The phosphate backbone indicated by the curvy lines is on the outside, and the bases are on the inside. Each base from one strand interacts via hydrogen bonding with a base from the opposing strand. Only certain types of base pairing are allowed. This means Adenine pairs with Thymine, and Guanine pairs with Cytosine. This is known as the base complementary rule because the DNA strands are complementary to each other.
Antiparallel Strands : In a double stranded DNA molecule, the two strands run antiparallel to one another so one is upside down compared to the other.
The phosphate backbone is located on the outside, and the bases are in the middle. Adenine forms hydrogen bonds or base pairs with thymine, and guanine base pairs with cytosine. At this time it is possible a mutation may occur. A mutation is a change in the sequence of the nitrogen bases. Most of the time when this happens the DNA is able to fix itself and return the original base to the sequence.
However, sometimes the repair is unsuccessful, resulting in different proteins being created. DNA packaging is an important process in living cells. Without it, a cell is not able to accommodate the large amount of DNA that is stored inside.
A eukaryote contains a well-defined nucleus, whereas in prokaryotes the chromosome lies in the cytoplasm in an area called the nucleoid. In prokaryotic cells, both processes occur together. What advantages might there be to separating the processes? What advantages might there be to having them occur together? Eukaryotic and prokaryotic cells : A eukaryote contains a well-defined nucleus, whereas in prokaryotes, the chromosome lies in the cytoplasm in an area called the nucleoid.
The size of the genome in one of the most well-studied prokaryotes, E. So how does this fit inside a small bacterial cell? The DNA is twisted by what is known as supercoiling. Supercoiling means that DNA is either under-wound less than one turn of the helix per 10 base pairs or over-wound more than 1 turn per 10 base pairs from its normal relaxed state. Some proteins are known to be involved in the supercoiling; other proteins and enzymes such as DNA gyrase 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 histones are evolutionarily conserved proteins that are rich in basic amino acids and form an octamer. The DNA which is negatively charged because of the phosphate groups is wrapped tightly around the histone core.
This nucleosome is linked to the next one with the help of a linker DNA. This is further compacted into a 30 nm fiber, which is the diameter of the structure. At the metaphase stage the chromosomes are at their most compact, approximately nm in width, and are found in association with scaffold proteins.
Eukaryotic chromosomes : These figures illustrate the compaction of the eukaryotic chromosome. In addition, they play roles in biological energy storage and transmission, signaling, regulation of various aspects of metabolism, and even an important role as an antioxidant. Mistakes or deficiencies in their synthesis usually lead to death. Overproduction or decreased elimination of nucleic acid derivates also lead directly to medical conditions.
Nucleosides have a nitrogenous base and a five-carbon carbohydrate group, usually a ribose molecule see Chapter 2. Nucleotides are simply a nucleoside with one or more phosphate groups attached Figure The resulting molecule is found in ribonucleic acid or RNA. If one hydroxyl OH group has been removed from the ribose, the deoxy versions of the nucleoside and nucleotide form the building blocks of deoxyribonucleic acid or DNA Figure Each component of nucleosides and nucleotides is discussed below.
Basic Structure of Nucleosides and Nucleotides. Five major nucleoside bases are common in human biology, including the purines two-ring structure adenine and guanine top and the pyrimidines one-ring structure cytosine, uracil, and thymine middle.
Nucleosides bottom are made of a nitrogenous base, usually either a purine or pyrimidine, and a five-carbon carbohydrate ribose. A nucleotide is simply a nucleoside with an additional phosphate group or groups blue ; polynucleotides containing the carbohydrate ribose are known as ribonucleotide or RNA. Nitrogenous base—The nitrogenous base of a nucleoside or nucleotide named because of the nitrogen atoms found in its structure may be either a purine or a pyrimidine.
Purines , including inosine I , adenine A , and guanine G , are two-ring structures and pyrimidines , including uracil U , cytosine C , and thymine T , have only one ring Figure Both purine and pyrimidine nitrogenous bases are made, in part, from amino acids as shown in Figures and Your MyAccess profile is currently affiliated with '[InstitutionA]' and is in the process of switching affiliations to '[InstitutionB]'. This div only appears when the trigger link is hovered over.
Otherwise it is hidden from view. Forgot Username? About MyAccess If your institution subscribes to this resource, and you don't have a MyAccess Profile, please contact your library's reference desk for information on how to gain access to this resource from off-campus. Of these carbons, the 5' carbon atom is particularly notable, because it is the site at which the phosphate group is attached to the nucleotide. Appropriately, the area surrounding this carbon atom is known as the 5' end of the nucleotide.
Opposite the 5' carbon, on the other side of the deoxyribose ring, is the 3' carbon, which is not attached to a phosphate group. This portion of the nucleotide is typically referred to as the 3' end Figure 1. When nucleotides join together in a series, they form a structure known as a polynucleotide. At each point of juncture within a polynucleotide, the 5' end of one nucleotide attaches to the 3' end of the adjacent nucleotide through a connection called a phosphodiester bond Figure 3.
It is this alternating sugar-phosphate arrangement that forms the "backbone" of a DNA molecule. Figure 3: All polynucleotides contain an alternating sugar-phosphate backbone. This backbone is formed when the 3' end dark gray of one nucleotide attaches to the 5' phosphate end light gray of an adjacent nucleotide by way of a phosphodiester bond.
How is the DNA strand organized? Figure 4: Double-stranded DNA consists of two polynucleotide chains whose nitrogenous bases are connected by hydrogen bonds. Within this arrangement, each strand mirrors the other as a result of the anti-parallel orientation of the sugar-phosphate backbones, as well as the complementary nature of the A-T and C-G base pairing. Figure Detail. Figure 6: The double helix looks like a twisted ladder.
How is DNA packaged inside cells? Figure 7: To better fit within the cell, long pieces of double-stranded DNA are tightly packed into structures called chromosomes. What does real chromatin look like?
Compare the relative sizes of the double helix, histones, and chromosomes. Figure 8: In eukaryotic chromatin, double-stranded DNA gray is wrapped around histone proteins red. Figure 9: Supercoiled eukaryotic DNA. How do scientists visualize DNA? Figure This karyotype depicts all 23 pairs of chromosomes in a human cell, including the sex-determining X and Y chromosomes that together make up the twenty-third set lower right.
Watch this video for a closer look at the relationship between chromosomes and the DNA double helix. What are karyotypes used for? Who is James Watson? What do we know about Francis Crick? Topic rooms within Genetics Close. No topic rooms are there. Browse Visually. Other Topic Rooms Genetics. Student Voices. Creature Cast. Simply Science. Green Screen.
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