When the replication process is complete, two DNA molecules — identical to each other and identical to the original — have been produced. Each strand of the original molecule has remained intact as it served as the template for the synthesis of a complementary strand.
Structure of cytosine with and without the 5-methyl group. Deamination converts 5-methylcytosine into thymine. Base modifications and DNA packaging Further information: DNA methylation and Chromatin remodeling The expression of genes is influenced by how the DNA is packaged in chromosomes, in a structure called chromatin.
Base modifications can be involved in packaging, with regions that have low or no gene expression usually containing high levels of methylation of cytosine bases.
DNA packaging and its influence on gene expression can also occur by covalent modifications of the histone protein core around which DNA is wrapped in the chromatin structure or else by remodeling carried out by chromatin remodeling complexes see Chromatin remodeling. There is, further, crosstalk between DNA methylation and histone modification, so they can coordinately affect chromatin and gene expression.
Mutagens include oxidizing agentsalkylating agents and also high-energy electromagnetic radiation such as ultraviolet light and X-rays. The type of DNA damage produced depends on the type of mutagen. For example, UV light can damage DNA by producing thymine dimerswhich are cross-links between pyrimidine bases.
Because of inherent limits in the DNA repair mechanisms, if humans lived long enough, they would all eventually develop cancer.
Although most of these damages are repaired, in any cell some DNA damage may remain despite the action of repair processes. These remaining DNA damages accumulate with age in mammalian postmitotic tissues.
This accumulation appears to be an important underlying cause of aging. Most intercalators are aromatic and planar molecules; examples include ethidium bromideacridinesdaunomycinand doxorubicin.
For an intercalator to fit between base pairs, the bases must separate, distorting the DNA strands by unwinding of the double helix. This inhibits both transcription and DNA replication, causing toxicity and mutations. The set of chromosomes in a cell makes up its genome ; the human genome has approximately 3 billion base pairs of DNA arranged into 46 chromosomes.
Transmission of genetic information in genes is achieved via complementary base pairing. Usually, this RNA copy is then used to make a matching protein sequence in a process called translationwhich depends on the same interaction between RNA nucleotides.
In alternative fashion, a cell may simply copy its genetic information in a process called DNA replication. The details of these functions are covered in other articles; here the focus is on the interactions between DNA and other molecules that mediate the function of the genome.
Genes and genomes Further information: In eukaryotes, DNA is located in the cell nucleuswith small amounts in mitochondria and chloroplasts. In prokaryotes, the DNA is held within an irregularly shaped body in the cytoplasm called the nucleoid.
A gene is a unit of heredity and is a region of DNA that influences a particular characteristic in an organism. Genes contain an open reading frame that can be transcribed, and regulatory sequences such as promoters and enhancerswhich control transcription of the open reading frame.
In many speciesonly a small fraction of the total sequence of the genome encodes protein. For example, only about 1. Telomeres and centromeres typically contain few genes but are important for the function and stability of chromosomes.
Genetic codeTranscription geneticsand Protein biosynthesis A gene is a sequence of DNA that contains genetic information and can influence the phenotype of an organism. Within a gene, the sequence of bases along a DNA strand defines a messenger RNA sequence, which then defines one or more protein sequences.
The relationship between the nucleotide sequences of genes and the amino-acid sequences of proteins is determined by the rules of translationknown collectively as the genetic code.
The genetic code consists of three-letter 'words' called codons formed from a sequence of three nucleotides e. These encode the twenty standard amino acidsgiving most amino acids more than one possible codon.
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.Editor’s Note: This is a guest post by Dee Shore, a writer in NC State’s College of Agriculture and Life Sciences.
DNA replication is among life’s most important processes, providing a way for an organism’s genetic material to be reproduced so it can be passed from cell to cell. Paul Andersen explains how DNA replication ensures that each cell formed during the cell cycle has an exact copy of the DNA.
He describes the Meselson-Stahl experiment and how it showed that DNA copies itself through a semi-conservative process.
DNA polymerases can only make DNA in the 5' to 3' direction, and this poses a problem during replication. A DNA double helix is always anti-parallel; in other words, one strand runs in the 5' to 3' direction, while the other runs in the 3' to 5' direction. This is the microbiology questions and answers section on "DNA Replication" with explanation for various interview, competitive examination and entrance test.
Solved examples with detailed answer description, explanation are given and it would be easy to understand. DNA replication begins with the "unzipping" of the parent molecule as the hydrogen bonds between the base pairs are broken.
Once exposed, the sequence of bases on each of the separated strands serves as a template to guide the insertion of a complementary set of bases on the strand being synthesized. Both strands of the DNA double helix act as templates for the new DNA strands.
Incoming DNA is unraveled by the enzyme helicase, resulting in the 3' strand and the 5' strand. The 3' strands and the 5' strands are replicated by a DNA polymerase enzyme but in different ways.