4.5 - Nucleic Acids

Nucleic acids are long chain-like molecules that carry genetic information in a cell. They contain elements such as carbon, hydrogen, oxygen, nitrogen and phosphorus. They are made from small building blocks called nucleotides. Each nucleotide has three characteristic components - a pentose sugar (5-carbon), a nitrogenous base and a phosphate group. There are two main classes of nucleic acids which are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).

Figure 17 - DNA (left) is double-stranded and helical in structure. RNA (right) only has one strand. DNA controls all activities within a cell while RNA is involved in protein synthesis.

Figure 18 - If you zoom into a DNA strand, you will see that one of the strands is joined with another strand via the hydrogen bonds. More specifically, the hydrogen bonding happens between the nitrogenous bases. In DNA, adenine (A) pairs with thymine (T) via 2 hydrogen bonds while guanine (G) pairs with cytosine (C) via 3 hydrogen bonds. In RNA, thymine (T) is absent, replaced by uracil (U).

Similarities and Differences Between DNA and RNA

• Similarities

  • Both use bases A, G and C
  • Contain sugar-phosphate backbone

• Differences

  • DNA

    • Pentose sugar: Deoxyribose
    • Base composition: Adenine (A)Guanine (G) Cytosine (C)Thymine (T)
    • Number of strands: Double stranded (double helix)

  • RNA

    • Pentose sugar: Ribose
    • Base composition: Adenine (A)Guanine (G)Cytosine (C)Uracil (U)
    • Number of strands: Single stranded

Importance of Nucleic Acids in a Cell

Figure 19 - Nucleic acids contain all genetic information needed by the cell to survive. 1. The bases are paired as such where A pairs with T while C pairs with G. 2. When the strands unwind during transcription, the sequence in DNA is transcribed into mRNA codons. 3. The mRNA then leaves the nucleus, carrying the genetic code for protein synthesis. 4. When the mRNA arrives at the ribosomes, translation occurs. The codons are read which are translated into specific amino acids. For example, the codon GAU is translated into amino acid leucine.

Formation of Chromosomes from DNA and Proteins

Figure 20 - Chromosome is a thread-like DNA molecule that can be found inside the nucleus. Histones are the proteins that the DNA strands wrap around in order to fit inside the nucleus. All the DNA strands are packed so compactly into the chromosome structure. If you unwind the chromosome and line up the DNA from all cells in your body, it can reach up to about 6000 million miles in length!