The size of the genome in one of the most well-studied prokaryotes, Escherichia coli, is 4. So how does this fit inside a small bacterial cell? The DNA is twisted beyond the double helix in what is known as supercoiling.
Some proteins are known to be involved in the supercoiling; other proteins and enzymes help in maintaining the supercoiled structure.
Eukaryotes, whose chromosomes each consist of a linear DNA molecule, employ a different type of packing strategy to fit their DNA inside the nucleus. At the most basic level, DNA is wrapped around proteins known as histones to form structures called nucleosomes.
The DNA is wrapped tightly around the histone core. This nucleosome is linked to the next one by a short strand of DNA that is free of histones.
This fiber is further coiled into a thicker and more compact structure. At the metaphase stage of mitosis, when the chromosomes are lined up in the center of the cell, the chromosomes are at their most compacted. They are approximately nm in width, and are found in association with scaffold proteins. In interphase, the phase of the cell cycle between mitoses at which the chromosomes are decondensed, eukaryotic chromosomes have two distinct regions that can be distinguished by staining.
There is a tightly packaged region that stains darkly, and a less dense region. The darkly staining regions usually contain genes that are not active, and are found in the regions of the centromere and telomeres. The lightly staining regions usually contain genes that are active, with DNA packaged around nucleosomes but not further compacted.
Concept in Action. Watch this animation of DNA packaging. The DNA molecule is a polymer of nucleotides. Each nucleotide is composed of a nitrogenous base, a five-carbon sugar deoxyribose , and a phosphate group. There are four nitrogenous bases in DNA, two purines adenine and guanine and two pyrimidines cytosine and thymine.
A DNA molecule is composed of two strands. Each strand is composed of nucleotides bonded together covalently between the phosphate group of one and the deoxyribose sugar of the next.
If there's a T on one side of the strand, there will always be an A on the other. And those nucleotides always pair. We also count DNA and the amount of DNA, or the length of DNA by using units of base pairs, so if we're discussing a gene and we want to describe how big is a gene, we might say that the gene is a thousand base pairs long.
If it's a really big gene, it may be 10, base pairs, or essentially 10 kilobases long. Lawrence C. The first chain contains 13 nucleotides, truncated after ddGTP addition. The second chain contains 11 nucleotides, also truncated after ddGTP addition. After complete analysis with all four ddNTPs, the final nucleotide sequence is shown in the right panel. Nucleotides are represented by different colored rectangles: red for thymine, blue for guanine, green for adenine, and orange for cytosine.
Below the sequenced strand, examples of truncated strands from the four reactions are shown. Reading the sequence: Now and then. How is DNA sequencing used by scientists? In recent years, DNA sequencing technology has advanced many areas of science.
For example, the field of functional genomics is concerned with figuring out what certain DNA sequences do, as well as which pieces of DNA code for proteins and which have important regulatory functions. An invaluable first step in making these determinations is learning the nucleotide sequences of the DNA segments under study.
Another area of science that relies heavily on DNA sequencing is comparative genomics, in which researchers compare the genetic material of different organisms in order to learn about their evolutionary history and degree of relatedness.
DNA sequencing has also aided complex disease research by allowing scientists to catalogue certain genetic variations between individuals that may influence their susceptibility to different conditions. How can all people benefit from DNA sequencing? More about sequencing. DNA sequencing technologies Sequencing the human genome. Watch this video for a summary of the Sanger sequencing process. How much does gene sequencing cost? How was the human genome sequenced? What happens during DNA replication?
Who, exactly, discovered DNA? How has the polymerase chain reaction PCR revolutionized biotechnology? What has genomics done for the biofuel industry? What ethical problems does DNA sequencing raise? How is sequencing done on a large scale? Who was Frederick Sanger? Key Concepts Human Genome Project bioinformatics genome.
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