Cell Biology

Step 1: Recall stages of cell cycle.
Cell cycle consists of interphase (cell growth and DNA replication) followed by mitosis (nuclear division) and cytokinesis.
Step 2: Stages of mitosis in proper order.
1. Interphase (D): Technically not part of mitosis itself, but a preparatory phase. DNA is replicated and cell gets ready for division.
2. Prophase (C): Chromosomes condense, spindle fibers start forming, and nuclear envelope breaks down.
3. Metaphase (A): Chromosomes align at the equatorial plate.
4. Anaphase (B): Sister chromatids separate and move toward opposite poles.
5. Telophase (E): Nuclear membranes reform, chromosomes de-condense, and cytokinesis begins.
Step 3: Verify the given options.
- (A) A, B, C, D, E: Wrong order, starts with metaphase instead of interphase.
- (B) E, B, C, A, D: Wrong, telophase cannot occur before prophase.
- (C) D, C, B, A, E: Wrong, metaphase must precede anaphase.
- (D) D, C, A, B, E: Correct, matches the biological sequence.
Step 4: Conclusion.
Hence, the correct sequence is Interphase → Prophase → Metaphase → Anaphase → Telophase.

Step 1: Recall stages of Prophase I in meiosis.
Prophase I is divided into 5 sub-stages: Leptotene → Zygotene → Pachytene → Diplotene → Diakinesis.
Step 2: Describe each stage.
- Leptotene: Chromosomes start condensing.
- Zygotene: Homologous chromosomes pair (synapsis).
- Pachytene: Crossing over of genetic material occurs.
- Diplotene: Homologous chromosomes begin to separate but remain attached at chiasmata.
- Diakinesis: Chromosomes fully condensed, nuclear envelope breaks down.
Step 3: Match with given codes.
Correct sequence is C (Leptotene) → E (Zygotene) → A (Pachytene) → B (Diplotene) → D (Diakinesis).
Step 4: Verify options.
Option (A) matches this correct order.
Step 5: Conclusion.
Thus, the correct sequence of stages in Meiosis I is Leptotene → Zygotene → Pachytene → Diplotene → Diakinesis.

Step 1: Recall cause of sickle cell anaemia.
Sickle cell anaemia is caused by a point mutation in the -globin gene of hemoglobin.
This mutation substitutes valine for glutamic acid at position 6 of the -chain.
Step 2: Analyze statements.
- (A) True: It is monogenetic due to mutation in -globin.
- (B) True: Valine replaces glutamic acid.
- (C) False: Describes Down’s syndrome (trisomy 21).
- (D) False: Describes Wilson’s disease (copper accumulation).
- (E) True: Mutation changes codon from GAG (glutamic acid) to GUG (valine).
Step 3: Correct group.
A, B, E are correct.
Step 4: Conclusion.
Correct answer = (B) A, B and E only.

Concept: The total erythrocyte count (RBC count) is commonly performed using a hemocytometer. To make counting possible, blood must be diluted with an RBC diluting fluid such as Hayem's fluid or Dacie's fluid. This dilution helps prevent cell clumping and ensures proper distribution of RBCs in the counting chamber.
Step 1: Identify the dilution used for RBC counting.
In manual RBC counting:

• Blood is diluted with RBC diluting fluid.
• The standard dilution used is 1:200.

Thus, the dilution factor used for total erythrocyte count is 1:200.

Concept: Pasteurization is a heat treatment process used to destroy pathogenic microorganisms in milk and other liquids. The Holder method of pasteurization involves heating milk to 63°C for 30 minutes. Among milk-borne pathogens, Coxiella burnetii, the causative agent of Q fever, is one of the most heat-resistant non-spore-forming bacteria. Pasteurization standards were designed specifically to ensure the destruction of this organism.
Step 1: Identify the bacterium associated with resistance to pasteurization.

• Mycobacterium tuberculosis – Destroyed by pasteurization.
• Coxiella burnetii – Highly heat-resistant and used as the standard for pasteurization effectiveness.
• Salmonella typhi – Destroyed by pasteurization.
• Brucella abortus – Destroyed by pasteurization.

Thus, the bacterium capable of surviving inadequate Holder pasteurization conditions is Coxiella burnetii.