Mind the Gap Grade 12 Life Sciences Study Guide

Comprehensive practice flashcards covering core concepts across all chapters of the Grade 12 Life Sciences CAPS curriculum.

What are the three structural components of a nucleotide?

A phosphate group ($\text{P}$), a pentose sugar (either deoxyribose or ribose, $\text{S}$), and a nitrogenous base ($\text{N}$).

What are the three structural differences between a DNA molecule and an RNA molecule?

1. DNA is double-stranded, whereas RNA is single-stranded.
2. DNA contains deoxyribose sugar, whereas RNA contains ribose sugar.
3. DNA contains the nitrogenous base thymine ($\text{T}$), whereas RNA contains uracil ($\text{U}$).

What is the biological significance of DNA replication?

It doubles the genetic material so it can be shared between the resulting daughter cells during cell division, and it results in the formation of genetically identical daughter cells during mitosis.

What are the two major stages of protein synthesis, and where in the cell does each stage occur?

Transcription takes place in the nucleus, and translation takes place in the cytoplasm at the ribosome.

What are the two primary reasons why meiosis is biologically important?

1. It halves the chromosome number from diploid ($2n$) to haploid ($n$), balancing the doubling effect of fertilisation.
2. Crossing over introduces genetic variation, producing offspring better adapted to survive environmental challenges.

What chromosomal abnormality causes Down syndrome?

Non-disjunction of chromosome pair $21$ during anaphase, resulting in an abnormal gamete with an extra copy of chromosome $21$. When fused with a normal gamete, the resulting zygote has three copies of chromosome $21$ ($47$ chromosomes).

Which phase of meiosis II is shown in the diagram?

Metaphase 2, where individual chromosomes line up at the equator of each cell with their centromeres attached to spindle fibres.

Which phase of meiosis II is shown in the diagram?

Anaphase 2, where centromeres split and daughter chromosomes (chromatids) are pulled to opposite poles of each cell.

How do precocial development and altricial development differ in hatchlings?

Precocial hatchlings are well-developed (eyes open, able to move and feed independently, brain size remains relatively constant), whereas altricial hatchlings are poorly developed, helpless, unable to feed or move, and require intense parental care.

What is the reproductive advantage of an amniote egg?

The shell and extraembryonic membranes protect the embryo from dehydration, allowing terrestrial animals to reproduce on land without requiring an aquatic environment.

What are the functions of the three accessory glands in the male reproductive system?

1. Seminal vesicle: Produces a nutrient-rich fluid that provides energy for sperm cells.
2. Prostate gland: Produces an alkaline fluid that neutralises vaginal acidity.
3. Cowper's gland: Produces mucus that aids sperm movement.

What are the functions of the acrosome and mitochondria in a human sperm cell?

The acrosome contains enzymes to digest the outer wall of the ovum for fertilisation, while the mitochondria provide the energy required for the tail to propel the sperm cell forward.

In which structure of the female reproductive system does fertilisation normally occur?

High up in the Fallopian tube.

What event is triggered on day $14$ of the human menstrual cycle, and by which hormones?

Ovulation (release of an egg cell from the mature Graafian follicle), triggered by high concentrations of LH (luteinising hormone) and FSH (follicle-stimulating hormone).

What is the difference between complete dominance, incomplete dominance, and co-dominance?

In complete dominance, the dominant allele completely masks the recessive allele in heterozygotes; in incomplete dominance, an intermediate phenotype is produced; in co-dominance, both alleles are equally and simultaneously expressed in the phenotype.

What states Mendel's Law of Independent Assortment?

Alleles of a gene for one characteristic segregate independently of the alleles of a gene for another characteristic during gamete formation, provided the genes are on different chromosomes.

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

The cerebrum controls voluntary actions, sensory perception, and higher thought processes; the cerebellum coordinates voluntary movements, balance, and muscle tone; the medulla oblongata regulates involuntary activities such as heartbeat and breathing rate.

What is the structural difference in signal direction between a dendrite and an axon?

Dendrites transmit nerve impulses towards the cell body, whereas the axon transmits nerve impulses away from the cell body.

What is the correct pathway of an impulse through a reflex arc?

$$\text{Receptor} \rightarrow \text{Sensory neuron} \rightarrow \text{Interneuron (connector)} \rightarrow \text{Motor neuron} \rightarrow \text{Effector}$$

How does the eye accommodate for near vision (objects closer than $6\text{ m}$)?

The ciliary muscles contract, causing the suspensory ligaments to slacken; this reduces tension on the lens, allowing it to become more convex (rounded), which refracts light rays more strongly onto the retina.

How do the iris muscles respond when moving from dim light to bright light?

The radial muscles of the iris relax and the circular muscles contract, causing the pupil to constrict (become smaller) to reduce the amount of light entering the eye.

What are the specific inner ear structures responsible for hearing and dynamic balance?

Hearing is detected by the organ of Corti in the cochlea; dynamic balance (changes in speed and direction) is detected by the cristae in the semicircular canals.

How does the negative feedback mechanism restore normal blood glucose when levels rise above normal?

The pancreas detects the increase and secretes insulin into the blood; insulin stimulates the liver and muscle cells to convert excess glucose into glycogen, lowering blood glucose back to normal.

What homeostatic response occurs in the skin on a hot day to increase heat loss?

Vasodilation occurs (blood vessels dilate, sending more warm blood to the skin surface to radiate heat) and sweat glands become more active, releasing more sweat whose evaporation cools the skin.

How do auxins cause a plant stem to bend towards unilateral light (phototropism)?

Light destroys or redistributes auxins towards the shaded side of the stem; higher auxin concentration on the shaded side stimulates greater cell elongation, causing the stem to bend towards the light source.

What two evolutionary 'laws' were proposed by Jean-Baptiste Lamarck?

1. The law of use and disuse (organs used more frequently become more developed, while unused organs shrink or disappear).
2. The inheritance of acquired characteristics (modifications acquired during an individual's lifetime are passed on to offspring).

What is the hypothesis of punctuated equilibrium?

Evolution involves long periods of time where species experience little or no change (equilibrium), punctuated by short periods of rapid speciation.

What are three skeletal differences between modern humans ($\text{Homo sapiens}$) and African apes that indicate bipedalism?

1. Modern humans have a more forward foramen magnum, while African apes have a backwardly positioned foramen magnum.
2. Modern humans have an S-shaped spine, while African apes have a C-shaped spine.
3. Modern humans have a short, wide pelvis, while African apes have a long, narrow pelvis.

What lines of evidence support the 'Out of Africa' hypothesis?

1. The oldest fossils of australopithecines, $\text{Homo habilis}$, and early bipedal hominids were found exclusively in Africa.
2. The oldest fossils of $\text{Homo erectus}$ were found in Africa.
3. Genetic analysis of mutations in mitochondrial DNA ($\text{mtDNA}$) traces the oldest female lineages back to Africa.

How does the destruction of the stratospheric ozone layer impact human health?

It allows more harmful ultraviolet ($\text{UV}$) radiation to reach the Earth's surface, which damages cellular DNA and leads to an increased incidence of skin cancer, cataract development, and permanent eye damage.

What are the time allocations and mark totals for each of the two Grade 12 Life Sciences examination papers?

Each paper has a total of $150$ marks and must be completed in $150\,\text{minutes}$ ($2\frac{1}{2}\,\text{hours}$).

What are the three chemical components that make up the basic nucleotide unit shown in the diagram?

The three components are a phosphate group (P), a deoxyribose or ribose sugar (S), and a nitrogenous base (N).

What are three structural differences between a DNA molecule and an RNA molecule?

1. DNA is a double-stranded molecule, whereas RNA is single-stranded.
2. DNA contains deoxyribose sugar, whereas RNA contains ribose sugar.
3. DNA contains the nitrogenous base thymine, whereas RNA contains uracil.

What is the biological significance of DNA replication occurring during interphase prior to cell division?

It doubles the genetic material so that it can be shared equally between resulting daughter cells during cell division, and it ensures the formation of genetically identical daughter cells during mitosis.

Which two main stages of protein synthesis are represented at A and B in this diagram?

Stage A represents transcription, which takes place in the nucleus, and Stage B represents translation, which takes place in the cytoplasm at the ribosome.

How is meiosis defined, and what ploidy change occurs during the process?

Meiosis is a type of cell division whereby a diploid somatic cell undergoes two divisions to produce four genetically dissimilar haploid cells (gametes or sex cells).

During which phase of meiosis does crossing over occur, and what is its biological significance?

Crossing over occurs during Prophase 1 when chromatids from homologous chromosomes touch at chiasmata and exchange genetic material, introducing genetic variation into the resulting offspring.

What is non-disjunction, and what genetic disorder results when it affects chromosome pair $21$ in humans?

Non-disjunction is the failure of homologous chromosomes (in Anaphase 1) or sister chromatids (in Anaphase 2) to separate correctly; in chromosome pair $21$, it produces an abnormal gamete with an extra copy of chromosome $21$, leading to Down syndrome (Trisomy $21$).

How do precocial and altricial reproductive strategies differ regarding hatchling development in vertebrates?

Precocial hatchlings are well-developed when they hatch (eyes open, able to move, and able to feed) requiring minimal parental care, whereas altricial hatchlings are poorly developed, unable to move or feed on their own, and require intensive parental care.

What distinguishes oviparous, viviparous, and ovoviviparous reproduction in animals?

In ovipary, eggs are laid and hatch outside the mother's body; in vivipary, young develop inside the mother's uterus receiving nourishment via a placenta and live young are born; in ovovivipary, fertilised eggs are retained inside the mother's body, obtaining nutrients from egg yolk, until hatching.

Why is the scrotum positioned outside the abdominal cavity in human males?

The scrotum holds the testes at a temperature approximately $2^\circ\text{C}$ below normal body temperature ($37^\circ\text{C}$), which is the optimum temperature for the production and maturation of sperm.

What are the structural functions of the acrosome, nucleus, and mitochondria of the sperm cell shown?

The acrosome contains enzymes to digest the outer wall of the ovum for fertilisation; the nucleus carries $23$ chromosomes; and the mitochondria provide energy for swimming.

What are the roles of oestrogen and progesterone in regulating the endometrium during the menstrual cycle?

Oestrogen stimulates the endometrium to thicken and develop blood vessels from day $7$ to $14$; progesterone causes further thickening, vascularisation, and glandular development to maintain the uterine lining for implantation.

What blood vessels are found in the human umbilical cord, and what does each transport?

The umbilical cord contains two umbilical arteries, which carry deoxygenated blood and wastes from the foetus to the placenta, and one umbilical vein, which carries oxygenated blood and nutrients from the placenta to the foetus.

What does Mendel's Law of Independent Assortment state regarding dihybrid inheritance?

Alleles of a gene for one characteristic segregate independently of alleles of a gene for another characteristic, so alleles come together randomly during gamete formation (provided the genes are located on different chromosomes).

How does incomplete dominance differ from co-dominance in monohybrid crosses?

In incomplete dominance, heterozygous offspring exhibit an intermediate phenotype distinct from both homozygous parents (e.g., red and white producing pink), whereas in co-dominance, both parental alleles are equally and simultaneously expressed in the heterozygous phenotype (e.g., red and white patches).

Why are sex-linked recessive conditions, such as haemophilia and colour-blindness, expressed more frequently in human males than females?

The recessive alleles are carried on the X chromosome; because males have only one X chromosome ($XY$), the presence of a single recessive allele causes the condition, whereas females have two X chromosomes ($XX$) and require two recessive alleles to express it.

What are the primary functions of the cerebrum, cerebellum, and medulla oblongata in the human brain shown?

The cerebrum controls voluntary actions, sensory perception, and higher thought; the cerebellum coordinates voluntary movements and controls muscle tone for balance; and the medulla oblongata controls involuntary actions such as heartbeat and breathing rate.

What is the correct pathway followed by a nerve impulse during the reflex arc shown in the diagram?

Receptor (A) $\rightarrow$ Sensory neuron (B) $\rightarrow$ Interneuron (C) in the spinal cord $\rightarrow$ Motor neuron (D) $\rightarrow$ Effector (E) (muscle).

What sequence of physiological events occurs in the eye during accommodation for near vision (closer than $6\,\text{m}$)?

Ciliary muscles contract, suspensory ligaments slacken, tension on the lens decreases, the lens becomes more convex (rounded), light rays are refracted more, and the image is focused clearly onto the retina.

How does the pupillary mechanism adjust the human eye in bright light?

Radial muscles of the iris relax, circular muscles of the iris contract, the pupil constricts (becomes smaller), and less light enters the eye.

How do sound waves travel from the outer ear to stimulate nerve impulses in the inner ear?

The pinna directs sound waves down the auditory canal to vibrate the tympanum; the ossicles amplify and pass vibrations to the oval window; pressure waves in inner ear fluid stimulate the organ of Corti in the cochlea, which generates nerve impulses transmitted via the auditory nerve to the cerebrum.

Describe the negative feedback response when blood thyroxin levels increase above normal limits.

Elevated thyroxin levels stimulate the pituitary gland to produce less TSH; lower TSH levels stimulate the thyroid gland to secrete less thyroxin, causing thyroxin concentrations to drop back to normal.

How do insulin and glucagon regulate normal blood glucose concentrations in humans?

When blood glucose rises, the pancreas secretes insulin, which travels to the liver to convert excess glucose into stored glycogen; when blood glucose falls, the pancreas secretes glucagon, which stimulates the liver to convert stored glycogen back into glucose.

How does antidiuretic hormone (ADH) restore water balance when the blood has less water than normal?

The hypothalamus stimulates the pituitary gland to secrete more ADH into the blood; ADH increases the permeability of the collecting ducts and distal convoluted tubules in the kidney, allowing more water to be reabsorbed into surrounding blood vessels.

What physiological changes occur in the skin on a hot day to regulate internal body temperature?

The hypothalamus stimulates cutaneous blood vessels to dilate (vasodilation), bringing more blood carrying heat to the skin surface to radiate away, and stimulates sweat glands to produce more sweat, which cools the skin through evaporation.

How does unilateral light cause a plant stem to bend towards the light source in phototropism?

Unilateral light causes auxins to accumulate on the dark side of the stem (as light destroys or repels auxins); the higher auxin concentration stimulates greater cell elongation on the dark side, causing the stem to bend towards the light.

What were the two laws of evolution proposed by Jean-Baptiste Lamarck, and why was his theory rejected?

Lamarck proposed the Law of Use and Disuse and the Inheritance of Acquired Characteristics; the theory was rejected because there is no experimental evidence that characteristics acquired during an organism's lifetime can be inherited genetically.

What is the hypothesis of punctuated equilibrium in evolutionary theory?

Punctuated equilibrium states that evolution is not always gradual; it consists of long periods of time where species show little or no change (equilibrium), alternating with short periods of rapid speciation, supported by the absence of transitional fossils during rapid change.

What sequence of events leads to speciation following geographic isolation?

A geographic barrier splits a population, preventing gene flow; each subpopulation experiences independent natural selection under different environmental conditions; over time, the populations become genotypically and phenotypically distinct until they can no longer interbreed to produce fertile offspring.

What anatomical features of this African ape skull distinguish it from a modern human skull?

It possesses a sagittal cranial ridge, prominent brow ridges, large canines, large protruding (prognathous) jaws, a smaller cranium, and a backwardly placed foramen magnum.

How does the shape of the vertebral column differ between African apes and modern humans as shown in the diagram?

African apes have a less curved (C-shaped) spine suited for quadrupedal posture, whereas modern humans have an S-shaped spine that enables upright posture and bipedal locomotion.

What three lines of scientific evidence support the 'Out of Africa' hypothesis of modern human origin?

1. The oldest fossils of bipedal hominids (australopithecines and Homo habilis) were found exclusively in Africa.
2. The oldest fossils of Homo erectus were discovered in Africa.
3. Analysis of mutations in mitochondrial DNA (mtDNA) indicates the oldest female ancestors of modern humans originated in Africa.

What causes the enhanced greenhouse effect, and how does it relate to global warming?

Increased emissions of greenhouse gases (carbon dioxide from burning fossil fuels and deforestation; methane from landfills, livestock, and coal mining) trap more long-wave infrared radiation emitted by Earth, leading to a significant rise in global average surface temperatures.

How does excessive agricultural fertilizer runoff cause eutrophication and oxygen depletion in freshwater bodies?

Excess nitrates and phosphates wash into dams and rivers, stimulating rapid algal blooms; when the dense algae die, decomposing bacteria multiply rapidly and consume dissolved oxygen, causing aquatic organisms to die from oxygen deprivation.

What is leachate in a solid-waste landfill, and how is groundwater protected from it?

Leachate is a toxic liquid formed when rainwater seeps through decomposing solid waste; groundwater pollution is prevented by placing an impermeable plastic liner beneath the dumpsite.