MCAT Biochemistry
DNA has two essential functions: stably storing the information of the cell and expressing this information through a multi-step process to eventually encode specific proteins. The structure of DNA is composed of three major components - nitrogenous bases, deoxyribose sugar, and phosphate groups.
The nitrogenous bases in DNA can be divided into two groups: pyrimidines (Thymine and Cytosine) with a single six-sided ring, and purines (Guanine and Adenine) with a hexagon and pentagon ring structure. These bases bond with the deoxyribose sugar to form nucleosides. The third component, the negatively charged phosphate group, attaches to the deoxyribose sugar, creating a nucleotide. DNA is a polymer of nucleotides, featuring strands with a 5' end and a 3' end. The two strands of DNA in a double helix are oriented antiparallel to one another, held together by hydrogen bonds of the complementary nitrogenous bases following Chargraff's rule, which links Adenine to Thymine and Guanine to Cytosine. The major and minor grooves in the DNA structure allow for protein binding and regulation of replication or expression.
Lesson Outline
<ul> <li>Two important functions of DNA</li> <ul> <li>Stably stores information of the cell</li> <li>Expressed by enzymes to encode specific proteins</li> </ul> <li>Structure of DNA</li> <ul> <li>Three major components</li> <ul> <li>Nitrogenous bases</li> <ul> <li>Pyrimidines: single 6-sided ring</li> <li>Purines</li> <ul> <li>Two ring structures (hexagon and pentagon)</li> <li>Aromatic, cyclic, planar, conjugated, and have 6 electrons</li> </ul> </ul> <li>Deoxyribose sugar</li> <ul> <li>Pentose sugar with five carbons and an oxygen</li> <li>Key difference between deoxyribose and ribose: hydroxyl group on second carbon</li> </ul> <li>Phosphate groups</li> <ul> <li>Negatively charged, binds to carbon 5</li> </ul> </ul> <li>Nitrogenous base, deoxyribose, and phosphate group make up a nucleotide</li> <li>DNA: polymers of nucleotides</li> </ul> <li>Three-dimensional structure: double-stranded helix</li> <ul> <li>Strands have 5' end (phosphate group) and 3' end (OH group)</li> <li>Two strands are antiparallel, run in opposite directions</li> <li>Phosphodiester bonds hold sugar phosphate backbones together</li> <li>Hydrogen bonds connect complementary nitrogenous bases</li> <ul> <li>Adenine to Thymine (2 hydrogen bonds)</li> <li>Guanine to Cytosine (3 hydrogen bonds)</li> </ul> </ul> <li>Complementary base pairing and strand polarity support DNA's functions</li> <ul> <li>Replication</li> <li>Expression</li> </ul> <li>Major and minor grooves in DNA double helix</li> <ul> <li>Targets for regulatory protein binding</li> </ul> </ul>
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FAQs
The basic structure of DNA is a double helix, which resembles a twisted ladder. It consists of two long chains of nucleotides, composed of a sugar (deoxyribose), a phosphate group, and a nitrogenous base. The two chains run in opposite, or antiparallel, directions and are connected by hydrogen bonds between the nitrogenous bases. The arrangement of these components creates the iconic double helix shape.
There are four nitrogenous bases in DNA: adenine (A), guanine (G), cytosine (C), and thymine (T). These bases can be classified into two groups: purines (A and G) and pyrimidines (C and T). According to Chargaff's rule, adenine always pairs with thymine (A-T) and guanine always pairs with cytosine (G-C). This is because of the specific arrangement of hydrogen bonds between the two complementary bases.
The antiparallel nature of the DNA strands plays a crucial role in DNA replication and transcription. The opposing directions of the two strands ensure that the DNA can be unzipped and replicated accurately by the enzymes responsible for these processes. Additionally, the polarity of the strands provides proper orientation, sequence, and structure for the genetic code to be preserved and expressed during protein synthesis.
Nucleotides are the building blocks of DNA and consist of a sugar (deoxyribose), a phosphate group, and a nitrogenous base. The order of nucleotides along a DNA strand forms the genetic code that carries the instructions for building proteins and other cellular components. Nucleotides also play a central role in DNA replication, as new nucleotides must be added to the growing strands during the process to ensure the accurate transmission of genetic information to daughter cells.
DNA serves several critical functions in a living organism. Firstly, it stores and preserves the genetic information that determines an organism's traits and characteristics. Secondly, it serves as the template for the synthesis of RNA during the process of transcription, which ultimately leads to the production of proteins through translation. Lastly, DNA plays a vital role in replication, ensuring that its genetic information is accurately passed on to offspring during cell division and reproduction.
