The smallest chromosome in the human genome, Chromosome 21, has around 48 million base pairs. Each strand of DNA is a polynucleotide composed of units called nucleotides. A nucleotide has three components: a sugar molecule, a phosphate group, and a nitrogenous base. RNA molecules use a different sugar, called ribose. A nitrogenous base is an organic molecule that contains nitrogen and has the chemical properties of a base.
RNA molecules contain cytosine, guanine, and adenine, but they have a different nitrogenous base, uracil U instead of thymine. Adenine pairs with thymine and cytosine pairs with guanine. Why do the nitrogenous bases pair in this specific way?
The bases on each strand are joined to the bases on the other strand with hydrogen bonds, but different bases have different chemical structures. Cytosine and thymine and uracil in RNA are pyrimidines, containing one ring. Adenine and guanine are purines, containing two rings.
The pyrimidines pair with the purines: cytosine and guanine form three hydrogen bonds, and adenine and thymine form two. Each strand of DNA is like a recipe book for synthesizing proteins. Certain sequences of nitrogenous bases along the strand encode particular RNA molecules.
These sequences are called genes. Chromosomes can vary widely in their number of base pairs and genes. 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. 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. Explanation : When determining complementary strands, there are a few important points to consider. Report an Error. Explanation : DNA is a double stranded molecule that is antiparallel, meaning the 5' end of one strand matches up with the 3' end of the other strand.
The correct answer presents two strands that are antiparallel and show proper binding patterns. Which of the following describes the components of a nucleotide?
Possible Answers: A phosphate group, a pentose sugar, and an amino acid. A phosphate group, a hexose sugar, and a nitrogenous base. A phosphate group, a pentose sugar, and a nitrogenous base. Correct answer: A phosphate group, a pentose sugar, and a nitrogenous base.
Which of the following DNA primers has the lowest melting point? Explanation : Cytosine and guanine form three hydrogen bonds with each other, while adenine and tyrosine only form two hydrogen bonds. Possible Answers:. More information is needed in order to answer the question. Correct answer:. Explanation : Guanine will pair with cytosine. The total composition of the DNA sample must be accounted for by the sum of all the bases.
Use the known values for guanine and cytosine to find the sum of adenine and thymine. Explanation : There are four nitrogenous bases found in DNA: adenine, thymine, cytosine, and guanine. Explanation : We can use Chargaff's rule to find the remaining compositional percentages. Since cytosine and guanine are present in equal amounts, we can simply divide their sum by 2. Uracil is only found in RNA. What is the strand's composition of the other bases? Correct answer: Cannot determine from the given information.
Explanation : Chargaff's rule only applies to DNA. RNA is single-stranded, and thus, no base pairing occurs. Possible Answers: Phosphate-sugar backbone. Correct answer: Nucleotides. Explanation : A polymer is a macromolecule that is made up of subunits that are repeated or very similar. Copyright Notice. View AP Biology Tutors. Jinia Certified Tutor. Manisha Certified Tutor. Sagar Certified Tutor. Report an issue with this question If you've found an issue with this question, please let us know.
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