DNA and protein synthesis

The genome is the complete set of genes in a cell, while the proteome refers to the full range of proteins a cell can produce. The genetic code for these proteins is stored in DNA, a long molecule located in the nucleus of eukaryotic cells. The main features of the genetic code include:

• Each amino acid is coded by a sequence of three bases, called a codon.

• As discussed in the DNA, genes and chromosomes subtopic, the code is degenerate, non-overlapping and universal across almost all organisms.

• There are three stop codons that signal the end of the polypeptide chain.

Types of RNA

To use the DNA code for protein synthesis, the cell utilises ribonucleic acid (RNA), which comes in three main forms: mRNA (messenger RNA), tRNA (transfer RNA), and rRNA (ribosomal RNA).

1. mRNA:

   • A long strand that forms a single helix.

   • Complementary to a DNA segment (usually a gene) and acts as a template for polypeptide synthesis.

   • Leaves the nucleus through nuclear pores to associate with ribosomes.

   • mRNA can be easily broken down after use.

2. tRNA:

   • A single-stranded RNA folded into a cloverleaf shape.

   • One end of the tRNA molecule has a site for an amino acid to attach.

   • The opposite end has an anticodon, which pairs with the complementary codon on the mRNA strand.

   • Each tRNA type has a unique anticodon and binds to a specific amino acid.

3. rRNA:

   • Combines with ribosomal proteins to form ribosomes, the sites of protein synthesis.

Transcription: synthesising mRNA

Transcription is the process of producing pre-mRNA using DNA as a template. It occurs in the nucleus and involves the following steps:

1. RNA polymerase binds to a DNA region known as the promoter, near the start codon.

2. RNA polymerase unwinds and breaks hydrogen bonds between complementary DNA strands, exposing the unpaired bases.

3. Free RNA nucleotides in the nucleus bind with the exposed template strand of DNA.

4. RNA polymerase moves along the DNA, forming strong phosphodiester bonds between adjacent RNA nucleotides.

5. The DNA rewinds behind the RNA strand as RNA polymerase continues to elongate the RNA molecule.

6. When RNA polymerase reaches a stop codon, it detaches, and the synthesis of pre-mRNA is complete.

Within pre-mRNA, only certain sequences, called exons, code for polypeptides, while non-coding sequences, known as introns, do not. Introns are removed in a process called splicing, leaving only exons in the mature mRNA. This splicing allows exons to be recombined in various ways, enabling one gene to code for multiple proteins.

Translation: synthsising polypeptides

Translation is the process of producing a polypeptide chain based on the sequence of codons in mRNA. This process occurs in the cytoplasm on ribosomes, either free-floating or attached to the rough endoplasmic reticulum (ER). The steps are as follows:

1. The mRNA exits the nucleus through nuclear pores and attaches to a ribosome at the start codon.

2. The ribosome binds to the start codon on the mRNA, and the complementary anticodon of a tRNA molecule carrying the specific amino acid binds to it.

3. Another tRNA with a matching anticodon binds to the next codon on the mRNA, bringing the next amino acid.

4. Enzymes, with the help of ATP, link the amino acids on adjacent tRNA molecules, forming a peptide bond.

5. The ribosome moves along the mRNA strand, releasing the first tRNA and allowing a new tRNA to bind.

6. Translation continues until the ribosome reaches a stop codon. At this point, the ribosome, mRNA, and tRNA molecules detach, leaving a complete polypeptide chain.

Post-tranlational modification

A protein may consist of one or more polypeptide chains. After translation, these chains undergo post-translational modifications:

1. Folding: The polypeptide folds into a secondary structure such as an alpha-helix or beta-sheet.

2. Assembly: Different polypeptide chains, along with any non-protein groups (e.g., lipids or carbohydrates), combine to form a quaternary structure.

This post-translational modification determines the final shape and function of the protein.

Definitions