The diameter of the DNA double helix is uniform throughout because a purine two rings always pairs with a pyrimidine one ring and their combined lengths are always equal.
Figure 9. There is a second nucleic acid in all cells called ribonucleic acid, or RNA. Each of the nucleotides in RNA is made up of a nitrogenous base, a five-carbon sugar, and a phosphate group. In the case of RNA, the five-carbon sugar is ribose, not deoxyribose. RNA nucleotides contain the nitrogenous bases adenine, cytosine, and guanine. Molecular biologists have named several kinds of RNA on the basis of their function. For this reason, the DNA is protected and packaged in very specific ways.
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. There still remains great controversy over whether their acquisition of her data was appropriate and whether personality conflicts and gender bias contributed to the delayed recognition of her significant contributions. Similarly, Barbara McClintock did pioneering work in maize corn genetics from the s through s, discovering transposons jumping genes , but she was not recognized until much later, receiving a Nobel Prize in Physiology or Medicine in Figure 9.
Today, women still remain underrepresented in many fields of science and medicine. In academia, the number of women at each level of career advancement continues to decrease, with women holding less than one-third of the positions of Ph.
Why do such disparities continue to exist and how do we break these cycles? The situation is complex and likely results from the combination of various factors, including how society conditions the behaviors of girls from a young age and supports their interests, both professionally and personally. Some have suggested that women do not belong in the laboratory, including Nobel Prize winner Tim Hunt, whose public comments suggesting that women are too emotional for science [9] were met with widespread condemnation.
Perhaps girls should be supported more from a young age in the areas of science and math Figure 9. Contributions by women in science should be made known more widely to the public, and marketing targeted to young girls should include more images of historically and professionally successful female scientists and medical professionals, encouraging all bright young minds, including girls and women, to pursue careers in science and medicine. Figure 9. Possibilities include bacterial infection e.
His physician orders a stool sample to identify possible causative agents e. The physician instructed Aamir to drink lots of fluids to replace what he was losing and discharged him from the hospital.
ETEC produces several plasmid-encoded virulence factors that make it pathogenic compared with typical E. These include the secreted toxins heat-labile enterotoxin LT and heat-stabile enterotoxin ST , as well as colonization factor CF. Both LT and ST cause the excretion of chloride ions from intestinal cells to the intestinal lumen, causing a consequent loss of water from intestinal cells, resulting in diarrhea.
CF encodes a bacterial protein that aids in allowing the bacterium to adhere to the lining of the small intestine. The work of Rosalind Franklin and R. Gosling was important in demonstrating the helical nature of DNA. Skip to main content. Biochemistry of the Genome. Search for:. Structure and Function of DNA Learning Objectives Describe the biochemical structure of deoxyribonucleotides Identify the base pairs used in the synthesis of deoxyribonucleotides Explain why the double helix of DNA is described as antiparallel.
Think about It Which scientists are given most of the credit for describing the molecular structure of DNA? Paving the Way for Women in Science and Health Professions Historically, women have been underrepresented in the sciences and in medicine, and often their pioneering contributions have gone relatively unnoticed. Key Concepts and Summary Nucleic acids are composed of nucleotides , each of which contains a pentose sugar, a phosphate group, and a nitrogenous base.
Deoxyribonucleotides within DNA contain deoxyribose as the pentose sugar. DNA contains the pyrimidines cytosine and thymine , and the purines adenine and guanine.
Chargaff discovered that the amount of adenine is approximately equal to the amount of thymine in DNA, and that the amount of the guanine is approximately equal to cytosine.
These relationships were later determined to be due to complementary base pairing. Watson and Crick, building on the work of Chargaff, Franklin and Gosling, and Wilkins, proposed the double helix model and base pairing for DNA structure. DNA is composed of two complementary strands oriented antiparallel to each other with the phosphodiester backbones on the exterior of the molecule.
The nitrogenous bases of each strand face each other and complementary bases hydrogen bond to each other, stabilizing the double helix. Heat or chemicals can break the hydrogen bonds between complementary bases, denaturing DNA. Cooling or removing chemicals can lead to renaturation or reannealing of DNA by allowing hydrogen bonds to reform between complementary bases.
DNA stores the instructions needed to build and control the cell. This information is transmitted from parent to offspring through vertical gene transfer.
These bonds are called phosphodiester bonds, and the sugar-phosphate backbone is described as extending, or growing, in the 5' to 3' direction when the molecule is synthesized.
In double-stranded DNA, the molecular double-helix shape is formed by two linear sugar-phosphate backbones that run opposite each other and twist together in a helical shape. 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.
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