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. Green Science. Bio 2. The Success Code. Why Science Matters. The Beyond. Replication occurs simultaneously at multiple places along a DNA strand. Because human DNA is so very long with up to 80 million base pairs in a chromosome it unzips at multiple places along its length so that the replication process is going on simultaneously at hundreds of places along the length of the chain.
Eventually these areas run together to form a complete chain. In humans, DNA is copied at about 50 base pairs per second. The process would take a month rather than the hour it actually does without these multiple places on the chromosome where replication can begin.
DNA replication is extraordinarily accurate. DNA polymerase makes very few errors, and most of those that are made are quickly corrected by DNA polymerase and other enzymes that "proofread" the nucleotides added into the new DNA strand. If a newly added nucleotide is not complementary to the one on the template strand, these enzymes remove the nucleotide and replace it with the correct one. With this system, a cell's DNA is copied with less than one mistake in a billion nucleotides. This is equal to a person copying large page dictionaries word for word, and symbol for symbol, with only one error for the whole process!
Figure 1. Even within genes, there are regions of noncoding DNA called introns. Noncoding regions of DNA are important because they provide binding sites for proteins that help activate or deactivate the process of transcription. They can also provide protection for the coding regions.
For instance, telomeres consist of repetitive sequences that protect the genetic information on each DNA molecule from being damaged during cell division. See more from our free eBook library. An article from Nature about the genome of Carsonella rudii. Eukaryotic Chromosomes. Prokaryotic Chromosomes. Eukaryotic vs. When you select "Subscribe" you will start receiving our email newsletter.
Use the links at the bottom of any email to manage the type of emails you receive or to unsubscribe. See our privacy policy for additional details. Learn Site. Zooming in on DNA Structure. A molecule of DNA consists of two strands that form a double helix structure. Each DNA strand is composed of nucleotides—units made up of a sugar deoxyribose , a phosphate group, and a nitrogenous base.
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