In 1958, Matthew Meselson and Franklin Stahl grew E. coli on a medium containing heavy nitrogen (nitrogen-15), then moved the bacteria to ordinary nitrogen-14 and let them divide. After one generation, every DNA molecule had an intermediate density: half heavy, half light. That single result ruled out the idea that the original double helix stays intact, and it fit semiconservative replication. Let's look closer at how a cell copies its DNA, reads it, and decides which genes to use.
DNA and RNA structure
Prokaryotes typically have a circular chromosome; eukaryotes have multiple linear chromosomes wrapped around histones. Both can carry plasmids, small circular DNA molecules outside the chromosome.
DNA works as hereditary material because of base pairing. Purines (adenine, guanine) have two rings; pyrimidines (cytosine, thymine, uracil) have one. A purine always pairs with a pyrimidine: A with T (U in RNA), G with C.
DNA replication
Replication is semiconservative: each strand serves as a template for a new complementary strand. The CED names four replication enzymes (plus RNA polymerase, for transcription):
- Helicase unwinds the strands.
- Topoisomerase relaxes supercoiling ahead of the replication fork.
- DNA polymerase builds DNA 5' to 3', and needs an RNA primer to start.
- Because the strands are antiparallel, the leading strand is made continuously and the lagging strand in fragments.
- Ligase joins the lagging-strand fragments.
Transcription and RNA processing
RNA polymerase reads the template strand 3' to 5' and builds RNA 5' to 3'. Three kinds of RNA matter here: mRNA carries the message, tRNA brings amino acids and has an anticodon, and rRNA is part of the ribosome.
In eukaryotes the transcript is processed before it leaves the nucleus. A 5' cap (the CED calls it a GTP cap) helps the ribosome recognize it, a poly-A tail makes it more stable, and introns are spliced out. Alternative splicing keeps different combinations of exons, so one gene can code for more than one protein.
Translation
Ribosomes in the cytoplasm or on the rough ER read mRNA in triplets called codons. Translation starts at AUG (methionine), tRNA anticodons match each codon, amino acids join the growing chain, and a stop codon ends it. In prokaryotes, translation can begin while the mRNA is still being transcribed. Nearly all organisms share the same genetic code, which is evidence for common ancestry. Retroviruses are the special case: reverse transcriptase copies their RNA into DNA.
Regulation of gene expression and cell specialization
Some genes are constitutive (always on); others are inducible. In prokaryotes, genes are often grouped in operons. In the lac operon of E. coli, a repressor sits on the operator and blocks transcription until lactose is present. When glucose is scarce, an activator protein (CAP) also helps RNA polymerase bind: positive regulation.
In eukaryotes, transcription factors bind promoters and enhancers, which can be upstream or downstream of the gene. Epigenetic changes, reversible modifications of DNA or histones, change expression without changing the sequence. Small RNAs such as microRNAs also regulate expression. Every cell in your body has essentially the same DNA; a muscle cell and a nerve cell differ because they express different genes.
Mutations
A point mutation substitutes one base. It can be silent (same amino acid), change one amino acid, or be a nonsense mutation (premature stop). Insertions or deletions cause frameshifts. Mutations come from replication or repair errors and from radiation and reactive chemicals, and whether one is beneficial, harmful or neutral depends on the environment. Prokaryotes also gain variation through transformation, transduction, conjugation and transposition.
Biotechnology
PCR copies DNA by cycles of denaturing, annealing primers and extending. Gel electrophoresis separates fragments by size: DNA is negative, so it moves toward the positive end, and small pieces travel farthest. Bacterial transformation inserts foreign DNA, and sequencing reads the order of bases.
Common mistakes students make
- Mixing up gene and allele. A gene is a stretch of DNA that codes for a product such as a protein; alleles are different versions of that gene.
- Saying point mutations cause frameshifts. A substitution keeps the reading frame; only insertions or deletions shift it.
- Saying a mutation "denatures" the protein. It changes the amino acid sequence, which can change folding and function.
- Assuming every mutation is harmful. Many are neutral, and some are beneficial in the right environment.
Frequently asked questions
What is the difference between transcription and translation?
Transcription copies DNA into RNA in the nucleus (in eukaryotes). Translation uses that mRNA at a ribosome to build a polypeptide.
Why is the lagging strand made in fragments?
DNA polymerase can only build 5' to 3', so on the strand running the other way it works in short pieces that ligase joins.
How does the lac operon work?
The repressor blocks the operator until lactose is present. Lactose (as allolactose) binds the repressor, releasing it so the genes can be transcribed.
Do all mutations change the protein?
No. A silent mutation changes a codon to another one for the same amino acid, so the protein is unchanged.
Try it
Work through each card and say the answer before you flip it. Then predict: what would you expect to see if a cell's ligase stopped working during replication?
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