Life Sciences: Evolution, DNA Code of Life, and Meiosis

Flashcards covering Life Sciences Grade 12 concepts across Evolution (theories, speciation, hominid evolution) and Genetics (DNA structure, replication, profiling, protein synthesis, and meiosis).

How is biological evolution defined in Grade 12 Life Sciences?

Biological evolution refers to the gradual change in the characteristics of a population or species over time, leading to the formation of new species.

What is the scientific distinction between a hypothesis and a theory?

A hypothesis is a testable prediction or tentative explanation of the relationship between variables, whereas a theory is a well-substantiated explanation of natural phenomena supported by facts, laws, and tested hypotheses.

What are the four primary lines of evidence that support the Theory of Evolution?

1. Fossil record (including transitional fossils), 2. Biogeography, 3. Modification by descent (homologous structures), and 4. Genetics.

What evolutionary concept is illustrated by the pentadactyl limb across bats, dolphins, anteaters, moles, monkeys, and horses?

Homologous structures (modification by descent), which possess a similar anatomical plan due to common ancestry despite performing different functions.

Which four mechanisms contribute to genetic variation among individuals within a species?

1. Meiosis (crossing over and random arrangement of chromosomes), 2. Mutations, 3. Random fertilisation, and 4. Random mating.

During which phase of meiosis does crossing over take place, and what is its significance?

Crossing over occurs during Prophase I at points called chiasmata, where non-sister chromatids of homologous chromosomes exchange genetic material, resulting in new combinations of alleles.

How do continuous and discontinuous variation differ in phenotypic distribution and genetic control?

Continuous variation displays a continuum of intermediate phenotypes (e.g., human height) controlled by multiple genes, whereas discontinuous variation falls into discrete categories with no intermediates (e.g., ABO blood groups) controlled by one or two genes.

What form of variation is represented by this graph displaying human ABO blood groups?

Discontinuous variation, because phenotypes fall into distinct, non-overlapping categories with no intermediate states.

What two laws did Jean-Baptiste de Lamarck use to explain evolution, and why was his theory rejected?

Lamarck proposed the 'Law of use and disuse' and the 'Law of inheritance of acquired characteristics.' His theory was rejected because acquired somatic changes do not alter an organism's genetic makeup and cannot be inherited by offspring.

How did Jean-Baptiste de Lamarck explain the evolution of long necks in giraffes as depicted in this diagram?

Originally all giraffes had short necks; by repeatedly stretching to reach high tree leaves, their necks elongated (use and disuse), and these acquired long necks were transmitted to subsequent generations.

How does Charles Darwin's theory of natural selection account for evolutionary change in a population?

Populations show phenotypic variation; under environmental pressures or competition, individuals possessing favourable characteristics survive and reproduce, passing the alleles for those traits to their offspring.

What evolutionary tempo is illustrated by this graph exhibiting long horizontal plateaus punctuated by rapid vertical shifts?

Punctuated Equilibrium (formulated by Eldredge and Gould in 1972), which proposes that species experience long periods of stasis (equilibrium) interrupted by rapid bursts of speciation.

What specific agricultural traits have been selected for in Jersey, Hereford, and Nguni cattle through artificial selection?

Jersey cattle were bred for high butterfat milk, Hereford cattle for high-yield meat production, and indigenous Nguni cattle for disease resistance and heat tolerance.

What are the essential steps of speciation caused by geographic isolation?

A geographic barrier splits an ancestral population into two; gene flow ceases; each sub-population experiences distinct environmental selection pressures; independent natural selection leads to genotypic and phenotypic divergence until interbreeding can no longer produce fertile offspring.

What are five reproductive isolation mechanisms that prevent closely related species from interbreeding?

1. Breeding at different times of the year, 2. Species-specific courtship behaviour, 3. Plant adaptation to different pollinators, 4. Production of infertile offspring (hybrid sterility), and 5. Prevention of fertilisation due to incompatible genitalia.

How did natural selection change the beak size of Galapagos ground finches on Daphne Major during the 1976–1978 drought?

The drought depleted small soft seeds, leaving mostly large, hard seeds; finches with larger, deeper beaks cracked these seeds successfully and survived to reproduce, increasing the average beak size of the population.

What is the taxonomic distinction between hominids and hominins?

Hominids encompass all modern and extinct Great Apes (humans, chimpanzees, gorillas, orangutans, and ancestors), while hominins include modern humans, extinct human species, and all direct bipedal ancestors (genera Homo, Australopithecus, and Ardipithecus).

In this comparison of pelvic girdles, which label identifies the human pelvis, and what is its functional significance?

Structure C is the human pelvis; it is short and wide to bear and balance upper body weight during upright bipedal walking, unlike the long, narrow ape pelvis (D).

How does the anatomical position of the foramen magnum differ between modern humans and African apes?

In modern humans, the foramen magnum is located in a forward, central position beneath the skull to allow vertical entry of the spinal cord for bipedalism, whereas in African apes, it is positioned more backward for quadrupedal posture.

What fossil and genetic evidence supports the Out-of-Africa hypothesis?

Fossil evidence shows Ardipithecus, Australopithecus, and Homo habilis fossils are found exclusively in Africa, alongside the oldest Homo erectus and Homo sapiens fossils. Genetic evidence from mitochondrial DNA (mtDNA) traces all modern humans to an ancestral female in East Africa approximately 150,000 years ago.

What three molecular subunits make up a single DNA nucleotide?

A deoxyribose pentose sugar, a phosphate ion, and one of four nitrogenous bases (Adenine, Thymine, Cytosine, or Guanine).

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

1. DNA contains deoxyribose sugar, while RNA contains ribose sugar. 2. DNA has the nitrogenous base thymine ($T$), whereas RNA contains uracil ($U$) instead. (DNA is also double-stranded while RNA is single-stranded).

What are the five consecutive steps in the process of DNA replication?

1. The DNA double helix unwinds. 2. Weak hydrogen bonds break and the two strands unzip. 3. Each original strand acts as a template. 4. Free nucleotides bind complementary bases ($A-T$ and $C-G$) to each template strand. 5. Two identical DNA molecules form, each consisting of one original and one new strand.

In this replication diagram, identify labels W, X, Y, and Z.

W represents a nucleotide, X is a phosphate group, Y is a deoxyribose sugar, and Z is a weak hydrogen bond.

Based on the DNA profiles of the Child, Mother, and four candidate males, which male is the biological father?

Male 3, because every DNA band present in the child's profile that was not inherited from the mother matches a band found in the profile of Male 3.

What occurs during the transcription phase of protein synthesis?

In the nucleus, DNA unwinds and unzips along a gene; one DNA strand serves as a template for free RNA nucleotides to form a complementary mRNA molecule, which carries codons through nuclear pores to ribosomes.

How does translation convert an mRNA transcript into a functional polypeptide?

mRNA attaches to a ribosome; tRNA molecules carrying specific amino acids bind their complementary anticodons to corresponding mRNA codons; adjacent amino acids join via peptide bonds to form a polypeptide chain.

Why does a gene mutation not necessarily result in a change to the synthesized protein?

Because the genetic code is degenerate; multiple codons can code for the same amino acid, so a base substitution may leave the final amino acid sequence unaltered.

What is the distinction between autosomes and gonosomes shown in this human karyotype?

Autosomes comprise chromosome pairs 1 to 22 (controlling general body characteristics), whereas gonosomes comprise pair 23 ($XX$ in females, $XY$ in males), which determines genetic sex.

How does non-disjunction during meiosis cause Down syndrome?

Chromosome pair 21 fails to separate during Anaphase I (or sister chromatids fail to separate during Anaphase II), producing a gamete with 24 chromosomes ($n + 1$); when fertilized by a normal gamete ($n = 23$), the resulting zygote has three copies of chromosome 21, totaling 47 chromosomes ($2n = 47$).

How does chromosome arrangement at the equator differ between Metaphase I and Metaphase II of meiosis?

In Metaphase I, homologous chromosome pairs arrange in two parallel rows along the equator, whereas in Metaphase II, individual replicated chromosomes line up singly along the equator.

Why is meiosis classified as a reduction division, and why is this essential for sexual reproduction?

Meiosis halves the chromosome number from diploid ($2n$) to haploid ($n$) during gamete formation, ensuring that upon fertilisation, the normal diploid number ($2n = 46$ in humans) is restored in the zygote.