You have enough DNA in your body to stretch to the sun and back over sixty times. And your cells need to copy it all! Episode 70 recaps the primary enzymes in DNA replication.
You have enough DNA in your body to stretch to the sun and back over sixty times. And your cells need to copy it all! Episode 70 recaps the primary enzymes in DNA replication: topoisomerase (3:53), helicase (4:24), DNA polymerase (5:11), and ligase (6:11).
The Question of the Day asks (7:36) When comparing pentose sugars, on which carbon does ribose have a hydroxyl group where deoxyribose does not?
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Hi and welcome to the APsolute Recap: Biology Edition. Today’s episode will recap Helicase and the other enzymes involved in DNA replication
Let’s Zoom out:
Unit 6 - Gene Expression and Regulation
Topic - 6.2
Big Idea: Information Storage and Transmission
Most cells inside your body have about 6 feet of DNA in them (if you uncoiled the strands of course). As a low estimate, you have approximately 10 trillion cells of all shapes, sizes, and types. So…. that's 60 trillion feet or about 10 million miles of DNA within your body. That's enough DNA to stretch to the sun and back over sixty times. And your cells need to copy it all - often.
Let’s Zoom in:
DNA replication occurs during the synthesis or S phase of the cell cycle within eukaryotic cells. In humans, this process is aided by multiple enzymes, from multiple origin sites, simultaneously, at each of the 46 chromosomes. And it's a good thing too, because at a rate of about 50 base pairs per second, it would take over a month to copy a single chromosome. Instead, the S phase takes only a few hours.
Beginning with a familiar definition: “Enzymes are biological catalysts that facilitate chemical reactions in cells by lowering the activation energy.” Enzymes reduce how much energy is required for a chemical reaction to occur and thus, makes metabolism more efficient. However, the difference in energy from reactants to products remains the same. Enzymes are not used up in a reaction and are substrate and condition specific. Why? Enzymes are made of proteins - each having a unique structure for a specific function. Remember, proteins are polymers made of amino acid monomers. These monomers differ in the chemistry of their R groups, which influences the manner in which the protein coils, bends and folds. Conventionally, many enzymes end in the suffix -ase and are named after the molecule or molecules they act upon. While there are several enzymes and intermediate steps of DNA replication - not all are specified in the CED. Let’s recap DNA replication through the function of those enzymes.
First topoisomerase. This enzyme relaxes the DNA supercoiling ahead of the replication fork. This not only allows hydrogen bonds to be more easily accessible but also reduces tension during replication. I always liken the processes to trying to take off ice skates or combat style boots. Those laces are so tight! And even as you loosen the top, you need to release the tension from the laces nearest the toes to accomplish the task of removing the shoe.
What do you and helicase have in common when changing outfits? You both need to unzip your genes! The enzyme helicase unwinds the DNA, breaking the hydrogen bonds between nitrogenous base pairs and forming a replication fork at the origin site. Now that the nitrogenous bases are exposed, semi-conservative replication can begin. Each original half of the parent strand serves as a template for new complementary nucleotides to be synthesized. In other words, half of the original strand is conserved or saved during replication. Multiple helicase enzymes bind to the DNA strand at origin sites, forming replication bubbles and multiple replication forks along a chromosome. Efficiency.
The next enzyme involved, I would argue, does most of the work - DNA Polymerase! As the name implies, this enzyme will have a role in creating the nucleic acid polymer from nucleotide monomers. DNA polymerase synthesizes the new strand in the 5’ to 3’ direction, joining new complementary nucleotides to the 3’ end and proofreading its own work as it goes! A and T with two hydrogen bonds, G and C with three hydrogen bonds. But remember - DNA strands are antiparallel. This creates a leading strand synthesized in one continuous piece and a lagging strand synthesized in short Okazaki fragments. Two things to take note of for DNA polymerase - it always needs a template strand to work off of and it always requires a short RNA primer to indicate a replication starting point.
The last enzyme for today’s episode is ligase, the “glue” of the operation. Ligase will seal any gaps left behind by the now removed RNA primers and ensures that Okazaki fragments are joined on the lagging strand. Ligase catalyzes a reaction where a 5’ phosphate group is linked to the 3’ hydroxyl group forming a seamless sugar-phosphate backbone.
Time for unit connections. Unit 1: Chemistry of life with nucleic acids, Unit 3: Cellular energetics with enzymes and later in Unit 6 for Biotechnology as ligase coupled with restriction enzymes creates recombinant plasmids. Alright - what about the exam? Make sure to review diagrams of DNA replication, paying special attention to directionality and the position of enzymes. The entire process is a domino effect of functionality, so be prepared to discuss or predict the effect of an altered enzyme or disrupted step.
To recap……
DNA replication is a semiconservative process that involves several enzymes in a series of steps. Topoisomerase uncoils, Helicase unzips, DNA polymerase synthesizes and Ligase closes the gaps.
Coming up next on the Apsolute RecAP Biology Edition: Origin of Life
Today’s question of the day is about nucleotide structure.
Question of the day: When comparing pentose sugars, on which carbon does ribose have a hydroxyl group where deoxyribose does not?