Grade 12 Life Sciences CAPS Revision Cheat Sheet

Comprehensive question-and-answer revision flashcards covering Grade 12 CAPS Life Sciences topics including nucleic acids, meiosis, reproduction, genetics, nervous and endocrine systems, homeostasis, plant responses, evolution, and hominid fossil evidence.

Who discovered the shape of the DNA molecule that is still recognized today?

Watson and Crick.

What are the three chemical components that form a nucleotide?

A nitrogenous base, a sugar (ribose in RNA or deoxyribose in DNA), and a phosphate group.

Which nitrogenous bases are found exclusively in DNA and exclusively in RNA?

Thymine ($\text{T}$) is found only in DNA, and uracil ($\text{U}$) is found only in RNA.

What type of bond connects complementary nitrogenous base pairs between two DNA strands?

Weak hydrogen bonds.

When and where does the process of DNA replication take place in eukaryotic cells?

During interphase in the nucleus.

How is a DNA profile interpreted and explained in a paternity test?

$50\%$ of the child's DNA markers match the mother and the remaining $50\%$ match the biological father, because a child inherits half their DNA from each parent.

How does template strand usage differ between transcription and DNA replication?

During transcription, only one DNA strand acts as a template to synthesize mRNA, whereas during DNA replication, both strands act as templates.

What specific chemical bond links amino acids together during translation at the ribosome?

A peptide bond.

What is the biological term for the point of contact where homologous chromosomes cross over and exchange genetic material?

Chiasmata (singular: chiasma).

What is non-disjunction and when does it occur during meiosis?

Non-disjunction is the failure of homologous chromosomes or sister chromatids to separate properly, occurring during Anaphase 1 or Anaphase 2.

How does non-disjunction at position 21 lead to Down syndrome?

Chromosomes fail to separate at position 21 during anaphase, forming an abnormal gamete with $24$ chromosomes ($2n+1$); fusion with a normal gamete of $23$ chromosomes ($n$) results in an offspring with $47$ chromosomes.

Define ovovivipary and state its primary advantages.

Fertilised eggs with large amounts of yolk are kept inside the female body and hatch internally, giving birth to live, fully developed young; this provides protection from predators and eliminates external egg vulnerability.

What is the specific function of the allantois in an amniotic egg?

It serves as a reservoir for storing nitrogenous metabolic waste produced by the developing embryo.

How do precocial and altricial offspring differ in development at birth or hatching?

Precocial offspring are born almost fully developed with eyes open, down feathers or hair, and are mobile; altricial offspring are born naked, blind, immobile, and entirely dependent on parents for food, warmth, and protection.

What are the reproductive functions of the prostate gland and the seminal vesicles?

The prostate gland neutralises the acidity of the vagina and nourishes sperm with activating enzymes; seminal vesicles secrete fructose for energy and lubricating fluid to enhance sperm motility.

How does the composition of blood in the umbilical vein differ from that in the umbilical artery?

The umbilical vein carries oxygenated and nutrient-rich blood from the placenta to the fetus, whereas the umbilical artery carries deoxygenated and waste-laden blood from the fetus to the placenta.

Which pituitary hormone exhibits a single prominent peak at day $14$ to induce ovulation and form the corpus luteum?

Luteinising Hormone (LH).

Why does the level of progesterone remain elevated instead of decreasing if pregnancy occurs?

Progesterone maintains a vascular and glandular endometrium for pregnancy and exerts negative feedback on the pituitary to inhibit FSH and LH, preventing new follicle maturation.

State Mendel's Law of Dominance.

When two homozygous organisms with contrasting characteristics are crossed, all individuals of the F1 generation will display the dominant trait; an individual that is heterozygous for a particular characteristic will express the dominant trait in its phenotype.

Define incomplete dominance and provide the expected phenotypic ratio when two heterozygous individuals interbreed.

Neither allele of a gene is dominant over the other, producing an intermediate phenotype in the heterozygous condition; crossing two heterozygotes yields a $1:2:1$ phenotypic ratio.

Why are human males more likely than females to inherit sex-linked recessive disorders?

Males have only one X chromosome, meaning a single recessive allele on the X chromosome is expressed; females possess two X chromosomes, requiring two recessive alleles to express the condition.

In reproductive cloning, why must a somatic cell nucleus be used instead of a sperm cell nucleus?

A somatic cell nucleus is diploid ($2n$) and contains the complete set of genetic instructions and desired characteristics, whereas a sperm cell is haploid ($n$) and carries only half the genome.

What are the primary functions of the cerebrum, cerebellum, and medulla oblongata?

Cerebrum: controls memory, intelligence, sight, hearing, taste, smell, and skin sensations; Cerebellum: co-ordinates voluntary movement, muscle tone, posture, and balance; Medulla oblongata: transmits impulses to and from the brain, regulates breathing, heartbeat, and autonomic reflexes like sneezing.

What is the sequence of structures involved in the pathway of a reflex arc?

Stimulus $\rightarrow$ sensory neuron $\rightarrow$ dorsal root $\rightarrow$ grey matter $\rightarrow$ interneuron $\rightarrow$ ventral root $\rightarrow$ motor neuron $\rightarrow$ effector.

Compare the underlying causes of Alzheimer's disease and Multiple Sclerosis.

Alzheimer's is characterized by the accumulation of amyloid plaques around neurons; Multiple Sclerosis is an autoimmune condition causing the progressive destruction and loss of the myelin sheath around axons.

Describe the changes that occur in the eye during accommodation for near vision ($< 6\,\text{m}$).

Ciliary muscles contract, suspensory ligaments slacken, the lens bulges and becomes more convex, refraction of light increases, and a clear focused image is formed on the retina.

How does the pupillary mechanism adjust the iris muscles in dim light conditions?

Radial muscles contract while circular muscles relax, causing the pupil to dilate and allow more light to enter the eye.

What structural defect causes myopia (short-sightedness), and how is it corrected?

The eyeball is too long or the cornea/lens is too convex, focusing light in front of the retina; it is corrected by wearing spectacles with concave lenses.

Name the auditory receptor found in the cochlea and identify where in the brain sound impulses are interpreted.

The organ of Corti converts pressure waves into nerve impulses, which travel via the auditory nerve to the cerebrum for interpretation.

Which specific receptor and ear structures are responsible for maintaining balance during changes in speed and direction of head movement?

Cristae located inside the ampullae of the semi-circular canals send impulses via the auditory nerve to the cerebellum.

Which specific receptor and ear structures detect changes in the position of the head in space to maintain posture?

Maculae located within the utriculus and sacculus send impulses via the auditory nerve to the cerebellum.

Describe the physiological response of the skin during thermoregulation on a hot day (vasodilation).

The hypothalamus stimulates dermal arterioles to relax their circular muscles and shunt vessels to contract; more blood flows to the skin surface, increasing heat loss via radiation, convection, and conduction, while sweat production increases.

How does the endocrine system respond via ADH when the hypothalamus detects low water levels in the blood?

The pituitary gland releases more ADH, increasing the permeability of the distal convoluted tubules and collecting ducts so more water is reabsorbed into the blood, producing a small volume of concentrated urine.

Describe the homeostatic mechanism that restores normal blood $\text{pH}$ when carbon dioxide levels rise.

Chemoreceptors in the aorta and carotid artery detect low $\text{pH}$ and stimulate the medulla oblongata, which accelerates heart rate and breathing muscle contractions to exhale $\text{CO}_2$ faster, restoring normal blood $\text{pH}$.

Explain how insulin and glucagon act antagonistically to regulate blood glucose levels.

Beta cells secrete insulin to lower high blood glucose by stimulating cells to absorb glucose and the liver to convert glucose to glycogen; alpha cells secrete glucagon to raise low blood glucose by stimulating the liver to break down glycogen into glucose.

Explain how auxin distribution causes a plant shoot to bend towards a unilateral light source in phototropism.

Auxins are light-sensitive and migrate to the shaded side of the stem tip, stimulating cell elongation on the dark side while illuminated cells stay short, causing the stem to curve toward the light.

How does the physiological effect of auxins differ between stems and roots during geotropism?

In horizontally placed roots, auxins settle on the lower side and inhibit cell elongation, causing downward curvature (positive geotropism); in stems, auxins on the lower side stimulate cell elongation, causing upward bending (negative geotropism).

Contrast the primary functions of gibberellins with those of abscisic acid in plants.

Gibberellins stimulate stem internode elongation and promote seed germination; abscisic acid is a growth inhibitor that promotes bud and seed dormancy, triggers stomatal closure during stress, and induces leaf/fruit abscission.

Distinguish between continuous and discontinuous variation, providing an example of each.

Continuous variation displays a complete spectrum of intermediate phenotypes governed by multiple genes (e.g., human height); discontinuous variation features distinct, separate categories with no intermediates governed by one gene (e.g., moth colour or ABO blood groups).

How does Lamarck's evolutionary mechanism differ fundamentally from Darwin's theory of natural selection?

Lamarck proposed that organisms intentionally adapt through the 'use and disuse' of organs and pass acquired phenotypic traits to offspring; Darwin proposed that inherited variation exists naturally, and environmental pressures select individuals possessing favourable traits to survive and reproduce.

What is punctuated equilibrium as formulated by Stephen Jay Gould?

A model stating that evolution involves long periods of species stability or gradual change (equilibrium), interrupted or punctuated by brief intervals of rapid natural selection during which new species arise.

Detail the process by which allopatric speciation occurs following geographic isolation.

A population is split by a physical barrier preventing gene flow; each subpopulation faces distinct environmental pressures and undergoes independent natural selection; over time, genetic and phenotypic differences accumulate until the groups can no longer interbreed to produce fertile offspring.

List four reproductive isolation mechanisms that prevent hybridization between different species.

Breeding at different times of the year (temporal/seasonal), adaptations to different pollinators, species-specific courtship behaviours (rituals), and mechanical incompatibility of genitalia (prevention of fertilisation).

State three distinct cranial differences between modern humans and African apes.

Humans have a forward-positioned foramen magnum, a non-prognathous flat face, a large cranium, small canines, absent brow ridges, and a parabolic palate; apes have a backward foramen magnum, a sloping prognathous jaw, a small cranium, large canines, prominent brow ridges, and a rectangular palate.

How does pelvic anatomy reflect bipedalism in humans compared to quadrupedalism in apes?

Humans have a short, broad pelvis that aligns and stabilizes body weight directly above the feet for bipedal locomotion; apes have a long, narrow pelvis adapted for quadrupedal knuckle-walking.

Identify the hominid species, discovery country, and discoverer for the fossils 'Ardi', 'Lucy', and 'Taung Child'.

Ardi: Ardipithecus ramidus, Ethiopia, Tim White; Lucy: Australopithecus afarensis, Ethiopia, Donald Johanson; Taung Child: Australopithecus africanus, South Africa, Raymond Dart.

Identify the hominid species, discovery country, and discoverer for the fossils 'Mrs Ples', 'Little Foot', and 'Turkana Boy'.

Mrs Ples: Australopithecus africanus, South Africa, Robert Broom; Little Foot: Australopithecus africanus, South Africa, Ronald Clarke; Turkana Boy: Homo ergaster, Kenya, Richard Leakey.

What fossil evidence specifically supports the 'Out of Africa' hypothesis for human origins?

Fossils of Ardipithecus, Australopithecus, and Homo habilis have been discovered in Africa only; furthermore, the oldest known fossils of Homo erectus and modern Homo sapiens are found exclusively in Africa, with younger specimens distributed abroad.

In scientific experimental design, how do investigators establish validity versus reliability?

Validity is established by identifying and keeping all extraneous variables identical/constant (such as species, age, and measuring equipment); reliability is demonstrated by repeating experimental trials, calculating average values, and testing large sample sizes.