Cell Cycle, Division & Chromosomes

The Cell Cycle
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Cells are not static structures, but are created and die. The life of a cell is called the cell cycle and has four phases:

  In different cell types the cell cycle can last from hours to years. For example bacterial cells can divide every 30 minutes under suitable conditions, skin cells divide about every 12 hours on average, liver cells every 2 years.

The mitotic phase can be sub-divided into four phases (prophase, metaphase, anaphase and telophase). Mitosis is strictly nuclear division, and is followed by cytoplasmic division, or cytokinesis, to complete cell division. The growth and synthesis phases are collectively called interphase (i.e. in between cell division). Mitosis results in two “daughter cells”, which are genetically identical to each other, and is used for growth and asexual reproduction.

Cell Division by Mitosis
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Mitosis is a type of cell division that produces genetically identical cells.  During mitosis DNA replicates in the parent cell, which divides into two new cells, each containing an exact copy of the DNA in the parent cell.  The only source of genetic variation in the cells is via mutations.

 Mitosis and Asexual Reproduction
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Asexual reproduction is the production of offspring from a single parent using mitosis. The offspring are therefore genetically identical to each other and to their “parent”- in other words they are clones. Asexual reproduction is very common in nature, and in addition we humans have developed some new, artificial methods. The Latin terms in vivo (“in life”, i.e. in a living organism) and in vitro (“in glass”, i.e. in a test tube) are often used to describe natural and artificial techniques.

Meiosis
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Meiosis is a form of cell division. It starts with DNA replication, like mitosis, but then proceeds with two divisions one immediately after the other. Meiosis therefore results in four daughter cells rather than the two cells formed by mitosis. It differs from mitosis in two important aspects:

• The chromosome number is halved from the diploid number (2n) to the haploid number (n). This is necessary so that the chromosome number remains constant from generation to generation. Haploid cells have one copy of each chromosome, while diploid cells have homologous pairs of each chromosome.

• The chromosomes are re-arranged during meiosis to form new combinations of genes. This genetic recombination is vitally important and is a major source of genetic variation. It means for example that of all the millions of sperm produced by a single human male, the probability is that no two will be identical.

You don’t need to know the details of meiosis at this stage (It's covered in module 5).

Sexual reproduction is the production of offspring from two parent using gametes. The cells of the offspring have two sets of chromosomes (one from each parent), so are diploid.  Sexual reproduction involves two stages:

• Meiosis- the special cell division that makes haploid gametes

• Fertilisation- the fusion of two gametes to form a diploid zygote

The Advantages of Sexual Reproduction
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For most of the history of life on Earth, organisms have reproduced only by asexual reproduction. Each individual was a genetic copy (or clone) of its “parent”, and the only variation was due to random genetic mutation. The development of sexual reproduction in the eukaryotes around one billion years ago led to much greater variation and diversity of life. Sexual reproduction is slower and more complex than asexual, but it has the great advantage of introducing genetic variation (due to genetic recombination in meiosis and random fertilisation). This variation allows species to adapt to their environment and so to evolve. This variation is clearly such an advantage that practically all species can reproduce sexually. Some organisms can do both, using sexual reproduction for genetic variety and asexual reproduction to survive harsh times.

DNA and Chromosomes
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The DNA molecule in a single human cell is 99 cm long, so is 10 000 times longer than the cell in which it resides (< 100mm). (Since an adult human has about 10¹⁴ cells, all the DNA is one human would stretch about 10¹⁴ m, which is a thousand times the distance between the Earth and the Sun.) In order to fit into the cell the DNA is cut into shorter lengths and each length is tightly wrapped up with histone proteins to form a complex called chromatin. During most of the life of a cell the chromatin is dispersed throughout the nucleus and cannot be seen with a light microscope. At various times parts of the chromatin will unwind so that genes on the DNA can be transcribed. This allows the proteins that the cell needs to be made.

Just before cell division the DNA is replicated, and more histone proteins are synthesised, so there is temporarily twice the normal amount of chromatin. Following replication the chromatin then coils up even tighter to form short fat bundles called chromosomes. These are about 100 000 times shorter than fully stretched DNA, and therefore 100 000 times thicker, so are thick enough to be seen under the microscope. Each chromosome is roughly X-shaped because it contains two replicated copies of the DNA. The two arms of the X are therefore identical. They are called chromatids, and are joined at the centromere. (Do not confuse the two chromatids with the two strands of DNA.) The complex folding of DNA into chromosomes is shown below.

 
Homologous Chromosomes
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If a dividing cell is stained with a special fluorescent dye and examined under a microscope during cell division, the individual chromosomes can be distinguished. They can then be photographed and studied. This is a difficult and skilled procedure, and it often helps if the chromosomes are cut out and arranged in order of size.

This display is called a karyotype, and it shows several features:

• Different species have different number of chromosomes, but all members of the same species have the same number. Humans have 46, chickens have 78, goldfish have 94, fruit flies have 8, potatoes have 48, and so on. The number of chromosomes does not appear to be related to the number of genes or amount of DNA.

• The chromosomes are numbered from largest to smallest.

• Chromosomes come in pairs, called homologous pairs ("same shaped"). So there are two chromosome number 1s, two chromosome number 2s, etc, and humans really have 23 pairs of chromosomes. Homologous chromosomes are a result of sexual reproduction, and the homologous pairs are the maternal (inherited from the mother) and paternal (inherited from the father) versions of the same chromosome, so they have the same sequence of genes

• One pair of chromosomes is different in males and females. These are called the sex chromosomes, and are non-homologous in one of the sexes. In humans the sex chromosomes are homologous in females (XX) and non-homologous in males (XY). (In birds it is the other way round!) The non-sex chromosomes are sometimes called autosomes, so humans have 22 pairs of autosomes, and 1 pair of sex chromosomes.

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Last updated 22/11/2004