D1.2 Protein Synthesis Flashcards
Define transcription
Synthesis of RNA using DNA as a template
Define translation
Synthesis of polypeptides from mRNA
Explain the process of transcription
- RNA polymerase binds to a site at the start of the DNA.
- The enzyme moves along the gene, which unwinds the DNA into two single strands.
- The strand that is read off is the antisense strand and the one that is identical to the RNA product is called the sense strand.
- RNA polymerase pairs the RNA nucleotides with its complementary bases. U binds with A.
- Hydrogen bonding between complementary base pairs occurs.
- RNA polymerase forms covalent bonds between RNA molecules.
- RNA separates from DNA and the double helix reforms.
- RNA molecule is released and leaves the nucleus through the nuclear pore.
What is the role of hydrogen bonding and complementary base pairing?
- The RNA will have a complementary base sequence to the DNA strand and will bind to the DNA using hydrogen bonds.
- A binds with U instead of T in RNA. G binds with C.
- Antisense strand: the template strand which has a complementary base sequence to the sense strand.
- Sense strand: DNA strand with the base sequence to be copied into RNA.
- Results in a strand of RNA with the same base sequence as the DNA strand.
What causes the stability of DNA templates?
- Ensured by hydrogen bonding and sugar-phosphate backbone of the DNA molecule.
- For somatic cells that don’t divide, the code must remain unchanged.
Why is transcription required for the expression of genes?
Not all genes in a cell are expressed at any given time, transcription determines which expression of a gene can be turned on and off.
What is the role of mRNA, tRNA, and ribosomes in translation?
- mRNA has a binding site for ribosomes:
– has a sequence of codons that specifies AA sequence of the polypeptide.
– can be translated many times - tRNA molecules translate base sequence of mRNA to an AA sequence.
– carries anticodons that bind to a codon via complementary base pairing. - Ribosomes act as the binding site for the tRNA to bind to:
– composed of large & small subunits of long strands of RNA.
– has a catalytic site to assemble the polypeptide.
Complementary base pairing between tRNA and mRNA
- Codons of three bases on mRNA correspond to one AA in a polypeptide.
- An anticodon is a sequence of three tRNA bases that are complementary to a codon.
- When the anticodon binds with the codon through complementary base pairing, the correct sequence of AA is synthesised into the polypeptide.
What are the features of the genetic code?
- Universal (all organisms use the same 4 letter code).
- Degeneracy of the genetic code: different combinations of codons can result in the expression of the same AA.
Define gene expression
Gene expression: the process by which info is carried by a gene is turned into an observable effect on an organism.
Explain the process of translation
- Translation is the synthesis of polypeptides from mRNA.
- mRNA leaves the nucleus through the nuclear pore into the cytoplasm.
- An activating enzyme binds to the tRNA and attaches a specific AA corresponding to its anticodon.
- That tRNA binds to the A site on the ribosome, with its anticodon linked by complementary base pairing to the next codon on the mRNA.
- The AA on the tRNA is linked to the end of a growing polypeptide by the formation of a peptide bond.
- tRNA now holds the whole of the growing polypeptide.
- tRNA moves to the P site as the ribosome moves along the mRNA by one codon. The anticodon of the tRNA is still paired with the codon of the mRNA.
- The polypeptide held by the tRNA is transferred to another tRNA that has arrived at the A site.
- tRNA moves to the E site as ribosome moves along by one codon. This causes separation between codon and anticodon, as well as, tRNA and ribosome.
- Cycle continues with an AA linked to a tRNA.
Outline an example of a mutation.
Sickle cell anemia: a mutation in the haemoglobin resulting in sickle cell anemia.
