Melanie recAPs DNA Replication by reviewing the role of enzymes. DNA Replication is semiconservative and occurs during the S phase...
Melanie recAPs DNA Replication by reviewing the role of enzymes. DNA Replication is semiconservative and occurs during the S phase of the cell cycle in eukaryotic cells (1:20). How does the structure of DNA influence the process of replication? (1:40) The CED requires you to know the function of the following enzymes: topoisomerase (2:38), helicase (2:48), DNA polymerase (3:25), and ligase (4:35). There are distinctions between replication patterns in prokaryotes and eukaryotes (5:00).
The Question of the Day asks (5:39) “What is the condensed, identical DNA strand called during mitosis?”
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Hi and welcome to the APsolute Recap: Biology Edition. Today’s episode will recap DNA Replication
Lets Zoom out:
Unit 6 - Gene Expression and Regulation
Topic - 6.2
Big Idea: Information Storage and Transmission
Sperm and egg came together at fertilization and the diploid number was restored - 46 chromosomes. Ever since then, the cell has copied that original DNA strand over and over again as the organism grew and differentiated. Imagine copying something that is three billion letters long - without making a mistake? Cells aren’t perfect, but they’ve got a few tricks up their sleeve.
Lets Zoom in:
Recall that eukaryotic cells go through phases of the cell cycle - interphase with G1, S, and G2 before cellular division with mitosis and cytokinesis. The S of interphase stands for synthesis - the time when DNA, in loose chromatin form, will be replicated with the assistance of enzymes. This process occurs in the nucleus of the cell.
A quick pause to connect to the big idea. How does the structure of DNA influence the process of replication? DNA is an antiparallel double helix, with a sugar phosphate backbone and nitrogenous bases joined through hydrogen bonds. The “meat and potatoes” of DNA is with the sequence of the bases, the As, Ts, Cc and Gs. In order to replicate the sequence, we will need to break the hydrogen bonds to access the code. Additionally, DNA has directionality, and just like the English language is read from the left side of the page to the right, enzymes will interact with DNA in a singular direction. Listen back to Episode 17 for a chemical recap of DNA directionality.
While there are several enzymes and intermediate steps of DNA replication - not all are specified in the CED. Let’s approach the process of DNA replication through the function of those enzymes. First up - topoisomerase. This enzymes relaxes the DNA supercoiling so that hydrogen bonds are more easily accessible. Next, the enzyme helicase unwinds the DNA, breaking the hydrogen bonds between nitrogenous base pairs and forming a replication fork. 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.
A short RNA primer is added to the template strands - indicating and supplying a replication starting point for our next enzyme, DNA polymerase. Enzymes aren’t named very creatively, and so literally this enzyme is involved in making a large polymer of DNA. DNA polymerase synthesizes the new strand in the 5’ to 3’ direction, laying down new complementary nucleotides and proofreading its own work as it goes! On the leading strand, this synthesis is continuous in one piece as DNA polymerase follows in the same direction that helicase unwinds. But remember - DNA strands are antiparallel. So DNA polymerase is synthesizing away from the direction of helicase on the, very appropriately named, lagging strand. This causes the lagging strand to be synthesized in short pieces, named Okazaki Fragments after the husband and wife team that discovered them.
We are almost done. Quick enzyme check in: Topoisomerase unwound, helicase unzipped, and DNA polymerase added new nucleotides. The last enzyme up to bat 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.
Since DNA is organized differently in prokaryotes and eukaryotes, it should be no surprise that their replication patterns have variation as well. Prokaryotic DNA is organized in a singular, circular chromosome and has only one origin site of replication. Eukaryotic chromosomes are linear, contain multiple origin sites of replication and often lose the ends of their chromosomes, called telomeres, with each replication. The shortening of telomeres is believed to influence the process of aging.
To recap….
DNA replication is semiconservative and occurs before the process of cellular division. A series of enzymes unwind, read, and synthesize complementary strands while accommodating for antiparallelism.
Today’s Question of the day is about cell division.
Question: What is the condensed identical DNA strand called during mitosis?