Expert: John Locke - 5/15/2007

Question
How many genes undergo DNA replication before meiosis?

Answer
Julie, thank you for using AllExperts and I'd be happy to answer your question.

All genes undergo replication before meiosis starts. Each chromosome replicates itself once before meiosis; a chromosome is a distinct long, coiled strand of DNA, as you may already know. Each of these long strands is copied once in the synthesis phase of meiosis (this is generally referred to as the S-phase), giving two copies of any one chromosome before the cell divisions of meiosis starts.

As you may also be aware, a gene consists at minimum of two different copies or alleles. This springs from the fact that half of one's genetic material derives from each parent--twenty-three chromosomes from one parent and twenty-three from the other, for total of twenty-three pairs or forty-six chromosomes (in humans). Since each gene is associated with a specific site on a chromosome, there are always two sites associated with a single gene--one on the maternal chromosome and one on the paternal chromosome that carries that gene.

From this perspective, it might be said that a gene replicates twice before meiosis, although the alleles themselves only replicate once.  

The only exception to this is the sex chromosomes in males, who have one X and one Y chromosome. The chromosomes themselves replicate like all the others, but in males there is only one copy of the genes on the X and Y chromosomes--there is no paired X or Y chromosome. Females have two copies of the X chromosome and thus have two copies of each gene on the X chromosome.