2nd Year NotesBiology class 12th

2nd Year Biology Chapter 20 Chromosomes & DNA

2nd Year Biology Chapter 20 Chromosomes & DNA Question an Answer

Short and Simple Question And Answer

Q 1. Who first observed chromosomes?

Ans. The German embryologist Walther Fleming first observed them in 1882 while examining the rapidly dividing cells of salamander larvae.

Q 2. What is euchromatin?

Ans. Euchromatin is the portion of the chromosome, excluding heterochromatin, which is condensed only during cell division and allows gene expression at other times.

Q 3. What is transformation?

Ans. Transformation is the transfer of genetic material from one cell to another, altering the recipient cell’s genetic makeup.

Q 4. What is nuclein?

Ans. Nuclein is a substance extracted by Miescher from the nuclei of human cells and fish sperm, associated with the nucleus.

Q 5. What is semi-conservative replication?

Ans. Semi-conservative replication is a DNA replication method where the daughter DNAs have one original strand and one newly synthesized strand.

Q 6. How does DNA polymerase III initiate DNA synthesis?

Ans. DNA polymerase III initiates DNA synthesis after an RNA primer is constructed by another enzyme called primase, recognizing the primer and adding DNA nucleotides to it.

Q 7. What are Okazaki fragments?

Ans. Okazaki fragments are short segments that make up the lagging strand during DNA replication.

Q 8. What is the length of Okazaki fragments?

Ans. In eukaryotes, Okazaki fragments are about 100-200 nucleotides long, while in prokaryotes, they are 1000-2000 nucleotides long.

Q 9. What is “one gene / one polypeptide”?

Ans. “One gene / one polypeptide” refers to the idea that each gene encodes a single polypeptide or protein subunit.

Q 10. What is transcription?

Ans. Transcription is the process of producing an mRNA copy of a gene, which is the first step in the central dogma of molecular biology.

Q 11. What are stop codons and why are they called nonsense codons?

Ans. Stop codons (UAA, UAG, UGA) do not code for any amino acid and are called nonsense codons. They signal the end of protein synthesis.

Q 12. What is the initiation codon?

Ans. Every gene starts with the initiation codon AUG, which encodes the amino acid methionine.

Q 13. How long would the DNA from all cells of an adult human stretch?

Ans. If the DNA from all cells of an adult human were lined up, it would stretch nearly 100 billion kilometers, equivalent to 60 times the distance from Earth to Jupiter.

Q 14. What is a primer?

Ans. A primer is a short sequence of about 10 RNA nucleotides complementary to the DNA template, synthesized by the enzyme primase.

Q 15. What are the three major classes of RNA?

Ans. The three major classes of RNA are mRNA (messenger RNA), tRNA (transfer RNA), and rRNA (ribosomal RNA).

Q 16. What is the function of RNA polymerase in transcription?

Ans. RNA polymerase initiates transcription by binding to the promoter region at the beginning of a gene.

Q 17. What is a triplet code?

Ans. The genetic code for specifying amino acids is based on a triplet code, where three consecutive bases represent one amino acid.

Q 18. What is an anticodon?

Ans. An anticodon is a sequence of three nucleotides in tRNA that is complementary to the mRNA codon during protein synthesis.

Q 19. What are chromosomes?

Ans. Chromosomes are thread-like structures found in the cell nucleus, first observed by Walther Fleming in 1882 during cell division.

Q 20. How many chromosomes do Penicillium and some ferns have?

Ans. Penicillium, a fungus, has one pair of chromosomes, while some ferns may have more than 500 pairs.

Q 21. What is a typical chromosome made of?

Ans. A typical chromosome is made up of chromatids, a centromere, and a secondary constriction.

Q 22. What is a Karyotype?

Ans. A Karyotype is the specific arrangement of an individual’s chromosomes, which can vary in size, staining properties, centromere location, and more.

Q 23. What are the different types of chromosomes based on the centromere’s location?

Ans. Chromosomes can be classified as telocentric, acrocentric, submetacentric, or metacentric, depending on the centromere’s position.

Q 24. What are the common shapes of chromosomes?

Ans. Chromosomes often have shapes resembling “i,” “j,” or “v.”

Q 25. What is the composition of chromosomes?

Ans. Chromosomes are composed of DNA and protein, with about 40% DNA and 60% protein.

Q 26. How many nucleotides are in a typical human chromosome?

Ans. A typical human chromosome contains approximately 140 million nucleotides.

Q 27. How much information is in one chromosome?

Ans. The information in one chromosome would fill around 280 printed books of 1000 pages each, with each nucleotide representing a word and 500 words per page.

Q 28. What is the length of a straight DNA strand from a single chromosome?

Ans. If a DNA strand from a single chromosome were stretched in a straight line, it would be about 5 centimeters long.

Q 29. What are nucleosomes?

Ans. Nucleosomes are complexes where DNA duplexes coil around a core of eight histone proteins, occurring every 200 nucleotides.

Q 30. Why are histones positively charged?

Ans. Histones are positively charged due to an abundance of basic amino acids like arginine and lysine, which attract them to the negatively charged phosphate groups of DNA.

Q 31. What are supercoils of DNA?

Ans. Supercoils are higher-order coils formed when a string of nucleosomes wraps up further, guided by histone cores.

Q 32. What is heterochromatin?

Ans. Heterochromatin refers to highly condensed portions of chromatin where DNA is not exposed.

Q 33. Define the chromosome theory of inheritance.

Ans. The chromosome theory of inheritance states that genes are physical units located on chromosomes, with one member of a gene pair on one homologous chromosome and the other on the other homologous chromosome.

Q 34. What is a sex-linked trait?

Ans. A trait determined by a gene on the X chromosome is known as a sex-linked trait.

Q 35. Who repeated Griffith’s experiments?

Ans. Oswald Avery, along with Colin Macleod and Maclyn McCarty, repeated Griffith’s experiments in 1944, characterizing the Transforming principle.

Q 36. Why are Hershey and Chase famous?

Ans. In 1952, Alfred Hershey and Martha Chase conducted experiments with bacteriophages T2, providing evidence supporting Avery’s conclusions.

Q 37. What are the main components of DNA?

Ans. The main components of DNA are phosphate (PO4) groups, five-carbon sugars, and nitrogen-containing bases, including purines (adenine, A, and guanine, G) and pyrimidines (thymine, T, and cytosine, C). RNA contains uracil (U) instead of thymine (T).

Q 38. What is the structure of a typical nucleotide?

Ans. A typical nucleotide consists of a nitrogen base attached to carbon 1 of a pentose sugar, with a phosphate group attached to carbon 5 and a free hydroxyl group (-OH) at carbon 3.

Q 39. What is a phosphodiester bond or linkage?

Ans. A phosphodiester bond is the linkage between two groups in a polynucleotide chain, formed by two ester (P-O-C) bonds with phosphate groups connecting two sugars.

Q 40. What did Erwin Chargaff’s work reveal?

Ans. Erwin Chargaff’s work showed that the amount of adenine in DNA always equals the amount of thymine, and the amount of guanine always equals the amount of cytosine.

Q 41. What is X-ray diffraction?

Ans. X-ray diffraction involves bombarding a molecule with X-rays, resulting in a diffraction pattern when X-rays interact with atoms. This pattern can reveal the three-dimensional structure of a molecule.

Q 42. Who prepared the X-ray diffraction of DNA?

Ans. Rosalind Franklin prepared the X-ray diffraction pattern of DNA in Maurice Wilkins’ laboratory, where DNA fibers were prepared.

Q 43. What does X-ray diffraction of DNA suggest?

Ans. X-ray diffraction of DNA suggests that the DNA molecule has a helical structure with a diameter of 2 nanometers (nm) and a complete helical turn every 3.4 nm.

Q 44. Who proposed the double helical structure of DNA?

Ans. James Watson and Francis Crick proposed the double helical structure of DNA in 1953.

Q 45. Define replication?

Ans. Replication is the process by which DNA gives rise to a copy of itself.

Q 46. What is Semi-conservative replication of DNA?

Ans. In semi-conservative replication, the two DNA strands separate, and each acts as a template for the assembly of new nucleotides, resulting in two new duplexes.

Q 47. Define Conservative Replication of DNA?

Ans. Conservative replication suggests that the parental double helix remains intact and generates entirely new DNA copies.

Q 48. What is Dispersive Replication of DNA?

Ans. Dispersive replication suggests that parental DNA becomes dispersed throughout the new copies, with each strand in the daughter molecules consisting of a mixture of old and new DNA.

Q 49. What was the work of Meselson and Stahl?

Ans. Meselson and Stahl in 1958 evaluated three hypotheses of DNA replication using their famous experiment.

Q 50. What is the role of DNA polymerase I?

Ans. DNA polymerase I plays a supporting role in DNA replication and is a relatively small enzyme.

Q 51. What is the role of DNA polymerase II?

Ans. DNA polymerase II plays a role in DNA repair.

Q 52. What is the role of DNA polymerase III?

Ans. DNA polymerase III is a dimmer enzyme responsible for replicating one DNA strand during replication.

Q 53. What is the direction of replication on a growing DNA strand?

Ans. Replication proceeds in the 5′ to 3′ direction on a growing DNA strand, as DNA polymerase III can only add nucleotides to the 3′ end.

Q 54. What is the role of DNA ligase?

Ans. DNA ligase attaches the repaired fragments of the lagging strand during DNA replication.

Q 55. What is Alkaptonuria?

Ans. Alkaptonuria is a disorder in which patients excrete urine containing homogentisic acid, turning the urine black upon exposure to air. It results from the inability to break down homogentisic acid, a process that occurs in normal individuals.

Q 56. Why are Beadle and Tatum famous?

Ans. Beadle and Tatum’s experiments with Neurospora (a fungus) led to the conclusion that each gene encodes the structure of one enzyme, known as the “one gene, one enzyme” hypothesis.

Q 57. Why is Frederick Sanger famous?

Ans. Frederick Sanger’s 1953 work involved describing the complete sequence of amino acids in insulin, demonstrating for the first time that proteins have definable sequences of amino acids.

Q 58. Why is Vernon Ingram famous?

Ans. Vernon Ingram is famous for discovering the molecular basis of sickle cell anemia in 1956, showing that it results from a change from glutamic acid to valine at position 6 in one chain of hemoglobin.

Q 59. What is a Promotor?

Ans. A promotor is a binding site at the beginning of a gene where RNA polymerase initiates transcription.

Q 60. What is translation?

Ans. Translation is the process of converting the nucleotide sequence of mRNA into an amino acid sequence in a polypeptide.

Q 61. What is a codon?

Ans. A codon is a sequence of three nucleotides on an mRNA strand that codes for a specific amino acid.

Q 62. What are the template and coding strands of DNA?

Ans. The template strand, also known as the antisense strand, is the DNA strand used for transcription. The coding strand, or sense strand, is the opposite strand.

Q 63. What is the role of RNA polymerase?

Ans. RNA polymerase is responsible for synthesizing RNA from DNA in the 5′ to 3′ direction during transcription.

Q 64. What is a core enzyme?

Ans. The core enzyme is RNA polymerase without its sigma subunit.

Q 65. How many binding sites are in the promoter of prokaryotes and eukaryotes?

Ans. In prokaryotes, the promoter has two binding sites, TTGACA and TATAAT, located at -35 and -10 positions. In eukaryotes, these sites are at -25 and -70 positions.

Q 66. What is the stop signal for transcription?

Ans. The stop signal for transcription is a series of GC base pairs followed by a series of AT base pairs, forming a GC hairpin followed by four or more U ribonucleotides, causing RNA polymerase to stop.

Q 67. Why are a cap and a tail added to mRNA?

Ans. A cap and a tail are added to mRNA to ensure stability during its journey to the ribosome.

Q 68. What is the genetic code?

Ans. The genetic code is a combination of three nucleotides in DNA that specifies a particular amino acid.

Q 69. What are chromosomal aberrations?

Ans. Chromosomal aberrations are changes in the number or structure of chromosomes.

Q 70. What are Point mutations?

Ans. Point mutations are mutational changes that affect the DNA sequence, typically involving a small number of base pairs.

Q 71. Can you provide examples of point mutations?

Ans. Examples of point mutations include Sickle cell anemia and Phenylketonuria.

Q 72. What is Sickle cell anemia?

Ans. Sickle cell anemia results from a point mutation that changes glutamic acid to valine at position 6 in the hemoglobin B chain, reducing its ability to carry oxygen.

Q 73. What is Phenylketonuria?

Ans. Phenylketonuria is a genetic disorder caused by a defective enzyme, phenylalanine hydroxylase, leading to the accumulation of phenylalanine in cells and resulting in mental retardation in infancy.

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