Illustration of DNA double helix replication and mRNA synthesis, highlighting key molecular structures.

- The image displays a DNA molecule undergoing replication, with two strands separating and new complementary strands being synthesized.
- On the left, the original double-stranded DNA is shown unwinding. The top strand is labeled D (deoxyribose) and P (phosphate), indicating the sugar-phosphate backbone.
- New nucleotides are being added to each separated strand to form two identical DNA molecules.
- On the right, a single strand of messenger RNA (mRNA) is shown being transcribed from one of the DNA strands.
- The mRNA strand has ribose sugar (labeled R) and phosphate (P) in its backbone, distinguishing it from DNA.
- Below the mRNA, four nucleotide structures are shown: guanine (G), adenine (A), cytosine (C), and thymine (T). Note that thymine is found in DNA, not RNA; uracil replaces thymine in RNA.
- The process depicted on the left is DNA replication, which occurs during the S phase of the cell cycle to duplicate genetic material before cell division.
- The process on the right is transcription, where a segment of DNA is copied into mRNA by RNA polymerase, which then carries the genetic code to the ribosome for protein synthesis.
- In both processes, base pairing rules apply: A pairs with T in DNA (or U in RNA), and G pairs with C.
- The directionality of nucleic acid synthesis is 5’ to 3’, meaning new nucleotides are added to the 3’ end of the growing chain.
- The diagram illustrates the central dogma of molecular biology: DNA → RNA → Protein, showing how genetic information flows from DNA to mRNA and eventually to proteins.