(a) What is Oparin concept? Comment on the chemical theory of origin of life. (20 marks) (b) Explain the functional elements of…

(a) Oparin's concept and the chemical theory of the origin of life (20 marks)

Oparin's concept. A. I. Oparin (1924), with J. B. S. Haldane (1929), proposed that life arose gradually from non-living matter by chemical evolution on the primitive Earth.

• The early atmosphere was reducing, with CH₄, NH₃, H₂ and water vapour and no free oxygen.
• Energy from ultraviolet light, lightning and volcanic heat converted these gases into simple organic molecules, which collected in the hot, dilute "primordial soup" of the oceans.
• The molecules polymerised, and Oparin proposed that they aggregated into coacervates (colloidal droplets bounded by a membrane-like layer), which absorbed substances and grew: the first protobionts, feeding on the soup, so they were heterotrophs. Autotrophs, photosynthesis and oxygen came later, followed by aerobes.

Chemical theory: stages. Formation of simple organic compounds, then of polymers, then of protobionts, and then of the first cell.

Experimental support.

• Miller and Urey (1953) sparked a mixture of CH₄, NH₃, H₂ and water and got amino acids (glycine, alanine and aspartic acid).
• Oró (1961) made adenine from HCN and ammonia. Fox heated amino acids to proteinoids, which formed microspheres.
• Amino acids have been found in the Murchison meteorite.
• Ribozymes (Cech and Altman) support an RNA world, in which RNA was both genetic material and catalyst before DNA and proteins.

Comment. The theory is testable and fits the heterotroph-first view. Its weaknesses are:

• The early atmosphere is now thought to have been less reducing (CO₂, N₂ and water), which gives lower yields.
• Products are racemic, and polymerisation in water is unfavourable.
• The origin of the genetic code and the leap from a protobiont to a cell are unexplained.

Alternatives. Hydrothermal-vent and iron–sulphur world hypotheses (Wächtershäuser), clay-mineral catalysis (Cairns-Smith) and panspermia. The RNA world hypothesis extends the theory.

(b) Functional elements of eukaryotic chromosomes (15 marks)

A chromosome needs three essential elements to replicate, segregate and stay stable. Murray and Szostak (1983) proved this by building an artificial yeast chromosome (YAC) from these three parts alone.

• Origins of replication (ARS): autonomously replicating sequences, about 100–150 bp in yeast, are bound by the origin recognition complex (ORC). Higher eukaryotes have many origins, which fire at different times in S phase, so that the long chromosome is copied in time.
• Centromere (CEN): the site of kinetochore assembly for attachment of spindle microtubules, and it is needed for segregation. Yeast has a point centromere of about 125 bp, and humans have alpha-satellite repeats and the histone variant CENP-A.
• Telomeres (TEL): repeats (TTAGGG in vertebrates) at the ends, added by telomerase, which protect the ends against degradation and fusion and solve the end-replication problem.

Organisational elements.

• Nucleosomes (146 bp of DNA on a histone octamer) fold into chromatin fibres.
• Chromatin loops are anchored on the scaffold at matrix attachment regions (MARs), which are AT-rich.
• The nucleolar organiser (NOR) carries rRNA genes and forms the nucleolus.
• Insulators (boundary elements, CTCF) block enhancers and separate chromatin domains.
• Heterochromatin, at centromeres and telomeres, is condensed and silenced, and euchromatin is active.

(c) Do mitochondria contribute to inheritance? (15 marks)

Yes. Mitochondria are semi-autonomous organelles with their own genome (mtDNA), so they show extranuclear (cytoplasmic) inheritance.

mtDNA. Human mtDNA is a circular molecule of 16,569 bp with 37 genes: 13 for proteins of oxidative phosphorylation, 22 for tRNAs and 2 for rRNAs. Each cell holds hundreds to thousands of copies.

Maternal transmission. The egg contributes almost all of the cytoplasm, and the sperm mitochondria are marked and destroyed after fertilisation. So mtDNA passes from mother to all offspring, and it does not follow Mendel's laws.

Heteroplasmy and threshold. A cell may hold a mixture of normal and mutant mtDNA (heteroplasmy). Mitochondria segregate at random in cell division, and disease appears only when the mutant fraction crosses a threshold (often 60–90%), which explains the variable expression within a family.

Example. Leber's hereditary optic neuropathy (LHON) is caused by mtDNA point mutations, such as m.11778G>A in the ND4 gene, and causes sudden loss of central vision.

• An affected mother passes it to all her children (with variable expression, and males are more often affected), and an affected father passes it to none.
• Other examples: MERRF and MELAS (mutations in mitochondrial tRNA genes), and cytoplasmic male sterility in maize, where the mitochondrial gene affects pollen.