2: Chromosomes, Mitosis, and Meiosis
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Chromosomes contain genetic information. We often take this fact for granted, but just over a century ago, even the best biologists in the world were uncertain of the function of these rod-shaped structures. We now know that most chromosomes contain a single molecule of double-stranded DNA that form a complex with proteins. This arrangement allows very long DNA molecules to be compacted into a small volume that can more easily be moved during mitosis and meiosis (Fig 2.1) and expressed during interphase. The compact structure also makes it easier for pairs of chromosomes to align with each other during meiosis. Finally, we shall see that chromosomal structure can affect whether genes are active or silent.
Figure 2.1: Moving chromosomes (blue) towards the poles at anaphase requires many proteins (red), all of which interact with microtubules (green). (Flickr: TheJCB-Zhang et al. (2007) J. Cell Biol. 177:231-242.- CC: ANS)
- 2.0: DNA is Packaged into Chromatin
- If stretched to its full length, the DNA molecule of the largest human chromosome would be 85mm. Yet during mitosis and meiosis, this DNA molecule is compacted into a chromosome approximately 5µm long. Although this compaction makes it easier to transport DNA within a dividing cell, it also makes DNA less accessible for other cellular functions such as DNA synthesis and transcription.
- 2.1: Mitosis
- Cell growth and division is essential to asexual reproduction and the development of multicellular organisms. Accordingly, the primary function of mitosis is to ensure that at division each daughter cell inherits identical genetic material, i.e. exactly one copy of each chromosome.
- 2.2: Meiosis
- Most eukaryotes replicate sexually - a cell from one individual joins with a cell from another to create the next generation. For this to be successful, the cells that fuse must contain half the number of chromosomes as in the adult organism. Otherwise, the number of chromosomes would double with each generation! The reduction in chromosome number is achieved by the process of meiosis.