Describe the di-hybrid cross carried on Drosophila melanogaster by Morgan and his group. How did they explain linkage, recombination and gene mapping on the basis of their observations?

OR

Describe the interaction of t-RNA, m-RNA and ribosome’s during the events of translation.

Morgan used Drosophila melanogaster (fruit flies) to explain how the process of sexual reproduction produced modifications. He chose fruit fly because it could be cultivated easily on an artificial laboratory, shows short lifespan, a single mating could generate multiple offspring, differentiable male and female sexes, and hereditary variations are of different types. Morgan used a di-hybrid cross between white-eyed and yellow-bodied females and red-eyed and brown-bodied males. Self-crossing of the F1 generation produced an F2 generation. The outcome exhibited a divergence from Mendel’s dihybrid cross in peas. Morgan noted that while crossing a set of characters, two genes did not follow Mendel’s law as they did not divide as per the law. Genes to be linked, when genes for different traits are located in similar chromosome; and thus, are tied to each other. A few genes have powerful linkage giving a limited chance of recombination whereas another linkage of genes is loosely linked and weak providing a greater chance of recombination. Once the linked genes were identified, the recurrence of linked genes also determined the expression of characteristics in the next generation. Linkages can be classified into two types established on the absence or presence of non-parental combinations or new combinations, complete linkage and incomplete linkage. A linkage is said to be complete when two or multiple characteristics are inherited and normally surface in two or further generations in their parental or original combinations, they are known as complete linkage. The incomplete linkage displayed by genes that generate some portion of non-parental combinations.

OR

In translation, the cell uses a transcribed mRNA as a template to construct proteins. The cell has transcribed mRNA strand from its DNA, it now translates the mRNA’s nucleotide sequence into amino acids. This is called a polypeptide, which forms the basic structure of a protein. A ribosome coordinates the translation process and it's where the protein was synthesized. It consists of two parts, a large and small subunit.

The ribosome attaches the mRNA together with tRNAs in the process of synthesizing polypeptide chain that the cell needs to build. Each tRNA is designed to carry a specific an amino acid that it can add to a polypeptide chain. Only the tRNA carrying the next amino acid in the polypeptide chain has the anticodon that binds to the appropriate location on the mRNA. This system ensures that amino acids are added to the chain in the correct order.

At the beginning of translation, the ribosome and a tRNA attach to the mRNA. This tRNAs anticodon is complementary to the mRNA’s start codon, where translation starts. The next mRNA codon is displayed in the ribosome's other attaching site. A tRNA with the complementary anticodon is attracted to the ribosome and binds to this codon. The tRNA carries the next amino acid in the polypeptide chain.

The first tRNA transfers its amino acid to the amino acid on the newly arrived tRNA, and a chemical bond is made between the two amino acids. The tRNA that has given up its amino acid is detached from the site. This bind to another molecule of amino acids and again used in the protein-making process.

Using this mechanism, the ribosome advances the mRNA through three nucleotides at a time. The ribosome also moves to the tRNA carrying the polypeptide chain into its recently abandoned attaching site. A tRNA anticodon is complementary to the next mRNA codon is attracted to the ribosome and the mRNA.

Again the polypeptide chain is transferred to the new tRNA, the empty tRNA is released. Translation continues until the ribosome encounters a stop codon in the mRNA. The nucleotide codon signals that the polypeptide chain is complete.