Skip to main content
Helicase

DNA & Genetics Lab

Genes are instructions written in DNA. Pair the letters of DNA, watch a real human gene become a protein, change a letter to see what happens, and predict how traits are inherited.

Work with a real human gene, beta-globin (HBB): complementary base pairing, transcription and translation, how point mutations and frameshifts change proteins, and Mendelian inheritance.

Use the RefSeq beta-globin coding sequence to explore base pairing, the genetic code, missense, nonsense and frameshift effects (including HbS), and independent assortment in mono- and dihybrid crosses.

GeneticsMolecular BiologyBiochemistry

The first 12 bases of the human beta-globin (HBB) gene. Type the partner for each base on the bottom strand.

5′3′3′5′

Type A, C, G or T. Arrow keys move between positions. Correct pairs show their hydrogen bonds as bars.

Why the strands match

DNA is made of two strands that zip together. A always pairs with T, and G always pairs with C, so one strand tells you exactly what the other must be.

The two strands of the double helix are held together by hydrogen bonds between bases: A pairs with T using two bonds, and G pairs with C using three. Because pairing is fixed, each strand can serve as a template for copying the other.

Watson and Crick proposed complementary, antiparallel strands, drawing on X-ray data from Franklin, Gosling and Wilkins. Specific purine-pyrimidine pairing (A-T with two hydrogen bonds, G-C with three) underlies semiconservative replication, later shown by Meselson and Stahl.

Notice the labels: the strands run in opposite directions (antiparallel), so the partner strand is written 3′ to 5′ underneath.

Check your understanding

Knowledge check

One strand of DNA reads ATGC. What does the matching stretch of the partner strand read, position by position?

Knowledge check

When a gene is transcribed, which base appears in the mRNA wherever the DNA coding strand has T?

Knowledge check

The sickle cell variant changes the codon GAG to GTG in the beta-globin gene. What kind of mutation is this?

Knowledge check

Why does deleting a single base near the start of a gene usually change the protein more than substituting a single base?

Your progress

Sources & further reading

The beta-globin sequence comes directly from NCBI's RefSeq records; translating it with the standard genetic code reproduces NCBI's protein exactly (checked by an automated test). Explanations are our own summaries of these sources. How we choose and check sources.

  12. The dependence of cell-free protein synthesis in E. coli upon naturally occurring or synthetic polyribonucleotides (opens the original in a new tab)

    Nirenberg MW, Matthaei JH (1961). Proceedings of the National Academy of Sciences 47:1588-1602.

    First codon assignment: poly-U RNA directs synthesis of polyphenylalanine.

    Historical paperPrimary research
Show 21 more sources

  15. Protection afforded by sickle-cell trait against subtertian malarial infection (opens the original in a new tab)

    Allison AC (1954). British Medical Journal 1:290-294.

    Early evidence that carrying one sickle cell allele protects against severe malaria.

    Historical paperPrimary research