Summary list D1.2 Protein synthesis
* Theme D - Continuity and change
* Level: Molecules
* Guide time: SL: 3 hours AHL: 3 hours
Guiding questions
How does a cell produce a sequence of amino acids from a sequence of DNA bases?
How is the reliability of protein synthesis ensured?
SL and HL
D1.2.1 - Transcription is the synthesis of RNA using a DNA template
The roles of RNA polymerase in this process, untwisting and separating the DNA strand, building a complementary RNA strand.
D1.2.2 - Hydrogen bonding and complementary base pairing in transcription, G-C and T-A
Include the pairing of adenine (A) on the DNA template strand with uracil (U) on the RNA strand.
D1.2.3 - DNA is a stable molecule. Single DNA strands can be used as a template for (RNA polymerase) transcribing a base sequence, without the DNA base sequence changing. In somatic (body) cells that do not divide, sequences must be conserved throughout the life of a cell.
D1.2.4 - Transcription is a process required for the expression of genes.
Not all genes in a cell are expressed at any given time.
Transcription, the first stage of gene expression, is a key stage where expression of a gene can be switched on and off.
D1.2.5 - Translation is the synthesis of polypeptides from mRNA
The base sequence of mRNA is translated into the amino acid sequence of a polypeptide.
D1.2.6 - Roles of mRNA, ribosomes and tRNA in translation
mRNA binds to the small subunit of the ribosome and that two tRNAs can bind simultaneously to the large subunit.
D1.2.7 - Complementary base pairing occurs between tRNA anticodon and mRNA codon
D1.2.8 - The genetic code is degenerate (sometimes there is more than one codon for an amino acid) and universal (all species use the same codons for the same amino acid)
Understand the reasons for a triplet code (there are 20 amino acids - there are not enough combinations of two of the four RNA bases (e.g. AU, CC, GA,) for all 20 amino acids - so three is ideal)
D1.2.9 - Use the genetic code expressed as a table of mRNA codons to deduce the sequence of amino acids coded by an mRNA strand.
D1.2.10 - Stepwise movement of the ribosome along mRNA and linkage of amino acids by peptide bonding to the growing polypeptide chain. Focus on elongation of the polypeptide, rather than on initiation and termination.
D1.2.11 - Mutations that change protein structure.
Include an example of a point mutation affecting protein structure.
Additional higher level
D1.2.12 - Directionality of transcription and translation
Understand what is meant by 5' to 3' transcription (by RNA polymerase) and 5' to 3' translation (in the ribosome).
D1.2.13 - Initiation of transcription is when transcription factors bind to the promoter (as an example)
Names of transcription factors not required.
D1.2.14 - Non-coding sequences in DNA do not code for polypeptides
Examples are: regulators of gene expression, introns, telomeres and genes for rRNAs and tRNAs in eukaryotes.
D1.2.15 - Post-transcriptional modification (of mRNA) in eukaryotic cells includes:
* removal of introns and splicing together of exons to form mature mRNA
* the addition of 5' caps and 3' polyA tails to stabilize mRNA transcripts.
D1.2.16 - Alternative splicing together different combinations of exons can produce variants of a protein from a single gene
It allows one gene to code for different polypeptides. (Specific examples are not required.)
D1.2.17- Initiation of translation is;
* the attachment of the small ribosome subunit to the 5' terminal of mRNA,
* movement to the start codon,
* the attachment of initiator tRNA and another tRNA
* attachment of the large subunit.
The roles of the three binding sites for tRNA on the ribosome during elongation (A - acceptor site, P peptidyl site and E - exit site).
D1.2.18 - Modification of polypeptides into their functional state
Many polypeptides must be modified before they can function. E.g. the two-stage modification of pre-proinsulin to insulin.
D1.2.19 - Recycling of amino acids by proteasomes to sustain a functional proteome requires constant protein breakdown
and synthesis.
Linking questions
How does the diversity of proteins produced contribute to the functioning of a cell?
What biological processes depend on hydrogen bonding?