Ok confusion setting in on whats the definition of a chromosome and why is it n instead of 2n at the end of telophase/cytokinesis I of meiosis I
1)
I read somewhere that a chromosome always has to have two chromatids. No one chromosome can exist without having two chromatids.
"by definition chromosomes containing just one chromatid do not exist"
But then I read somewhere else that a chromosome is just a chromatid hanging out when it is in the G1 phase (not replicating)
2)
At the end of telophase I/Cytokinesis I you form two cells right. I get that homologous chromosomes get separated and each cell gets sister chromatids (Replicated chromatids of chromosome either from father or mother) But since you get two sister chromatids of one chromosome aren't you technically still 2n because there are 2 copies of the chromosome even though they are the same
Ploidy refers to the # of sets of chromosomes in a cell. Humans are diploid because we have 23 homologous pairs of chromosomes, two complete haploid sets (one set from the father and one set from the mother).
At metaphase each of the 46 chromosomes (members of the 23 pairs) consists of two sister chromatids that had formed after replication in S phase. So there is a total of 92 chromatids, but that does not mean there is 92 chromosomes. It's easy to follow chromosome number by keeping track of the number of centromeres. Sister chromatids become individual chromosomes during anaphase, when the centromeres split.
So you start with 23 pairs (46 chromosomes) in interphase/mitosis, and each member of the pair replicates itself. Technically speaking, you have double the amount of genetic material but still 46 chromosomes since the replicated material shares a centromere. The only time you really have "92 chromosomes" is the moment after anaphase. The sister chromatids that were previously held together by the centromere split, and each become their own chromosome -- 46 chromosomes are pulled to one side and 46 to the other side. After cytokinesis, there are two new cells with 46 chromosomes (23 pairs like at the beginning), and this ends the cycle. The cell is never 4n because there is never 4 homologs (or members of a homologous family).
Consider meiosis. The goal is to get 4 haploid cells from one diploid cell. With your 23 pairs (46 chomosomes) in your diploid cell, you replicate to 23 tetrads. A tetrad consists of 4 chromatids, two sets of sisters, and technically 46 chromosomes because the sisters are held together by centromeres.
Meiosis I happens, and the tetrads are pulled apart (but no centromeres are pulled apart). So a cell with 46 chromosomes becomes 2 cells, each with 23 chromosomes (reductive division). This cell is haploid because there is no longer a homologous pair in any cell. Each of the two new cells only consists of the sister chromatids that were a result of replication. Meosis II serves to split the sister chromatids apart. Remember, this splitting results in each chromatid becoming its own chromosome, so you go from 23 chromosomes in one cell to 23 chromosomes in each cell. This happens in each of the two cells that resulted from meiosis I, so you have 4 cells that contain 23 chromosomes = haploid.
Credit goes to attixx ::
I was really confused on damn chromosomes and chromatids; not anymore..
