Topic 4: Genetic Information: Protein synthesis

Triplet code

Every three bases in a strand of DNA codes for one amino acid. Coding = gene → non-coding = control gene → coding = gene ( these are between genes and control whether genes are expressed or not)

Transcription

Initial step in the synthesis of proteins. In eukaryotic cells, transcription takes place within the nucleus. mRNA then exits the nucleus and enters the cytoplasm where translation occurs

Process of transcription

1. Transcription begins with RNA polymerase enzyme binding to DNA

2. The hydrogen bonds between the DNA bases break, and the two strands of the double helix separate

3. The antisense strand acts as the template for mRNA synthesis

4. Free RNA nucleotides align with the DNA template through complementary base pairing

5. In the RNA molecule , uracil pairs with adenine, adenine with thymine and cytosine with guanine.

6. RNA polymerase catalyses the formation of phosphodiester bonds between adjacent RNA nucleotides

7. As complementary mRNA strand is formed, carrying the same base sequence as the DNA sense strand.

8. The process ends when RNA polymerase reaches a stop codon, detaches from DNA and terminates transcription

9. mRNA is released, detaches from DNA, and DNA rewinds into its double helix structure

Introns, exons and splicing

1. Both introns and exons of DNA are transcribed into initial mRNA transcript (pre- mRNA)

2. Pre-mRNA undergoes RNA splicing, introns removed and exons joined together

3. Produces mature mRNA consisting of only exon sequences

4. This mature mRNA then travels out of the nucleus for translation

In prokaryotes , mRNA is directly produced from transcription without any splicing

In eukaryotes, pre-mRNA is spliced together after transcription, before it is transported to the cytoplasm

Pre- mRNA

Is made from two different sections; introns and exons

Introns

Sections that do not code for proteins

Exons

The parts that do code for proteins. Exons are spliced to make mRNA

Translation

Is the process of decoding the information in messenger RNA ( mRNA) to synthesise a polypeptide chain, with the help of transfer RNA (tRNA). This chain then folds into a functional protein.

Process of translation

1. Ribosome attaches to the mRNA strand at start codon (AUG)

2. The tRNA molecule, carrying a specific amino acid and with an anti-codon (UAC) that is complementary to the start codon, binds to the tRNA

3. A second tRNA molecule with an anti-codon complementary to the next mRNA codon, and also carrying a specific amino acid, attaches to mRNA.

4. The amino acid carried by the first two tRNA molecules are linked together via a peptide bond using ATP.

5. The first tRNA molecule detaches from mRNA and is free to collect amino acid for future use.

6. The ribosome moves along mRNA, allowing another tRNA molecule, which carries the next amino acid, to bind to the next codon on mRNA

7. The process from step 4 to 6 is repeated, which elongates the polypeptide chain.

8. At any point during this process, two tRNA molecules can be attached to the ribosome

9. The sequence continues until the ribosome reaches a stop codon on mRNA

10. The completed polypeptide chain detaches from the ribosome

A random change or alteration in a DNA base sequence is a mutation

They are most frequent during DNA replication

Carcinogens such as UV, arsenic and alcohol can affect the incidence of mutations

e.g. Cystic fibrosis, polydactyl and sickle cell anemia

Substitution

Where one base is swapped for another

Deletion

Where a base is removed

Addition

Where a base is added

These mutations could be silent (degenerate) or minor or major (as it could change the R group which affects a bond)

A mutation that causes a stop codon would shorten the protein and may cause it not to form at all

Frame shift

A deletion or addition mutation would cause a domino effect. Which means each triplet base after this would be different.