What did the cell say when her sister stepped on her foot? MY TOE, SIS! Episode 62 recaps the process of nuclear division
What did the cell say when her sister stepped on her foot? MY TOE, SIS! Episode 62 recaps the process of nuclear division which creates two identical diploid cells. (1:12) Mitosis has four main sequential events - prophase (3:57), metaphase (4:31), anaphase (5:10) and telophase (5:36).
The Question of the Day asks (7:20) What protein filament creates the contractile ring in animal cell cytokinesis?
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Hi and welcome to the APsolute Recap: Biology Edition. Today’s episode will recap mitosis
Zoom out:
Unit 4 - Cell Communication and Cell Cycle
Topic - 4.6
Big idea - Information Storage and Transmission
What did the cell say when her sister stepped on her foot? MY TOE, SIS! Corny I know, but appropriate pronoun association. Whereas we use masculine terms with genetics problems (paternal, F1 Generation for filius, or son) - it's all about the ladies with cell division. We will have daughter cells and sister chromatids! Cell’s don’t have gender, but this terminology may stem from early cell studies in which larger sized egg cells were used.
Let’s Zoom in:
Mitosis: the process of nuclear division which creates two identical diploid cells. (Think the T of mitosis for two). Remember that diploid cells, 2n, are somatic body cells with chromosomes in homologous pairs - one inherited from mom and one from dad. Sandwiched between Interphase and cytokinesis in the cell cycle, the entire process takes approximately two hours with a series of coordinated chromosome movements. Mitosis can also be referred to as karyokinesis, or division of the karyotype. A karyotype is a mapping of chromosomes, organized into homologous pairs. Karyo somes from the Greek meaning kernel, or nut and has the same association to the nuclear material as the term eukaryote. So mitosis, diving the nut.
Mitosis has four main sequential events - prophase, metaphase, anaphase and telophase. These can be recalled simply with the mnemonic PMAT. These events arrange and divide the chromosomes evenly (if all goes well) towards opposite poles of the cell. Each of these phases has discernible features when viewed under a microscope, but the cell doesn’t “pause” when it reaches each step for a snapshot. This is why you will often see terms like prometaphase or meta-anaphase, showing transitional images in diagrams.
Before we recap each step, let’s remind ourselves what the cell has accomplished thus far. Interphase is complete, with cellular growth and organelle duplication achieved in G1 and G2. DNA has also been copied during S phase, and the cell has passed all checkpoints. It's safe to assume that the cell is ready to begin nuclear division.
P. First is prophase - in which the nuclear membrane and nucleolus break down. DNA becomes more visible, as chromosomes coil and condense from loose chromatin into their familiar dense X shape. The extra copy of DNA that was formed in the S phase can now be identified. Each half of the X is an identical sister chromatid, connected to each other at the centromere. Additionally, spindle fibers form from centrioles at centrosomes (if you’re an animal cell). Plant cells will have an alternate version of microtubule organization for division.
PM. Next is metaphase where the goal is to line up chromosomes in the middle of the cell, known as the metaphase plate. Not an actual plate, just an identified cellular location. During metaphase, some spindle fibers extend from the centrioles and attach to kinetochores at each centromere. The centrioles have migrated to opposite poles of the cell, and through a tug of war motion, align the chromosomes to the middle or equator of the cell. This is a big checkpoint for the cell - and division will not continue unless all spindle fibers are attached. Other spindle fibers extend to the opposite pole, giving structure to the division process.
PMA. During anaphase, sister chromatids are pulled apart. Separated at their centromere as spindle fibers shorten, the identical copies migrate to opposite poles of the cell. Other spindle fibers extend and elongate the cell’s shape. And since they are being pulled from the middle of a chromosome, each end tends to bend, and so you can even see little capital A’s in diagrams if you look from the right angle.
PMAT. Finally, telophase. We need to “undo” the steps of prophase, so we will uncoil DNA back into its loose chromatin form, reduce spindle fibers and centrioles, and reform, what is now two, nuclear membranes and nucleoli. Oftentimes, cytokinesis begins while anaphase and telophase occur. Remember - cleavage furrow for animal cells and cell plate for plant cells.
Ok great - so we have two new cells. What was all that for? Growth, tissue repair, and asexual reproduction. Each new daughter cell has a complete genome from a parent cell.
Time for unit connections. Big compare and contrast with Unit 5 and Meiosis. Also, strong connection to Unit 6 - Gene expression and regulation - watching out for errors and their effects on phenotype. Alright - what about the exam? Be prepared to explain how mitosis results in the transmission of chromosomes from one generation to the next. You may be expected to predict the effect on a cell if there is a disruption in the cell cycle, or checkpoints. A common exam error is forgetting to explain the purpose and timing of the cell cycle checkpoints. You should also be prepared to answer a comparative question about mitosis and meiosis. I highly recommend using diagrams that illustrate chromosomes movement - better yet, sketch and caption your own!
To recap……
Mitosis is the process of nuclear division that creates two identical daughter cells. Through PMAT and the splitting of sister chromatids, more cells are created for growth, repair and asexual reproduction.
Coming up next on the Apsolute RecAP Biology Edition: Meiosis
Today’s question of the day is about cytokinesis
Question of the day: What protein filament creates the contractile ring in animal cell cytokinesis?