The Cell Cycle, Mitosis & Meiosis

You think you’ve got a lot of chores? Our bodies are tirelessly undergoing cell division, replacing the skin cells that we’re constantly shedding to make sure we don’t disappear in a poof of smoke. There are two types of cell division - mitosis, which produces genetically identical cells for growth and repair and meiosis which produces genetically unique cells for sexual reproduction.

Mitosis and the Cell Cycle

Mitosis is a type of cell division where cells produce identical copies of themselves and is used for growth and repair and asexual reproduction. It differs from meiosis, which is the type of cell division used to produce gametes.

Mitosis occurs as part of the cell cycle which consists of four distinct phases. First, interphase takes place which is made up of three growth phases (called G1 phase, S phase and G2 phase), followed by mitosis.

• Gap Phase 1 (G1) - cell grows bigger and replicates its organelles. A high amount of protein synthesis is taking place in order to build new organelles.

• Synthesis Phase (S) - the cell replicates its DNA

• Gap Phase 2 (G2) - the cell keeps growing until all of the organelles have duplicated.

There are two ‘checkpoints’ in the cell cycle - one before S phase and one straight after S phase. During these checkpoints, the cell is checking its DNA for errors. This minimises the chances of duplicating any mutated DNA into the replicated cell.

The Stages of Mitosis

Mitosis can be divided into a series of stages depending on what’s going on with the chromosomes in the cell. You can use the acronym IPMAT to help you remember the order.

Interphase - the cell prepares for mitosis by growing larger, replicating its organelles and synthesising new DNA (see above). Once the DNA has replicated, each chromosome now consists of two sister chromatids, connected by a structure called the centromere. The mitochondria produce more ATP which will provide the energy for cell division.

Prophase - the chromosomes condense (they become shorter and fatter) and the nuclear envelope disintegrates. The centrioles move to opposite poles of the cell and form spindle fibres.

Metaphase - the chromosomes line up along the middle of the cell. They attach to the spindle fibre by their centromere.

Anaphase - the centromere splits and the chromatids are pulled to opposite poles of the cell.

Telophase & cytokinesis - the two groups of chromsomes decondense (they become long and thin) and a nuclear envelope reforms around them, forming two new nuclei. The cytoplasm divides (cytokinesis) and the plasma membrane pinches off to form two new, genetically-identical cells.

Mitotic Index

The mitotic index is a measure of the proportion of cells which are undergoing mitosis. You may be asked to calculate it in the exam. To do this, you need to count the number of cells with visible chromosomes and divide this by the total number of cells.

Meiosis

Meiosis is the type of cell division which produces gametes for sexual reproduction. Unlike mitosis, the daughter cells are genetically different from the parent cell and contain just half the number of chromosomes (i.e. they are haploid). When two haploid gametes join during fertilisation, a diploid cell called a zygote is formed. Meiosis involves two rounds of cell division which are referred to as meiosis I and meiosis II. It takes place in the following stages:

Meiosis I

• Interphase: the DNA replicates so there are now two identical copies of each chromosome (referred to as chromatids).

• Prophase I: chromatids condense and arrange themselves into homologous pairs (called bivalents). Crossing over occurs (see below). The nuclear envelope disintegrates and spindle fibres form.

• Metaphase I: homologous chromosomes line up along the equator and attach to the spindle fibre by their centromeres.

• Anaphase I: homologous chromosomes are separated

• Telophase I: chromosomes reach opposite poles of the cell. Nuclear envelope reforms around the chromosomes. Cytokinesis results in the formation of two daughter cells.

Meiosis II

• Prophase II: chromosomes condense, nuclear envelope disintegrates and spindle fibres form.

• Metaphase II: chromosomes attach to the spindle fibre by their centromeres.

• Anaphase II: sister chromatids are separated.

• Telophase II: chromatids reach opposite poles of the cell. Nuclear envelope reforms and cytokinesis takes places. Four genetically unique daughter cells are produced.

Meiosis increases genetic variation

From an evolutionary point of view, it is important that organisms produce offspring that show as much genetic variation as possible. Imagine if a mother duck gave birth to a group of ducklings that were all had very similar genes - these ducklings will all be equally vulnerable to the same diseases and other threats to their survival. Meiosis increases genetic variation in two ways - crossing over and independent assortment.

Crossing Over

During prophase I of meiosis, a process called crossing over occurs. This is when the homologous chromosomes move towards each other and exchange genetic material. When the pair of chromosomes have come together, we call this a bivalent. A chromatid from the maternal chromosome becomes twisted around the paternal chromosome and they connect through a structure called the chiasmata. Pieces of chromosomes are exchanged and the chromatids separate, forming chromosomes with different combinations of alleles.

Independent assortment

Depending on the order in which chromosomes line up along the equator of the cell during metaphase, different combinations of chromosomes will end up in each gamete. The way in which the chromosomes align themselves on the spindle fibre is completely random, resulting in a huge number of possibilities of chromosomal combinations in the gametes.

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