All Free Biology MCQs with Answers

1591. Which one of the following stage preceeds mitosis during cell cycle?

• A. G1 phase
• B. S phase
• C. G2 phase
• D. M phase

Explanation: The G2 phase directly precedes the M phase in the cell cycle and allows the cell to prepare for mitosis after DNA replication has occurred in S phase. G1 occurs before S phase, whereas M phase is mitosis itself. Students may choose S phase because DNA replication is essential before mitosis, but G2 is the immediate preceding stage.

1592. Which one of the following syndrome is the result of meiotic non dysjunction?

• A. Klinefelters syndrome
• B. Turners syndrome
• C. Downs syndrome
• D. All of these

Explanation: Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate during meiosis, producing gametes with abnormal chromosome numbers. It can produce an extra X chromosome in Klinefelter syndrome, a missing X chromosome in Turner syndrome, or an extra chromosome 21 in Down syndrome. Students may choose only Down syndrome because it is the most commonly cited example, but all three can result from meiotic nondisjunction.

1593. Which one of the following is correct sequence of stages of prophase 1 of meiosis

• A. Diakinesis diplotene pachytene leptotene zygotene
• B. Zygotene leptotene pachytene diplotene diakinesis
• C. Leptotene zygotene pachytene diplotene diakinesis
• D. Pachytene leptotene diplotene zygotene diakinesis

Explanation: The five substages of prophase I occur in the order leptotene, zygotene, pachytene, diplotene and diakinesis. Pairing begins in zygotene, crossing over occurs during pachytene, and chromosomes become maximally condensed by diakinesis. Students may choose option b because zygotene is associated with pairing, but leptotene comes first.

1594. In humans the number of tetrads formed during mitosis is___________?

• A. 23
• B. 46
• C. 0
• D. 4

Explanation: A tetrad is formed during prophase I of meiosis when homologous chromosomes pair, with each homolog consisting of two chromatids. Mitosis does not involve pairing of homologous chromosomes, so no tetrads are formed in human mitosis. Students may choose 23 or 46 by confusing chromosome number with the number of homologous pairs involved in tetrad formation.

1595. The basic structural unit of a chromosome is____________?

• A. The centromere
• B. Nuceleosome
• C. Telomere
• D. Histone

Explanation: A nucleosome is the basic structural unit of chromatin, formed when DNA winds around a core of histone proteins, helping package DNA into chromosomes. Students often choose histone because it is part of the nucleosome, but histone alone is not the complete repeating unit.

1596. Which of the following is an autosomal disorder______________?

• A. Down's syndrome
• B. Klinifelter's syndrome
• C. Turner's syndrome
• D. Jacob's syndrome

Explanation: Down syndrome is usually caused by trisomy 21, meaning there are three copies of autosomal chromosome 21. Klinefelter syndrome, Turner syndrome and Jacobs syndrome involve sex chromosomes, so they are sex-chromosome abnormalities. The common trap is Klinefelter syndrome, but its XXY chromosome arrangement is not autosomal.

1597. Un-separated replica of chromosome is called___________?

• A. Chromatin
• B. Kinetochore
• C. Chromatid
• D. Centromere

Explanation: After DNA replication, each chromosome consists of two identical sister chromatids joined at the centromere until they separate during cell division. Students may choose centromere because it joins the copies, but the unseparated replicated units themselves are chromatids.

1598. Mitosis is divided into_____________?

• A. Karyokinesis
• B. Cytokinesis
• C. Interphase
• D. Both a & b

Explanation: Mitosis is commonly considered to include karyokinesis, the division of the nucleus, followed by cytokinesis, the division of the cytoplasm. Interphase is a growth and DNA-replication stage between divisions, not a stage of mitosis itself, so students may incorrectly include option c.

1599. Exact replica of chromosome refers to ___________?

• A. Chromatid
• B. Centromere
• C. Kinetochore
• D. Arms

Explanation: After DNA replication, each identical copy of a chromosome is called a sister chromatid, and the two chromatids remain joined at the centromere. Students may choose the arms because they are visible chromosome regions, but arms are only parts of a chromatid and are not complete replicas.

1600. The science of crop production and soil management.

• A. Soil Science
• B. Agronomy
• C. Metrology
• D. Ecology

Explanation: Agronomy is the applied science of crop production, including the management of soil, crops and their productivity. Soil science focuses specifically on soil properties and formation, so students may choose option a when they overlook the crop-production part. The required topic has no dedicated agronomy slug, so this item is placed under the closest available general science category.