CBSE Class 12 Biology Most Repeated Questions for Exam

Preparing for CBSE Class 12 Biology can be less stressful when you focus on the most repeated questions, as these highlight key concepts frequently tested in the exam. By understanding these essential topics, you can enhance your chances of scoring high marks. Whether it's human physiology, genetics, evolution, or ecology, knowing what areas are likely to be covered in the exam will help you study more effectively. In this blog, we will walk you through some of the most repeated questions in CBSE Class 12 Biology, provide tips on how to tackle them, and break down the exam pattern to ensure you are fully prepared. Let's dive in!

Key Topics in CBSE Class 12 Biology

The CBSE Class 12 Biology syllabus is vast, but certain topics are repeatedly tested in exams. Here are the key areas you should focus on:

1. Human Physiology

Human physiology is one of the most important and frequently tested topics. Questions often focus on the circulatory system (e.g., heart structure, blood circulation, ECG) and the excretory system (e.g., nephron structure, kidney function, urine formation).

2. Genetics and Evolution

Genetics is a high-yield topic, particularly Mendel's laws of inheritance, DNA replication, and genetic disorders. Evolutionary biology also features heavily, with questions related to Darwin’s theory of evolution and the Hardy-Weinberg principle.

3. Ecology

Ecology and environmental science often have several questions in the exam. Topics like biodiversity, conservation, ecosystem dynamics, and the carbon cycle are key areas of focus. You can also expect questions on environmental pollution and biogeochemical cycles.

4. Biotechnology

This topic includes questions on genetic engineering, cloning, and biotechnology tools like PCR and gel electrophoresis. While not as frequent as physiology or genetics, it still appears in exams.

5. Reproduction

In this section, you should focus on human reproduction, reproductive health, and the structure of the male and female reproductive systems. This is a crucial topic for your exam preparation.

6. Plant Physiology

Understanding photosynthesis, respiration in plants, and transpiration is essential. Also, the movement of water and minerals in plants is often tested in various formats.

Studying Diagrams and Labeling

A significant part of the CBSE Class 12 Biology exam involves understanding and labeling diagrams. Here are a few important diagrams that are frequently asked in the exam:

• Human Heart: You should be able to label and explain its structure, functions, and blood circulation process.
• Nephron: Understand the parts of the nephron and its role in urine formation.
• DNA Structure: Be able to label the parts of a DNA molecule and explain DNA replication.

These diagrams often come with questions like "Label the parts" or "Explain the process", so practicing them thoroughly will boost your marks.

CBSE Class 12 Biology Exam Pattern 2024-25

The CBSE Class 12 Biology Exam follows a structured pattern designed to test students on various topics in-depth. Here is a breakdown of the exam pattern:

Section A:

• 16 questions (1 mark each). These questions are typically objective or very short-answer questions.

Section B:

• 5 questions (2 marks each). These require concise but explanatory answers.

Section C:

• 7 questions (3 marks each). These demand detailed explanations or illustrations.

Section D:

• 2 case-based questions (4 marks each). These test your analytical skills and the application of knowledge.

Section E:

• 3 long-answer questions (5 marks each). These require in-depth understanding and elaboration, often integrating multiple concepts.

CBSE Class 12 Biology Most Repeated Questions for Exam

Here are some of the most repeated questions that often appear in the CBSE Class 12 Biology exam, along with sample answers. Practicing these will ensure you are well-prepared for your exam:

CBSE Class 12 Biology Most Repeated 1 Mark Questions

1. Which one of the following processes results in the production of recombinants in future generations?
   1. (i) Mutation
   2. (ii) Independent assortment during meiosis I
   3. (iii) Independent assortment during meiosis II
   4. (iv) Crossing over of bivalents
2. Study the DNA profiles obtained as a result of DNA fingerprinting of a child ‘X’ and three individuals 1, 2, and 3. Which one of the following options shows the possible parents of the child ‘X’?
3. The organism used in the construction of the first artificial recombinant DNA by Cohen and Boyer in 1972 was:
   1. (a) E. coli
   2. (b) Salmonella typhimurium
   3. (c) Agrobacterium tumefaciens
   4. (d) Bacillus thuringiensis
4. Assertion (A): A patient of ADA deficiency undergoing treatment for gene therapy requires periodic infusion of genetically engineered lymphocytes. Reason (R): Lymphocytes are immortal.

CBSE Class 12 Biology Most Repeated 2 Mark Questions

1. A human being suffering from Down’s syndrome shows trisomy of the 21st chromosome. Mention the cause of this chromosomal abnormality.
2. In a dihybrid cross, when would the proportion of the parental gene combinations be much higher than non-parental types, as experimentally shown by Morgan and his group?
3. A tall Pea plant with yellow seeds (heterozygous for both) is crossed with a dwarf Pea plant with green seeds. Using a Punnett square, work out the cross to show the phenotypes and genotypes of F1 generation.
4. The length of a DNA molecule in a typical mammalian cell is approximately 2.2 meters. How is this long molecule packaged to accommodate it within the nucleus of the cell?
5. Name any two vertebrate body parts that are homologous to human forelimbs.
6. Name the type of human cell HIV attacks at its entry into the body. Explain the events that occur in the cell which further lead to immunodeficiency syndrome.

CBSE Class 12 Biology Most Repeated 3 Mark Questions

1. Assisted Reproductive Technologies (a) (i) Expand the abbreviations:
   1. ZIFT: Zygote Intrafallopian Transfer
   2. ICSI: Intracytoplasmic Sperm Injection
   3. IUT: Intrauterine Transfer
   4. GIFT: Gamete Intrafallopian Transfer
   (ii) Which one of them cannot be considered a procedure of IVF?
   1. GIFT cannot be considered a part of IVF because, in GIFT, gametes (sperm and ovum) are directly transferred to the fallopian tube for fertilization, unlike IVF where fertilization occurs outside the body.
2. Differences (a) Differentiate between: (i) Perisperm and Pericarp
   1. Perisperm: Remnant of nucellus in seeds (e.g., black pepper).
   2. Pericarp: Fruit wall derived from the ovary wall after fertilization.
   (ii) Syncarpous pistil and Apocarpous pistil
   1. Syncarpous: Carpels are fused (e.g., mustard).
   2. Apocarpous: Carpels are free (e.g., lotus).
   (iii) Plumule and Radicle
   1. Plumule: Develops into the shoot.
   2. Radicle: Develops into the root.
3. IUDs as Contraceptives (a) Expand IUD: Intrauterine Device (b) Hormone-releasing IUDs are considered good contraceptives as they:
   1. Release hormones like progestin to prevent ovulation.
   2. Thicken cervical mucus, hindering sperm entry.
   3. Reduce endometrial receptivity to implantation.
4. Natural Selection and Fitness (a) Natural Selection operates when nature selects for fitness. Explain. (b) The rate of appearance of new forms is linked to the lifespan of an organism. Explain with the help of a suitable example.

CBSE Class 12 Biology Most Repeated 5 Mark Questions

1. (i) Which two types of nitrogen were used by them in their experiment and why?
   • Meselson and Stahl used two types of nitrogen: Nitrogen-15 (¹⁵N) and Nitrogen-14 (¹⁴N). These isotopes were used to distinguish between the old (parental) and new (daughter) strands of DNA during replication.
   • ¹⁵N (Heavy nitrogen): This is a stable isotope of nitrogen, which has an extra neutron. It was used to label the DNA molecules in the original generation of E. coli cells.
   • ¹⁴N (Light nitrogen): This is the more common isotope of nitrogen, used to label the newly synthesized DNA strands after the DNA was transferred to a medium containing ¹⁴N.
   • The purpose of using these two isotopes was to track the incorporation of nitrogen into the DNA and observe how it was distributed in the newly replicated DNA strands.
2. (ii) Why did they take samples of E. coli at definite time intervals for their observation?
   • Meselson and Stahl took samples of E. coli at definite time intervals to track the progression of DNA replication. By collecting samples at various points in time, they were able to observe how the nitrogen isotopes were distributed between the parental and newly synthesized strands.
   • Immediate sampling: At the beginning of the experiment, when the bacteria were grown in a medium containing ¹⁵N, the DNA became fully labeled with heavy nitrogen.
   • After one round of replication: The DNA would contain one strand with heavy nitrogen (¹⁵N) and one strand with light nitrogen (¹⁴N).
   • Further intervals: With more replication cycles, they could determine how the proportion of heavy and light strands changed.
3. (iii) State the role of caesium chloride density gradient in their experiment.
   • The caesium chloride (CsCl) density gradient was used to separate DNA strands based on their density.
   • After extracting the DNA, it was placed in a solution of caesium chloride, and when the solution was centrifuged, the DNA molecules formed a gradient where heavier DNA (labeled with ¹⁵N) settled at the bottom, and lighter DNA (labeled with ¹⁴N) remained at the top.
   • This allowed Meselson and Stahl to distinguish between DNA strands that contained different isotopes of nitrogen and observe how the DNA was distributed after each replication cycle.
4. (iv) Write the conclusions they arrived at.
   • Meselson and Stahl concluded that DNA replication is semi-conservative.
   • This means that during DNA replication, each of the two strands of the original DNA molecule serves as a template for the synthesis of a new strand, resulting in two DNA molecules, each containing one original (parental) strand and one newly synthesized (daughter) strand.
   • Their experiment provided clear evidence that DNA replication does not result in completely new molecules or use the old DNA to replicate it entirely but instead conserves one strand from the parent molecule.

Tips for Effective Preparation

1. Practice Previous Years' Papers: Reviewing past exam papers will give you a better understanding of the question format and the type of questions asked.
2. Focus on NCERT Textbooks: Make sure to thoroughly study your NCERT textbook as it forms the basis of the CBSE exam.
3. Create a Study Schedule: Allocate time to focus on each section of the syllabus and revise consistently.
4. Take Mock Tests: Taking mock tests will help you gauge your preparation level and improve time management.

By focusing on the most repeated questions and following these tips, you can enhance your preparation and boost your performance in the CBSE Class 12 Biology exam.