1. GC content: the higher the GC%, the higher the T_m
2. Salt: the higher the [salt], the higher the T_m (ions shield charges and lessen repulsion between phosphates).
3. Low or high pH decrease T_m (ionization of the bases)
4. Organic compounds destabilize the double helix by competing as H-bond partners or by disrupting the water shell around the bases.

[Several Kinds of RNA]

Ribosomal RNA (rRNA)-

- An integral part of ribosomes (intracellular ribonucleoproteins that are the sites of protein synthesis).

- Ribosomal RNA is the most abundant class of ribonucleic acid, accounting for about 80% of the total cellular RNA.

[Several Kinds of RNA]

 Transfer RNA (tRNA)

-Molecules carry activated amino acids to the ribosomes for incorporation into growing peptide chains during protein synthesis.

- tRNA molecules are only 73 to 95 nucleotide residues long.

- Account for about 15% of the total cellular RNA.

[Several Kinds of RNA]

 Messenger RNA (mRNA)

- molecules encode the sequences of amino acids in proteins.

- They are the "messengers" that carry information from DNA to the translation complex where proteins are synthesized.

- In general, mRNA accounts for only 3% of the total cellular RNA. These molecules are the least stable of the cellular ribonucleic acids.

[Several Kinds of RNA]

 Small RNA molecules

- are present in all cells.

- some have catalytic activity or contribute to catalytic activity in association with proteins.

- Many of these RNA molecules are associated with processing events that modify RNA after it has been synthesized.

- The major proteins of chromatin

- H1, H2A, H2B, H3, and H4

-Positively charged histones bind to negatively charged sugar-phosphates of DNA

- Enzymes produced by bacterial cells to degrade or "restrict" foreign DNA molecules.

- Recognize specific base sequences in double stranded DNA andcatalyze hydrolytic cleavageof the two strands in or near that region.