Cell Cycle and Cell Division

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The Cell Cycle

As far as the AS specification goes, the cell cycle consists of 3 phases:

Interphase (the longest phase)

Mitosis (it should be shorter than that diagram shows)

Cytokinesis (the shortest phase).

You need to know what events take place in each of these phases.

Interphase

Several important things occur during interphase:

=> The cell grows, in preparation for mitosis.

=> New organelles are synthesised.

=> DNA is replicated.

=> There are several checkpoints to make sure that everything is going smoothly. When this control goes wrong, cancer ensues.

Mitosis

The nucleus (not the cell - common misconception here!) divides into two separate, identical nuclei. (This is covered in detail on the next page.)

Cytokinesis

The cell itself then splits in two, with one nucleus in each daughter cell. (That's all you need to know about cytokinesis.)

Mitosis

Cells constantly need to divide, both during our development and growth, and to replace damaged or dead cells. The purpose of mitosis is to produce two genetically identical nuclei. The cell then splits into two cells during cytokinesis.

Mitosis consists of 4 subphases:

PROPHASE

METAPHASE

ANAPHASE

TELOPHASE

1. In prophase the replicated DNA is wrapped around proteins called histones to form visible chromosomes. These consist of two identical (sister) chromatids joined at a centromere. The nuclear envelope breaks down, and spindle fibres form a structure called a spindle.

2. In metaphase the chromosomes reach the centre (or "equator") of the cell. At this point the chromatids are still held together.

3. In anaphase, the sister chromatids split and begin to move towards opposite ends (poles) of the spindle. Since one identical

copy of each chromosome ends up in one daughter cell, the cells are identical to each other and to the original cell.

4. In telophase, the separated chromatids (now called chromosomes) reach the poles of the spindle. The spindle breaks down,

and two nuclear envelopes form, one around each set of chromosomes. Thus, two distinct, identical nuclei are formed.

When considering meiosis, always bear in mind what its purposes are:

To produce haploid cells (with just one full set of chromosomes)

To introduce genetic variation.

In meiosis, two divisions occur, resulting in four non-identical daughter cells. Each of these divisions, like mitosis, consists of a prophase, metaphase, anaphase and telophase, but slightly different things happen to the chromosomes:

During prophase of the first division (Prophase I), homologous chromosomes align to form a bivalent. Some DNA is then swapped between adjacent chromosomes (genetic crossover or crossing over):

(You don't need to know the word synapsis.)

During metaphase I, the chromosomes line up in pairs, each aligned with its homologous chromosome (held together by chiasmata). In anaphase I, the homologous chromosomes separate and move to opposite poles, but the chromatids themselves do not separate and are still connected at the centromere:

Pair of homologous chromosomes. The black and red tips indicate where genetic crossover has occurred.

The important feature of this separation is that it is random: one daughter cell might have, say, 10 paternal and 13 maternal chromosomes. This random segregation (also known as independent assortment), combined with genetic crossover, gives rise to huge genetic variation in daughter cells.

Meiosis I therefore halves the chromosome number, as each daughter cell contains 23 chromosomes (or whatever the haploid number is in the particular organism). However, each of the two daughter cells has two almost identical copies of each chromosome (not quite identical due to genetic crossover). This is unwanted, so the daughter cells divide again; this second division (meiosis II) is essentially identical to mitosis.

The Cell Cycle

Mitosis

Meiosis