That seems like an easy question, doesn't it? There are 23 pairs of chromosomes. There are about 20,000-25,000 genes. So a quick answer to your question would be about 1,000 genes.

Here are a few wrinkles, though. A gene is basically a recipe for a protein. A lot of each chromosome is DNA that is not recipes. This part of the collection of DNA (the genome) may have directions on which part of the chromosomes to read. These would be sort of like switches that turn on to get the cell to use a particular recipe. So every chromosome has more DNA than is contained in the recipes.

The chromosomes are also different sizes. Chromosome 1 is much larger than chromosome 22, for example. There's even a pair of chromosomes that may be different sizes. In females, chromosome pair 23 is two X chromosomes. For males, pair 23 is one X and one Y. The X is much larger and contains around 2,000 genes. The Y is quite small and only has about 86 genes that make about 23 proteins, which basically say be male.

Look up the number of genes that a few different species have. You will notice that the number of genes or chromosomes will not let you predict how large or complex something is.

Do you think there's a lower limit to the size of a chromosome? What about an upper limit? Why don't we just have one or two big chromosomes? Think about how cell division works and see if that gives you some ideas.

Actually, chromosomes are just one very long strand of DNA wound together in a manageable shape. Chromatids, a pair of identical chromosomes, contain two strands of DNA.

You've got it the wrong way around: chromosomes are composed of DNA (and other things). That said, each chromosome consists of one very long DNA strand, which loops back on itself many times to make the chromosome.

Actually, a chromosome is a bundled up strand of DNA! If you take a double-helix of DNA, and wind it up really tightly into itself, you get a chromosome!

Actually, when we talk about "strands of DNA", this can refer to DNA molecules of any length (measured in single A,T,G, or C nucleotides known as "base pairs"). Human cells contain two sets of 23 chromosomes, one set inherited from each parent (46 total). These chromosomes are large "chunks" of DNA wound tightly together, and range in size from 51 million to 245 million base pairs each. As a whole, the whole human genome is 3.3 billion base pairs long! If you put all this sequence information into a text file, it would just barely fit onto one CD (~800 megabytes).

One chromosome has 2 strands of DNA in a double helix. But the 2 DNA strands in chromosomes are very, very long. One strand of DNA can be very short - much shorter than even a small chromosome. Strands of DNA are made by joining together the 4 DNA bases in strings. The 4bases are A, C, G, and T. So a short strand of DNA might be AAGCCTTGCAAT - or any other string of those 4 DNA bases.

Keep asking questions!
Best wishes,