Meiotic division of the secondary oocyte is
completed:
Explanation:
The secondary oocyte is arrested in metaphase II and completes meiosis II only after fertilization. When a sperm fuses with the ovum, the oocyte finishes meiosis II, forming the mature ovum and a second polar body. Hence, the meiotic division is completed at the time of fusion of a sperm with an ovum (option 4).
🧠 Did You Know?
Fertilization triggers the last meiotic division, ensuring the haploid number is restored in the zygote.
💡 NCERT Memory Line:
The secondary oocyte completes meiosis II only upon fertilization, forming the mature ovum.
Identify the wrong statement with reference to
the gene 'I' that controls ABO blood groups.
Explanation:
The wrong statement is option 3. In the ABO system, IA and IB are codominant; when both are present (IAIB), the person has AB blood type with both A and B antigens. The statement says they express the same sugar, which is incorrect because A and B antigens are different sugars on RBC surfaces.
🧠 Did You Know?
Allele i is recessive and does not code for A or B antigens; individuals with ii blood type have no A or B antigens (O blood group).
💡 NCERT Memory Line:
The gene I has three alleles (IA, IB, i); a person carries two alleles, and IA and IB show codominance to yield AB type when present together.
Option 3 is correct. Sickle cell anaemia is an autosomal recessive disorder caused by a mutation in the beta-globin gene on chromosome 11. Children must inherit two faulty copies to express the disease. Options 1, 2 and 4 are incorrect according to NCERT: haemophilia is X-linked recessive (not Y-linked); phenylketonuria is autosomal recessive (not dominant); thalassemia is also autosomal recessive (not X-linked).
🧠 Did You Know?
Carriers (heterozygotes) for sickle cell trait usually do not show full disease but may have some abnormal Hb; this trait is more common in populations where malaria is or was prevalent.
💡 NCERT Memory Line:
“Sickle cell anaemia is an autosomal recessive disorder due to mutation in beta-globin gene on chromosome 11.”
If the distance between two consecutive base pairs
is 0.34 nm and the total number of base pairs of a
DNA double helix in a typical mammalian cell is
6.6×109 bp, then the length of the DNA is
approximately:
