In previous sessions I've told you that a gene is a segment of DNA that specifies the sequence of a particular protein (of course there are some exceptions: Some genes are templates for structural RNA's like tRNA and rRNA). In addition, promoters and enhancers, regulatory sequences that help control the level of transcription, are also parts of genes.
In most organisms, hundreds or thousands or more of these coding and regulatory sequences are arranged in a specific order on enormously long molecules of DNA. As we've said before, long molecules of DNA (in combination with the proteins that are bound to them) are called chromosomes.
Most organisms bear more than one chromosome per cell. In fact, each species is characterized by a specific number of chromosomes. For example, Drosophila, the fruit fly, has eight chromosomes in each of its cells, tomatoes have 24, and humans have 46. A table showing the chromosome number of a few selected organisms is shown below. Each chromosome, in turn, has a characteristic size and shape, and each may stain with a distinctive pattern. The illustration below the table shows the size and shapes of the chromosomes of humans.
|
camel |
70 |
|
goat |
60 |
|
guinea pig |
64 |
|
armidillo |
64 |
|
opossum |
22 |
|
bat |
44 |
|
king crab |
208 |
|
petunia |
14 |
|
tomato |
24 |
|
rice |
24 |
|
corn |
20 |
|
yeast |
32 |
Each chromosome, in turn, bears a particular set of genes, and these genes are arranged in a specific order on each chromosome. When a cell divides in two, the process of mitosis ensures that both daughter cells get the precise complement of chromosomes of their parent. Because of mitosis, each cell in a multicellular organism contains an entire set of chromosomes, and thereby gets to carry all the genes for making an entire organism.
