Melanie recaps DNA in all its coiled and condensed forms during Episode 17. Beginning on the most microscopic level Melanie reviews the nucleotide monomer structure.
Melanie recaps DNA in all its coiled and condensed forms during Episode 17. Beginning on the most microscopic level, (1:27) Melanie reviews the nucleotide monomer structure. You can create an antiparallel model with your own arms (2:45). Do you prefer a history or chemistry themed memory hack for nitrogenous bases? (3:40). DNA wraps around histones to form chromatin before condensing into chromosomes. The episode concludes with a contrast between prokaryotic and eukaryotic chromosome structure. (6:22)
The Question of the Day asks (6:19) “Which nitrogenous base is unique to RNA?”
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Hi and welcome to the APsolute Recap: Biology Edition. Today’s episode will recap DNA
Lets Zoom out:
DNA is incorporated into several areas of the CED, or Course Exam Description, published by the College Board. We are in Unit 1, Chemistry of life with Topic 4, Nucleic Acids. Also in Unit 6, Gene Expression and Regulation with topic 6.1, DNA and RNA Structure - and in Unit 5 for Heredity. Our Big idea is Information Storage and Transmission.
Deoxyribonucleic acid. Quite a mouthful to say. Discovered through the work of many great scientists, such as Franklin, Wilkins, Hershey, Griffith, Chargaff, and most remembered - Watson and Crick. This polymer of nucleotides holds all of the instructions for life. Today’s episode will focus on DNA structure and its MANY nicknames as it coils and condenses in the cell.
Lets Zoom in:
Literally. We are going to start on the most microscopic level and gradually zoom out with our descriptions. DNA is a polymer made up of nucleotide monomers. For a recap of all four biological molecules, look back to Episode 4. There are about three billion nucleotides within one human cell, and stretched out - would be about 2 meters long! A nucleotide has three parts - the pentose sugar from which DNA gets its “Deoxyribose” name, the phosphate group which gives DNA its acidic quality, and a nitrogenous base encoding the genetic information. The sugar and phosphate, commonly referred to as the “backbone” of DNA, are covalently bonded together at the 5’ carbon. The nitrogenous base is arranged perpendicular to the backbone and covalently bonded to the sugar at the 1’ carbon. This numbering of carbons on the pentose rings gives DNA directionality. This will be important when discussing orientation during processes such as replication, transcription and translation.
Zooming out a bit, we can see that DNA is structured as a twisted ladder or antiparallel double helix. Lets model this structure. Place your arms out in front of you and allow your right hand to be closest to your left elbow and left hand to be closest to your right elbow. Space your arms out so that they’re not touching. They are now antiparallel! Hands and elbows pointing in opposite directions, but not intersecting. If you are feeling up to the challenge, you can twist your arms together. For DNA, the sides of the ladder are made of alternating sugars and phosphates, each running in the opposite 5’ to 3’ direction. Another way of viewing this is to note that the point of the sugar (where an oxygen is located) will face in opposite directions on a diagram for each backbone. The nitrogenous bases face inward making up the rungs of the ladder, and join through hydrogen bonds.
There are four different types of nitrogenous bases in DNA. Yes, only four - and with only four letters, cells are able to form an abundance of unique instructions for life. These four bases are Adenine, Guanine, Thymine, and Cytosine. Here’s a memory hack. “Tombs and Crypts are Found in the Pyramids.” Translated into Biology speak. Pyrimidine bases are made of one ring (just like pyramids have one point). The two pyrimidines are Thymine (the tombs) and Cytosine, (the crypts). “Tombs and Crypts are Found in the Pyramids.” No good? Prefer a chemistry themed memory hack? How about “Gold is always pure.” Translated into biology speak. Guanine (gold) and Adenine (always) are both two-ringed bases - classified as Purines. I think of gold wedding bands for the two ring associations. This is tomato tomAHto - but a study strategy doesn’t hurt. Adenine will always bond with Thymine with two hydrogen bonds and Guanine will always bond with Cytosine with three hydrogen bonds.
We are still VERY zoomed in. How are we going to fit all 3 billion nucleotides into one cell? And not get tangled? Coiling and condensing is essential. DNA begins to wrap around proteins called histones much like beads on a string. This loosest form is referred to as chromatin and is found during most of the cell cycle. DNa then begins to coil further inward, before supercoiling, and forming loops. These loops are arranged in rosette structures, which stack together in a very condensed manner that can be seen under a microscope. We call these condensed strands of DNA chromosomes.
It should be noted that prokaryotic organisms, like bacteria, typically have circular chromosomes while eukaryotic organisms have multiple linear chromosomes. There are 46 individual chromosomes structures in human somatic cells. In this condensed form, chromosomes can be moved around by spindle fibers and accounted for during processes like mitosis and meiosis. Both prokaryotes and eukaryotes can also contain plasmids - small extra chromosomal DNA molecules that serve a variety of functions for the cell.
To recap….
DNA is a nucleic acid of nucleotide monomers, which differ only in their nitrogenous base. It forms an antiparallel double helix, that coils and condenses into chromosomes when preparing for cellular division.
Today’s Question of the day is about RNA.
Question:Which nitrogenous base is unique to RNA?