus. Implantation follows, where the zygote burrows into the endometrium for nutrient and gas exchange. The embryo develops alongside the amniotic sac (cushioning and temperature control), the placenta (oxygen/nutrient supply and waste removal), and the umbilical cord.
Pregnancy and birth are regulated by specific hormones. Oestrogen, secreted by ovaries and then the placenta after week , stimulates uterine growth, mammary gland development, and increases blood flow. Progesterone, produced by the corpus luteum and then the placenta after week , thickens the endometrial lining, relaxes smooth muscles to prevent early contractions, and suppresses the maternal immune response. Near birth, progesterone levels drop. Oxytocin, released by the posterior pituitary, stimulates uterine contractions and milk ejection. It operates on a positive feedback loop: contractions stimulate more oxytocin, leading to stronger contractions until birth. Relaxin softens the cervix and pelvic ligaments for easier passage. Prolactin, from the anterior pituitary, stimulates milk production, though it is inhibited by progesterone until after the birth.
Manipulation of Reproduction in Agriculture
Scientific knowledge has allowed humans to manipulate reproduction for agricultural benefit. In animals, natural breeding involves selected pairs based on traits like speed or size. Artificial insemination (AI) involves inserting collected semen from a genetically desirable male into multiple females. Advantages of AI include the ability to freeze and store semen for international exchange and spreading productive traits (like milk yield or disease resistance) quickly. However, it risks reduced genetic diversity. In plants, artificial pollination ensures specific traits like drought resistance or size are inherited. While cheap and effective, it is labor-intensive and reduces long-term adaptability.
Cloning and tissue culture produce genetically identical offspring. Tissue culture involves breaking small pieces of tissue, like root tips, into individual cells and growing them in sterile nutrient media. This enables mass propagation of elite plants and preservation of endangered species but is expensive. Hybridization, or heterosis (hybrid vigour), involves mating different varieties to create offspring with enhanced qualities, such as hybrid corn or broiler chickens. This has transformed agricultural output but requires continuous breeding programs.
Cell Replication and DNA Models
Cell replication is crucial for genetic continuity. Mitosis is nuclear division in somatic cells producing two identical diploid cells for growth and repair. It consists of: 1) Interphase (DNA replication), 2) Prophase (chromatin condenses, spindle forms), 3) Metaphase (chromosomes align at equator), 4) Anaphase (centromeres split, chromatids move to poles), and 5) Telophase/Cytokinesis (cytoplasm divides). Meiosis produces four unique haploid gametes in germline cells through two divisions. Meiosis I includes crossing over (prophase I) and independent assortment (metaphase I), which generate genetic variation. Meiosis II separates sister chromatids similarly to mitosis.
The Watson and Crick DNA model describes a double helix made of nucleotides. Each nucleotide has a nitrogenous base (G, C, A, T), a deoxyribose sugar, and a phosphate group. DNA replication occurs before division: 1) DNA helicase unwinds the double helix and breaks hydrogen bonds. 2) DNA polymerase catalyzes the addition of free nucleotides to separated template strands via complementary base pairing. 3) DNA ligase secures the new strands. 4) The strands wind back into a stable helix. Exact replication ensures that daughter cells receive a full complement of genetic instructions. Mutations occur if base pairing is incorrect.
Polypeptide Synthesis and Genetic Variation
Polypeptide synthesis translates genetic code into functional proteins. In eukaryotes, DNA is linear and wrapped around histones in the nucleus, containing non-coding introns and coding exons. Prokaryotic DNA is a single circular chromosome in the cytoplasm with minimal non-coding regions and plasmids. Synthesis involve two steps: 1) Transcription in the nucleus, where RNA polymerase binds to a promoter, unzips DNA, and forms mRNA using a 3 to 5 template direction. mRNA then enters the cytoplasm. 2) Translation at the ribosome, where mRNA is read in codons (triplets). tRNA with matching anticodons brings specific amino acids, which are joined by peptide bonds. Translation stops at a stop codon (UAA, UAG, UGA), and the polypeptide folds into a 3D protein. mRNA carries instructions, tRNA matches codons to amino acids, and rRNA forms the ribosome structure.
Phenotypic expression is the result of interaction between genes (genotype) and the environment. Epigenetics describes how environmental factors like nutrition or temperature activate or silence genes (e.g., hydrangeas are blue in acidic soil and pink in alkaline soil). Proteins are made of polypeptide chains with four levels of structure: primary (linear sequence), secondary (helices/pleated sheets), tertiary (3D folding), and quaternary (multiple chains like haemoglobin). Fibrous proteins (collagen) provide structure, while globular proteins (enzymes, hormones, antibodies) perform functional roles.
Genetic variation is fostered by crossing over, independent assortment, random segregation, and mutations. Fertilization combines two unique haploid genomes. Inheritance patterns include autosomal (traits on chromosomes to ), sex-linkage (traits on X chromosome, more common in males), co-dominance (both alleles expressed equally, e.g., roan cattle), and incomplete dominance (phenotypes blend, e.g., pink snapdragons). Multiple alleles also exist, such as the blood types A, B, and O.
Population Genetics and Biotechnology
Population genetics studies allele frequencies in gene pools. The frequency of an allele is calculated as: . Frequencies change due to mutation, gene flow (migration), selection, and genetic drift (random fluctuations in small populations). Single Nucleotide Polymorphisms (SNPs) are base pair changes in at least of the population, used as genetic markers for disease mapping. High-scale projects like the "Out of Africa" theory use mitochondrial DNA (mtDNA) to trace human lineage, suggesting humans originated in Africa due to high mtDNA diversity in that region.
Technologies like PCR (Polymerase Chain Reaction) amplify DNA, while Gel Electrophoresis separates fragments by size using an electric current. DNA sequencing methods include Sanger chain termination (using dideoxynucleotides) and Maxam-Gilbert (chemical cleavage). DNA profiling compares non-coding short tandem repeats (STRs) for forensics and paternity. Biotechnology utilizes biological systems for products. Past applications include selective breeding and fermentation. Present tools include CRISPR and cloning. Future directions include synthetic biology and de-extinction. This field raises ethical concerns regarding animal welfare, environmental impact, and social inequality. Examples like Bt Cotton (pest resistance) and Golden Rice (vitamin A enrichment) demonstrate the agricultural benefits of transgenic species.
Infectious Diseases and Pathogens
Infectious diseases are caused by pathogens: transmission occurs between hosts. Pathogens include: 1) Bacteria (Unicellular prokaryotes like Mycobacterium tuberculosis). 2) Viruses (Non-cellular genetic material in a capsid, like Influenza A). 3) Prions (Infective proteins causing neurodegeneration, like CJD). 4) Protozoans (Eukaryotic unicellular organisms like Plasmodium/malaria). 5) Fungi (Eukaryotic saprophytes or parasites like Athlete’s foot). 6) Macroparasites (Multicellular organisms like tapeworms). Transmission can be direct (physical contact), indirect (airborne/contaminated objects), or via vectors (arthropods like mosquitoes).
Robert Koch established postulates to link specific microbes to diseases: 1) Pathogen present in all diseased individuals. 2) Isolated and grown in pure culture. 3) Causes disease in healthy host. 4) Re-isolated from second host. Louis Pasteur disproved spontaneous generation with his swan-neck flask experiment and developed pasteurization (heating to to ) and vaccines. Agricultural disease impacts include revenue loss and food insecurity, as seen in the Irish Potato Famine caused by Phytophthora infestans. Animal diseases like Marek’s disease in birds cause paralysis and significant economic loss.
Host responses include three lines of defense: 1) Physical/chemical barriers (skin, mucous, stomach acid). 2) Innate immunity (phagocytosis by macrophages, inflammation via histamines, fevers). 3) Adaptive immunity (T cells and B cells). Helper T cells activate B cells to produce specific antibodies and cytotoxic T cells to kill infected cells. Immunity can be actively acquired (vaccines/exposure) or passively acquired (breast milk/antibody injections).
Homeostasis and Non-Infectious Disease
Homeostasis maintains a constant internal state via negative feedback loops. The process involves a stimulus, receptors (chemoreceptors, thermoreceptors), a control centre (hypothalamus), and effectors (muscles, glands). Body temperature is maintained around and blood glucose between and . Endotherms use internal mechanisms for regulation; for example, the Red Kangaroo pants for heat loss. Plants like xerophytes adapt to arid conditions via leaf curling, thick waxy cuticles, and CAM photosynthesis to conserve water.
Non-infectious diseases include: 1) Genetic (Tay-Sachs, caused by HEXA gene mutation). 2) Environmental (Skin cancer from UV; Heavy metal poisoning). 3) Nutritional (Iron deficiency anaemia; Vitamin D deficiency). 4) Cancer (caused by mutations in oncogenes or tumour suppressor genes). Epidemiology analyzes disease patterns to identify risk factors. Significant studies, like the Framingham Heart Study, identified high blood pressure and smoking as risk factors for cardiovascular disease. Prevention methods include public health campaigns like "Slip, Slop, Slap" or genetic engineering like gene therapy (Luxturna for blindness).
Technologies assisting with disorders include: Cochlear implants and bone conduction implants for hearing loss, and spectacles or laser surgery for refractive errors (myopia/hyperopia). Kidney function loss is managed via dialysis (haemodialysis or peritoneal dialysis) which artificially filters waste from the blood. Cochlear implants are particularly effective for profound sensorineural hearing loss as they bypass damaged hair cells to stimulate the auditory nerve directly, though they require surgery and training.