AQA 4.1.2.1 Chromosomes

Welcome to GCSE Science Unlocked, everyone! I'm Lottie, here with Mr H. And Mr H, I want to start with a measurement that completely short-circuits my brain. If you took all the DNA out of just one human cell and stretched it out end to end, it would be about TWO METRES long. TWO METRES of genetic code crammed into a space I can't even see without a microscope. Two metres is precisely right. But we must be absolutely clear about where that two metres is stored. We are talking exclusively about eukaryotic cells here. Right, the ones with a true nucleus. So we aren't talking about bacteria today. Exactly. In eukaryotic cells, the genetic material is enclosed within the nucleus. But it isn't just swimming around like two metres of loose spaghetti. If it were, the cell would never be able to divide without tearing it to pieces. It is METICULOUSLY coiled and packaged into structures called chromosomes. Chromosomes. And those chromosomes are made of DNA molecules. But when I look at a textbook, they usually draw them looking like little X shapes. They do, and it is a persistent frustration of mine because that X shape ONLY appears right before the cell divides. But more importantly, we need to understand what is ACTUALLY on those chromosomes. Each one carries a large number of genes. A large number -- like dozens, or hundreds? THOUSANDS, Lottie. Chromosome 1, the largest human chromosome, carries over three thousand genes. And each gene is a small section of DNA that codes for a specific sequence of amino acids, to make a SPECIFIC protein. Over three thousand genes on just ONE chromosome. Okay, that makes the packaging system make sense. But body cells don't just have one of each chromosome, do they? They come in pairs. Spot on. In ordinary body cells -- what we call somatic cells -- chromosomes are normally found in pairs. In humans, that means 23 pairs. 46 chromosomes in total. Twenty-three pairs. Because you inherit one set of 23 from your biological mother, and the other set of 23 from your biological father. They partner up. Precisely. One maternal, one paternal. And understanding that paired structure is the ABSOLUTE prerequisite for understanding how cells divide. If you don't know the starting inventory, you cannot get the marks for explaining how a cell duplicates it. Okay, so let's talk about that duplication. The cell cycle. Because cells don't live forever, and we are constantly growing or repairing damage. Like if I scrape my knee, my body needs to make new skin cells to heal it. Yes, growth and repair. Those are the two critical keywords for the exam. Multicellular organisms use the cell cycle to grow and to replace cells that are old or damaged. But here is the Mr H Mark Scheme Warning: [clears throat] students often think cell division is JUST a cell pinching itself in half. It is a three-stage process, and the actual splitting is ONLY the end of it. Right, because if you just chop a cell in half, each new cell only gets HALF the DNA. Which means half of that 2-metre blueprint is missing. So stage one HAS to be about preparation. Exactly. Stage one is the LONGEST part of the cell cycle. The cell grows, increasing its mass. It increases the number of sub-cellular structures -- things like ribosomes and mitochondria. And most crucially... the DNA REPLICATES to form two copies of each chromosome. So instead of 46 chromosomes, for a brief window, we've got a double set ready to be divided. Correct. The genetic material is doubled. Once that preparation is complete, we enter stage two: mitosis. Mitosis. This is where the REAL action happens. This is where those double copies of the chromosomes get separated. Yes. In mitosis, one set of chromosomes is pulled to EACH end of the cell. Then, [short pause] and ONLY then, the nucleus divides. Two distinct nuclei form inside the single cell, each containing a complete, IDENTICAL set of the DNA. Pulled to each end of the cell. I always picture it like two teams playing tug-of-war, pulling the chromosomes to opposite poles. So now we have one cell with two identical nuclei sitting inside it. Which brings us to the third and final stage. The cytoplasm and the cell membrane divide. [snaps fingers] They physically separate to form two identical cells. Two identical cells. So my new skin cells are perfect CLONES of the original ones. I'll accept 'clones' for our conversation, but on the exam paper, you MUST write 'genetically identical daughter cells'. [sternly but kindly] That is the terminology that secures the mark. Mitosis guarantees that the new cells have the exact same 23 pairs of chromosomes as the original parent cell. NOTHING lost, nothing altered. Genetically identical daughter cells. Got it. TWO METRES of DNA, flawlessly copied and divided, millions of times a day, just to keep us functioning. Next time we are looking at Stem Cells. See you then.