Photosynthesis, chemosynthesis and protein biosynthesis
Lessons 9–10 · 2 lessons · A. G‘afurov, A. Abdukarimov, J. Tolipova, O. Ishankulov, M. Umaraliyeva, I. Abdurahmonova. Biology, Grade 10, 1st edition. “Sharq” publishing and printing joint-stock company, Tashkent, 2017
10
The cell – the hereditary unit of life
Textbook: pp. 47–49
GoalExplain template-synthesis reactions, the properties of the genetic code and the stages of protein biosynthesis.
New words
replication · reduplikatsiyatranscription · transkripsiyatranslation · translatsiyagenetic code · genetik kod
Explanation
The cell is the hereditary unit: hereditary information is stored in DNA as a sequence of nucleotides. Biopolymers are made on a template from this information by the principle of complementarity – template synthesis. In replication (S phase) DNA unwinds and each strand is a template on which DNA polymerase builds a new complementary strand; the result is two identical DNA molecules, each with one old and one new strand. A gene is a segment of DNA that determines the sequence of one polypeptide or RNA. In transcription RNA polymerase makes mRNA on a gene (A–U, T–A, G–C). In translation the mRNA is read on a ribosome: the tRNA anticodon is complementary to the codon and brings an amino acid, and peptide bonds form. The genetic code is the encoding of each amino acid by triplets (codons). Four nucleotides give 4³ = 64 triplets: 61 code for amino acids and 3 (UAA, UAG, UGA) are stop codons; AUG is the start codon (methionine). The code is triplet, one codon specifies one amino acid, one amino acid may have several codons, and the code is (almost) universal across organisms.
Worked examples
If the DNA template strand is TACGGT, the mRNA is AUGCCA; the tRNA anticodons UAC and GGU match the codons.
When the human insulin gene is placed into a bacterium, the bacterium reads it and makes the human protein: this is possible because the genetic code is universal.
Class activity
“Translators”. In pairs, one student writes a DNA strand and the other finds the mRNA and tRNA anticodons and identifies the amino acids from the code table.
Practice
1
State the difference between transcription and translation.
Transcription is the synthesis of mRNA on DNA (in the nucleus); translation is the synthesis of a protein on mRNA at the ribosome.
2
The template strand is TAC CTT TTT AAA CCA ATT. Write the mRNA and, using the table (AUG – Met, GAA – Glu, AAA – Lys, UUU – Phe, GGU – Gly, UAA – stop), find the amino acids.
mRNA: AUG GAA AAA UUU GGU UAA; chain: Met–Glu–Lys–Phe–Gly, then stop.
3
How many nucleotides does the mRNA region coding a protein of 150 amino acids have (without the stop codon)? (Number only.)
450
4
What does the universality of the genetic code mean and why is it important?
In almost all organisms the same codon means the same amino acid. This is evidence of common origin of life and allows genetic engineering to use one species’ gene in the cell of another.