LGS A-Level OCR Biology - Unit 2 - The Cell Cycle, Mitosis, Meiosis Part 2
This deck covers key concepts from Unit 2 of the OCR Biology A-Level syllabus, focusing on the cell cycle, mitosis, and meiosis. It includes definitions, processes, and functions related to cellular division.
Define chromosomes
Key Terms
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| Term | Definition |
|---|---|
Define chromosomes | Structures of condensed and coiled DNA molecules in the form of chromatin |
Define sister chromatid. | Two identical DNA molecules joined by a common centromere |
Define centromere. | Region at which two sister chromatids are held together
Area of constriction of cells chromosomes
Point at which the chromosomes short arm (P) and long arm (Q) are separated.
Point where mitotic spindle fibers attach to pull sister chromatids apart during cell division. |
Define centrioles. | Component of the cytoskeleton made of microtubules and associated proteins, involved in the development of spindle fibres. |
Define spindle fibres | A structure made of microtubules and associated proteins that assemble from the centrosome to provide the structure that moves chromosomes. |
Define homologous pairs | A pair of chromosomes, one maternal and one paternal
Contain the same genes on the same loci
Do not necessarily have the same alleles |
How is DNA packaged? | Wrapped around histone proteins to form chromatin Coiled around more proteins to form chromosomes |
When is DNA found as visible chromosomes? | Only during cell division. |
Why does DNA form chromosomes for cell division? | Helps protect DNA from damage when it is being moved and replicated |
Why does DNA wrap around histones? | Makes it more compact Makes it easy to access |
What are the 4 stages of mitosis in order? | Prophase Metaphase Anaphase Telophase |
What happens in prophase of mitosis? | Chromatin fibres become more tightly coiled, condensing into discrete chromosomes visible with a light microscope Nuclear membrane begins to break down and nucleolus disappears Each duplicated chromosome appears as two identical sister chromatids joined at their centromeres and, in some species, all along their arms by cohesins The mitotic spindle begins to form from centrosomes and the microtubules from them. N.b. shorter, radial arrays of microtubules are called asters 'stars'. |
What happens in early metaphase (prometaphase) of mitosis? | The nuclear envelope fragments Microtubules extending from each chromosome can now invade the nuclear area The chromosomes have become even more condensed Each of the two chromatids of each chromosome now has a kinetochore, a specialised protein structure at the centromere. Some of the microtubule attach to the kinetochores becoming 'kinetochore microtubules' which jerk the chromosomes back and forth Non-kinetochore fibres interact with those from the opposite pole of the spindle. |
What happens in (late) metaphase of mitosis? | Centrosomes are now at opposite poles of the cell Chromosomes are moved by spindle fibres to line up along the equatorial plate of the cell known as the 'metaphase plate' Each chromosome has the kinetochore of the sister chromatid attached to the kinetochore microtubule from the opposite pole. |
What happens in anaphase of mitosis? | Shortest phase lasting only a few minutes Cohesion proteins are cleaved allowing the sister chromatids to be pulled apart by spindle fibres Daughter chromosomes move towards the poles of the cell as their kinetochore microtubules shorten Chromosomes move centromere first at ~1μm/min The cell elongates as the non-kinetochore microtubules lengthen Results in two poles of cell having equivalent, and complete, collections of chromosomes. |
What happens in telophase of mitosis? | Two daughter nuclei form in the cell - nuclear membrane forms from fragments of parents and portions of endomembrane system Nucleoli reappears Chromatids now called chromosomes which become less condensed and indistinct Remaining spindle fibres are depolymerized Mitosis, the division of one nucleus into two genetically identical nuclei, is now complete. |
Why are centrioles important in cell division? | Pairs of centrioles called centrosomes are the subcellular region which organises the cell's microtubules
During mitosis the spindle microtubules (fibres) start at the centrosomes
Spindle fibres are responsible for the movement of chromosomes within the cell |
What happens in cytokinesis of animal cells? | A cleavage furrow forms Microfilaments form a ring around the edge of the plasma membrane pulling the membrane inwards pinching the cell in two The plasma membrane fuses in the middle to form two cells |
What happens in the cytokinesis of plants? | Vesicles carry cell membrane and cell wall components assemble along the plane of division The cell membrane first forms as vesicles fuse Cell wall forms alongside |
What do single celled organisms use mitosis for? | To reproduce asexually |