☰ Contents · Biology (Cytology and genetics)

Protein functions, nucleic acids and the amylase experiment

Lessons 27–29 · 3 lessons · A. Zikiryayev, A. To‘xtayev, I. Azimov, N. Sonin. Biology. Basics of Cytology and Genetics, Grade 9, revised and expanded 5th edition. “Yangiyul Poligraph Service”, Tashkent, 2019
28

Nucleic acids

Textbook: pp. 72–75
GoalKnow the structure of DNA, RNA and ATP, the rule of complementarity and the importance of nucleic acids.
New words
nucleotide · nukleotidcomplementarity · komplementarlikDNA double helix · DNK qo‘sh spiraliATP · ATF
Explanation

Nucleic acids were isolated from the nuclei of white blood cells in 1869 by the Swiss physician F. Miescher. DNA (deoxyribonucleic acid) is found in the nucleus and also in mitochondria and plastids; RNA is in the nucleus, cytoplasm, ribosomes and organelles. Both are polymers of nucleotides; a nucleotide = a nitrogenous base + a five-carbon sugar + a phosphate residue. In DNA the sugar is deoxyribose and the bases are A (adenine), T (thymine), G (guanine), C (cytosine); it is a two-stranded double helix (described in 1953 by J. Watson and F. Crick, using X-ray data of R. Franklin and M. Wilkins). The strands are held by the rule of complementarity: A always pairs with T (2 hydrogen bonds) and G always with C (3 hydrogen bonds); therefore A = T and G = C. RNA is single-stranded, its sugar is ribose and it has uracil (U) instead of thymine; it has three kinds: messenger (mRNA), transfer (tRNA) and ribosomal (rRNA). ATP consists of adenine, ribose and three phosphate residues; when a phosphate is split off, energy (about 40 kJ/mol) is released, which is why ATP is the cell’s universal energy currency. DNA stores and transmits hereditary information.

Worked examples
If one DNA strand reads A–T–G–C–C, the complementary strand is T–A–C–G–G.
If adenine is 20 % of DNA, thymine is also 20 %; the remaining 100 − 20 − 20 = 60 % is shared equally by G and C: 30 % each.
Class activity

“Complementary pairs”. Cards carry the letters A, T, G, C. One student “reads” a DNA strand and a partner builds the complementary strand; they count the hydrogen bonds (2 for A–T, 3 for G–C).

Practice
1
What is a DNA nucleotide made of?
2
If one DNA strand is ATTGCA, write the complementary strand.
3
If guanine is 30 % of DNA, what percent is adenine? (G = C = 30 %, the rest A and T)
4
State two differences between DNA and RNA. Why is ATP called the “energy currency”?