Chapter 12.2, Problem 22AQP

a.

Summary Introduction

To determine:

Cells in G1, before switching to medium with 14N.

Explanation of Solution

Pictorial representation: fig 1: Pair of homologous chromosomes in a mouse cell.

Fig 1: Pair of homologous chromosomes in a mouse cell.

A mouse which has been grown in a ${}^{\text{15}}\text{N}$ containing medium for many generations would contain only ${}^{\text{15}}\text{N}$ isotopes in the nucleotides of DNA. As nitrogen is a basic component which can produce the nucleotides of DNA and RNA, and given in the mouse cells medium that contain ${}^{\text{15}}\text{N}$ is the only source of nitrogen.

As in, new DNA molecules that are being replicated as the cell takes place division, only the heavier nitrogen isotope is present to incorporate in it. Thus, the cells which are growing continuously in a ${}^{\text{15}}\text{N}$ containing medium will contain only ${}^{\text{15}}\text{N}$ in their DNA. Cells in G1, before switching to medium with ${}^{\text{14}}\text{N}$, will contain only ${}^{\text{15}}\text{N}$ in their DNA.

b.

Summary Introduction

To determine:

Cells in G2, after switching to medium with 14N.

Explanation of Solution

Pictorial representation: fig 2: Pair of homologous chromosomes in a mouse cell in ${\text{G}}^{\text{2}}$, after continuing to medium rich in ${}^{\text{14}}\text{N}$.

Fig 2: Pair of homologous chromosomes in a mouse cell in ${\text{G}}^{\text{2}}$, after continuing to medium rich in ${}^{\text{14}}\text{N}$.

After continuing to medium rich in ${}^{\text{14}}\text{N}$ a pair of homologous chromosomes in a mouse cell in ${\text{G}}^{\text{2}}$ will only contain both ${}^{\text{15}}\text{N}$ and ${}^{\text{14}}\text{N}$. Because DNA replication is semi-conservative, so, each of the chromatids of the homologous pair of chromosomes has one strand containing ${}^{\text{15}}\text{N}$ and the new strand will contain ${}^{\text{14}}\text{N}$.

c.

Summary Introduction

To determine:

Cells in anaphase of mitosis, after switching to medium with 14N.

Explanation of Solution

Pictorial representation: fig 3: Pair of homologous chromosomes in a mouse cell in ${\text{G}}^{\text{2}}$, in anaphase of mitosis, after continuing to medium rich in ${}^{\text{14}}\text{N}$

Fig 3: Pair of homologous chromosomes in a mouse cell in ${\text{G}}^{\text{2}}$, in anaphase of mitosis, after continuing to medium rich in ${}^{\text{14}}\text{N}$

If the cell enters into next phase after S phase in the cell cycle, it does not influenced by the surrounding medium. Thus, after the S phase of the cell cycle, the DNA composition is remained same until the next S phase. Therefore, pair of homologous chromosomes in a mouse cell in ${\text{G}}^{\text{2}}$, in anaphase of mitosis, after continuing to medium rich in ${}^{\text{14}}\text{N}$, will appear as above figure.

d.

Summary Introduction

To determine:

Cells in metaphase I of meiosis, after switching to medium with 14N.

Explanation of Solution

Pictorial representation: fig 3: Pair of homologous chromosomes in a mouse cell in ${\text{G}}^{\text{2}}$, in metaphase I of meiosis, after continuing to medium rich in ${}^{\text{14}}\text{N}$.

Fig 3: Pair of homologous chromosomes in a mouse cell in ${\text{G}}^{\text{2}}$, in metaphase I of meiosis, after continuing to medium rich in ${}^{\text{14}}\text{N}$.

Since, after the S phase of the cell cycle, the DNA composition is remained same until the next S phase. Therefore, pair of homologous chromosomes in a mouse cell in ${\text{G}}^{\text{2}}$, in metaphase I of meiosis, after continuing to medium rich in ${}^{\text{14}}\text{N}$ appears as given n the above diagram.

e.

Summary Introduction

To determine:

Cells in anaphase II of meiosis, after switching to medium with 14N.

Explanation of Solution

Pictorial representation: fig 3: Pair of homologous chromosomes in a mouse cell in ${\text{G}}^{\text{2}}$, in anaphase II of meiosis, after continuing to medium rich in ${}^{\text{14}}\text{N}$.

Fig 3: Pair of homologous chromosomes in a mouse cell in ${\text{G}}^{\text{2}}$, in anaphase II of meiosis, after continuing to medium rich in ${}^{\text{14}}\text{N}$.

Since, after the S phase of the cell cycle, the DNA composition is remained same until the next S phase. Thus, Pair of homologous chromosomes in a mouse cell in ${\text{G}}^{\text{2}}$, in anaphase II of meiosis, after continuing to medium rich in ${}^{\text{14}}\text{N}$ will appear as given in the above picture.

Conclusion

Nitrogen is a basic component which can produce the nucleotides of DNA and RNA, and given in the mouse cell medium which have ${}^{\text{15}}\text{N}$ ,the only source of nitrogen.