In addition, DNA molecules can be very long. Stretched end-to-end, the DNA molecules in a single human cell would come to a length of about 2 meters. Thus, the DNA for a cell must be packaged in a very ordered way to fit and function within a structure the cell that is not visible to the naked eye. The chromosomes of prokaryotes are much simpler than those of eukaryotes in many of their features Figure 9. Most prokaryotes contain a single, circular chromosome that is found in an area in the cytoplasm called the nucleoid.
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 sugar-phosphate backbone is negatively charged and hydrophilic, which allows the DNA backbone to form bonds with water. A sugar-phosphate backbone alternating grey-dark grey joins together nucleotides in a DNA sequence. Copyright , Nature Education. Related Concepts 9. You have authorized LearnCasting of your reading list in Scitable.
They deliver their "messages" to the protein synthesizing machinery in the cytoplasm of the cell. There are several kinds of rRNA, some are found in the large ribosomal subunit and some in the small subunit. There are more than 20 types of tRNA. They act as the link between the genetic code and the process of joining amino acids together to form polypeptides. Deoxyribose is the pentose sugar found in this type of polynucleotide, hence its name D eoxyribo n ucleic A cid, or DNA.
The nitrogenous bases found in DNA are, adenine, guanine, cytosine and thymine. DNA molecules have two polynucleotide chains, held together in a ladderlike structure. The sugar phosphate backbones of the two chains run parallel to each other in opposite directions.
Each "rung" of the ladder is a pair of nitrogenous bases, one purine and one pyrimidine extending into the center of the molecule.
0コメント