Q.What would be the molar concentration of human DNA in a human cell? Consult your teacher.

The question asks for the total number of amino acids in human insulin. This is a classic fact from biochemistry, but understanding why the number is what it is makes it stick.

Insulin is a peptide hormone. It is not a single, straight chain of amino acids. Instead, it is made of two separate chains — an A chain and a B chain — that are held together by disulfide bridges. The A chain is shorter, the B chain is longer. Their lengths are fixed and well-known.

1. The A chain of human insulin contains 21 amino acids. This chain has an internal disulfide bond as well.

2. The B chain of human insulin contains 30 amino acids. This chain connects to the A chain via two disulfide bonds. …

In biology, the number of chromosomes in an organism's cells is a fundamental characteristic. We distinguish between two main types of chromosome numbers: haploid and diploid.

• Haploid number ($n$): This refers to the number of unique chromosomes in a single set. Gametes (sperm and egg cells) are haploid, containing one set of chromosomes.
• Diploid number ($2n$): This refers to the total number of chromosomes in a somatic (body) cell. Diploid cells contain two sets of chromosomes, one inherited from each parent. Therefore, the diploid number is always twice the haploid number ($2n = 2 \times n$).

To solve this problem, we need to know the chromosome numbers for both Ophioglossum and Zea mays.

1. Determine the diploid number of Ophioglossum:

   • Ophioglossum reticulatum, commonly known as the adder's-tongue fern, is remarkable for having the highest known chromosome number among all living organisms. Its diploid chromosome number ($2n$) is $1260$.

2. Determine the haploid number of Zea mays:

   • Zea mays, or corn/maize, has a diploid chromosome number ($2n$) of $20$. …

The structure of DNA is fundamental to solving this problem. DNA exists as a double helix, where two polynucleotide strands are coiled around each other. The key principles we need are:

• Base Pairing: Adenine (A) always pairs with Thymine (T) via two hydrogen bonds ($A=T$), and Guanine (G) always pairs with Cytosine (C) via three hydrogen bonds ($G \equiv C$). This is known as Chargaff's rules, which state that in a double-stranded DNA molecule, the amount of A equals the amount of T, and the amount of G equals the amount of C.
• Length per Base Pair: The distance between two consecutive base pairs in a typical B-DNA double helix is $3.4 \text{ Å}$ (Angstroms).
• Nucleotides and Base Pairs: Each base pair consists of two nucleotides (one on each strand). Therefore, the total number of nucleotides in a double-stranded DNA molecule is twice the number of base pairs.

Let's break down the calculation step-by-step.

1. Calculate the total number of base pairs (bp): The total length of the DNA molecule is given as $510 \text{ Å}$. We know that each base pair occupies a length of $3.4 \text{ Å}$ along the helix.

2. Calculate the total number of nucleotides: Since each base pair consists of two nucleotides (one on each strand), the total number of nucleotides is twice the number of base pairs.

3. Determine the number of each type of base: We are given that 6-aminopurine (which is Adenine, A) constitutes $20\%$ of the DNA. This percentage refers to the proportion of Adenine among all bases.
   • Number of Adenine (A) nucleotides:

*   **Number of Thymine (T) nucleotides:**
    According to Chargaff's rules, the number of A equals the number of T.

*   **Number of Guanine (G) and Cytosine (C) nucleotides:**
    The total number of A and T nucleotides is $60 + 60 = 120$.
    The remaining nucleotides must be G and C. …

The key idea here is that DNA’s double helix is held together by hydrogen bonds between complementary nitrogenous bases. Adenine (A) pairs with thymine (T) via 2 hydrogen bonds, while guanine (G) pairs with cytosine (C) via 3 hydrogen bonds. The total number of hydrogen bonds in a given helix therefore depends entirely on the ratio of A–T to G–C pairs. The question asks for the minimum and maximum possible number of hydrogen bonds in one helix — but what does “one helix” mean? In standard molecular biology, a single turn of B‑DNA contains about 10 base pairs. So we assume the question refers to one complete turn (10 base pairs) of a DNA double helix.

Let’s work through it step by step.

1. Understand the base‑pair bonding rules

   • A–T pair: 2 hydrogen bonds
   • G–C pair: 3 hydrogen bonds This is a fixed biochemical fact. No other pairing occurs in natural DNA.

2. Determine the range of possible hydrogen bonds for 10 base pairs

   • Minimum: Use only A–T pairs. Each contributes 2 bonds. $10 \times 2 = 20$ hydrogen bonds.
   • Maximum: Use only G–C pairs. Each contributes 3 bonds. $10 \times 3 = 30$ hydrogen bonds.

3. Interpret “one helix” correctly

   A common pitfall is to think “one helix” means a single strand. But a single strand has no hydrogen bonds — they form only between the two strands. “One helix” here means one double‑helical turn (10 base pairs). The problem is standard in biology textbooks: the minimum and maximum H‑bonds per turn are 20 and 30.

4. Match to the options …

The question asks about the density threshold that defines "toxic heavy metals" in the context of water pollution. This is a standard environmental chemistry concept, not a calculation problem. The key is knowing the accepted definition used in pollution studies and regulatory frameworks.

Heavy metals are a group of metallic elements that have a relatively high density compared to common metals like iron or aluminum. In environmental science, the threshold is set at a density greater than $5~\mathrm{g/cm^3}$. This includes well-known toxic pollutants like lead ($11.3~\mathrm{g/cm^3}$), cadmium ($8.6~\mathrm{g/cm^3}$), mercury ($13.5~\mathrm{g/cm^3}$), and chromium ($7.2~\mathrm{g/cm^3}$). The density cutoff helps distinguish these from lighter, less toxic metals.

1. The definition is not arbitrary — it comes from the fact that metals with density above $5~\mathrm{g/cm^3}$ tend to be more toxic and persist in the environment, bioaccumulating in food chains. For example, lead and mercury are notorious for causing neurological damage even at low concentrations.

2. Let's check the options: $12.5~\mathrm{g/cm^3}$ is too high — it would exclude cadmium and chromium, which are definitely heavy metals. $15~\mathrm{g/cm^3}$ is even more restrictive and incorrect. $7.5~\mathrm{g/cm^3}$ would exclude cadmium ($8.6~\mathrm{g/cm^3}$) but include it, so it's not the standard threshold. …

The confusion here hinges on distinguishing between chromosome number and DNA content—two related but distinct concepts that change differently through the cell cycle.

A chromosome is defined by its centromere. During S phase (synthesis phase), DNA replication occurs: each chromosome's single DNA molecule is copied, producing two identical sister chromatids joined at a common centromere. Crucially, these two sister chromatids still count as one chromosome because they share one centromere. The chromosome number therefore remains $2n$ from $G_1$ through $S$ and into $G_2$. Only at anaphase, when sister chromatids separate and each gains its own independent existence, does the count change.

DNA content, however, doubles. If a $G_1$ cell has $2C$ amount of DNA (where $C$ is the haploid DNA content), after S phase it has $4C$—twice as much DNA packaged into the same $2n$ chromosomes, now each consisting of two chromatids instead of one.

Let me evaluate each statement:

1. Assertion (A): "If the chromosome number is $2n$ during $G_1$, after S phase the chromosome number is the same"

   This is true. The cell enters $G_1$ with $2n$ chromosomes (each a single chromatid). After replication in S phase, it still has $2n$ chromosomes (now each composed of two sister chromatids). The defining feature—the centromere—has not duplicated; sister chromatids are held together at one centromere.

2. Reason (R): "After S phase there is no increase in DNA per cell" …

Concept: The dental formula lists, per half of upper/lower jaw, the number of incisors (I), canines (C), premolars (PM) and molars (M). Humans are thecodont, diphyodont and heterodont.

Permanent dentition per jaw-half:

• Incisors = 2
• Canines = 1
• Premolars = 2
• Molars = 3 (including the third molar / wisdom tooth, which erupts 17–25 years) …

The concept here is per capita death rate — a fundamental measure in population ecology. It tells us the probability that an average individual will die in a given time interval. The key is to divide the total number of deaths by the initial population size, then by the time interval (if it's not one unit). This normalises the rate so we can compare across populations of different sizes.

A common mistake is to just divide deaths by time, ignoring the population size — that gives a raw death count, not a rate per individual. Another is to forget that the time interval is already one week, so no further division by time is needed.

Let’s work through it.

1. Identify the given data.

   • Initial population: $N = 40$ fruit flies
   • Deaths during the week: $D = 4$ individuals
   • Time interval: $t = 1$ week

2. Recall the definition of per capita death rate.

   It is the number of deaths per individual per unit time. Mathematically:

3. Plug in the numbers. $\text{Death rate} = \frac{4}{40 \times 1} = \frac{4}{40} = 0.1$ …

Concept & Intuition

DNA’s sugar-phosphate backbone gives it a strong negative charge (each phosphate group carries a negative oxygen). In the cell nucleus, DNA must be tightly packed to fit, so it associates with proteins that have a positive charge—histones, rich in basic amino acids like lysine and arginine. This DNA–protein complex is called chromatin. The question tests this fundamental pairing: negative DNA + positive proteins = chromatin.

Step-by-step reasoning

1. DNA charge – The phosphate groups in DNA’s backbone are deprotonated at physiological pH, giving DNA a net negative charge. So the first blank is “negatively” (option B in the answer choices).

2. Associated proteins – Histones are the primary proteins that bind DNA. Their high content of basic (positively charged) amino acids makes them positively charged, allowing electrostatic attraction to DNA. So the second blank is “positively” (option A).

3. Region name – The complex of DNA and histone proteins is called chromatin (option E). This is the material that makes up chromosomes and is found in the nucleus. …

The question tests a specific historical fact: who first discovered and named the substance we now call DNA. This is a common early topic in molecular biology, and the answer hinges on knowing the timeline of DNA research.

Friedrich Miescher, a Swiss physician and biologist, was the first to isolate nucleic acids. In 1869, working with pus cells from discarded surgical bandages, he extracted a substance from the cell nuclei that was unlike proteins or fats — it was rich in phosphorus and acidic. He called it nuclein (from the Latin nucleus). This happened nearly a century before Watson and Crick's model of the double helix.

The other scientists listed made later, distinct contributions:

• James Watson and Francis Crick (1953) proposed the double-helix structure of DNA.
• Maurice Wilkins and Rosalind Franklin used X-ray crystallography to produce images of DNA (Franklin's Photo 51 was critical for Watson and Crick's model).

None of them named it 'Nuclein' — that credit belongs solely to Miescher.