A1.2 Nucleic acids

The percentage of thymine in the DNA of an organism is approximately`30 %`. What is the percentage of guanine? 

A. `70%`.

B. `30%`.

C.`40%`.

D. `20%`.

Answer: D

The structure of DNA is a double helix in which the bases pair complementarily (complementary base pairing). Chargaff’s rule states that in DNA:

1. Adenine (A) always pairs with Thymine (T).
2. Guanine (G) always pairs with Cytosine (C).
3. Therefore, the percentage of A must equal the percentage of T (A = T), and the percentage of G must equal the percentage of C (G = C).
4. The total percentage of all four bases must equal 100% (A+T+G+C=`100%`)

A. Incorrect. If G = `70%`, then the total G + C would be 140% (since G = C), which exceeds the total of `100%`.

B. Incorrect. `30%` is the percentage of T (or A). The percentage of G must be`20%`. (Although Levene once proposed the incorrect hypothesis that each base made up `25%`, Chargaff’s data disproved this)

C. Incorrect. `40%` is the total percentage of G + C, not the percentage of G alone.

D. Correct. The percentage of G is calculated as
G=`100%` - `frac{A + T}{2}`=`20%`

What does a nucleosome consist of? 

A. DNA and histones.

B. DNA and chromatin.

C. Chromatin and nucleotides.

D. Mature RNA and histones.

Answer: A

A. Correct. A nucleosome is the basic structural unit of a chromosome in eukaryotic cells. A nucleosome consists of DNA (deoxyribonucleic acid) wrapped around a protein core. This protein core is made up of histones. Specifically, the nucleosome core usually consists of eight histone molecules (two copies each of four different histones) forming a bead-like structure. The double-stranded DNA is wrapped around this protein core about two turns.

B. Incorrect. Although a nucleosome contains DNA, it does not contain chromatin. On the contrary, chromatin is the nuclear material composed of DNA and histone proteins. The nucleosome is actually the repeating subunit of chromatin (the “beads-on-a-string” structure that makes up chromatin). 

C. Incorrect.

• Nucleotides are the monomers that make up DNA. A nucleosome consists of DNA (a polymer of nucleotides) and histone proteins—not chromatin and nucleotides.
• Chromatin is a larger structure made up of DNA and histone proteins, whereas the nucleosome is a structural subunit of chromatin. Defining the nucleosome using the larger structure it helps form (chromatin) is biologically inaccurate. 

D. Incorrect. A nucleosome is a structure composed of DNA and histone proteins. Mature RNA (mRNA) is an RNA molecule that has undergone post-transcriptional processing (intron removal—RNA splicing) and is transported to the ribosome; it is not a structural component of the nucleosome.

Hershey and Chase used a bacteriophage (a virus that infects bacteria) to investigate the chemical nature of genes. The diagram shows a bacteriophage 

[Source: Graham Knott and Christel Genoud, ‘Commentary: is EM dead?’, Journal of Cell Science (2013), 126: 4545–4552, reproduced with permission. http://jcs.biologists.org/content/126/20/4545.figures-only doi: 10.1242/jcs.124123 http://www.biologists.com/journal-of-cell-science]

The sulphur in the protein and the phosphorus in the DNA of the bacteriophage were radioactively labelled. The data obtained after bacterial infection and centrifugation are shown in the table.

What did Hershey and Chase conclude from their experiment? 

A. DNA was mainly outside the bacterial cells 

B. Viruses infect bacterial cells with proteins.

C. Viral DNA was found within the bacterial cells..

D. Neither protein nor DNA were chemicals making up genes in viruses.

Answer: C

Experimental Mechanism (Hershey–Chase Experiment):

- Used the bacteriophage T2, a type of virus consisting of a protein coat surrounding a nucleic acid core (DNA).

 - Took advantage of the chemical differences between DNA and protein: DNA contains phosphorus (P) but no sulfur (S), whereas protein contains sulfur (S) but no phosphorus (P).

- Prepared two batches of viruses labeled with radioactive isotopes:

• Batch 1: The viral protein was labeled with radioactive sulfur (`³⁵S`).
• Batch 2: The viral DNA was labeled with radioactive phosphorus (`³²P`).

- After the viruses infected the bacteria, the mixture was centrifuged to separate the bacteria (pellet – settled at the bottom) from the viral particles (supernatant – remaining on top).

A. Incorrect. The experiment showed that DNA (labeled with `³²P`) entered the bacterial cells (found in the pellet).

B. Incorrect. The protein (labbled with `³⁵S`) remained mostly outside the cells (in the supernatant). Therefore, the virus infects by injecting DNA, not protein.

C. Correct. Since `³²P` (the DNA label) was found inside the cells (in the pellet), they concluded that DNA was the material injected and carried the genetic information..

D. Incorrect. The results disproved the idea that protein is the genetic material but provided strong evidence that DNA is the genetic material. 

What provides evidence for the universal nature of the genetic code? 

A. Uracil replaces thymine in RNA.

B. The amount of A is equal to the amount of T and the amount of G is equal to the amount of C.

C. Nucleic acids contain the same bases in all species.

D. mRNA codons are assigned to the same amino acids in different species.

Answer: D

A. Incorrect. This is a structural difference between DNA and RNA. It does not provide evidence for the universality of the genetic code.

B. Incorrect. This is Chargaff’s rule, which serves as evidence for the double-helix structure of DNA (A pairs with T, G pairs with C), but it does not directly prove the universality of the genetic code.

C. Incorrect. This is not entirely true. DNA contains A, T, C, and G, while RNA contains A, U, C, and G. Moreover, the fact that different species share these bases is not sufficient to prove the universality of the genetic code. Universality is defined by how codons are interpreted.

D. Correct. The universality of the genetic code means that mRNA codons specify the same amino acids in nearly all organisms. This allows gene transfer between species and provides strong evidence that all life on Earth shares a common ancestor. 

What links the pairs of complementary bases in a DNA double helix? 

A. Covalent bonds

B. Hydrogen bonds

C. Ionic bonds

D. Peptide bonds

Answer: B

A. Incorrect. Covalent bonds are strong bonds that form the sugar-phosphate backbone along each DNA strand. They do not connect the complementary bases.

B. Correct: Complementary base pairs (Adenine with Thymine, Cytosine with Guanine) are held together by hydrogen bonds. A and T form two hydrogen bonds, while C and G form three hydrogen bonds. 

C. Incorrect. Ionic bonds are not the primary type of bond that holds the two DNA strands together.

D. Incorrect. Peptide bonds are covalent bonds that link amino acids together to form a polypeptide.

What are components of DNA and RNA? 

Answer: D

A. Incorrect. DNA contains deoxyribose, while RNA contains ribose. Uracil is found only in RNA.

B. Incorrect. DNA contains deoxyribose, and RNA contains ribose.

C. Incorrect. Uracil is found only in RNA and is not a common component of both.

D. Correct. DNA contains deoxyribose, RNA contains ribose, and adenine is a base common to both types of nucleic acids.

Which molecules form the nucleotide marked in the diagram?

A. phosphate, deoxyribose and nitrogenous base

B. phosphorus, ribose and nitrogenous base

C. phosphorus, deoxyribose and guanosine

D. phosphate, ribose and guanine

Answer: A

A. Correct. The components of a DNA nucleotide are a phosphate group, the pentose sugar deoxyribose, and a nitrogenous base (A, T, C, or G). The base attaches to the C1′ carbon of the sugar, and the phosphate group attaches to the C5′ carbon.

B. Incorrect. Although a nucleotide contains phosphorus (within the phosphate group), a nitrogenous base, and a sugar, DNA nucleotides do not contain ribose. Moreover, the correct component is the phosphate group (which includes P and O), not elemental phosphorus.

C. Incorrect. Guanosine is a nucleoside (a base, guanine, plus a sugar), not a nitrogenous base itself.

D. Incorrect. The sugar ribose is a component of RNA, not DNA.

What is a nucleosome? 

A. DNA wrapped around histone proteins in a eukaryotic cell

B. RNA bound to ribosomes in a eukaryotic cell

C. A region of the nucleus in a eukaryotic cell

D. Genetic material within a viral capsule

Answer: A

A. Correct. A nucleosome is the basic and repeating structural unit of chromatin in eukaryotic cells. A nucleosome consists of a segment of double-stranded DNA wrapped about two turns around a protein core, which is made up of eight histone molecules. This structure helps supercoil and compact the DNA.

B. Incorrect. Although ribosomes contain RNA (rRNA), a nucleosome is a complex of DNA and proteins, not RNA.

C. A region of the nucleus in a eukaryotic cell: Incorrect. The nucleolus is the region of the nucleus where ribosomes are synthesized. The nucleosome, on the other hand, is a physical structure, not a region within the nucleus.

D. Genetic material within a viral capsule: Incorrect. The genetic material (DNA or RNA) inside a viral shell is called the capsid or nucleocapsid, and it is not associated with histones to form nucleosomes.

State two differences between RNA and DNA nucleotides. [2]

Any two of the following:

a. RNA nucleotides contain ribose and DNA nucleotides contain deoxyribose;

b. (some) RNA nucleotides contain uracil and (some) DNA nucleotides contain thymine;

Sample answer:

RNA nucleotides contain ribose [1] and DNA nucleotides contain deoxyribose [1].

Most of the DNA of a human cell is contained in the nucleus. Distinguish between unique and highly repetitive sequences in nuclear DNA. [5]

Award [1] for each pair of statements in the table and [1] for any statement below the table.

satellite DNA is repetitive;

repetitive sequences are used for profiling;

prokaryotes do not (usually) contain repetitive sequences; 

Sample answer: 

Most of the DNA in human cells is contained within the nucleus, and it is categorized into two main types of sequences: unique sequences and highly repetitive sequences. Unique sequences usually occur only once in the genome [1] and include most of the protein-coding genes, which make up a small proportion (about 1.5%) of our total DNA [1]. In contrast, highly repetitive sequences occur many times [1] and are typically short [1], non–protein-coding sequences. These repetitive sequences (such as variable number tandem repeats – VNTRs) make up a large portion [1] of non-coding DNA and can vary greatly [1] between individuals, which is why they are used in DNA profiling techniques.