Replication of DNA - protein synthesis

What are the two stages of protein synthesis?

Transcription (where the strand of mRNA is formed) and translation (forming the protein)

What are the steps of protein synthesis?

Transcribing DNA to mRNA, processing the mRNA, translating the mRNA to make a protein

Do eukaryotes have continuous genes?

No as they often have sections of non-coding DNA (introns) which are copied when DNA is replicated, then removed from the pre-mRNA to leave functional mRNA (with only coding sections of DNA) as the coding sequence. The remaining exons make up the code for the polypeptide. Eukaryotic genes are referred to as discontinuous.

Do prokaryotes have continuous genes?

Yes as they do not have non-coding sections, so no post-transcriptional processing is necessary

How is the strand of mRNA synthesised?

DNA helicase binds to the DNA double helix, causing it to unwind as it breaks the hydrogen bonds between the bases and exposes the unpaired bases. Only one of the DNA strands is used as a template (the 3 prime to 5 prime section). RNA polymerase moves along the strand and binds free complimentary mRNA nucleotides onto the exposed DNA bases to form a molecule of mRNA (according to the complimentary base pairing). The mRNA leaves the template DNA when it reaches the ‘stop’ codon. Once the mRNA moves away, the DNA reforms its double helix. The mRNA strand is processed to remove introns (non-coding sections) from the code so the polypeptide can be produced

Describe genetic code

A linear, triplet, non-overlapping, degenerate, unambiguous, universal code for the production of polypeptides

What does linear mean?

Each molecule of DNA has a start and an end point (it is not circular)

What does triplet mean?

Codons make up DNA - there are three nucleotides per codon

What does non overlapping mean?

A base can only form one codon

What does degenerate mean?

Multiple codons can code for the same amino acid during protein synthesis, since there are 64 different codons and only 20 amino acids (3 of which are stop signal codons)

What does unambiguous mean?

Each codon specifies only one amino acid (although there is redundancy as multiple codons can code for the same amino acid, only one amino acid is coded for by a codon)

What does universal mean?

All species use the same four bases; adenine, guanine, cytosine, thymine, and each base sequence codes for the same amino acid in all species

What happens during translation?

tRNA carries an amino acid to a ribosome. The tRNA has anticodon bases to determine the specific amino acid attached to the tRNA, and it has anticodon bases amino acid attachment site which requires energy. The large subunit (with two attachment sites for tRNA) and small subunit (with only one attachment site for tRNA) of the ribosome come together to form this ribosome. mRNA attaches to the smaller subunit on the ribosome and two mRNA codons enter the information processing region of the ribosome. A tRNA binds to the first codon using its anticodon. A second tRNA anticodon complimentary to the second DNA codon attaches to the other attachment site. A peptide bond is formed between the two amino acids. The first tRNA molecule detaches, leaving its amino acid behind and the ribosome moves along the mRNA and a third tRNA binds onto the third codon. A second polypeptide bond forms and the process repeats until a stop codon is reached to form the polypeptide chain

What is a polysome?

A group of ribosomes moving along a strand of mRNA apart from each other to form multiple polypeptide chains

What is the one gene one polypeptide hypothesis?

The idea that each gene is responsible for a single type of polypeptide - it was modified from the original purely enzyme based hypothesis

How can polypeptides be further modified?

The addition of carbohydrates, lipids or phosphates, or the combination of multiple polypeptides to form one protein (like haemoglobin)