(a) Give structure of t-RNA. Write role of different RNAs in the translation process. (20 marks) (b) Describe the principle and…

(a) Structure of tRNA and the roles of different RNAs in translation

Structure of tRNA: a single chain of about 73 to 93 nucleotides, folded by base pairing into a cloverleaf in two dimensions. Acceptor stem: a stem of seven base pairs whose 3′ end carries the unpaired sequence CCA, to which the amino acid is joined at the 3′-OH of the terminal adenosine. D arm: stem and loop containing dihydrouridine. Anticodon arm: stem and loop carrying the anticodon of three bases, which pairs with the mRNA codon. Variable loop: small loop between the anticodon and T arms. TΨC arm: stem and loop with ribothymidine, pseudouridine and cytosine. Tertiary structure: the cloverleaf folds into a compact L shape, with the acceptor end at one end and the anticodon at the other. Unusual bases such as inosine and methylated bases are common. Roles of RNAs in translation: mRNA: carries the genetic message from DNA as a series of codons that the ribosome reads. In eukaryotes the 5′ cap and poly-A tail help binding and stability, and in bacteria the Shine-Dalgarno sequence positions the ribosome. tRNA: the adaptor. An aminoacyl-tRNA synthetase attaches the correct amino acid, and the anticodon pairs with the codon in the ribosome so that amino acids join in the right order. rRNA: with proteins forms the ribosome. The small subunit rRNA (16S or 18S) binds mRNA and checks codon-anticodon pairing. The large subunit rRNA (23S or 28S) forms the peptidyl transferase centre, so it acts as a ribozyme that forms peptide bonds. 5S and 5.8S rRNAs give structure.

(b) Principle and method of whole animal cloning

Principle: a nucleus of a specialised body cell holds the entire genome. When placed in an enucleated egg, the egg cytoplasm reprograms it to the totipotent state, so the egg develops into an animal genetically identical to the nuclear donor. This is somatic cell nuclear transfer. Method: Step 1: A somatic cell, such as a skin or mammary cell, is taken from the donor animal and grown in culture, often held in a resting (G0) state. Step 2: An unfertilised egg in metaphase II is collected and its nucleus removed. Step 3: The donor cell is put next to the enucleated egg and fused with a short electric pulse, so that its nucleus enters the egg. Step 4: The egg is stimulated by electric pulse or chemicals to start dividing, and cultured to the blastocyst stage. Step 5: The embryo is transferred into the uterus of a surrogate mother, who gives birth to the clone. Examples: Dolly the sheep (1997) from a mammary cell nucleus; Garima II, the cloned buffalo of NDRI, Karnal (2009). Features: the clone has the nuclear genes of the donor, while its mitochondrial DNA comes from the egg donor. Success is low, with many abnormal or early deaths.

(c) The Miller experiment and the origin of life

Experiment: Stanley Miller, working with Harold Urey in 1953, made a closed glass apparatus that modelled the early earth. Set-up: a flask of boiling water gave water vapour, which circulated through a second flask containing the gases methane, ammonia and hydrogen. These were treated as the reducing early atmosphere. Energy: electric sparks between electrodes imitated lightning. Collection: a condenser cooled the vapours and the liquid collected in a trap, which was heated again and again for a week. Result: the liquid became brown and contained simple organic compounds, mainly the amino acids glycine and alanine, together with organic acids. Support to theory: the experiment supported the Oparin-Haldane theory of chemical evolution, which says that life arose gradually from simple inorganic substances in a hot, reducing atmosphere through organic molecules forming a primordial soup. It showed that the building blocks of life can form without life, though it did not produce a living cell.