Himachal Pradesh PSC Botany Notes: Replication, Transcription and Translation

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Deoxyribonucleic Acid (DNA)

In very general terms, what does a chromosome contain?

  • Information, genetic information to carry out the characteristics of life precise self replication, ability to exchange energy with the environment, etc.

In very general terms, what are the two related functions of DNA?

  • Information storage
  • DNA replication
  • Information transfer
  • DNA transcribed into RNA
  • DNA's function in information transfer

What is the Central Dogma associated with information storage and retrieval?

  • Central Dogma: DNA → RNA → unfolded protein → native, folded protein

What are the three processes of the central dogma?

How does DNA function as an information molecule?

  • replication, DNA → DNA
  • transcription, DNA → RNA
  • translation, RNA → unfolded protein → folded protein

In terms of molecular conformation, what occurs through the central dogma?

  • Translation of linear information, a sequence of nucleotides, into 3 − D information, the structure of a protein.

What are the differences between DNA and RNA?

  • base composition: RNA = AGCU, DNA = AGCT
  • carbohydrate: RNA = ribose, DNA = deoxyribose
  • structure: RNA = single stranded, DNA = double helix

What is the major replication enzyme?

  • DNA polymerase III, a DNA-directed DNA polymerase
  • Synthesis is 5' to 3
  • Substrates are deoxynucleoside triphosphates (to make deoxyribonucleic acid)
  • Proof reading, errors removed by 3' to 5 exonuclease
  • Processivity is very high the ability of the enzyme to replicate a large tract of DNA before falling off
  • Replication requires DNA unwinding by enzymes termed helicases: These enzymes unwind the DNA helix before the replication fork and wind it up again afterwards.
  • There are large numbers of different enzymes and proteins involved at the replication fork in the replisome.
  • DNA damage by UV radiation or chemicals is repaired by other DNA polymerases. UV-damage results in adjacent T residues in one strand becoming covaletly linked to each other, producing a thymine dimer. This causes the double helix to become distorted--kinky. Xeroderma pigmentosa is a genetic disorder in which patients cannot carry out UV-radiation repair. They are very prone to skin cancer from an early age.

Ribonucleic Acid (RNA)

  • It is usually single stranded
  • It is linear polymer of ribonucleotides.
  • Some secondary and tertiary structure but often ill-defined.

What are the different types of RNA? What are the functions of the different types of RNA?

  • messenger RNA = mRNA, information transfer
  • transfer RNA = tRNA, information transfer
  • ribosomal RNA = rRNA, structural
  • small nuclear RNA = snRNA, ribozymes, RNA processing.

What is replication?

  • Transfer of genetic information from one generation to the next. DNA-directed DNA synthesis: Replication of the genome.

What is the structural basis for the precise duplication of the genome?

  • The Watson-Crick structure of DNA: The strands are complementary, the nucleotide sequence in one automatically specifies the other.
  • The enzyme, DNA polymerase III, is very accurate: It has proof reading capabilities.

Is replication conservative or semi-conservative? What does that mean? Is the parental genome of double stranded DNA fully conserved in the parental cell or is it split equally (semi-conserved) between two daughter cells?

  • Replication is semi-conservative.

What is the evidence for semi-conservative replication?

  • Classical experiments of Meselson and Stahl. Label DNA with * heavy isotope * N15 and allow replication in light N14: Distinguish heavy, light and hybrid DNA by centrifugation.
  • Results: After 1 generation, each genome contains a hybrid N15 − N14 DNA; after 2 generations, there are 2 hybrid and 2 light (N14 − N14) genomes.
  • Each strand of DNA serves as a template for the synthesis of its complement.
  • The strands separate and each is used as a template for the synthesis of a daughter strand.
  • The two new double helices each contain half the parental DNA.
  • This process produces a replication fork

Is replication uni-directional or bi-directional?

  • Bi-directional
  • Two replication forks proceeding from the origin.