Structurally, nucleic acids are giant polymers composed of units called nucleotides, each of which consists of a sugar, a phosphate, and a base. The sugar in DNA is deoxyribose?. The four bases are adenine?, cytosine, thymine?, and guanine. Each can readily form hydrogen bonds to one of the others--A with T, C with G--so that two matching strands of DNA will group together in a double helix. This form is called B-DNA; there are a few others.
Reasons why thymine? is used in DNA, and uracil? in RNA need some explanation. Thymine is created from uracil in a methylation? process, which is energetically expensive. In other words, uracil is cheaper for a cell that thymine. Unfortunatelly cytosine becomes deaminated? with significant speed and becomes uracil?. If uracil were used in DNA, that would be source of many mutations. With thymine, all uracils created in such way can be removed, and the proper fragment of DNA can be copied from the other strand. Cheaper uracil is used in RNA because RNA doesn't need to last a long time anyway.
DNA does not code for any proteins directly. Instead, matching strands of messenger RNA (mRNA) are formed by enzymes called [RNA polymerases]?--this is called "transcription"--which are sent off into the cytoplasm for translation in the ribosomes. Other kinds of RNA are also formed to help in the process--tRNA, rRNA, and snRNA. The genetic code used by these molecules to encode the sequence of proteins is highly conserved, nearly universally so. This suggests that all life on Earth is descended from a single original strain of living matter.
In bacteria there is a single main strand of DNA, copied over when the cell replicates. There may also be small pieces of circular DNA called plasmids floating in the cytoplasm that can be exchanged between cells. In eukaryotes there are typically multiple pieces of DNA contained in a central nucleus, separated out in cell division by a process called mitosis. There may be a single copy of the genome (whence the organism is haploid), two copies (diploid), or many copies (polyploid?).
In preparation for [sexual reproduction]?, haploid cells are produced from a diploid precursor in a process called meiosis. Eventually some haploid cells unite together to form a new diploid cell (syngamy?) with half its genome from each diploid ancestor. When there are different versions of the same gene, their effects may be blended or a single version may be dominant.
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