Which of the following statements is true?
A DNA molecule is made up of two long polynucleotide strands forming a double helical structure (double helix) just like a spiral staircase.
Understanding Genetics and DNA Structure
This question asks us to identify the true statement among the given options related to genetics and the structure of DNA. Let's examine each statement carefully.
Analyzing Each Statement
Let's break down each option and evaluate its accuracy:
-
Statement 1: The term 'Genetics’ was coined by J. D. Watson in 1906.
- Analysis: This statement is incorrect. While James D. Watson is famous for co-discovering the structure of DNA, the term 'Genetics' was coined much earlier by William Bateson in 1905.
-
Statement 2: In human beings, there are 46 chromosomes. Of these, 42 (21 pairs) are autosomes and 4 (2 pairs) are sex chromosomes.
- Analysis: This statement is also incorrect. Human beings do have 46 chromosomes (23 pairs). However, the breakdown is different. There are 44 autosomes (22 pairs) and 2 sex chromosomes (1 pair).
-
Statement 3: Mendel was the first scientist to visualise a gene as an inheritance in 1886.
- Analysis: This statement is incorrect. Gregor Mendel's groundbreaking work on inheritance using pea plants was published in the 1860s, well before 1886. While he is considered the father of genetics and described the basic principles of inheritance (what we now understand to be genes), he did not "visualise" a gene in the modern sense. The concept of a gene evolved over time, and the term 'gene' itself was coined by Wilhelm Johannsen in 1909.
-
Statement 4: A DNA molecule is made up of two long polynucleotide strands forming a double helical structure (double helix) just like a spiral staircase.
- Analysis: This statement is correct. The structure of DNA was famously described by Watson and Crick (with contributions from Rosalind Franklin and Maurice Wilkins) as a double helix. It consists of two polynucleotide strands wound around each other, resembling a spiral staircase, where the sugar-phosphate backbones form the sides and the base pairs form the steps.
Identifying the True Statement
Based on the analysis, the only statement that accurately describes a known biological fact is the fourth statement regarding the structure of a DNA molecule.
| Statement | Accuracy | Explanation |
|---|---|---|
| Term 'Genetics' coined by J.D. Watson in 1906. | False | Coined by William Bateson in 1905. |
| Human chromosomes: 42 autosomes (21 pairs), 4 sex chromosomes (2 pairs). | False | Human chromosomes: 44 autosomes (22 pairs), 2 sex chromosomes (1 pair). |
| Mendel visualised gene as inheritance in 1886. | False | Mendel's work in 1860s. Concept evolved, 'gene' coined later (1909). |
| DNA is a double helix of two polynucleotide strands. | True | Correct description of DNA structure. |
Conclusion
The true statement among the given options is the one describing the structure of a DNA molecule as a double helix made of two polynucleotide strands.
Revision Table: Key Genetic Concepts
| Term/Concept | Description | Key Figures/Events |
|---|---|---|
| Genetics | The study of heredity and variation. | William Bateson (coined the term, 1905) |
| Chromosome | Thread-like structures in the nucleus carrying genetic information in the form of genes. Humans have 46 (23 pairs). | Walther Flemming (observed mitosis, 1882), Walter Sutton & Theodor Boveri (Chromosome Theory of Inheritance, ~1902) |
| Autosomes | Any chromosome that is not a sex chromosome. Humans have 22 pairs (44). | |
| Sex Chromosomes | Chromosomes (X and Y) that determine the sex of an individual. Humans have 1 pair (2). | |
| Gene | A basic unit of heredity; a segment of DNA that carries instructions for building one or more molecules (proteins or RNA). | Gregor Mendel (concept of heritable factors, 1860s), Wilhelm Johannsen (coined the term 'gene', 1909) |
| DNA Double Helix | The structure of deoxyribonucleic acid (DNA), consisting of two spiraling strands of nucleotides. | James Watson, Francis Crick, Rosalind Franklin, Maurice Wilkins (described structure, 1953) |
Additional Information on DNA Structure
The discovery of the double helix structure of DNA by Watson and Crick in 1953 is considered one of the most significant scientific breakthroughs of the 20th century. Their model, based on X-ray diffraction data primarily from Rosalind Franklin and Maurice Wilkins, revealed how genetic information could be stored and replicated.
- Each strand of DNA is a polymer of nucleotides.
- Each nucleotide consists of a deoxyribose sugar, a phosphate group, and one of four nitrogenous bases: Adenine (A), Guanine (G), Cytosine (C), and Thymine (T).
- The two strands are held together by hydrogen bonds between complementary bases: A pairs with T (A-T pair), and G pairs with C (G-C pair). This complementary base pairing is crucial for DNA replication.
- The strands run in opposite directions, meaning they are antiparallel. One strand runs 5' to 3', and the other runs 3' to 5'.
- The double helix structure allows DNA to be a stable molecule capable of carrying the genetic code and being accurately copied during cell division.
Similar Questions
Name the vitamin whose deficiency causes excessive bleeding?
Important Questions from Biomolecules
The chemical name of Vitamin D 3is:
Which of the following food comes under complex carbohydrate?
I. Potatoes
II. Sweets
III. Pasta
IV. Wheat
Following are the pKa's of the ionizable groups in lysine
pKa 1 = 2.16 (α - carboxylic group)
pKa 2 = 9.06 (α - amino group)
pKa 3 = 10.54 (ε - amino group)
Which one of the following options represents the pl of lysine?
How many hydrogen bonds involving the backbone CO and NH can be observed in an α‐helix consisting of 15 amino acid residues?
In eukaryotic cells, covalently attached lipids help to anchor some water soluble proteins to the plasma membrane. One group of cytosolic proteins are anchored to the cytosolic face of membrane by a fatty acyl group (e.g. myristate or palmitate). These groups are generally covalently attached to which amino acids present at the N‐terminus of the polypeptide chain?
