Grade 12 Life Sciences Comprehensive Study Guide

Introducing Life Sciences

• Definition and Core Concepts: The term Life Sciences combines two fundamental ideas:

  • Life: Includes all living things, from basic molecules to the interactions of organisms with each other and their environments.

  • Science: Refers to the methodology used to study the subject, aiming to increase existing knowledge and discover new things. This involves proposing hypotheses (predicted outcomes of investigations) and conducting experiments to test them.

• Benefits of Studying Life Sciences: It encourages the ability to think critically, solve problems, and understand the world. It provides skills helpful for everyday life and exposes students to sub-fields like microbiology, genetics, environmental studies, and biotechnology.

• Primary Skills: Life Sciences teaches biological concepts, processes, systems, and theories. It enables students to evaluate scientific issues, understand the negative human impact on the environment, be responsible citizens, and appreciate South Africa's unique biomes and scientific contributions.

• Knowledge Strands:

  • Strand 1: Life at a Molecular, Cellular, and Tissue Level.

  • Strand 2: Life Processes in Plants and Animals.

  • Strand 3: Diversity, Change, and Continuity.

• The Three Broad Aims:

  • Aim 1 (Theory): Understanding scientific ideas, connecting them, and describing processes. It involves interpreting data and linking it to established theories.

  • Aim 2 (Practical Work): Handling equipment safely, making observations (drawings, measurements), recording data, and designing investigations. Planning an experiment involves identifying a problem, hypothesizing, identifying variables, and selecting apparatus.

  • Aim 3 (Application and Context): Understanding the history of science and indigenous knowledge systems. Modern science and traditional knowledge are viewed as complementary dynamics rather than opposing forces.

DNA: The Code of Life

• Nucleic Acid Structure: Nucleic acids are organic compounds made of building blocks called nucleotides. Every nucleotide consists of:

  • A Phosphate group ($P$).

  • A Sugar molecule ($S$): Deoxyribose in DNA; Ribose in RNA.

  • A Nitrogenous base ($NB$).

• DNA (Deoxyribonucleic Acid):

  • Structure: A double helix consisting of two strands joined and twisted spirally. The strands are polymers of nucleotides linked by strong bonds between sugars and phosphates, while nitrogenous bases are held by weak hydrogen bonds.

  • Nitrogenous Bases: Adenine ($A$), Thymine ($T$), Guanine ($G$), and Cytosine ($C$). Complementary pairing: $A$ always links to $T$; $G$ always links with $C$.

  • Discovery History:

    • 1952: Rosalind Franklin and Maurice Wilkins used X-ray diffraction images to research DNA structure.

    • 1953: Watson and Crick proposed the 3-D double helix model based on Franklin's images.

    • 1962: Watson, Crick, and Wilkins received the Nobel Prize (Franklin had passed away).

  • Location: Mostly in the nucleus (nuclear DNA), but small amounts exist in mitochondria (mitochondrial DNA) and chloroplasts (chloroplastic DNA).

  • Role: Carries hereditary information in segments called genes. Much of DNA is non-coding DNA, which does not code for proteins.

• RNA (Ribonucleic Acid):

  • Structure: Single-stranded and not coiled. Contains the sugar Ribose.

  • Bases: Adenine ($A$), Uracil ($U$), Guanine ($G$), and Cytosine ($C$). Uracil replaces Thymine.

  • Types:

    • Messenger RNA (mRNA): Carries the code from DNA to the ribosome.

    • Ribosomal RNA (rRNA): Forms the site of protein synthesis.

    • Transfer RNA (tRNA): Brings amino acids to the ribosome.

• DNA Replication: The process by which DNA makes an identical copy of itself during interphase.

  1. The double helix unwinds.

  2. Weak hydrogen bonds break (unzipping).

  3. Each strand serves as a template.

  4. Free nucleotides build complementary strands ($A-T$ and $G-C$).

  5. Result: Two identical DNA molecules, each with one original and one new strand.

• Mutations: Errors during replication (additions, deletions, or substitutions) that change the nitrogen base sequence and gene structure.

• DNA Profiling: A pattern of lines on X-ray film used to identify suspects, prove paternity, determine genetic defects, or identify relatives. Challenges include human error, laboratory inconsistencies, cost, and potential privacy/prejudice issues.

• Protein Synthesis:

  • Transcription (Stage 1): Occurs in the nucleus. DNA unzips, one strand acts as a template to form mRNA using free RNA nucleotides.

  • Translation (Stage 2): mRNA attaches to a ribosome in the cytoplasm. tRNA with specific anti-codons brings complementary amino acids. Amino acids link via peptide bonds to form a protein.

Meiosis

• Definition: Cell division that halves the chromosome number from diploid ($2n$) to haploid ($n$), producing four genetically different daughter cells.

• Key Terminology:

  • Chromosome: Threadlike structure of DNA and protein.

  • Chromatid: One of two identical strands of a replicated chromosome.

  • Centromere: Joins two chromatids.

  • Homologous Chromosomes: Pairs with similar shape, size, and genes.

  • Bivalent: A pair of homologous chromosomes in physical contact.

  • Crossing Over: Exchange of genetic material during Prophase I at the chiasma.

• The Process:

  • Meiosis I (Reduction Division):

    • Prophase I: Chromatin condenses, homologous pairs form bivalents, and crossing over occurs.

    • Metaphase I: Homologous pairs arrange randomly at the equator.

    • Anaphase I: Spindle fibers pull whole chromosomes to opposite poles.

    • Telophase I: Two haploid cells form via cytokinesis.

  • Meiosis II (Division of Chromatids):

    • Similar to mitosis; centromeres split in Anaphase II, and chromatids move to opposite poles.

• Significance: Produces gametes, maintains constant chromosome number across generations, and introduces variation through crossing over and random arrangement.

• Non-disjunction: Failure of chromosomes to separate in Anaphase I or II. If it occurs in pair $21$, it leads to Down Syndrome (Trisomy 21), resulting in $47$ chromosomes in the zygote.

Reproductive Strategies in Vertebrates

• Fertilization:

  • External: Occurs outside the body, usually in water. Requires many gametes due to high mortality rates (e.g., fish).

  • Internal: Occurs inside the female body. Fewer gametes needed due to increased protection (e.g., birds, mammals).

• Developmental Strategies:

  • Ovipary: Eggs are laid outside the body (yolk provides nutrition).

  • Ovovivipary: Eggs are retained inside until hatching; nutrition from yolk.

  • Vivipary: Young develop in the uterus; nutrition through the placenta.

• The Amniotic Egg: Allowed for land reproduction. Structures include:

  • Amnion: Produces fluid to cushion and protect from injury/dehydration.

  • Allantois: Collects nitrogenous waste and assists gas exchange.

  • Chorion: Allows gas exchange; forms the placenta in mammals.

  • Yolk sac: Contains food reserves.

• Precocial and Altricial Development:

  • Precocial: Hatchlings are well-developed, mobile, and can feed themselves (e.g., ducklings).

  • Altricial: Hatchlings are underdeveloped, naked, and helpless, requiring heavy parental care (e.g., songbirds, rats).

Human Reproduction

• Male System:

  • Testes: Produce sperm and testosterone. Suspended in the scrotum at a temperature $2-3\,^\circ\text{C}$ lower than body temp for healthy sperm production.

  • Ducts: Epididymis (storage), Vas deferens (transport), Urethra (passage for semen and urine).

  • Glands: Seminal vesicles (alkaline fluid), Prostate (nutrients), Cowper’s glands (mucus for mobility).

• Female System:

  • Ovaries: Produce eggs, oestrogen, and progesterone.

  • Fallopian tubes: Site of fertilization; use cilia to move egg cells.

  • Uterus: Houses the fetus; endometrium is the site of implantation.

• Gametogenesis:

  • Spermatogenesis: Occurs in seminiferous tubules under testosterone influence. One diploid germinal epithelial cell produces four haploid spermatids.

  • Oogenesis: Occurs in ovaries. One diploid cell produces one mature ovum and three degenerating polar bodies every $28$ days.

• The Menstrual Cycle:

  • Ovarian Cycle: FSH stimulates follicle growth into a Graafian follicle. Ovulation occurs at Day $14$ due to LH. The follicle becomes the corpus luteum, secreting progesterone.

  • Uterine Cycle: If no fertilization occurs, the endometrium breaks down (menstruation). If fertilized, the corpus luteum remains active to maintain the lining.

• Gestation: Lasts about $280$ days ($40$ weeks). The placenta allows nutrient/gas diffusion and secretes progesterone after $12$ weeks. The umbilical cord has two arteries (deoxygenated) and one vein (oxygenated).

Genetics and Inheritance

• Key Concepts:

  • Genotype: Genetic composition ($TT, Tt, tt$).

  • Phenotype: Physical appearance (Tall, short).

  • Allele: Different forms of a gene at the same locus.

  • Heterozygous: Two different alleles ($Tt$).

  • Homozygous: Two identical alleles ($TT$ or $tt$).

• Mendel’s Laws:

  • Segregation: Alleles separate during gamete formation.

  • Dominance: One allele can mask another in the phenotype.

  • Independent Assortment: Alleles of different traits sort independently.

• Inheritance Patterns:

  • Complete Dominance: Dominant allele completely masks the recessive.

  • Incomplete Dominance: Blending of phenotypes (e.g., Red x White = Pink).

  • Co-dominance: Both alleles expressed equally (e.g., Blood group $AB$; Red and White patches in cattle).

  • Sex-linked: Carried on the $X$ chromosome (e.g., Haemophilia, Colour-blindness). Males are more affected as they only have one $X$.

  • Multiple Alleles: e.g., Blood groups controlled by $I^A, I^B, i$.

• Biotechnology:

  • Genetic Engineering: Recombinant DNA technology used to make human insulin in $E. coli$.

  • GMOs: Benefits include pest/disease resistance; disadvantages include ethical concerns and potential loss of biodiversity.

  • Stem Cells: Undifferentiated cells from embryos, bone marrow, or umbilical cord used to treat diseases like leukaemia.

  • Cloning: Artificial process creating identical copies (e.g., Dolly the sheep in 1996).

Human Responses to the Environment

• The Nervous System:

  • Central (CNS): Brain and spinal cord.

  • Peripheral (PNS): Cranial ($12$ pairs) and Spinal nerves ($31$ pairs).

• Brain Structure:

  • Cerebrum: Largest part; controls voluntary actions, thought, and memory.

  • Cerebellum: Coordinates muscles and balance.

  • Medulla Oblongata: Controls involuntary actions (heartbeat, breathing).

  • Hypothalamus: Regulates temperature, thirst, and emotions.

• Neurons and Synapse:

  • Sensory neurons: From receptors to CNS.

  • Motor neurons: From CNS to effectors.

  • Synapse: Gap between neurons crossed by neurotransmitters. Ensures one-way impulse travel.

• Reflex Action: Fast, automatic response involving a Reflex Arc (Receptor → Sensory Neuron → Interneuron → Motor Neuron → Effector).

• Sense Organs:

  • The Eye: Pupil size regulated by iris muscles (pupillary mechanism). Accommodation allows focusing on near vs distant objects by changing lens convexity.

  • The Ear: Outer ear collects sound. Middle ear (ossicles) amplifies vibrations. Inner ear contains the Organ of Corti (hearing) and Maculae/Cristae (balance).

The Human Endocrine System and Homeostasis

• Secretory Glands: Exocrine glands have ducts (e.g., sweat); Endocrine glands are ductless and release hormones into the blood.

• Homeostasis: Maintaining a constant internal environment ($CO_2$, glucose, water, salt, and temperature).

• Specific Regulation Mechanisms:

  • Glucose: Pancreas releases Insulin (lowers glucose) and Glucagon (raises glucose).

  • Water (Osmoregulation): Hypothalamus detects low water; Pituitary releases ADH, increasing kidney tubule permeability to reabsorb water.

  • Salt: Aldosterone from the adrenal cortex regulates sodium levels.

  • Temperature (Thermoregulation): Hypothalamus stimulates vasodilation and sweating to lose heat, or vasoconstriction and shivering to conserve heat.

  • Thyroxin: Regulated via negative feedback with TSH from the pituitary gland to control metabolism.

Plant Responses to the Environment

• Hormones:

  • Auxins: Responsible for cell elongation. High concentrations stimulate stems but inhibit roots. High concentration at the tip causes apical dominance.

  • Gibberellins: Promote seed germination and flowering.

  • Abscisic Acid: Promotes dormancy and inhibits growth.

• Tropisms: Growth responses to stimuli.

  • Phototropism: Stem bends toward unilateral light as auxins move to the shaded side.

  • Geotropism: Roots grow toward gravity; stems grow against it.

• Defense Mechanisms: Chemical compounds (poison/tannins) and mechanical structures (thorns, prickles, spines).

Evolution

• Theories:

  • Lamarckism: Proposed "Use and Disuse" and "Inheritance of Acquired Characteristics" (proven wrong as acquired traits are not genetic).

  • Darwinism: Evolution by Natural Selection based on variation, competition, and survival of the fittest.

  • Punctuated Equilibrium: Long periods of stability followed by rapid change.

• Speciation: Occurs when a population is split by a geographical barrier (geographic speciation), leading to reproductive isolation.

• Reproductive Isolation Mechanisms: Breeding at different times, species-specific courtship, different pollinators, or hybrid infertility.

Human Evolution

• Classification: Humans are mammals in the Order Primates and Family Hominidae.

• Primate Shared Characteristics: Large brains, binocular vision, rotating arms, opposable thumbs, nails instead of claws.

• Differences (Humans vs. Apes): Bipedalism (foramen magnum in forward position, S-shaped spine, wide pelvis), larger cranium, smaller canines, and non-prognathous jaws.

• Fossil Sites:

  • Cradle of Humankind (SA): Sterkfontein (Mrs Ples, Little Foot), Malapa (A. sediba), Rising Star (H. naledi).

  • East Africa: Rift Valley (Lucy - A. afarensis).

• Out of Africa Hypothesis: States Homo sapiens evolved in Africa ($150,000-200,000$ years ago) and migrated outward. Supported by fossil records and mtDNA studies ("Mitochondrial Eve").