All Free Biology MCQs with Answers

• A. Centromere of the chromosome
• B. Kinetosome of the chromosome
• C. Telomere of the chromosome
• D. Kinetochore of the chromosome

Explanation: Small disc-­shaped structures at the surface of the centromeres are called kinetochores. These structures serve as the sites of attachment of spindle fibres to the chromosomes that are moved into position at the centre of the cell.

• A. (i), (ii), (iii), (iv)
• B. (ii), (iii), (iv), (i)
• C. (ii), (i), (iv), (iii)
• D. (ii), (i), (iii), (iv)

Explanation: Prophase ­I of meiosis has been divided into five sub­stages which occur in the following sequence:Leptotene ➡ Zygotene ➡ Pachytene ➡ Diplotene ➡ Diakinesis. Synapsis i.e., the pairing of homologous chromosomes occurs during zygotene. Crossing over i.e., exchange of chromatid segments occurs during pachytene. Terminalisation of chiasmata i.e., shifting of chiasmata towards the ends of chromosomes and complete disappearance of nucleolus take place during diakinesis.

• A. Twice the number of chromosomes and four times the amount of DNA
• B. Four times the number of chromosomes and twice the amount of DNA
• C. Twice the number of chromosomes and the amount of DNA
• D. Same number of chromosomes but twice the amount of DNA

Explanation: In diploid cells (somatic cells) during G 1 phase, DNA content is 2C and chromosome number is 2n whereas in haploid cells (gamete) during G 1 phase, DNA content is 1C and chromosome number is n. S phase is marked by replication of DNA and the amount of DNA per cell is doubled i.e., it becomes 4C in somatic cells, whereas chromosome number remains same i.e., 2n. Thus, a somatic cell which has just completed S phase, will have 4C DNA content but 2n chromosome number, while the gamete cell has 1C DNA content and n chromosome number.

• A. G0 and G1
• B. G1 and S
• C. Only G2
• D. G2 and M

Explanation: The G2 phase holds the maximum 4C DNA content. The M phase (mitosis) also maintains the 4C DNA level throughout prophase, metaphase, and anaphase because the chromosomes separate but the cell has not yet fully divided (cytokinesis). The content drops back to 2C only after cytokinesis is complete. Thus, Option D is correct. Other options included G1, which has 2C, or G0, which also contains 2C.

• A. Amount of DNA doubles in each cell
• B. Amount of DNA remains same in each cell
• C. Chromosome number is decrease
• D. Amount of DNA is reduced to half in each cell

Explanation: S phase (Synthesis Phase) is the phase of the cell cycle in which DNA is replicated, occurring between G1 phase and G2 phase. To produce two similar daughter cells, the complete DNA instructions in the cell must be duplicated. DNA replication occurs during this S (synthesis) phase. So, the correct answer is (a)'Amount of DNA doubles in each cell'.

• A. Pachytene
• B. Zygotene
• C. Diplotene
• D. Diakinesis

Explanation: (a) : Pachytene is characterized by the appearance of recombination nodules, the sites at which crossing over occurs between non­sister chromatids of the homologous chromosomes. Nodules contain multienzyme complex called recombinase. Recombinase is made of endonuclease, exonuclease, unwindase, R­protein, etc.

• A. Bivalent
• B. Axoneme
• C. Equatorial plate
• D. Kinetochore

Explanation: During the zygotene stage, chromosomes start pairing together and this process of association is called synapsis. Such paired chromosomes are called homologous chromosomes. Electron micrographs of this stage indicate that chromosome synapsis is accompanied by the formation of a complex structure called the synaptonemal complex. The complex formed by a pair of synapsed homologous chromosomes is called a bivalent or a tetrad.

• A. Cytokinesis: Cell plate formed, mitochondria distributed between two daughter cells
• B. Telophase: Endoplasmic reticulum and nucleolus not reformed yet
• C. Telophase: Nuclear envelope reforms, Golgi complex reforms
• D. Late anaphase: Chromosomes move away from equatorial plate, Golgi complex not present

Explanation: The figure given in the question shows a stage of mitotic cell division called the telophase stage. The individual chromosomes are no longer seen and chromatin material tends to collect in a mass at the two poles. This is the stage that shows the following key events: -Chromosomes cluster at opposite spindle poles and their identity is lost as discrete elements. - Nuclear envelope assembles around the chromosome clusters. - Nucleolus, Golgi complex and ER reform.

• A. Zygotene
• B. Pachytene
• C. Diplotene
• D. Leptotene

Explanation: During zygotene or zygonema of meiotic prophase I the chromosomes become shorter and thicker. The homologous chromosomes come to lie side ­by ­side in pairs. This pairing of homologous chromosomes is known as synapsis or syndesis. A pair of homologous chromosomes lying together is called a bivalent.

• A. Kinetochore
• B. The centromere and the kinetochore
• C. Centromere, kinetochore and the areas adjoining the centromere.
• D. Centromere

Explanation: In metaphase, chromosomes consisting of two sister chromatids get arranged at equator. Discontinuous fibres radiate out from two spindle poles and get connected to the disc-shaped structure at the surface of the centromere called kinetochores. These are known as chromosome fibres or tractile fibrils. A kinetochore is a complex protein structure that is analogous to a ring for the microtubule hook; it is the point where microtubules attach themselves to the chromosome.