Eukaryotes accomplish this feat by wrapping their DNA around special proteins called histones , thereby compacting it enough to fit inside the nucleus Figure 8. Together, eukaryotic DNA and the histone proteins that hold it together in a coiled form is called chromatin. It is impossible for researchers to see double-stranded DNA with the naked eye — unless, that is, they have a large amount of it. Modern laboratory techniques allow scientists to extract DNA from tissue samples, thereby pooling together miniscule amounts of DNA from thousands of individual cells.
When this DNA is collected and purified, the result is a whitish, sticky substance that is somewhat translucent. To actually visualize the double-helical structure of DNA, researchers require special imaging technology, such as the X-ray diffraction used by Rosalind Franklin. However, it is possible to see chromosomes with a standard light microscope, as long as the chromosomes are in their most condensed form.
To see chromosomes in this way, scientists must first use a chemical process that attaches the chromosomes to a glass slide and stains or "paints" them. Staining makes the chromosomes easier to see under the microscope. In addition, the banding patterns that appear on individual chromosomes as a result of the staining process are unique to each pair of chromosomes, so they allow researchers to distinguish different chromosomes from one another. Then, after a scientist has visualized all of the chromosomes within a cell and captured images of them, he or she can arrange these images to make a composite picture called a karyotype Figure This page appears in the following eBook.
Aa Aa Aa. What components make up DNA? Figure 1: A single nucleotide contains a nitrogenous base red , a deoxyribose sugar molecule gray , and a phosphate group attached to the 5' side of the sugar indicated by light gray. Opposite to the 5' side of the sugar molecule is the 3' side dark gray , which has a free hydroxyl group attached not shown. Figure 2: The four nitrogenous bases that compose DNA nucleotides are shown in bright colors: adenine A, green , thymine T, red , cytosine C, orange , and guanine G, blue.
Although nucleotides derive their names from the nitrogenous bases they contain, they owe much of their structure and bonding capabilities to their deoxyribose molecule. The central portion of this molecule contains five carbon atoms arranged in the shape of a ring, and each carbon in the ring is referred to by a number followed by the prime symbol '. 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. Your body is made of billions of cells.
Cells are the very small units that make up all living things. A cell is so tiny that you can only see it using a strong microscope. Chromosomes come in matching sets of two or pairs and there are hundreds — sometimes thousands — of genes in just one chromosome. Most cells have one nucleus say: NOO-clee-us. The nucleus is a small egg-shaped structure inside the cell which acts like the brain of the cell.
It tells every part of the cell what to do. But, how does the nucleus know so much? It contains our chromosomes and genes. As tiny as it is, the nucleus has more information in it than the biggest dictionary you've ever seen.
In humans, a cell nucleus contains 46 individual chromosomes or 23 pairs of chromosomes chromosomes come in pairs, remember? Half of these chromosomes come from one parent and half come from the other parent. Under the microscope, we can see that chromosomes come in different lengths and striping patterns. When they are lined up by size and similar striping pattern, the first twenty two of the pairs these are called autosomes; the final pair of chromosomes are called sex chromosomes, X and Y.
The sex chromosomes determine whether you're a boy or a girl: females have two X chromosomes while males have one X and one Y. But not every living thing has 46 chromosomes inside of its cells. For instance, a fruit fly cell only has four chromosomes! Each gene has a special job to do. They believe these could be linked to an increased lifespan in mammals. The genetic switches that they have discovered involve enzymes that are ramped up after mild stress during early development. This could lead to a breakthrough in the goal to develop drugs that can flip these switches to improve human metabolic function and increase longevity.
The marks can be passed on from cell to cell as they divide, and they can even be passed from one generation to the next. Specialized cells can control many functions in the body. For example, specialized cells in red blood cells make proteins that carry oxygen from air to the rest of the body.
The epigenome controls many of these changes within the genome. Lifestyle and environmental factors such as smoking, diet and infectious diseases can bring about changes in the epigenome. They can expose a person to pressures that prompt chemical responses. These responses can lead to direct changes in the epigenome, and some of these changes can be damaging.
Cancer can result from changes in the genome, the epigenome or both. Changes in the epigenome can switch on or off the genes that are involved in cell growth or the immune response. These changes can cause uncontrolled growth, a feature of cancer, or a failure of the immune system to destroy tumors.
Researchers in The Cancer Genome Atlas TCGA network are comparing the genomes and epigenomes of normal cells with those of cancer cells in the hope of compiling a current and complete list of possible epigenomic changes that can lead to cancer. Researchers in epigenomics are focused on trying to chart the locations and understand the functions of all the chemical tags that mark the genome. This information may lead to a better understanding of the human body and knowledge of ways to improve human health.
Gene therapy uses sections of DNA to treat or prevent disease. This science is still in its early stages, but there has been some success. For example, in , scientists reported that they had managed to improve the eyesight of 3 adult patients with congenital blindness by using gene therapy. In , a reproductive endocrinologist, named John Zhang, and a team at the New Hope Fertility Center in New York used a technique called mitochondrial replacement therapy in a revolutionary way.
They announced the birth of a child to a mother carrying a fatal genetic defect. Researchers combined DNA from two women and one man to bypass the defect. The result was a healthy baby boy with three genetic parents. This type of research is still in the early stages, and much is still unknown, but results look promising.
Scientists are looking at different ways of treating cancer using gene therapy. In one study, 82 percent of patients had their cancer shrink by at least half at some point during treatment.
Women with the BRCA1 gene have a significantly higher chance of developing breast cancer. A woman can have a test to find out whether she carries that gene. BRCA1 carriers have a 50 percent chance of passing the anomaly to each of their children.
Scientists say that one day we will be able to test a patient to find out which specific medicines are best for them, depending on their genetic makeup. DNA is a double helix formed by base pairs attached to a sugar-phosphate backbone. What is a gene? What is a chromosome? How many chromosomes do people have? What is noncoding DNA?
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