Genes are DNA
Loading structuresGenes are DNA
All the hereditary information in an organism is its genome. It is divided into chromosomes, each a very long molecule of duplex DNA carrying many genes.
The genome is physically divided into chromosomes, discrete units that each carry many genes. Each consists of a very long molecule of duplex DNA and, in eukaryotes, an approximately equal mass of proteins. A chromosome is visible as a morphological entity only during cell division.
A gene is a sequence of DNA that encodes a single type of RNA, and that RNA may encode a polypeptide. While all genes are made up of DNA, not all DNA is part of a gene: about 98% of the human genome does not encode protein.
The place on a chromosome at which a gene lies is its locus, and an allele is a variant form of a gene. In a diploid organism each gene has two alleles, one inherited from each parent, at the same locus on a pair of homologous chromosomes.
Protein-coding genes encode RNAs that are translated into polypeptides; non-protein-coding genes encode RNAs that function without being translated. All genes code for an RNA, but not all genes code for a protein, and only one of a gene's two strands is used to code for its RNA.
Regulator genes encode products that regulate the expression of other genes, and may be protein-coding or encode a non-coding RNA. The final products of gene expression, RNAs or polypeptides, may themselves regulate the expression of other gene products.
The genome carries information but plays no active role itself: cellular machinery acts on specific sequences in the DNA to produce products. Genes must be switched on and off as the cell needs, so the genome also carries regulatory elements.
Neither the size of a genome nor its number of genes follows the complexity of an organism: rice has far fewer base pairs than humans, but more genes.
| Organism | Genome size | Chromosomes | Genes |
|---|
In prokaryotes such as bacteria the genome is often a single, circular chromosome, often with plasmids beside it. In eukaryotes it is chromosomal DNA together with organellar DNA, in mitochondria and chloroplasts.
1 kb (kilobase) = 1,000 bases · 1 Mb (megabase) = 1,000,000 bases · 1 Gb (gigabase) = 1,000,000,000 bases. The human genome's 3,200 Mb is 3.2 Gb.
Through gene expression, the DNA sequence directs the production of all the RNAs in the cell, which in turn are used to produce its proteins. This flow of information, from DNA to RNA to protein, is the central dogma of molecular biology.
Information in nucleic acid can be perpetuated or transferred, but its transfer into a polypeptide is irreversible: information never passes from a polypeptide back to a nucleic acid.
No cellular organism has a genome made of anything other than double-stranded DNA; the genomes of some viruses are single-stranded DNA, or double- or single-stranded RNA.
Smooth and rough pneumococci injected into mice, alone, heat-killed and mixed: predict each result, then watch the transforming principle and DNA from S cells turn R cells into S.
Play ↗ Experiment · 10 stepsPhage T2 infects E. coli, its DNA entering the cell while its coat stays outside; labelled with ³²P and ³⁵S, only the DNA's label goes in and on to the progeny.
Play ↗ Experiment · 6 stepsCultured cells that lack thymidine kinase take up a plasmid carrying its gene; the cell whose plasmid joins a chromosome grows into a colony under selection.
Play ↗ Scene · 8 stepsA base, a sugar and a phosphate atom by atom: the glycosidic bond, nucleosides and nucleotides, deoxyribose and ribose, purines and pyrimidines joined at N9 or N1, uracil in RNA, and a chain's 5′ and 3′ ends.
Play ↗ Scene · 10 stepsA crystal structure of B-DNA: G–C and A–T pairs, antiparallel strands, a constant width, flat base pairs about ten to a turn, the two grooves and the charged backbone.
Play ↗ Scene · 6 stepsA replication fork moving along a parental duplex, leaving two daughter duplexes that each keep one parental strand, the DNA helicase and the DNA polymerases at work, and the leading and lagging strands.
Play ↗ Scene · 6 stepsHeavy DNA in a density gradient, one and then two generations in light medium, and why the bands fit semiconservative replication and not conservative or dispersive replication.
Play ↗ Scene · 7 stepsViral genomes of double- or single-stranded DNA or RNA, how DNA and RNA differ, and the central dogma, with an RNA-dependent RNA polymerase and a reverse transcriptase at work.
Play ↗ Scene · 6 stepsA DNA duplex parted by heat and paired again, an RNA that pairs with itself and with another RNA, and a DNA–RNA heteroduplex atom by atom.
Play ↗ Scene · 6 stepsA base pair changed in a gene, cytosine turned into uracil by nitrous acid atom by atom, the change copied at replication into a transition, and transitions and transversions between purines and pyrimidines.
Play ↗ Scene · 6 stepsInsertions and deletions shifting a gene's reading frame codon by codon, and which mutations can revert: point mutations and insertions can, deletions cannot.
Play ↗ Library · 8 moleculesThe double helix, transfer RNA and a DNA–RNA hybrid, and the enzymes that replicate DNA, copy RNA genomes and, in transfection, rescue cells: thymidine kinase.
Open ↗ ReadingGenes are DNA in full: the genome, the evidence that genes are DNA, nucleotides, the double helix, replication, genomes of DNA or RNA, hybridisation and mutations.
Read ↗Its structures load over HTTP. From the repository root, run
pixi run serve-drawings
then open http://127.0.0.1:8080/genes_are_dna/.