Meiosis starts with what?

[Teleologist]

If meiosis can be summed up as occurring in sex cells and is a 2n -> n process, what cells does meiosis start in? Wouldn't the sex cells themselves be "n" as well?

Or is this why there is interphase (S) and replication of DNA?

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[The Brown Knight]

meiosis starts with precursor cells (specifically, primary spermatocytes and primary oocytes - derived from spermatogonia/oogonia) that are diploid 2n. it's products are the sex cells (specifically, spermatozoa and ova) haploid n.
The precursor cells are just like most other body cells and actually divide mitotically before undergoing meiosis at puberty. And yes the precursor cells undergo S phase DNA replication as well before going into meiosis.

 

[justadream]

You start with 2n The replication during S phase makes sister chromatids (but it's still 2n).

 

[type12]

If meiosis can be summed up as occurring in sex cells and is a 2n -> n process, what cells does meiosis start in? Wouldn't the sex cells themselves be "n" as well?

Or is this why there is interphase (S) and replication of DNA?

2n. It's all about the wording they choose.

As you know, the n refers to the number of homologous pairs of chromosomes (e.g., chromosome 1 from dad, chromosome 1 from mom makes 2n). During the S phase of interphase, this genetic code is duplicated, but the number of homologous pairs isn't considered doubled because chromosome 1 from dad and chromosome 1 from dad are considered a value of 1n (same genetic information). Instead, this difference is noted as having two sister chromatids.

During Meiosis 1, it is the HOMOLOGOUS chromosomes that line up and get separated (e.g., (sis chromatids from dad lined up against sis chromatids from mom). After Meiosis 1, the cells are now at n, because while you have chromosome 1 from dad and chromosome 1 from dad (aka sister chromatids), there's only one variance of genetic code for each gene (NOTE: (you could have chromosome 2 from mom and chromosome 2 from mom as the next set of sister chromatids). Meiosis 2 is when sister chromatids separate.

 

[Marleywh]

Doubling or halving the amount of DNA (like what occurs in mitosis) does not determine your ploidy! You still have the same genetic information (both homologous chromosomes). In meiosis 1 you separate those homologs and that is where the 2n --> n occurs. You still have double DNA before meiosis 2, but you are haploid nonetheless.