Evidence for Dna and Its Uses

Evidence for DNA & its uses

Nucleic Acids

RNA is composed of nucleotides, but differs from DNA in that...

* The pyrimidine base, uracil, replaces the base thymine. When intramolecular (RNA-RNA) or intermolecular (t-RNA--m-RNA or DNA-RNA) base pairing occurs uracil hydrogen bonds to adenine.

* The pentose ribose, which has an -OH group at the 2' carbon atom, replaces 2'-deoxyribose.

...and RNA is usually single-stranded, although it may take on complex structures depending on its function. RNA is always used as a primer in replication of DNA and RNA may possibly catalyse its own preparation for translation (i.e. ribozyme is an RNA-enzyme).

There are three different types of DNA which have slightly different functions:-

1. Messenger RNA (mRNA) - is a single-stranded RNA formed by transcription of a single strand of double-stranded DNA molecule. Transcription occurs in the nucleus, and allows the flow of genetic information (in the form of triplets of nucleotide bases) from the nucleus to the sites of protein synthesis at the ribosomes in the cytoplasm.

2. Transfer RNA (tRNA) - is single-stranded, but has a complex three-dimensional shape formed by intramolecular hydrogen bonding. (Anticodon: nucleotide sequence complimentary to codon on mRNA e.g. tRNA anticodon CUU, mRNA codon GAA.) Transfer RNA 'reads' the genetic code in that it is able to pick up specific amino acids and deliver them to the ribosomes where the amino acids may be assembled into proteins according to the instructions carried on messenger RNA. This assembly process is called translation.

3. Ribosomal RNA (rRNA) - is single-stranded but folded into a complex series of shapes which form aggregates with ribonuclear proteins in the ribosomes. The rRNA molecules are synthesized in the nucleolus, and the many ribosomal subunits are transported via nuclear pores to be assembled in the cytoplasm. The ribosomes are the sites of the translation stage of protein synthesis.

Retroviruses such as HIV have RNA as their genetic material. In order to insert their genome into the host cell (typically a T-lymphocyte) these viruses use the enzyme reverse transcriptase to convert the viral RNA template into DNA. The drug zidovudine (formerly AZT) blocks this reaction and thus prevents the viral genome being inserted into host DNA.

Evidence that DNA is a genetic material - questions

1. Which strain of the S. pneumoniae caused the mice to die?

The S strain; this killed the rat in the first column, whereas the rat injected with the R strain remained healthy.

2. Suggest why this scientist used mice in his experiment.

Mice have similar physiology to humans, so the