DNA is produced by replication of two existing strands when they are unwound, NOT transcription.

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Transcription is the process of making mRNA. They are doubled-stranded due to the hydrogen bonds and hydrophobic forces of non-polar molecules. Single stranded DNA also exists in the lab by heating them so that they are denatured.

correction there, crazydavid: transcription is the process of making RNA. you can make tRNA, mRNA, rRNA, miRNA, SRP-RNA, snRNA, snoRNA, telomerase RNA, etc etc and it’s still called transcription.

Thanks for correction. I want to ask if there are any difference in the transcription between tRNA, mRNA and rRNA? I just know something about mRNA transcription. I am wondering wherther there will be big differences between them.

indeed there are. Eukaryotic cells have three different RNA polymerase enzymes. RNA Polymerase II is the best studied one, the one that transcribes mRNA. RNA polymerase I transcribes most of the ribosomal RNA. RNA polymerase III transcribes one type of ribosomal RNA (the 5S gene), tRNAs, and most other coding RNAs that I listed above (with the small exception of small nucleolar RNA). Biochemical differences arise from the fact that there are different enzymes transcribing the different types of RNA, but I am guessing that you are not a molecular biologist and so it would be best not to overwhelm you.

Which is referred to as replication and is the normal process of DNA production using DNA polymerases.

Actually it is possible to produce DNA from transcription, via reverse transcription. This happens when certain viruses such as HIV invades a human cell. As the genome of HIV consists of RNA, in order for it replicate using the host machinery, it must produce a DNA copy of its genome to insert into the genome of the host, which is termed a provirus. It does so by the reverse transcriptase it encodes, which is a DNA dependant RNA polymerase. All living things (therefore not including viruses) such as animals and bacteria, store their genetic information as DNA, so transcription can only produces messager RNA which leaves the nucleus to pass on the info needed to make proteins.