Nucleic acids

All living organisms do have DNA as genetic material. Some viruses use RNA for that purpose.

After Griffith’s experiment in 1928 showing that dead bacteria could transform living ones, Avery, MacLeod and McCarty discovered that this transforming principle was indeed DNA (using different set up deteriorating specifically proteins, DNA or RNA). Those findings, combined with those of Hershey and Chase on bacteriophages demonstrate that DNA is carrying inheritance information.
Some viruses (HIV for example) are using RNA as genetic material and do have an enzyme converting RNA into DNA. However, as all viruses, they cannot reproduce on their own and need the host machinery to replicate.

DNA is associated with different types of proteins to form chromosomes. DNA coiling saves space but doesn’t help for replication or transcription.

In eukaryotes, DNA wraps around cores of 8 histone proteins (nucleosomes). This structure (11 nm in diameter) is maintained by another protein, the H1 histone. Nucleosomes can be supercoiled again and again to form a 700nm structure during mitosis or meiosis.
However, to be replicated or transcribed during interphase DNA needs to be decondensed. Different mechanisms regulate this condensation such as the methylation state of DNA. That is a very good example of epigenetics as the environment can modify that methylation state and be inherited over generations.

For a long time, all DNA sequences not giving proteins were considered as junk DNA. Now scientists know that reality is far more complex.

In addition to DNA sequences found to be giving part or a whole protein, different sequences can be identified for their role. Introns in eukaryotes are part of pre-mRNA and are involved in RNA processing even if they’re not translated into amino acids.
Other parts of the genome are keys to that protein expression such as regulating sequences where proteins can bind and modulate transcription.
It is the same for sequences giving rRNA and tRNA involved in translation (for ribosomes and amino acids transfer respectively). Telomeres, located at the end of chromosomes are not fully replicated at each cycle but protect genes to be lost for that reason.