Chapter 5, Problem 5Q

A.

Summary Introduction

To find: The sequence of the complementary strand of the single-stranded DNA sequence.

5′-GGATTTTTGTCCACAATCA-3′

Concept introduction: Deoxyribonucleic acid (DNA) is the hereditary material found in humans and many other organisms. It is located in every cell nucleus; hence it is called as nuclear DNA. But a little amount of DNA can be also found in the mitochondria, where it is called mitochondrial DNA. The typical structure of a DNA molecule consists of two strands twisting around each other like a twisted ladder. The rails of the ladder are made up of sugar and phosphate molecules in alternative way. The steps are made of two bases linked together with either two or three weak hydrogen bonds. There is only one way the nitrogenous bases can pair up on the rungs of the DNA ladder. An adenine molecule only pairs with a thymine and cytosine only with guanine.

Answer to Problem 5Q

5′-TGATTGTGGACAAAAATCC-3′

Explanation of Solution

Complementary base pairing describes the way in which the nitrogenous bases of the DNA molecules align with each other. Complementary base pairing is also responsible for the double-helix structure of DNA. The double helical structure of DNA consists of A, G, T, C, and the proportion of each of these bases in DNA is not random. The number of adenine bases in the DNA would be always equal to the number of cytosine bases. The rule %A=%T and %C=%G is known as the Chargaff’s rule.

Adenine and guanine are purines, while thymine and cytosine are pyrimidines. A complementary strand of DNA is constructed on the basis of nucleobase complementarity. The base pairs A=T and C=G take up all over the strand enabling a twisted DNA double helix formation. When DNA replicates or is transcribed into RNA, the two strands are unpaired, and the polymerization occurs 5′>3′ by copying the opposite strand from 3′>5′.

B.

Summary Introduction

To find: The percentage of nucleotides other than adenine in the DNA of bacterial cells.

Concept introduction: Deoxyribonucleic acid (DNA) is the hereditary material found in humans and many other organisms. It is located in every cell nucleus; hence it is called as nuclear DNA. But a little amount of DNA can be also found in the mitochondria, where it is called mitochondrial DNA. The typical structure of a DNA molecule consists of two strands twisting around each other like a twisted ladder. The rails of the ladder are made up of sugar and phosphate molecules in alternative way. The steps are made of two bases linked together with either two or three weak hydrogen bonds. There is only one way the nitrogenous bases can pair up on the rungs of the DNA ladder. An adenine molecule only pairs with a thymine and cytosine only with guanine.

Explanation of Solution

Every DNA strand is made up of four nucleotides. They are adenine, thymine, guanine, and cytosine. The rules of base pairing explain the phenomenon that whatever the amount of adenine (A) is present in the DNA of an organism, the amount of thymine (T) is the same. Similarly, whatever the amount of guanine (G) is present, the amount of cytosine (C) is the same.

Out of the cent percentage of nucleotide bases in the DNA, it has 13%of adenine nucleotide. As told earlier, the percentage of adenine and thymine will be equal in proportion. So, the bacterial DNA contains 13% thymine. Then remain74%, out of this, half will be guanine and the other half will be cytosine, that is, 37% G and 37% C.

$\text{13\%}\text{adenine+}\text{13\%}\text{thymine}\text{+}\text{37\%}\text{guanine}\text{+}\text{37\%}\text{cytosine=100\%}\text{nucleotide}\text{bases}$

Conclusion

The percentage of nucleotides other than adenine in the DNA of bacterial cells:

Thymine = 13%

Cytosine = 37%

Guanine = 37%

C. (a)

Summary Introduction

To find:  How many nucleotide sequences would be there in a stretch of DNA of N nucleotides long if the strand is single

Answer to Problem 5Q

4^N Nucleotide sequences.

Explanation of Solution

The nucleotide triplet that encodes an amino acid is called as a codon. Since there are 64 combinations of 4 nucleotides taken three at s time and only 20 amino acids. A triplet code could make a genetic code for 64 different combinations $\text{(4×4×4=64)}$, for a strand of N nucleotide length would possess 4^N nucleotide sequences in it.

C. (b)

Summary Introduction

To find:  How many nucleotide sequences would be there in a stretch of DNA of N nucleotides long if the strand is double

Explanation of Solution

To calculate the number of nucleotide sequences in a double-stranded DNA is much more difficult. Many of the 4^N single-stranded sequences will formits complement pair. If N is an odd number, every single-stranded DNA sequence will be complement with another sequence, so that the number of double-stranded sequences will be $\text{0}{\text{.5×4}}^{\text{N}}$. If N is an even number, then there will be $\text{0}{\text{.5×4}}^{\text{N}}\text{+0}{\text{.5×4}}^{\frac{\text{N}}{\text{2}}}$ nucleotide sequences.

Conclusion

If N is an odd number; $\text{0}{\text{.5×4}}^{\text{N}}$ nucleotide sequences would be there in it; and if N is an even number, $\text{0}{\text{.5×4}}^{\text{N}}\text{+0}{\text{.5×4}}^{\frac{\text{N}}{\text{2}}}$ nucleotide sequences.

D.

Summary Introduction

To find: The average length of the nucleotide sequences, such that the sequence is in an order to make just one cut in a bacterial genome of ${\text{3×10}}^{\text{6}}$ nucleotide pairs.

Concept introduction: Deoxyribonucleic acid (DNA) is the hereditary material found in humans and many other organisms. It is located in every cell nucleus; hence it is called as nuclear DNA. But a little amount of DNA can be also found in the mitochondria, where it is called mitochondrial DNA. The typical structure of a DNA molecule consists of two strands twisting around each other like a twisted ladder. The rails of the ladder are made up of sugar and phosphate molecules in alternative way. The steps are made of two bases linked together with either two or three weak hydrogen bonds. There is only one way the nitrogenous bases can pair up on the rungs of the DNA ladder. An adenine molecule only pairs with a thymine and cytosine only with guanine.

Explanation of Solution

For a DNA with N nucleotides long, 4^N has to be larger than the bacterial genome of ${\text{3×10}}^{\text{6}}$ nucleotide base pairs, that is, ${\text{4}}^{\text{N}}{\text{>3×10}}^{\text{6}}$.

By solving N,

$\begin{array}{c}\text{N>ln}\frac{{\text{(3×10}}^{\text{6}}\text{)}}{\text{ln}\text{(4)}}\text{=10}\text{.7}\\ \end{array}$

From this, it is clear that DNA sequence withan average length of 11 nucleotides can be unique in the bacterial genome. A relatively small sequence can mark an exclusive position in the genome and is enough to serve as an identity mark for one specific genome.

Conclusion

Sequence with 11-nucleotide length.