Depiction of the adenine–thymine Watson–Crick base pair
Figure 1. During meiosis, homologous recombination can produce new combinations of genes as shown here between similar but not identical copies of human chromosome 1.
Figure 2. An early illustration of crossing over from Thomas Hunt Morgan
Figure 3. Homologous recombination repairs DNA before the cell enters mitosis (M phase). It occurs only during and shortly after DNA replication, during the S and G2 phases of the cell cycle.
Figure 4. Double-strand break repair models that act via homologous recombination
Figure 5. The DSBR and SDSA pathways follow the same initial steps, but diverge thereafter. The DSBR pathway most often results in chromosomal crossover (bottom left), while SDSA always ends with non-crossover products (bottom right).
Figure 6. Recombination via the SSA pathway occurs between two repeat elements (purple) on the same DNA duplex, and results in deletions of genetic material. (Click to view animated diagram in Firefox, Chrome, Safari, or Opera web browsers.)
Figure 7. Crystal structure of a RecA protein filament bound to DNA. A 3' overhang is visible to the right of center.
Figure 8A. Molecular model for the RecBCD pathway of recombination. This model is based on reactions of DNA and RecBCD with ATP in excess over Mg2+ ions. Step 1: RecBCD binds to a double-stranded DNA end. Step 2: RecBCD unwinds DNA.  RecD is a fast helicase on the 5’-ended strand, and RecB is a slower helicase on the 3'-ended strand (that with an arrowhead) [ref 46 in current Wiki version].  This produces two single-stranded (ss) DNA tails and one ss loop.  The loop and tails enlarge as RecBCD moves along the DNA.  Step 3:  The two tails anneal to produce a second ss DNA loop, and both loops move and grow.  Step 4:  Upon reaching the Chi hotspot sequence (5' GCTGGTGG 3'; red dot) RecBCD nicks the 3’-ended strand.  Further unwinding produces a long 3'-ended ss tail with Chi near its end.  Step 5:  RecBCD loads RecA protein onto the Chi tail.  At some undetermined point, the RecBCD subunits disassemble.  Step 6:  The RecA-ssDNA complex invades an intact homologous duplex DNA to produce a D-loop, which can be resolved into intact, recombinant DNA in two ways.  Step 7:  The D-loop is cut and anneals with the gap in the first DNA to produce a Holliday junction.  Resolution of the Holliday junction (cutting, swapping of strands, and ligation) at the open arrowheads by some combination of RuvABC and RecG produces two recombinants of reciprocal type.  Step 8:  The 3' end of the Chi tail primes DNA synthesis, from which a replication fork can be generated.  Resolution of the fork at the open arrowheads produces one recombinant (non-reciprocal) DNA, one parental-type DNA, and one DNA fragment.
Figure 8B. Beginning of the RecBCD pathway. This model is based on reactions of DNA and RecBCD with Mg2+ ions in excess over ATP. Step 1: RecBCD binds to a DNA double strand break.  Step 2: RecBCD initiates unwinding of the DNA duplex through ATP-dependent helicase activity.  Step 3: RecBCD continues its unwinding and moves down the DNA duplex, cleaving the 3' strand much more frequently than the 5' strand. Step 4: RecBCD encounters a Chi sequence and stops digesting the 3' strand; cleavage of the 5' strand is significantly increased. Step 5: RecBCD loads RecA onto the 3' strand. Step 6: RecBCD unbinds from the DNA duplex, leaving a RecA nucleoprotein filament on the 3' tail.
Schematic representation of the s2m RNA secondary structure, with tertiary structural interactions indicated as long range contacts.
Figure 9. Joining of single-ended double strand breaks could lead to rearrangements
Figure 10. Protein domains in homologous recombination-related proteins are conserved across the three main groups of life: archaea, bacteria and eukaryotes.
Figure 11. As a developing embryo, this chimeric mouse had the agouti coat color gene introduced into its DNA via gene targeting. Its offspring are homozygous for the agouti gene.

These sites are non-randomly located on the chromosomes; usually in intergenic promoter regions and preferentially in GC-rich domains These double-strand break sites often occur at recombination hotspots, regions in chromosomes that are about 1,000–2,000 base pairs in length and have high rates of recombination.

- Homologous recombination

Mismatched base pairs can be generated by errors of DNA replication and as intermediates during homologous recombination.

- Base pair
Depiction of the adenine–thymine Watson–Crick base pair

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Overall

The structure of the DNA double helix. The atoms in the structure are colour-coded by element and the detailed structures of two base pairs are shown in the bottom right.

DNA

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Polymer composed of two polynucleotide chains that coil around each other to form a double helix carrying genetic instructions for the development, functioning, growth and reproduction of all known organisms and many viruses.

Polymer composed of two polynucleotide chains that coil around each other to form a double helix carrying genetic instructions for the development, functioning, growth and reproduction of all known organisms and many viruses.

The structure of the DNA double helix. The atoms in the structure are colour-coded by element and the detailed structures of two base pairs are shown in the bottom right.
Chemical structure of DNA; hydrogen bonds shown as dotted lines. Each end of the double helix has an exposed 5' phosphate on one strand and an exposed 3' hydroxyl group (—OH) on the other.
A section of DNA. The bases lie horizontally between the two spiraling strands ([[:File:DNA orbit animated.gif|animated version]]).
DNA major and minor grooves. The latter is a binding site for the Hoechst stain dye 33258.
From left to right, the structures of A, B and Z DNA
DNA quadruplex formed by telomere repeats. The looped conformation of the DNA backbone is very different from the typical DNA helix. The green spheres in the center represent potassium ions.
A covalent adduct between a metabolically activated form of benzo[a]pyrene, the major mutagen in tobacco smoke, and DNA
Location of eukaryote nuclear DNA within the chromosomes
T7 RNA polymerase (blue) producing an mRNA (green) from a DNA template (orange)
DNA replication: The double helix is unwound by a helicase and topo­iso­merase. Next, one DNA polymerase produces the leading strand copy. Another DNA polymerase binds to the lagging strand. This enzyme makes discontinuous segments (called Okazaki fragments) before DNA ligase joins them together.
Interaction of DNA (in orange) with histones (in blue). These proteins' basic amino acids bind to the acidic phosphate groups on DNA.
The lambda repressor helix-turn-helix transcription factor bound to its DNA target
The restriction enzyme EcoRV (green) in a complex with its substrate DNA
Recombination involves the breaking and rejoining of two chromosomes (M and F) to produce two rearranged chromosomes (C1 and C2).
The DNA structure at left (schematic shown) will self-assemble into the structure visualized by atomic force microscopy at right. DNA nanotechnology is the field that seeks to design nanoscale structures using the molecular recognition properties of DNA molecules.
Maclyn McCarty (left) shakes hands with Francis Crick and James Watson, co-originators of the double-helix model.
Pencil sketch of the DNA double helix by Francis Crick in 1953
A blue plaque outside The Eagle pub commemorating Crick and Watson
Impure DNA extracted from an orange

The nitrogenous bases of the two separate polynucleotide strands are bound together, according to base pairing rules (A with T and C with G), with hydrogen bonds to make double-stranded DNA.

The most common form of chromosomal crossover is homologous recombination, where the two chromosomes involved share very similar sequences.

DNA damage resulting in multiple broken chromosomes

DNA repair

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Collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome.

Collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome.

DNA damage resulting in multiple broken chromosomes
Structure of the base-excision repair enzyme uracil-DNA glycosylase excising a hydrolytically-produced uracil residue from DNA. The uracil residue is shown in yellow.
Double-strand break repair pathway models
DNA ligase, shown above repairing chromosomal damage, is an enzyme that joins broken nucleotides together by catalyzing the formation of an internucleotide ester bond between the phosphate backbone and the deoxyribose nucleotides.
DNA repair rate is an important determinant of cell pathology
Most life span influencing genes affect the rate of DNA damage
A chart of common DNA damaging agents, examples of lesions they cause in DNA, and pathways used to repair these lesions. Also shown are many of the genes in these pathways, an indication of which genes are epigenetically regulated to have reduced (or increased) expression in various cancers. It also shows genes in the error-prone microhomology-mediated end joining pathway with increased expression in various cancers.

Three mechanisms exist to repair double-strand breaks (DSBs): non-homologous end joining (NHEJ), microhomology-mediated end joining (MMEJ), and homologous recombination (HR).

Pol η is known to add the first adenine across the T^T photodimer using Watson-Crick base pairing and the second adenine will be added in its syn conformation using Hoogsteen base pairing.

Escherichia coli

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Gram-negative, facultative anaerobic, rod-shaped, coliform bacterium of the genus Escherichia that is commonly found in the lower intestine of warm-blooded organisms.

Gram-negative, facultative anaerobic, rod-shaped, coliform bacterium of the genus Escherichia that is commonly found in the lower intestine of warm-blooded organisms.

Model of successive binary fission in E. coli
Redistribution of fluxes between the three primary glucose catabolic pathways: EMPP (red), EDP (blue), and OPPP (orange) via the knockout of pfkA and overexpression of EDP genes (edd and eda).
A colony of E. coli growing
E. coli on sheep blood agar.
E. coli growing on basic cultivation media.
Scanning electron micrograph of an E. coli colony.
An image of E. coli using early electron microscopy.
Escherichia coli bacterium, 2021, Illustration by David S. Goodsell, RCSB Protein Data Bank This painting shows a cross-section through an Escherichia coli cell. The characteristic two-membrane cell wall of gram-negative bacteria is shown in green, with many lipopolysaccharide chains extending from the surface and a network of cross-linked peptidoglycan strands between the membranes. The genome of the cell forms a loosely-defined "nucleoid", shown here in yellow, and interacts with many DNA-binding proteins, shown in tan and orange. Large soluble molecules, such as ribosomes (colored in reddish purple), mostly occupy the space around the nucleoid.
Helium ion microscopy image showing T4 phage infecting E. coli. Some of the attached phage have contracted tails indicating that they have injected their DNA into the host. The bacterial cells are ~ 0.5 µm wide.

It is a circular DNA molecule 4.6 million base pairs in length, containing 4288 annotated protein-coding genes (organized into 2584 operons), seven ribosomal RNA (rRNA) operons, and 86 transfer RNA (tRNA) genes.

For instance, recA is named after its role in homologous recombination plus the letter A. Functionally related genes are named recB, recC, recD etc. The proteins are named by uppercase acronyms, e.g. RecA, RecB, etc. When the genome of E. coli strain K-12 substr.