International A Level Biology / Mitosis, meiosis and reproduction

The cell cycle

Edexcel International A Level Biology (YBI11), chapter 8: Mitosis, meiosis and reproduction, specification point 3.14. Free lesson.

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The cell cycle is the sequence of events from one cell division to the next. Understanding how DNA is copied and shared explains how mitosis produces genetically identical cells for growth, repair and asexual reproduction.

What happens during the cell cycle

The cell cycle is everything a cell does from one division to the next. It is a loop because each new cell can begin the cycle again. Most of the cycle is interphase, when the cell grows and copies its DNA. Interphase is not a resting stage. G1: the cell grows and makes proteins and organelles. S phase: the DNA is replicated, so the amount of DNA in the cell doubles. G2: the cell grows more, checks the copied DNA for errors and makes proteins needed for division, such as spindle proteins. Mitosis: the nucleus divides into two genetically identical nuclei. Cytokinesis:…

How S phase prepares chromosomes for mitosis

When DNA is replicated in S phase, each chromosome becomes two identical copies joined together. Each copy is a chromatid, which is one long DNA molecule with its proteins. The two identical copies are called sister chromatids and are held together at the centromere. The sister chromatids have the same base sequence because they were copied from the same DNA molecule. During mitosis, they separate and move to opposite poles of the cell. Each new nucleus receives one copy of every chromosome. This is why the daughter cells are genetically identical to each other and to the parent…

How DNA content changes through the cycle

A graph of DNA per cell stays level during G1, rises gradually during S phase, then stays high through G2 and mitosis. The rise takes time because the DNA is replicated over hours, rather than all at once. The amount per cell falls at cytokinesis because the copied DNA is shared between two cells. In a human cell grown in a dish, one cycle takes about 24 hours. The DNA content rises from two arbitrary units to four during S phase. It returns to two units per cell at cytokinesis. DNA doubles in S phase, not in mitosis. Mitosis shares the copies out equally.

What mitosis is for

A role means what mitosis is for, not the stages of how it works. Each role needs genetically identical cells, so the new cells have the same genes as the original cell. Growth: a zygote, the fertilised egg, grows into an organism. In plants, growth by mitosis happens mostly at root and shoot tips. Repair: new cells help close wounds and replace worn-out cells. They can do the same job as the cells they replace. Asexual reproduction: one parent produces offspring without gametes. For example, a strawberry plant can grow new plants on runners. The offspring are clones of the…

How to explain why daughter cells are identical

Link DNA replication in S phase to the separation of sister chromatids in mitosis. Each new nucleus must receive one copy of every chromosome.

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