Chromosomal and chromatid numbers

Question

Mitosis:

Prophase. Somes= 2 Tids= 4

Metaphase. Somes = 2. Tids= 4

Anaphase. Somes= 4. Tids = 4

Telophase. Somes = 4. Tids =4

Cytokinesis.Somes = 2. Tids= 2

Meiosis i:

Prophase. Somes= 2 Tids= 4

Metaphase. Somes = 2 Tids= 4

Anaphase. Somes= 4 Tids = 2

Telophase. Somes= 2 Tids = 2

Cytokinesis.Somes = 1 Tids= 2.

Meiosis ii:

Prophase. Somes= 1 Tids= 2

Metaphase. Somes =1 Tids= 2

Anaphase. Somes= 2 Tids = 2

Telophase. Somes = 2 Tids = 2

Cytokinesis.Somes = 1 Tids= 1.

Am I correct in assuming that these are the simplest numbers of chromosomes and chromatids per each subsequent phase?

Answer 1

You are mostly correct with just a few things I’d alter. Let’s walk through the processes of mitosis and meiosis.

Imagine a cell in interphase. There are 2 chromosomes in total. DNA has not been replicated yet, so each chromosome is composed of just 1 chromatid (2 chromosomes, 2 chromatids). Now, the cell replicates its DNA in S phase and we have doubled the amount of DNA in the cell. The DNA in each chromosome has been duplicated. Each chromosome is now composed of 2 identical sister chromatids, but we still have the same number of chromosomes. So we have 2 chromosomes and 4 chromatids. 

During mitosis, the cell divides and produces 2 identical daughter cells with the same DNA present as that original cell. As the cell enters mitosis, it contains 2 chromosomes with 4 chromatids (representing that the DNA of each chromosome has been duplicated/“doubled”)

Prophase: DNA condenses. 2 chromosomes, 4 chromatids. 

Metaphase: Chromosomes align along metaphase plate. 2 chromosomes, 4 chromatids. 

Anaphase: Sister chromatids begin to separate. This is a tricky point - once the chromatids separate but the cell has not divided, we actually have 4 chromosomes each consisting of a single chromatid (4 chromatids). We have doubled the number of chromosomes because each chromosome of 2 chromatids had split - but we haven’t actually “added” any DNA. 

Telophase and cytokinesis: The cell divides. Each new cell has 2 chromosomes and 2 chromatids. 

Meiosis is the division of gametes and only occurs in germ cells (ie, sperm cells). We start with an interphase cell, 2 chromosomes and 2 chromatids. S phase occurs and we now have 2 chromosomes with 4 chromatids. 

In meiosis, the cell must distribute chromatids into 2 daughter cells as before. However, it must also separate homologous chromosomes, which are the similar but no identical pair of chromosomes inherited from each parent. 

Meiosis I: Separation of homologous chromosomes

Prophase I: DNA condenses as in mitosis, but now homologous chromosomes “pair up”. 2 chromosomes and 4 chromatids. (Note that this is where the process of “crossing over” occurs between homologous chromosomes, resulting in new, unique chromosomes with unique patterns of alleles.)

Metaphase I: Each pair of homologous chromosomes (rather than single chromosomes) align along metaphase plate. 2 chromosomes 4 chromatids.

Anaphase I: Homologous chromosomes (rather than sister chromatids) begin to separate. 

Telophase and cytokinesis: Cells divide with homologous chromosomes equally distributed to each cell. Each new cell contains 1 chromosome with 2 chromatids.

Meiosis II: Separation of sister chromatids

Prophase II: Chromosome DNA condenses. One chromosome 2 chromatids.

Metaphase II: Single chromosome aligns along metaphase plate. 1 chromosome 2 chromatids.

Anaphase II: Sister chromatids separate. As in mitosis, we now have 2 chromosomes, 2 chromatids, as each sister chromatid that are now separate are also an entire chromosome.

Telophase/Cytokinesis: Each new cell contains 1 chromosome, 1 chromatid (haploid). When combined with another germ cell during reproduction, they will make a full diploid cell of 2 chromosomes.