HL 1 Biology

3.3 & 7.1 DNA Structure

<<Nucleotides and Nucleic Acids>>

Nucleotides: molecules that consist of three parts = sugar, phosphate and nitrogenous base

  • DNA: polymer of nucleotides
  • 2 types of Nitrogen bases in DNA:
      • Purines: adenine and guanine
      • Pyrimidines: cytosine and thymine
      • **Uracil- pyrimidine in RNA

Nucleic Acid: polymer of nucleotides

  • in polymer: a covalent bond links the phosphate of one nucleotide to the sugar of the next nucleotide
  • e.g. ribonucleic acid/RNA, deoxyribonucleic acid/DNA
  • Sugar
      • in DNA: deoxyribose
      • in RNA: ribose

<<DNA Structure>>

DNA strands are complementary (one strand’s sequence determines the sequence of the other strand). This enables DNA to self replicate and guide to produce RNA

  • Nucleotides within each strand joined by covalent bonds
  • Backbone for each strand formed by alternating sugar and phosphate molecules
  • Hydrogen bonds between the nitrogenous bases hold the two strands together
  • two strands arranged in anti-parallel fashion
  • 4 Nitrogeous bases: adenine (A), thymine (T), guanine (G), cytosine (C)
      • Always paired up as: Adenine + Thymine, Guanine + Cytosine

<<Discovery of DNA Structure>>

  • Double helix structure: Composed of 2 polymers of nucleotides (coiled around each other)
  • One strand oriented in direction 5′ to 3′, and the other  3′ to 5′ = run opposite directions; antiparallel
  • Pyrimidine always opposite purine
  • 3 hydrogen bonds between G + C, 2 bonds between A + T

*Double helix: 2 polymers coiled around the other in a structure

  • X-rays crystallograph shows that DNA is a double helix
  • produced by Rosalind Franklin
  • when x-rays pass through a strand of DNA = bend/diffract

<<The Nucleosome>>

Histones: most common proteins

  • help to super coil the DNA to package chromatin
  • help to regulate the expression of genes

Nucleosome: group of 8 histone proteins arranged to form octomer

**DNA naked in bacteria, DNA with protein in eukaryote.

======================DBQ ============================

Chargaff’s Data (p.58)

(1) The Mycobacterium has more guanine and cytosine yet less adenine and thymine compared to the base composition of eukaryotes. In both the Mycobacterium and eukaryotes, the amount of adenine and thymine are equal.

(2) A + G/T + C  For humans: 31.0 + 19.1/ 31.5 + 18.4 ≈ 1.00   For Mycobacterium tuberculosis: 15.1 + 34.9/ 14.6 + 35.4 ≈ 1.00

(3) Because of the experimental errors and uncertainties, the amount of adenine and thymine are not exactly equal. (Same goes for the guanine and cytosine). The uncertainties indicate the slight alternation is the preciseness of the measurements.

(4) The ratio between the amount of bases should be shown in an equal amount between the A and T as well as the C and G due to the complementary base pairing. Each of these base pairs are always seen to be together, with the help of hydrogen bond between the two bases.

(5) The difference in the base composition of bacteriophage T2 and the polio virus is apparent due to the reasons the bacteriophage T2 may be composed of double strands whereas the polio virus may be single stranded. The Bacteriophage T2 is presented to be greater in adenine and thymine compared to the Polio virus.

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One thought on “3.3 & 7.1 DNA Structure

  1. Grade 6 A consistent and thorough understanding of the required knowledge and skills, and the ability to apply them in a wide variety of situations.

    DBQ p. 58, Chargaff’s Data
    1. The quantities of the 4 bases are similar across all of the eukaryotes:
    • The relative quantities of bases in Mycobacterium are distinct from the eukaryotes
    • Mycobacterium has less adenine and thymine, but more guanine and cytosine
    • The amount of adenine approximately equals the amount of thymine in both
    • The amount of guanine is approximately equal to the amount of cytosine in both

    2. 1.00 in both cases

    3. Within experimental error, the data supports the hypothesis.

    4. Complementary base pairing between A and T would mean that they would need to be present in equal quantities; same argument for C and G

    5. • Polio virus is a single-stranded RNA virus
    • Bacteriophage T2 is a double-stranded DNA virus

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