Question: What are the changes in the DNA sequence of a cell's genome?
About the article "Evolution", section "Mutation".

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A transposable element (TE, transposon or retrotransposon) is a DNA sequence that can change its position within the genome, sometimes creating or reversing mutations and altering the cell's genome size.
Transposition often results in duplication of the TE.
Barbara McClintock's discovery of these jumping genes earned her a Nobel prize in 1983.TEs make up a large fraction of the C-value of eukaryotic cells.
They are generally considered non-coding DNA, although it has been unambiguously shown that TEs are important in genome function and evolution.
In "Oxytricha", which has a unique genetic system, they play a critical role in development.
They are also very useful to researchers as a means to alter DNA inside a living organism. 
"Retroviridae" is a family of enveloped viruses that replicate in a host cell through the process of reverse transcription.
A retrovirus is a single-stranded RNA virus that stores its nucleic acid in the form of an mRNA genome (including the 5' cap and 3' PolyA tail) and targets a host cell as an obligate parasite.
Once inside the host cell cytoplasm the virus uses its own reverse transcriptase enzyme to produce DNA from its RNA genome, the reverse of the usual pattern, thus "retro" (backwards).
This new DNA is then incorporated into the host cell genome by an integrase enzyme, at which point the retroviral DNA is referred to as a provirus.
The host cell then treats the viral DNA as part of its own genome, translating and transcribing the viral genes along with the cell's own genes, producing the proteins required to assemble new copies of the virus.
It is difficult to detect the virus until it has infected the host.
At that point the infection will persist indefinitely.
In most viruses, DNA is transcribed into RNA, and then RNA is translated into protein.
However, retroviruses function differently – their RNA is reverse-transcribed into DNA, which is integrated into the host cell's genome (when it becomes a provirus), and then undergoes the usual transcription and translational processes to express the genes carried by the virus.
So, the information contained in a retroviral gene is used to generate the corresponding protein via the sequence: RNA → DNA → RNA → polypeptide.
This extends the fundamental process identified by Francis Crick, (one gene-one peptide), in which the sequence is: DNA → RNA → peptide, (proteins are made of one or more polypeptide chain e.g.
haemoglobin is a four chain peptide).
Retroviruses are proving to be valuable research tools in molecular biology and have been used successfully in gene delivery systems. 
When sequencing a genome, there are usually regions that are difficult to sequence (often regions with highly repetitive DNA).
Thus, 'completed' genome sequences are rarely ever complete, and terms such as 'working draft' or 'essentially complete' have been used to more accurately describe the status of such genome projects.
Even when every base pair of a genome sequence has been determined, there are still likely to be errors present because DNA sequencing is not a completely accurate process.
It could also be argued that a complete genome project should include the sequences of mitochondria and (for plants) chloroplasts as these organelles have their own genomes.
It is often reported that the goal of sequencing a genome is to obtain information about the complete set of genes in that particular genome sequence.
The proportion of a genome that encodes for genes may be very small (particularly in eukaryotes such as humans, where coding DNA may only account for a few percent of the entire sequence).
However, it is not always possible (or desirable) to only sequence the coding regions separately.
Also, as scientists understand more about the role of this noncoding DNA (often referred to as junk DNA), it will become more important to have a complete genome sequence as a background to understanding the genetics and biology of any given organism.
In many ways genome projects do not confine themselves to only determining a DNA sequence of an organism.
Such projects may also include gene prediction to find out where the genes are in a genome, and what those genes do.
There may also be related projects to sequence ESTs or mRNAs to help find out where the genes actually are. 
ZFN technology licensed from Sigma-Aldrich utilises purpose engineered restriction enzymes called Zinc Finger Nucleases which bind to specific unique DNA sequences in the target genome.
Through the creation of double stranded breaks in the target sequence and the utilisation of endogenous DNA repair mechanisms, ZFN technology can create targeted gene deletions, modifications or integrations.
When a double-strand break is introduced through heterodimerisation of the DNA in a cell, it stimulates the cell's own DNA repair mechanisms, causing homologous recombination of the DNA strands in the presence of a Repair Template to integrate the gene of interest.
In the absence of a repair template, the DNA will undergo Non-Homologous End Joining, resulting in gene deletion at the target site.
Nuclease based technologies are particularly efficient at rapid deletion or integration of any allele, as well as the knockout 2 or more alleles and they require no antibiotic selection for cell screening. 
Two different kinds of genetic material exist: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).
Cells use DNA for their long-term information storage.
The biological information contained in an organism is encoded in its DNA sequence.
RNA is used for information transport (e.g., mRNA) and enzymatic functions (e.g., ribosomal RNA).
Transfer RNA (tRNA) molecules are used to add amino acids during protein translation.
Prokaryotic genetic material is organized in a simple circular DNA molecule (the bacterial chromosome) in the nucleoid region of the cytoplasm.
Eukaryotic genetic material is divided into different, linear molecules called chromosomes inside a discrete nucleus, usually with additional genetic material in some organelles like mitochondria and chloroplasts (see endosymbiotic theory).
A human cell has genetic material contained in the cell nucleus (the nuclear genome) and in the mitochondria (the mitochondrial genome).
In humans the nuclear genome is divided into 46 linear DNA molecules called chromosomes, including 22 homologous chromosome pairs and a pair of sex chromosomes.
The mitochondrial genome is a circular DNA molecule distinct from the nuclear DNA.
Although the mitochondrial DNA is very small compared to nuclear chromosomes, it codes for 13 proteins involved in mitochondrial energy production and specific tRNAs.
Foreign genetic material (most commonly DNA) can also be artificially introduced into the cell by a process called transfection.
This can be transient, if the DNA is not inserted into the cell's genome, or stable, if it is.
Certain viruses also insert their genetic material into the genome.