Biology Chapter 4 Quiz

General Instructions

• The exam is comprised of 16 questions.
• All questions are compulsory.
• There are 10 objective, 2 very short answer, 2 short answer and 2 long answer questions.
• Each objective and very short answer question is worth 2 marks.
• Each short answer question is worth 3 marks.
• Each long answer question is worth 5 marks.
• There is no negative marking for incorrect answers.

Section A: Objective Questions

• The total number of chromosomes in a normal human cell is 46 (23 pairs).
• Failure of segregation of chromatids during cell division cycle results in the gain or loss of chromosomes called aneuploidy.
• Sickle cell anaemia is caused by the substitution of glutamic acid by valine at the sixth position of the beta globin chain of the haemoglobin molecule.
• Thalassemia is an autosome linked recessive blood disease.
• Down syndrome was discovered by Langdon Down.
• Turner’s Syndrome is caused by the absence of one of the X chromosomes, resulting in a 45 XO configuration.
• DNA was first identified as an acidic substance present in the nucleus by Friedrich Miescher.
• The haploid content of human DNA is 3.3×109 base pairs.
• The sugar present in DNA is deoxyribose.
• Adenine pairs with thymine, and guanine pairs with cytosine.

Section B: Very Short Answer Questions

• Mutation is a permanent change in the DNA sequence.
• Central Dogma describes the flow of genetic information from DNA to RNA to protein.

Section C: Short Answer Questions

• Dominance: The expression of one allele masks the expression of another allele.
• Recessive: An allele whose expression is masked by a dominant allele.
• Homozygous: An individual with two identical alleles for a particular trait.
• Heterozygous: An individual with two different alleles for a particular trait.
• Monohybrid cross: A cross between two individuals that differ in only one trait.
• Dihybrid cross: A cross between two individuals that differ in two traits.
• Co-dominance: The expression of both alleles in a heterozygous individual.
  • Example: AB blood group in humans.
• Incomplete dominance: Neither allele completely masks the other, resulting in a blended phenotype.
  • Example: Pink flower color in snapdragons.

Section D: Long Answer Questions

• Law of Dominance: In a monohybrid cross, one allele (dominant) masks the expression of another allele (recessive).

  • This is demonstrated using a cross between two heterozygous individuals, resulting in a 3:1 phenotypic ratio.

• Double-helix structure of DNA:

  • DNA is a double-stranded helix with two antiparallel strands.
  • The two strands are held together by hydrogen bonds between complementary base pairs: adenine with thymine, and guanine with cytosine.
  • The sugar-phosphate backbone is on the outside of the helix, while the bases are on the inside.
  • The DNA helix is right-handed, meaning it twists in a clockwise direction.
  • The diameter of the DNA helix is about 2 nanometers.