Interactive Double Helix

Hover over parts of the model to see what each piece does.

The helix has a major groove and minor groove — grooves of different widths created by the twist. Proteins that regulate genes (transcription factors) recognize specific base sequences by fitting into the major groove, where more of each base pair's chemical surface is exposed.

You can think of the two backbone rails as the sides of a ladder, and each base pair as a rung — the whole thing is just twisted into a spiral.

DNA Replication

Before a cell divides, it must copy its entire DNA so each new cell gets a full set.

This is called semi-conservative replication — each daughter molecule retains one parental strand. DNA polymerase performs the synthesis, always extending in the 5'→3' direction, which is why the lagging strand is synthesized in short Okazaki fragments.

From Gene to Trait: How DNA Becomes You

A gene is just an instruction. To actually build something — like a protein that determines eye color or hair texture — the cell has to read and execute that instruction. This happens in two main steps.

This overall pathway — DNA → RNA → protein — is known as the Central Dogma of molecular biology. Not all genes code for proteins directly; some produce functional RNA molecules (like rRNA or tRNA) that never get translated at all.

Think of DNA as a recipe book, mRNA as a photocopy of one recipe taken to the kitchen, and the protein as the finished dish.

Transcription: DNA → RNA

The DNA instructions are copied into a related molecule called messenger RNA (mRNA), so the message can leave the nucleus.

RNA polymerase binds to a promoter region and synthesizes a complementary mRNA strand from the DNA template. RNA uses uracil (U) instead of thymine (T), so adenine pairs with uracil during this process. In eukaryotes, the pre-mRNA is then spliced (introns removed, exons joined) before leaving the nucleus.

Translation: RNA → Protein

The mRNA message travels to a ribosome, which reads it three letters (a "codon") at a time and assembles a chain of amino acids — a protein.

Transfer RNA (tRNA) molecules, each carrying a specific amino acid, recognize codons via complementary anticodons. The ribosome links amino acids together in the order specified until it hits a stop codon. The resulting polypeptide folds into a functional protein.

Codon Table (sample)

This is a small sample — the full genetic code has 64 codons for 20 amino acids plus stop signals.

Mendelian Inheritance

Gregor Mendel discovered that traits are passed from parents to offspring in predictable patterns, through discrete units we now call genes.

This describes autosomal traits with complete dominance — Mendel's original pea-plant experiments. Real-world inheritance also includes sex-linked genes, incomplete dominance, codominance, and polygenic traits, covered further down.

A simple way to picture it: if Dad gives a "brown eye" instruction and Mom gives a "blue eye" instruction, the brown one usually wins — but the blue instruction doesn't disappear, it's still there for the next generation.

Punnett Square Calculator

Pick a genotype for each parent to see the possible offspring combinations.

Sex Chromosomes: XX vs XY

Among your 46 chromosomes, one pair determines biological sex. Females typically have two X chromosomes (XX); males typically have one X and one Y (XY).

Every egg cell carries an X. Every sperm cell carries either an X or a Y. So it's the father's contribution that determines whether a child develops as XX or XY.

The Y chromosome is much smaller than the X and carries far fewer genes — but includes the SRY gene, which triggers male development in the embryo. Because males have only one X, any X-linked recessive allele they inherit is expressed — they have no second X to potentially mask it. This is why X-linked recessive conditions (like red-green color blindness or hemophilia) are far more common in males than females.

Sex-Linked Punnett Example: Color Blindness (X-linked recessive)

Here, X^B = normal vision (dominant), X^b = color blindness (recessive). Males (X^?Y) only need one copy to show the trait; females (X^?X^?) need two.

Beyond Simple Dominance

Not every trait follows the strict "one wins" pattern above — inheritance has some interesting variations.

Other patterns include codominance (both alleles fully expressed, e.g. AB blood type), polygenic inheritance (many genes contributing to one trait, like human height), and epistasis (one gene masking the effect of another, unrelated gene).

Where Genetic Variation Comes From

If children only ever got an exact copy of one parent's DNA, siblings would be identical. They're not — because of two key processes.

Meiosis

Meiosis is the special cell division that creates sex cells (sperm and egg), each carrying only half the normal number of chromosomes — so when two combine at fertilization, the full number is restored.

Meiosis involves two rounds of division (Meiosis I and II) producing four haploid cells from one diploid cell. Homologous chromosomes pair up and separate in Meiosis I; sister chromatids separate in Meiosis II.

Crossing-Over

During meiosis, paired chromosomes can swap segments of DNA with each other before separating — shuffling combinations of alleles that get passed on.

This exchange happens at points called chiasmata. Combined with independent assortment (random orientation of chromosome pairs during Meiosis I) and random fertilization, crossing-over is a major source of the genetic diversity seen within a species.

Mutation: A Basic Introduction

A mutation is simply a change in a DNA sequence — a base gets swapped, added, or removed. Mutations are the ultimate source of all genetic variation; without them, there would be no new alleles at all.

Mutations can occur spontaneously (replication errors) or be induced by environmental factors. Most are neutral or harmless, some are harmful, and a small fraction are beneficial — the raw material natural selection acts on.

Ask AI — Your Genetics Tutor

Ask anything about DNA, genes, or inheritance. Answers are tailored to the level you picked above (Basic).

Get a free key at aistudio.google.com/apikey. The key is stored only in your browser's localStorage — never sent anywhere but Google's API.