DNA→ RNA→Protein

Read below for a written explanation of DNA and protein. Or watch these videos. The top 2 videos is are fun raps, and also a great explanation of how DNA leads to protein. Below that is a visually beautiful and thorough explanation of DNA and protein, but it’s a tad dry.

DNA: DNA is the genetic code. It’s stored information made of sugar building blocks (bases) that have a phosphorus atom attached as a phosphate group, with a base attached. The sugar is essentially structurally the same for every part of DNA. The unique part of the nucleotide comes from the nitrogen-containing base. There are four that naturally occur: adenine (A), thymine (T), cytosine (C), and guanine (G). One strand of DNA will have a backbone that’s a long chain of connected phosphates. To each phosphate a sugar is attached. To each sugar a base (A, G, C, or T) is attached. Again, that’s one strand of DNA. Most eukaryotic DNA is double stranded. This occurs by the pairing of the bases. Adenines are weakly bonded to thymines and cytosines are weakly bonded to guanines. So on the DNA would be in the order of AGCT. Now, matching each of those bases with its pair would mean the other strand, going down the same direction as the opposite strand, would have the sequence TCGA. This base pairing is also referred to as complementation, so that when one DNA strand is made that pairs up with another DNA strand, they are referred to as each other’s complement. Double-stranded DNA twists to form a helix, and then bundles and winds tighter and smaller until it forms a compact structure called a chromosome. A genome is a complete set of all the DNA in an organism, and is typically divided up into individual portions we call chromosomes. So different genes can exist on different chromosomes. DNA typically exists in the nucleus of the cell, but has also been found in mitochondria and chloroplasts.

RNA: There are many different kinds of RNA, but we only need to go over two kinds now, messenger (mRNA) and transfer (RNA). mRNA is the information in process, and is essentially what parts of the genetic code (DNA) the cells decides to process at that time. It’s like having a multi-volume set of thick recipe books (the genome made of DNA), and our friend wants a copy of the book’s recipe for vegetable soup. In order for her to use the recipe, we need to transcribe it or make a copy she can understand. Instead of copying the entire recipe book for her, we instead just copy the recipe she needs, for vegetable soup. Now let’s say that our friend is used to the metric system of measuring volume in milliliters, but our recipe book lists volumes in cups and pints. We know how to convert the recipe to the volume measurements our friend understands, but she doesn’t, so we do it for her. This an analogy of why mRNA is made. There’s so much genetic information stored in our genome that it would be wasteful for cell machinery to make all of the proteins it codes for every time protein is made. Not all proteins are needed all the time. Some are needed much more often than others. To be efficient, the cell only makes a copy of what it needs when it needs it, which is why RNA is transcribed. RNA is made of sugars very similar to DNA, but has an extra hydroxyl (an oxygen-hydrogen group). When transcribed into RNA, the thymine base is substituted for a uracil, the RNA equivalent, so that AGTCTT becomes AGUCUU. The reason RNA is made instead of just another DNA copy of the gene is because that extra hydroxyl is necessary for the working activity of RNA.  It’s thought that uracil is used in RNA because it takes less energy for the cell to make it. It’s not used in DNA because cytosine can degrade into uracil and there needs to be some way for the cell to detect the C/U difference from thymine in order to catch and correct DNA mutations. Another feature distinguishing mRNA from DNA is that mRNA is usually single-stranded, which means that the bases don’t pair. Once DNA is transcribed into mRNA copies in the nucleus, it’s moved from the nucleus back into the cytosol, where it is placed near a ribosome, and translation (protein manufacturing) begins.

While DNA is an information store, RNA (the multiple types) can be both an information store and an active molecule in itself. tRNA does not serve the same purpose as mRNA. It has shorter sequences than mRNA, and it forms a clover-leaf like structure. It serves directly to bring amino acids to a growing peptide sequence, which we’ll cover more below.

Protein: Proteins are the workers of the cellular processes. They make things happen. At the beginning of a gene, a “Reading frame” begins. Every three nucleotides (a codon) makes an individual message to the translation machinery. These three nucleotides become their own code that calls for an individual amino acid, which is a relatively small biomolecule.  The codon is recognized by the tRNA that brings the individual amino acid to the ribosome and puts it into a position so that the amino acids can be attached to the peptide chain. Twenty amino acids occur naturally in most living organisms. SO AGT becomes AGU, which calls/codes for a methione (M). When I worked at a fast food drive in, we used short codes for our food items. So when I saw S BURR on the ticket, I knew the customer asked for a sausage burrito, so that’s what I took them. I acted like the tRNA there. Now each tRNA is actually unique to the amino acid it delivers, but there are actually a few combinations/ codons that can code for one amino acid, so the tRNA can recognize one to all of these codons. As the codons are read from the RNA sequence, the amino acids corresponding to each codon are joined to the growing protein chain one by one. The type of amino acids, the placement within the protein sequence, as well as the number of each amino acid each protein contains affects the protein identity. Each amino acid has a unique structure, causing it to have its own properties, which can add to and form the total properties of the protein. These properties affect the function of a protein. One such property is the amino acid polarity, and how that contributes to the overall polarity of the protein. Polarity can affect where in the cell the protein can exist, and thus affects what functions it may have. For example, a protein with a lot of nonpolar residues will be relatively hydrophobic, and so it won’t be stable in the cytosol. One place it could be stable is within the plasma membrane, which has a hydrophobic core because of the hydrophobic, nonpolar lipid tails. Proteins that are embedded in the membrane can serve a variety of purposes, such as forming a channel that molecules can pass through to enter the cell.

Seriously, why are so many science videos so dry. Also, the ones that try to be a little fun are also some of the best explanations for the general person.


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