DNA Structure and Double Helix
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Questions and Answers

What is the most common form of DNA?

  • A form
  • m form
  • B form (correct)
  • Z form
  • Histone proteins usually have a high proportion of which type of amino acids?

  • Positively-charged (correct)
  • Aromatic
  • Negatively-charged
  • Hydrophobic
  • Which of the following correctly describes a nucleotide?

  • Nit.Base + sugar + phosphate (correct)
  • Purine base + phosphate
  • Nitrogenous base
  • Nit.Base + sugar
  • What type of sugar molecule is present in a nucleotide?

    <p>Pentose</p> Signup and view all the answers

    Which purine base is found in RNA?

    <p>Guanine</p> Signup and view all the answers

    What is the primary structure of DNA characterized by?

    <p>Two strands of polynucleotides with alternating sugar-phosphate backbone</p> Signup and view all the answers

    Which base pairs with adenine in the process of base pairing?

    <p>Thymine</p> Signup and view all the answers

    In the Watson-Crick model, how are the two DNA strands oriented?

    <p>Anti-parallel to each other</p> Signup and view all the answers

    What helps stabilize the DNA double helix structure?

    <p>Hydrogen bonds between purine and pyrimidine bases</p> Signup and view all the answers

    Which of the following correctly describes the DNA's physical dimensions?

    <p>2 nm wide with 10.4 base pairs per turn</p> Signup and view all the answers

    Which component directly carries genetic information in DNA?

    <p>Nitrogenous bases</p> Signup and view all the answers

    Where is the majority of human DNA located?

    <p>In the nucleus</p> Signup and view all the answers

    What is the function of the major and minor grooves in DNA?

    <p>They serve as binding sites for regulatory proteins</p> Signup and view all the answers

    What is the primary charge of histones that allows them to stabilize DNA?

    <p>Positively charged due to high content of positively charged amino acids</p> Signup and view all the answers

    What is the diameter of a nucleosome?

    <p>10 nm</p> Signup and view all the answers

    How many pairs of chromosomes are found in a diploid somatic cell?

    <p>23 pairs</p> Signup and view all the answers

    What structure forms when 10-nm chromatin fibrils coil into a higher order arrangement?

    <p>30-nm fiber</p> Signup and view all the answers

    What connects neighboring nucleosomes within a chromatin structure?

    <p>Linker DNA</p> Signup and view all the answers

    What is the combined length of DNA contained within the human chromosomes?

    <p>Approximately 1.74 meters</p> Signup and view all the answers

    Which type of cell contains a haploid number of chromosomes?

    <p>Germ cell</p> Signup and view all the answers

    What is the role of Histone H1 in chromatin structure?

    <p>To bind to the linker DNA between nucleosomes</p> Signup and view all the answers

    Study Notes

    DNA Structure

    • Deoxyribonucleic acid (DNA) is the primary storage and expression mechanism for genetic information.
    • Function: DNA directs its own replication during cell division and the synthesis of RNA (Transcription).
    • Location: Human cells primarily contain DNA in the nucleus, with a small amount found in the mitochondria.
    • Structure: DNA is a polymer of deoxyribonucleotides linked by 3'→5' phosphodiester bonds.
    • Primary structure: DNA is composed of 2 strands (chains) of polynucleotides.
      • Each strand contains an alternating sugar-phosphate backbone with nitrogenous bases projecting inwards.
      • The sequence of bases determines the coding information of DNA.
      • Polarity: Each strand has a 5' end with a free phosphate group and a 3' end with a free hydroxyl (OH) group.

    DNA Double Helix

    • Secondary structure: Commonly known as the Watson-Crick structure (B form - most common).
    • Configuration: Two DNA strands pair and wind to form a right-handed double helix.
    • Antiparallel: The two strands run in opposite directions: one from 5' to 3' and the other from 3' to 5'.
    • Base pairing: Nitrogenous bases are located in the center of the double helix, following base-pairing rules:
      • Adenine (A) pairs with Thymine (T) (A=T) - two hydrogen bonds
      • Guanine (G) pairs with Cytosine (C) (GΞC) - three hydrogen bonds
    • Dimensions: The helix has 10.4 base pairs per turn and is 2nm wide.
    • Grooves: The spatial relationship between the strands creates major and minor grooves.
      • These grooves are of unequal size and serve as binding sites for regulatory proteins.

    DNA Packing

    • Human DNA from 23 pairs of chromosomes (22 autosomes + 2 sex chromosomes) has a total length of approximately 1.74 meters.
    • It must be packed into a nucleus with a diameter of 5-10 μm.
    • Chromatin network: All chromosomes within a nucleus form this network, a complex of:
      • Double-stranded DNA molecule
      • Histone proteins

    Histones

    • Five main classes: H1, H2A, H2B, H3, H4
    • Charge: Histones are positively charged due to their high content of positively charged amino acids (Arginine and Lysine).
    • DNA interaction: Histones form ionic bonds with the negatively charged DNA (due to phosphate groups) to stabilize the chromatin structure.

    Levels of Eukaryotic DNA Organization

    • Nucleosomes: The simplest unit of DNA-protein interaction.

      • A segment of DNA (approximately 146 bp) wraps around a histone octamer (core histones: 2 molecules each of H2A, H2B, H3, and H4) to form a nucleosome (10-nm diameter & 5-nm height).
      • Neighboring nucleosomes are joined by "linker" DNA (about 50 bp).
      • Histone H1 binds to the linker DNA chain between the nucleosome beads.
    • 10-nm fibril: Consists of nucleosomes arranged in a "Beads-on-string" shape.

    • 30-nm fiber: The 10-nm fibril is further supercoiled with 6-7 nucleosomes per turn to form the 30-nm chromatin fiber.

    • 700-nm chromatid: The 30-nm fibril is coiled into loops:

      • Looped structures are wrapped to form rosettes (each containing 6 loops).
      • Rosettes are arranged into coils (30 rosettes per coil).
      • 10 coils form a chromatid.
    • 1400-nm chromosome: Each chromosome is composed of two identical sister chromatids connected at a centromere.

      • The end of each chromatid is marked by a tandem repeat of the 5-TTAGGG-3 sequence (many kilobases long) called a telomere.
      • Centromere: AT-rich region. Its position determines the "arms of the chromosome":
        • "p": shorter arm
        • "q": longer arm

    Chromosome Number

    • Somatic cells: Diploid. Contain two copies of each chromosome (23 pairs).
    • Germ cells (gametes): Haploid. Contain only one copy of each chromosome (23 total; 22 autosomes + 1 sex chromosome).

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    Description

    This quiz explores the fundamental concepts of DNA, including its structure, function, and the mechanisms of replication and transcription. It covers the primary and secondary structures of DNA, detailing the features of the double helix model. Dive into the essential aspects that govern genetic information storage and expression.

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