Question

You have studied mitosis in onion root tip cells. It has 14 chromosomes in each cell. Can you tell how many chromosomes will the cell have at the G1 phase, after the S phase, and after the M phase? Also, what will be the DNA content of the cells at G1, after S, and at G2, if the content after M phase is 2C?

Answer 1

Hint: G1 is an intermediate phase occupying the time between the end of cell division in mitosis and the beginning of DNA replication during the S phase. During this point, the cell grows in preparation for DNA replication, and certain intracellular components, like the centrosomes, undergo replication.

Complete answer:
At G1 it'll have 14 chromosomes
At S also it'll have 14 chromosomes
G2 also has 14 chromosomes
After M phase 7 chromosomes
The content will be 2C at G1 and it will be 4C at G2 and S phase.
One significant difference between growth phases G1 and G2 is that the first growth phase is about cell growth while G2 is about cell division. cell division occurs during the M phase, which consists of nuclear division (mitosis) followed by cytoplasmic division (cytokinesis). The DNA is replicated within the preceding S phase; the 2 copies of every replicated chromosome (called sister chromatids) remain glued together by cohesins.

Note:
- Interphase consists of the G1 phase (cell growth), followed by the S phase (DNA synthesis), followed by the G2 phase (cell growth). At the end of interphase comes the mitotic phase, which is formed from mitosis and cytokinesis and results in the formation of two daughter cells.
- The use of onion tip for mitosis study:
An onion root tip is the rapidly growing portion of the onion and thus many cells can grow in different stages of mitosis. The onion root tips are often prepared and squashed in a way that permits them to be flattened on a microscopic slide in order that the chromosomes of individual cells are often observed easily.